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158-411: The respiratory system (also respiratory apparatus , ventilatory system ) is a biological system consisting of specific organs and structures used for gas exchange in animals and plants . The anatomy and physiology that make this happen varies greatly, depending on the size of the organism, the environment in which it lives and its evolutionary history. In land animals , the respiratory surface

316-454: A diving chamber or decompression chamber . However, as one rises above sea level the density of the air decreases exponentially (see Fig. 14), halving approximately with every 5500 m rise in altitude . Since the composition of the atmospheric air is almost constant below 80 km, as a result of the continuous mixing effect of the weather, the concentration of oxygen in the air (mmols O 2 per liter of ambient air) decreases at

474-427: A fibrinolytic system that dissolves clots that may have arrived in the pulmonary circulation by embolism , often from the deep veins in the legs. They also release a variety of substances that enter the systemic arterial blood, and they remove other substances from the systemic venous blood that reach them via the pulmonary artery. Some prostaglandins are removed from the circulation, while others are synthesized in

632-403: A partial pressure of carbon dioxide of 5.3 kPa (40 mmHg) (i.e. the same as the oxygen and carbon dioxide gas tensions as in the alveoli). As mentioned in the section above , the corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively. This marked difference between

790-434: A respiratory acidosis , or a respiratory alkalosis will occur. In the long run these can be compensated by renal adjustments to the H and HCO 3 concentrations in the plasma ; but since this takes time, the hyperventilation syndrome can, for instance, occur when agitation or anxiety cause a person to breathe fast and deeply thus causing a distressing respiratory alkalosis through the blowing off of too much CO 2 from

948-620: A cell are determined by whether the cell is a eukaryote or prokaryote . Gas exchange Gas exchange is the physical process by which gases move passively by diffusion across a surface. For example, this surface might be the air/water interface of a water body, the surface of a gas bubble in a liquid, a gas-permeable membrane , or a biological membrane that forms the boundary between an organism and its extracellular environment. Gases are constantly consumed and produced by cellular and metabolic reactions in most living things, so an efficient system for gas exchange between, ultimately,

1106-416: A diffusion rate in air 10,000 times greater than in water. The use of sac-like lungs to remove oxygen from water would therefore not be efficient enough to sustain life. Rather than using lungs, gaseous exchange takes place across the surface of highly vascularized gills . Gills are specialised organs containing filaments , which further divide into lamellae . The lamellae contain capillaries that provide

1264-544: A gas exchange dilemma: gaining enough CO 2 without losing too much water. Therefore, water loss from other parts of the leaf is minimised by the waxy cuticle on the leaf's epidermis . The size of a stoma is regulated by the opening and closing of its two guard cells : the turgidity of these cells determines the state of the stomatal opening, and this itself is regulated by water stress. Plants showing crassulacean acid metabolism are drought-tolerant xerophytes and perform almost all their gas-exchange at night, because it

1422-519: A gas exchange surface without the need for a specialised gas exchange organ. Flatworms therefore lack gills or lungs, and also lack a circulatory system. Other multicellular organisms such as sponges (Porifera) have an inherently high surface area, because they are very porous and/or branched. Sponges do not require a circulatory system or specialised gas exchange organs, because their feeding strategy involves one-way pumping of water through their porous bodies using flagellated collar cells . Each cell of

1580-457: A given time will be in rough proportion to the volume of its cytoplasm . The volume of a unicellular organism is very small; thus, it produces (and requires) a relatively small amount of gas in a given time. In comparison to this small volume, the surface area of its cell membrane is very large, and adequate for its gas-exchange needs without further modification. However, as an organism increases in size, its surface area and volume do not scale in

1738-422: A large surface area and short diffusion distances, as their walls are extremely thin. Gill rakers are found within the exchange system in order to filter out food, and keep the gills clean. Gills use a countercurrent flow system that increases the efficiency of oxygen-uptake (and waste gas loss). Oxygenated water is drawn in through the mouth and passes over the gills in one direction while blood flows through

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1896-415: A lesser extent, the oxygen tension rises: the rate and depth of breathing are reduced until blood gas normality is restored. Since the blood arriving in the alveolar capillaries has a P O 2 {\displaystyle P_{{\mathrm {O} }_{2}}} of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13 kPa (100 mmHg), there will be

2054-407: A much more even distribution of blood flow to the lungs than occurs at sea level. At sea level, the pulmonary arterial pressure is very low, with the result that the tops of the lungs receive far less blood than the bases , which are relatively over-perfused with blood. It is only in the middle of the lungs that the blood and air flow to the alveoli are ideally matched . At altitude, this variation in

2212-412: A net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has a P C O 2 {\displaystyle P_{{\mathrm {CO} }_{2}}} of also about 6 kPa (45 mmHg), whereas that of the alveolar air is 5.3 kPa (40 mmHg), there

2370-479: A normal mammal, the lungs cannot be emptied completely. In an adult human, there is always still at least 1 liter of residual air left in the lungs after maximum exhalation. The automatic rhythmical breathing in and out, can be interrupted by coughing, sneezing (forms of very forceful exhalation), by the expression of a wide range of emotions (laughing, sighing, crying out in pain, exasperated intakes of breath) and by such voluntary acts as speech, singing, whistling and

2528-428: A partial pressure of CO 2 of also about 6 kPa (45 mmHg), whereas that of the alveolar air is 5.3 kPa (40 mmHg), there is a net movement of carbon dioxide out of the capillaries into the alveoli. The changes brought about by these net flows of individual gases into and out of the alveolar air necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds or so. This

2686-453: A pivotal role in the determination and maintenance of the pH of the extracellular fluids . The carbon dioxide that is breathed out with each breath could probably be more correctly be seen as a byproduct of the body's extracellular fluid carbon dioxide and pH homeostats If these homeostats are compromised, then a respiratory acidosis , or a respiratory alkalosis will occur. In the long run these can be compensated by renal adjustments to

2844-409: A process called buccal pumping . The lower floor of the mouth is moved in a "pumping" manner, which can be observed by the naked eye. All reptiles breathe using lungs. In squamates (the lizards and snakes ) ventilation is driven by the axial musculature , but this musculature is also used during movement, so some squamates rely on buccal pumping to maintain gas exchange efficiency. Due to

3002-460: A rise in arterial blood pressure . Large amounts of the angiotensin-converting enzyme responsible for this activation are located on the surfaces of the endothelial cells of the alveolar capillaries. The converting enzyme also inactivates bradykinin . Circulation time through the alveolar capillaries is less than one second, yet 70% of the angiotensin I reaching the lungs is converted to angiotensin II in

3160-403: A similar way. Unlike the invertebrates groups mentioned so far, insects are usually terrestrial, and exchange gases across a moist surface in direct contact with the atmosphere, rather than in contact with surrounding water. The insect's exoskeleton is impermeable to gases, including water vapor, so they have a more specialised gas exchange system, requiring gases to be directly transported to

3318-424: A single trip through the capillaries. Four other peptidases have been identified on the surface of the pulmonary endothelial cells. The movement of gas through the larynx , pharynx and mouth allows humans to speak , or phonate . Vocalization, or singing, in birds occurs via the syrinx , an organ located at the base of the trachea. The vibration of air flowing across the larynx ( vocal cords ), in humans, and

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3476-476: A small piece of the plant. The mechanism of gas exchange in invertebrates depends their size, feeding strategy, and habitat (aquatic or terrestrial). The sponges (Porifera) are sessile creatures, meaning they are unable to move on their own and normally remain attached to their substrate . They obtain nutrients through the flow of water across their cells, and they exchange gases by simple diffusion across their cell membranes. Pores called ostia draw water into

3634-407: A typical biological system, where two compartments ('inside' and 'outside'), are separated by a membrane barrier, and where a gas is allowed to spontaneously diffuse down its concentration gradient: Gases must first dissolve in a liquid in order to diffuse across a membrane , so all biological gas exchange systems require a moist environment. In general, the higher the concentration gradient across

3792-430: A variety of different combinations. The relative importance of these structures differs according to the age, the environment and species of the amphibian. The skin of amphibians and their larvae are highly vascularised, leading to relatively efficient gas exchange when the skin is moist. The larvae of amphibians, such as the pre-metamorphosis tadpole stage of frogs , also have external gills . The gills are absorbed into

3950-399: A variety of molecules that aid in the defense of the lungs. These include secretory immunoglobulins (IgA), collectins , defensins and other peptides and proteases , reactive oxygen species , and reactive nitrogen species . These secretions can act directly as antimicrobials to help keep the airway free of infection. A variety of chemokines and cytokines are also secreted that recruit

4108-405: A very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin . The oxygen is held on the hemoglobin by four ferrous iron -containing heme groups per hemoglobin molecule. When all the heme groups carry one O 2 molecule each the blood is said to be "saturated" with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase

4266-417: A watery surface (the water-air interface) tends to make that surface shrink. When that surface is curved as it is in the alveoli of the lungs, the shrinkage of the surface decreases the diameter of the alveoli. The more acute the curvature of the water-air interface the greater the tendency for the alveolus to collapse . This has three effects. Firstly, the surface tension inside the alveoli resists expansion of

4424-558: A wide range of circumstances, at the expense of the arterial partial pressure of O 2 , which is allowed to vary within a very wide range of values, before eliciting a corrective ventilatory response. However, when the atmospheric pressure (and therefore the partial pressure of O 2 in the ambient air) falls to below 50-75% of its value at sea level, oxygen homeostasis is given priority over carbon dioxide homeostasis. This switch-over occurs at an elevation of about 2500 m (or about 8000 ft). If this switch occurs relatively abruptly,

4582-436: Is a net movement of carbon dioxide out of the capillaries into the alveoli. The changes brought about by these net flows of individual gases into and out of the functional residual capacity necessitate the replacement of about 15% of the alveolar air with ambient air every 5 seconds or so. This is very tightly controlled by the continuous monitoring of the arterial blood gas tensions (which accurately reflect partial pressures of

4740-453: Is a sign of, illness.) It ends in the microscopic dead-end sacs called alveoli , which are always open, though the diameters of the various sections can be changed by the sympathetic and parasympathetic nervous systems . The alveolar air pressure is therefore always close to atmospheric air pressure (about 100  kPa at sea level) at rest, with the pressure gradients because of lungs contraction and expansion cause air to move in and out of

4898-442: Is at sea level). This reduces the partial pressure of oxygen entering the alveoli to 5.8 kPa (or 21% of [33.7 kPa – 6.3 kPa] = 5.8 kPa). The reduction in the partial pressure of oxygen in the inhaled air is therefore substantially greater than the reduction of the total atmospheric pressure at altitude would suggest (on Mt Everest: 5.8 kPa vs. 7.1 kPa). A further minor complication exists at altitude. If

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5056-604: Is breathed back into the alveoli before environmental air reaches them. At the end of inhalation, the airways are filled with environmental air, which is exhaled without coming in contact with the gas exchanger. The lungs expand and contract during the breathing cycle, drawing air in and out of the lungs. The volume of air moved in or out of the lungs under normal resting circumstances (the resting tidal volume of about 500 ml), and volumes moved during maximally forced inhalation and maximally forced exhalation are measured in humans by spirometry . A typical adult human spirogram with

5214-426: Is breathed in or out, either through the mouth or nose or into or out of the alveoli are tabulated below, together with how they are calculated. The number of breath cycles per minute is known as the respiratory rate . An average healthy human breathes 12–16 times a minute. In mammals , inhalation at rest is primarily due to the contraction of the diaphragm . This is an upwardly domed sheet of muscle that separates

5372-412: Is brought to the alveoli in small doses (called the tidal volume ), by breathing in ( inhalation ) and out ( exhalation ) through the respiratory airways , a set of relatively narrow and moderately long tubes which start at the nose or mouth and end in the alveoli of the lungs in the chest. Air moves in and out through the same set of tubes, in which the flow is in one direction during inhalation, and in

5530-404: Is determined by the blood gas homeostat , which regulates the partial pressures of oxygen and carbon dioxide in the arterial blood. This homeostat prioritizes the regulation of the arterial partial pressure of carbon dioxide over that of oxygen at sea level. That is to say, at sea level the arterial partial pressure of CO 2 is maintained at very close to 5.3 kPa (or 40 mmHg) under

5688-417: Is dominated by the roles of carbon dioxide, oxygen and water vapor . CO 2 is the only carbon source for autotrophic growth by photosynthesis , and when a plant is actively photosynthesising in the light, it will be taking up carbon dioxide, and losing water vapor and oxygen. At night, plants respire , and gas exchange partly reverses: water vapor is still lost (but to a smaller extent), but oxygen

5846-448: Is fatal. Basic scientific experiments, carried out using cells from chicken lungs, support the potential for using steroids as a means of furthering the development of type II alveolar cells. In fact, once a premature birth is threatened, every effort is made to delay the birth, and a series of steroid injections is frequently administered to the mother during this delay in an effort to promote lung maturation. The lung vessels contain

6004-569: Is fresh warm and moistened air. Since this 350 ml of fresh air is thoroughly mixed and diluted by the air that remains in the alveoli after a normal exhalation (i.e. the functional residual capacity of about 2.5–3.0 liters), it is clear that the composition of the alveolar air changes very little during the breathing cycle (see Fig. 9). The oxygen tension (or partial pressure) remains close to 13–14 kPa (about 100 mm Hg), and that of carbon dioxide very close to 5.3 kPa (or 40 mm Hg). This contrasts with composition of

6162-425: Is internalized as linings of the lungs . Gas exchange in the lungs occurs in millions of small air sacs; in mammals and reptiles, these are called alveoli , and in birds, they are known as atria . These microscopic air sacs have a very rich blood supply, thus bringing the air into close contact with the blood. These air sacs communicate with the external environment via a system of airways, or hollow tubes, of which

6320-402: Is internalized to form lungs, as it is in most of the larger land animals. Gas exchange occurs in microscopic dead-end air-filled sacs called alveoli , where a very thin membrane (called the blood-air barrier ) separates the blood in the alveolar capillaries (in the walls of the alveoli) from the alveolar air in the sacs. The membrane across which gas exchange takes place in the alveoli (i.e.

6478-441: Is no unidirectional through-flow as there is in the bird lung ). This typical mammalian anatomy combined with the fact that the lungs are not emptied and re-inflated with each breath (leaving a substantial volume of air, of about 2.5–3.0 liters, in the alveoli after exhalation), ensures that the composition of the alveolar air is only minimally disturbed when the 350 ml of fresh air is mixed into it with each inhalation. Thus

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6636-401: Is now taken up and carbon dioxide released. Plant gas exchange occurs mostly through the leaves. Gas exchange between a leaf and the atmosphere occurs simultaneously through two pathways: 1) epidermal cells and cuticular waxes (usually referred as ' cuticle ') which are always present at each leaf surface, and 2) stomata , which typically control the majority of the exchange. Gases enter into

6794-453: Is only during the night that these plants open their stomata. By opening the stomata only at night, the water vapor loss associated with carbon dioxide uptake is minimised. However, this comes at the cost of slow growth: the plant has to store the carbon dioxide in the form of malic acid for use during the day, and it cannot store unlimited amounts. Gas exchange measurements are important tools in plant science: this typically involves sealing

6952-455: Is only, on average, about 2 μm thick. The gas pressures in the blood will therefore rapidly equilibrate with those in the alveoli , ensuring that the arterial blood that circulates to all the tissues throughout the body has an oxygen tension of 13−14 kPa (100 mmHg), and a carbon dioxide tension of 5.3 kPa (40 mmHg). These arterial partial pressures of oxygen and carbon dioxide are homeostatically controlled . A rise in

7110-496: Is particularly important for respiration , which involves the uptake of oxygen ( O 2 ) and release of carbon dioxide ( CO 2 ). Conversely, in oxygenic photosynthetic organisms such as most land plants , uptake of carbon dioxide and release of both oxygen and water vapour are the main gas-exchange processes occurring during the day. Other gas-exchange processes are important in less familiar organisms: e.g. carbon dioxide, methane and hydrogen are exchanged across

7268-423: Is restored. Since the blood arriving in the alveolar capillaries has a partial pressure of O 2 of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 13–14 kPa (100 mmHg), there will be a net diffusion of oxygen into the capillary blood, changing the composition of the 3 liters of alveolar air slightly. Similarly, since the blood arriving in the alveolar capillaries has

7426-527: Is the first air to re-enter the alveoli during inhalation. Only after the dead space air has returned to the alveoli does the remainder of the tidal volume (500 ml - 150 ml = 350 ml) enter the alveoli. The entry of such a small volume of fresh air with each inhalation, ensures that the composition of the FRC hardly changes during the breathing cycle (Fig. 5). The alveolar partial pressure of oxygen remains very close to 13–14  kPa (100 mmHg), and

7584-491: Is therefore almost the same at the end of exhalation as at the end of inhalation. Thirdly, the surface tension of the curved watery layer lining the alveoli tends to draw water from the lung tissues into the alveoli. Surfactant reduces this danger to negligible levels, and keeps the alveoli dry. Pre-term babies who are unable to manufacture surfactant have lungs that tend to collapse each time they breathe out. Unless treated, this condition, called respiratory distress syndrome ,

7742-424: Is therefore strictly speaking untrue that the primary function of the respiratory system is to rid the body of carbon dioxide "waste". In fact the total concentration of carbon dioxide in arterial blood is about 26 mM (or 58 ml per 100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml per 100 ml blood). This large concentration of carbon dioxide plays

7900-478: Is very tightly controlled by the monitoring of the arterial blood gases (which accurately reflect composition of the alveolar air) by the aortic and carotid bodies , as well as by the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain. There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries , and are therefore responsible for directing

8058-401: The alveoli . The branching airways of the lower tract are often described as the respiratory tree or tracheobronchial tree (Fig. 2). The intervals between successive branch points along the various branches of "tree" are often referred to as branching "generations", of which there are, in the adult human, about 23. The earlier generations (approximately generations 0–16), consisting of

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8216-452: The amount of gas that can diffuse in a given time. This is because the amount of gas diffusing per unit time (d q /d t ) is the product of J and the area of the gas-exchanging surface, A : Single-celled organisms such as bacteria and amoebae do not have specialised gas exchange surfaces, because they can take advantage of the high surface area they have relative to their volume. The amount of gas an organism produces (or requires) in

8374-427: The arterial blood . This information determines the average rate of ventilation of the alveoli of the lungs , to keep these pressures constant . The respiratory center does so via motor nerves which activate the diaphragm and other muscles of respiration . The breathing rate increases when the partial pressure of carbon dioxide in the blood increases. This is detected by central blood gas chemoreceptors on

8532-459: The endothelial cells of the alveolar capillaries (Fig. 10). This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick). It is folded into about 300 million small air sacs called alveoli (each between 75 and 300 μm in diameter) branching off from the respiratory bronchioles in the lungs , thus providing an extremely large surface area (approximately 145 m) for gas exchange to occur. The air contained within

8690-516: The lungs of mammals. In a cocurrent flow system, the blood and gas (or the fluid containing the gas) move in the same direction through the gas exchanger. This means the magnitude of the gradient is variable along the length of the gas-exchange surface, and the exchange will eventually stop when an equilibrium has been reached (see upper diagram in Fig. 2). Cocurrent flow gas exchange systems are not known to be used in nature. The gas exchanger in mammals

8848-425: The operculum (gill cover). Although countercurrent exchange systems theoretically allow an almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood. Amphibians have three main organs involved in gas exchange: the lungs, the skin, and the gills, which can be used singly or in

9006-409: The partial pressure of carbon dioxide varies minimally around 5.3 kPa (40 mmHg) throughout the breathing cycle (of inhalation and exhalation). The corresponding partial pressures of oxygen and carbon dioxide in the ambient (dry) air at sea level are 21 kPa (160 mmHg) and 0.04 kPa (0.3 mmHg) respectively. This alveolar air, which constitutes the FRC, completely surrounds

9164-417: The red blood cells . The reaction can go in either direction depending on the prevailing partial pressure of carbon dioxide. A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as carbamino groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO 2 , and carbamino groups) in arterial blood (i.e. after it has equilibrated with

9322-408: The red blood cells . The reaction can go in both directions depending on the prevailing partial pressure of CO 2 . A small amount of carbon dioxide is carried on the protein portion of the hemoglobin molecules as carbamino groups. The total concentration of carbon dioxide (in the form of bicarbonate ions, dissolved CO 2 , and carbamino groups) in arterial blood (i.e. after it has equilibrated with

9480-508: The trachea or nose , respectively. In this manner, irritants caught in the mucus which lines the respiratory tract are expelled or moved to the mouth where they can be swallowed . During coughing, contraction of the smooth muscle in the airway walls narrows the trachea by pulling the ends of the cartilage plates together and by pushing soft tissue into the lumen. This increases the expired airflow rate to dislodge and remove any irritant particle or mucus. Respiratory epithelium can secrete

9638-428: The ventilation/perfusion ratio of alveoli from the tops of the lungs to the bottoms is eliminated, with all the alveoli perfused and ventilated in more or less the physiologically ideal manner. This is a further important contributor to the acclimatatization to high altitudes and low oxygen pressures. The kidneys measure the oxygen content (mmol O 2 /liter blood, rather than the partial pressure of O 2 ) of

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9796-459: The 19.7 kPa of oxygen entering the alveolar air. (The tracheal partial pressure of oxygen is 21% of [100 kPa – 6.3 kPa] = 19.7 kPa). At the summit of Mt. Everest (at an altitude of 8,848 m or 29,029 ft), the total atmospheric pressure is 33.7 kPa , of which 7.1 kPa (or 21%) is oxygen. The air entering the lungs also has a total pressure of 33.7 kPa, of which 6.3 kPa is, unavoidably, water vapor (as it

9954-400: The 23 number (on average) of branchings of the respiratory tree in the adult human, the mouse has only about 13 such branchings. The alveoli are the dead end terminals of the "tree", meaning that any air that enters them has to exit via the same route. A system such as this creates dead space , a volume of air (about 150 ml in the adult human) that fills the airways after exhalation and

10112-402: The H and HCO 3 concentrations in the plasma; but since this takes time, the hyperventilation syndrome can, for instance, occur when agitation or anxiety cause a person to breathe fast and deeply thus blowing off too much CO 2 from the blood into the outside air, precipitating a set of distressing symptoms which result from an excessively high pH of the extracellular fluids. Oxygen has

10270-436: The actions of the intercostal muscles (Fig. 8). These accessory muscles of inhalation are muscles that extend from the cervical vertebrae and base of the skull to the upper ribs and sternum , sometimes through an intermediary attachment to the clavicles . When they contract, the rib cage's internal volume is increased to a far greater extent than can be achieved by contraction of the intercostal muscles alone. Seen from outside

10428-422: The adult human has a volume of about 2.5–3.0 liters (Fig. 3). Resting exhalation lasts about twice as long as inhalation because the diaphragm relaxes passively more gently than it contracts actively during inhalation. The volume of air that moves in or out (at the nose or mouth) during a single breathing cycle is called the tidal volume . In a resting adult human, it is about 500 ml per breath. At

10586-434: The alveolar air) is about 26 mM (or 58 ml/100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml/100 ml blood). The dissolved oxygen content in fresh water is approximately 8–10 milliliters per liter compared to that of air which is 210 milliliters per liter. Water is 800 times more dense than air and 100 times more viscous. Therefore, oxygen has

10744-450: The alveolar air) is about 26 mM (or 58 ml/100 ml), compared to the concentration of oxygen in saturated arterial blood of about 9 mM (or 20 ml/100 ml blood). Ventilation of the lungs in mammals occurs via the respiratory centers in the medulla oblongata and the pons of the brainstem . These areas form a series of neural pathways which receive information about the partial pressures of oxygen and carbon dioxide in

10902-460: The alveolar partial pressure of carbon dioxide has returned to 5.3 kPa (40 mmHg). It is therefore strictly speaking untrue that the primary function of the respiratory system is to rid the body of carbon dioxide "waste". The carbon dioxide that is breathed out with each breath could probably be more correctly be seen as a byproduct of the body's extracellular fluid carbon dioxide and pH homeostats If these homeostats are compromised, then

11060-402: The alveoli after the last exhalation. This relatively large volume of air that is semi-permanently present in the alveoli throughout the breathing cycle is known as the functional residual capacity (FRC). At the beginning of inhalation the airways are filled with unchanged alveolar air, left over from the last exhalation. This is the dead space volume, which is usually about 150 ml. It

11218-409: The alveoli causes carbon dioxide to move into the alveoli. The exchange of gases occurs as a result of diffusion down a concentration gradient. Gas molecules move from a region in which they are at high concentration to one in which they are at low concentration. Diffusion is a passive process , meaning that no energy is required to power the transport, and it follows Fick's law : In relation to

11376-423: The alveoli during inhalation (i.e. it makes the lung stiff, or non-compliant). Surfactant reduces the surface tension and therefore makes the lungs more compliant , or less stiff, than if it were not there. Secondly, the diameters of the alveoli increase and decrease during the breathing cycle. This means that the alveoli have a greater tendency to collapse (i.e. cause atelectasis ) at the end of exhalation than at

11534-454: The alveoli has a semi-permanent volume of about 2.5–3.0 liters which completely surrounds the alveolar capillary blood (Fig. 12). This ensures that equilibration of the partial pressures of the gases in the two compartments is very efficient and occurs very quickly. The blood leaving the alveolar capillaries and is eventually distributed throughout the body therefore has a partial pressure of oxygen of 13–14 kPa (100 mmHg), and

11692-400: The ambient atmospheric pressure is about 100 kPa, the moistened air that flows into the lungs from the trachea consists of water vapor (6.3 kPa), nitrogen (74.0 kPa), oxygen (19.7 kPa) and trace amounts of carbon dioxide and other gases (a total of 100 kPa). In dry air the partial pressure of O 2 at sea level is 21.0 kPa (i.e. 21% of 100 kPa), compared to

11850-413: The animal is provided with a very special "portable atmosphere", whose composition differs significantly from the present-day ambient air . It is this portable atmosphere (the functional residual capacity ) to which the blood and therefore the body tissues are exposed – not to the outside air. The resulting arterial partial pressures of oxygen and carbon dioxide are homeostatically controlled . A rise in

12008-452: The anterior surface of the medulla oblongata . The aortic and carotid bodies , are the peripheral blood gas chemoreceptors which are particularly sensitive to the arterial partial pressure of O 2 though they also respond, but less strongly, to the partial pressure of CO 2 . At sea level, under normal circumstances, the breathing rate and depth, is determined primarily by the arterial partial pressure of carbon dioxide rather than by

12166-424: The antero-posterior diameter is increased by the so-called pump handle movement shown in Fig. 4. The enlargement of the thoracic cavity's vertical dimension by the contraction of the diaphragm, and its two horizontal dimensions by the lifting of the front and sides of the ribs, causes the intrathoracic pressure to fall. The lungs' interiors are open to the outside air and being elastic, therefore expand to fill

12324-438: The arterial P C O 2 {\displaystyle P_{{\mathrm {CO} }_{2}}} , and, to a lesser extent, a fall in the arterial P O 2 {\displaystyle P_{{\mathrm {O} }_{2}}} , will reflexly cause deeper and faster breathing until the blood gas tensions return to normal. The converse happens when the carbon dioxide tension falls, or, again to

12482-437: The arterial partial pressure of oxygen , which is allowed to vary within a fairly wide range before the respiratory centers in the medulla oblongata and pons respond to it to change the rate and depth of breathing. Exercise increases the breathing rate due to the extra carbon dioxide produced by the enhanced metabolism of the exercising muscles. In addition, passive movements of the limbs also reflexively produce an increase in

12640-433: The arterial blood. When the oxygen content of the blood is chronically low, as at high altitude, the oxygen-sensitive kidney cells secrete erythropoietin (EPO) into the blood. This hormone stimulates the red bone marrow to increase its rate of red cell production, which leads to an increase in the hematocrit of the blood, and a consequent increase in its oxygen carrying capacity (due to the now high hemoglobin content of

12798-447: The arterial partial pressure of CO 2 and, to a lesser extent, a fall in the arterial partial pressure of O 2 , will reflexly cause deeper and faster breathing until the blood gas tensions in the lungs, and therefore the arterial blood, return to normal. The converse happens when the carbon dioxide tension falls, or, again to a lesser extent, the oxygen tension rises: the rate and depth of breathing are reduced until blood gas normality

12956-538: The belly to bulge outwards to the front and sides, because the relaxed abdominal muscles do not resist this movement (Fig. 7). This entirely passive bulging (and shrinking during exhalation) of the abdomen during normal breathing is sometimes referred to as "abdominal breathing", although it is, in fact, "diaphragmatic breathing", which is not visible on the outside of the body. Mammals only use their abdominal muscles during forceful exhalation (see Fig. 8, and discussion below). Never during any form of inhalation. As

13114-432: The blood in the alveolar capillaries (Fig. 6). Gas exchange in mammals occurs between this alveolar air (which differs significantly from fresh air) and the blood in the alveolar capillaries. The gases on either side of the gas exchange membrane equilibrate by simple diffusion. This ensures that the partial pressures of oxygen and carbon dioxide in the blood leaving the alveolar capillaries, and ultimately circulates throughout

13272-450: The blood into the outside air. Oxygen has a very low solubility in water, and is therefore carried in the blood loosely combined with hemoglobin . The oxygen is held on the hemoglobin by four ferrous iron -containing heme groups per hemoglobin molecule. When all the heme groups carry one O 2 molecule each the blood is said to be “saturated” with oxygen, and no further increase in the partial pressure of oxygen will meaningfully increase

13430-447: The blood). In other words, at the same arterial partial pressure of O 2 , a person with a high hematocrit carries more oxygen per liter of blood than a person with a lower hematocrit does. High altitude dwellers therefore have higher hematocrits than sea-level residents. Irritation of nerve endings within the nasal passages or airways , can induce a cough reflex and sneezing . These responses cause air to be expelled forcefully from

13588-516: The blood-air barrier) is extremely thin (in humans, on average, 2.2 μm thick). It consists of the alveolar epithelial cells , their basement membranes and the endothelial cells of the pulmonary capillaries (Fig. 4). The large surface area of the membrane comes from the folding of the membrane into about 300 million alveoli, with diameters of approximately 75-300 μm each. This provides an extremely large surface area (approximately 145 m ) across which gas exchange can occur. Air

13746-427: The body during metamorphosis , after which the lungs will then take over. The lungs are usually simpler than in the other land vertebrates , with few internal septa and larger alveoli; however, toads, which spend more time on land, have a larger alveolar surface with more developed lungs. To increase the rate of gas exchange by diffusion, amphibians maintain the concentration gradient across the respiratory surface using

13904-429: The body, are the same as those in the FRC. The marked difference between the composition of the alveolar air and that of the ambient air can be maintained because the functional residual capacity is contained in dead-end sacs connected to the outside air by long, narrow, tubes (the airways: nose , pharynx , larynx , trachea , bronchi and their branches and sub-branches down to the bronchioles ). This anatomy, and

14062-425: The body, the lifting of the clavicles during strenuous or labored inhalation is sometimes called clavicular breathing , seen especially during asthma attacks and in people with chronic obstructive pulmonary disease . During heavy breathing, exhalation is caused by relaxation of all the muscles of inhalation. But now, the abdominal muscles, instead of remaining relaxed (as they do at rest), contract forcibly pulling

14220-443: The breathing cycle. Air exiting the lungs during exhalation joins the air being expelled from the anterior air sacs (both consisting of "spent air" that has passed through the gas exchanger) entering the trachea to be exhaled (Fig. 10). Selective bronchoconstriction at the various bronchial branch points ensures that the air does not ebb and flow through the bronchi during inhalation and exhalation, as it does in mammals, but follows

14378-416: The breathing rate. Information received from stretch receptors in the lungs' limits tidal volume (the depth of inhalation and exhalation). The alveoli are open (via the airways) to the atmosphere, with the result that alveolar air pressure is exactly the same as the ambient air pressure at sea level, at altitude, or in any artificial atmosphere (e.g. a diving chamber, or decompression chamber) in which

14536-422: The case of a single-celled organism, a typical cell membrane is only 10 nm thick; but in larger organisms such as roundworms (Nematoda) the equivalent exchange surface - the cuticle - is substantially thicker at 0.5 μm. In multicellular organisms therefore, specialised respiratory organs such as gills or lungs are often used to provide the additional surface area for the required rate of gas exchange with

14694-470: The cell membrane of methanogenic archaea . In nitrogen fixation by diazotrophic bacteria, and denitrification by heterotrophic bacteria (such as Paracoccus denitrificans and various pseudomonads ), nitrogen gas is exchanged with the environment, being taken up by the former and released into it by the latter, while giant tube worms rely on bacteria to oxidize hydrogen sulfide extracted from their deep sea environment, using dissolved oxygen in

14852-406: The composition of the alveolar air and that of the ambient air can be maintained because the functional residual capacity is contained in dead-end sacs connected to the outside air by fairly narrow and relatively long tubes (the airways: nose , pharynx , larynx , trachea , bronchi and their branches down to the bronchioles ), through which the air has to be breathed both in and out (i.e. there

15010-593: The coral, including oxygen. The roundworms (Nematoda), flatworms (Platyhelminthes), and many other small invertebrate animals living in aquatic or otherwise wet habitats do not have a dedicated gas-exchange surface or circulatory system. They instead rely on diffusion of CO 2 and O 2 directly across their cuticle. The cuticle is the semi-permeable outermost layer of their bodies. Other aquatic invertebrates such as most molluscs (Mollusca) and larger crustaceans (Crustacea) such as lobsters , have gills analogous to those of fish, which operate in

15168-399: The diaphragm contracts, the rib cage is simultaneously enlarged by the ribs being pulled upwards by the intercostal muscles as shown in Fig. 4. All the ribs slant downwards from the rear to the front (as shown in Fig. 4); but the lowermost ribs also slant downwards from the midline outwards (Fig. 5). Thus the rib cage's transverse diameter can be increased in the same way as

15326-418: The diaphragmaticus - but this muscle helps create a unidirectional flow of air through the lungs rather than a tidal flow: this is more similar to the air-flow seen in birds than that seen in mammals. During inhalation, the diaphragmaticus pulls the liver back, inflating the lungs into the space this creates. Air flows into the lungs from the bronchus during inhalation, but during exhalation, air flows out of

15484-431: The dry outside air at sea level, where the partial pressure of oxygen is 21 kPa (or 160 mm Hg) and that of carbon dioxide 0.04 kPa (or 0.3 mmHg). During heavy breathing ( hyperpnea ), as, for instance, during exercise, inhalation is brought about by a more powerful and greater excursion of the contracting diaphragm than at rest (Fig. 8). In addition, the " accessory muscles of inhalation " exaggerate

15642-417: The end of exhalation, the airways contain about 150 ml of alveolar air which is the first air that is breathed back into the alveoli during inhalation. This volume air that is breathed out of the alveoli and back in again is known as dead space ventilation, which has the consequence that of the 500 ml breathed into the alveoli with each breath only 350 ml (500 ml – 150 ml = 350 ml)

15800-404: The end of inhalation. Since surfactant floats on the watery surface, its molecules are more tightly packed together when the alveoli shrink during exhalation. This causes them to have a greater surface tension-lowering effect when the alveoli are small than when they are large (as at the end of inhalation, when the surfactant molecules are more widely spaced). The tendency for the alveoli to collapse

15958-403: The entrance of airflow take up more O 2 than capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final P O 2 {\displaystyle P_{{\mathrm {O} }_{2}}} of the mixed pulmonary venous blood is higher than that of the exhaled air, but lower than that of the inhaled air. Gas exchange in plants

16116-453: The environment into the alveoli or atria by the process of breathing which involves the muscles of respiration . In most fish , and a number of other aquatic animals (both vertebrates and invertebrates ), the respiratory system consists of gills , which are either partially or completely external organs, bathed in the watery environment. This water flows over the gills by a variety of active or passive means. Gas exchange takes place in

16274-511: The example given. The differences between the atmospheric and intrapulmonary pressures, driving air in and out of the lungs during the breathing cycle, are in the region of only 2–3 kPa. A doubling or more of these small pressure differences could be achieved only by very major changes in the breathing effort at high altitudes. All of the above influences of low atmospheric pressures on breathing are accommodated primarily by breathing deeper and faster ( hyperpnea ). The exact degree of hyperpnea

16432-532: The external environment. However the distances between the gas exchanger and the deeper tissues are often too great for diffusion to meet gaseous requirements of these tissues. The gas exchangers are therefore frequently coupled to gas-distributing circulatory systems , which transport the gases evenly to all the body tissues regardless of their distance from the gas exchanger. Some multicellular organisms such as flatworms (Platyhelminthes) are relatively large but very thin, allowing their outer body surface to act as

16590-427: The fact that the lungs are not emptied and re-inflated with each breath, provides mammals with a "portable atmosphere", whose composition differs significantly from the present-day ambient air . The composition of the air in the FRC is carefully monitored, by measuring the partial pressures of oxygen and carbon dioxide in the arterial blood. If either gas pressure deviates from normal, reflexes are elicited that change

16748-425: The flow of air and blood to different parts of the lungs. It is only as a result of accurately maintaining the composition of the 3 liters of alveolar air that with each breath some carbon dioxide is discharged into the atmosphere and some oxygen is taken up from the outside air. If more carbon dioxide than usual has been lost by a short period of hyperventilation , respiration will be slowed down or halted until

16906-471: The gas-exchange surface, with the blood-flow in the gill capillaries beneath flowing in the opposite direction. Although this theoretically allows almost complete transfer of a respiratory gas from one side of the exchanger to the other, in fish less than 80% of the oxygen in the water flowing over the gills is generally transferred to the blood. Alternative arrangements are cross current systems found in birds. and dead-end air-filled sac systems found in

17064-404: The gas-exchanging surface, the faster the rate of diffusion across it. Conversely, the thinner the gas-exchanging surface (for the same concentration difference), the faster the gases will diffuse across it. In the equation above, J is the flux expressed per unit area, so increasing the area will make no difference to its value. However, an increase in the available surface area, will increase

17222-559: The gills which consist of thin or very flat filaments and lammellae which expose a very large surface area of highly vascularized tissue to the water. Other animals, such as insects , have respiratory systems with very simple anatomical features, and in amphibians , even the skin plays a vital role in gas exchange. Plants also have respiratory systems but the directionality of gas exchange can be opposite to that in animals. The respiratory system in plants includes anatomical features such as stomata , that are found in various parts of

17380-402: The hyperpnea at high altitude will cause a severe fall in the arterial partial pressure of carbon dioxide, with a consequent rise in the pH of the arterial plasma . This is one contributor to high altitude sickness . On the other hand, if the switch to oxygen homeostasis is incomplete, then hypoxia may complicate the clinical picture with potentially fatal results. There are oxygen sensors in

17538-414: The increased space, pleura fluid between double-layered pleura covering of lungs helps in reducing friction while lungs expansion and contraction. The inflow of air into the lungs occurs via the respiratory airways (Fig. 2). In a healthy person, these airways begin with the nose . (It is possible to begin with the mouth, which is the backup breathing system. However, chronic mouth breathing leads to, or

17696-418: The individual is breathing freely. With expansion of the lungs the alveolar air occupies a larger volume, and its pressure falls proportionally , causing air to flow in through the airways, until the pressure in the alveoli is again at the ambient air pressure. The reverse happens during exhalation. This process (of inhalation and exhalation) is exactly the same at sea level, as on top of Mt. Everest , or in

17854-469: The industry of man." Inspired in the work of Adam Smith , Milne-Edwards wrote that the "body of all living beings, whether animal or plant, resembles a factory ... where the organs, comparable to workers, work incessantly to produce the phenomena that constitute the life of the individual." In more differentiated organisms, the functional labor could be apportioned between different instruments or systems (called by him as appareils ). The exact components of

18012-636: The interior of the cell(s) and the external environment is required. Small, particularly unicellular organisms, such as bacteria and protozoa , have a high surface-area to volume ratio . In these creatures the gas exchange membrane is typically the cell membrane . Some small multicellular organisms, such as flatworms , are also able to perform sufficient gas exchange across the skin or cuticle that surrounds their bodies. However, in most larger organisms, which have small surface-area to volume ratios, specialised structures with convoluted surfaces such as gills , pulmonary alveoli and spongy mesophylls provide

18170-410: The lamellae in the opposite direction. This countercurrent maintains steep concentration gradients along the entire length of each capillary (see the diagram in the "Interaction with circulatory systems" section above). Oxygen is able to continually diffuse down its gradient into the blood, and the carbon dioxide down its gradient into the water. The deoxygenated water will eventually pass out through

18328-436: The large area needed for effective gas exchange. These convoluted surfaces may sometimes be internalised into the body of the organism. This is the case with the alveoli, which form the inner surface of the mammalian lung , the spongy mesophyll, which is found inside the leaves of some kinds of plant , or the gills of those molluscs that have them, which are found in the mantle cavity. In aerobic organisms , gas exchange

18486-461: The largest is the trachea , which branches in the middle of the chest into the two main bronchi . These enter the lungs where they branch into progressively narrower secondary and tertiary bronchi that branch into numerous smaller tubes, the bronchioles . In birds , the bronchioles are termed parabronchi . It is the bronchioles, or parabronchi that generally open into the microscopic alveoli in mammals and atria in birds. Air has to be pumped from

18644-430: The lower edges of the rib cage downwards (front and sides) (Fig. 8). This not only drastically decreases the size of the rib cage, but also pushes the abdominal organs upwards against the diaphragm which consequently bulges deeply into the thorax (Fig. 8). The end-exhalatory lung volume is now well below the resting mid-position and contains far less air than the resting "functional residual capacity". However, in

18802-476: The lungs and released into the blood when lung tissue is stretched. The lungs activate one hormone. The physiologically inactive decapeptide angiotensin I is converted to the aldosterone -releasing octapeptide, angiotensin II , in the pulmonary circulation. The reaction occurs in other tissues as well, but it is particularly prominent in the lungs. Angiotensin II also has a direct effect on arteriolar walls , causing arteriolar vasoconstriction , and consequently

18960-435: The lungs during breathing rarely exceeding 2–3 kPa. During exhalation, the diaphragm and intercostal muscles relax. This returns the chest and abdomen to a position determined by their anatomical elasticity. This is the "resting mid-position" of the thorax and abdomen (Fig. 7) when the lungs contain their functional residual capacity of air (the light blue area in the right hand illustration of Fig. 7), which in

19118-405: The lungs into the bronchus by a different route: this one-way movement of gas is achieved by aerodynamic valves in the airways. Birds have lungs but no diaphragm . They rely mostly on air sacs for ventilation . These air sacs do not play a direct role in gas exchange, but help to move air unidirectionally across the gas exchange surfaces in the lungs. During inhalation, fresh air is taken from

19276-537: The lungs. It is only as a result of accurately maintaining the composition of the 3 liters alveolar air that with each breath some carbon dioxide is discharged into the atmosphere and some oxygen is taken up from the outside air. If more carbon dioxide than usual has been lost by a short period of hyperventilation , respiration will be slowed down or halted until the alveolar P C O 2 {\displaystyle P_{{\mathrm {CO} }_{2}}} has returned to 5.3 kPa (40 mmHg). It

19434-636: The names given to the various excursions in volume the lungs can undergo is illustrated below (Fig. 3): Not all the air in the lungs can be expelled during maximally forced exhalation ( ERV ). This is the residual volume (volume of air remaining even after a forced exhalation) of about 1.0–1.5 liters which cannot be measured by spirometry. Volumes that include the residual volume (i.e. functional residual capacity of about 2.5–3.0 liters, and total lung capacity of about 6 liters) can therefore also not be measured by spirometry. Their measurement requires special techniques. The rates at which air

19592-459: The opposite direction during exhalation. During each inhalation, at rest, approximately 500 ml of fresh air flows in through the nose. It is warmed and moistened as it flows through the nose and pharynx . By the time it reaches the trachea the inhaled air's temperature is 37 °C and it is saturated with water vapor. On arrival in the alveoli it is diluted and thoroughly mixed with the approximately 2.5–3.0 liters of air that remained in

19750-455: The opposite direction, through orifices in the pelvic floor. The abdominal muscles contract very powerfully, causing the pressure inside the abdomen and thorax to rise to extremely high levels. The Valsalva maneuver can be carried out voluntarily but is more generally a reflex elicited when attempting to empty the abdomen during, for instance, difficult defecation, or during childbirth. Breathing ceases during this maneuver. The primary purpose of

19908-424: The oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as HCO 3 ions in the plasma. However the conversion of dissolved CO 2 into HCO 3 (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and alveolar capillaries on the other. The reaction is therefore catalyzed by carbonic anhydrase , an enzyme inside

20066-444: The oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as bicarbonate ions (HCO 3 ) in the plasma. However the conversion of dissolved CO 2 into HCO 3 (through the addition of water) is too slow for the rate at which the blood circulates through the tissues on the one hand, and through alveolar capillaries on the other. The reaction is therefore catalyzed by carbonic anhydrase , an enzyme inside

20224-415: The paths described above. The unidirectional airflow through the parabronchi exchanges respiratory gases with a crosscurrent blood flow (Fig. 9). The partial pressure of O 2 ( P O 2 {\displaystyle P_{{\mathrm {O} }_{2}}} ) in the parabronchioles declines along their length as O 2 diffuses into the blood. The capillaries leaving the exchanger near

20382-402: The photosynthetic condition of the plants. Simpler methods can be used in specific circumstances: hydrogencarbonate indicator can be used to monitor the consumption of CO 2 in a solution containing a single plant leaf at different levels of light intensity, and oxygen generation by the pondweed Elodea can be measured by simply collecting the gas in a submerged test-tube containing

20540-421: The photosynthetic tissue of the leaf through dissolution onto the moist surface of the palisade and spongy mesophyll cells. The spongy mesophyll cells are loosely packed, allowing for an increased surface area, and consequently an increased rate of gas-exchange. Uptake of carbon dioxide necessarily results in some loss of water vapor, because both molecules enter and leave by the same stomata, so plants experience

20698-555: The plant (or part of a plant) in a chamber and measuring changes in the concentration of carbon dioxide and water vapour with an infrared gas analyzer . If the environmental conditions ( humidity , CO 2 concentration, light and temperature ) are fully controlled, the measurements of CO 2 uptake and water release reveal important information about the CO 2 assimilation and transpiration rates. The intercellular CO 2 concentration reveals important information about

20856-433: The plant. In humans and other mammals , the anatomy of a typical respiratory system is the respiratory tract . The tract is divided into an upper and a lower respiratory tract . The upper tract includes the nose , nasal cavities , sinuses , pharynx and the part of the larynx above the vocal folds . The lower tract (Fig. 2.) includes the lower part of the larynx , the trachea , bronchi , bronchioles and

21014-473: The playing of wind instruments. All of these actions rely on the muscles described above, and their effects on the movement of air in and out of the lungs. Although not a form of breathing, the Valsalva maneuver involves the respiratory muscles. It is, in fact, a very forceful exhalatory effort against a tightly closed glottis , so that no air can escape from the lungs. Instead, abdominal contents are evacuated in

21172-457: The principal functions - and consequently of the systems - remained almost the same since Antiquity, but the classification of them has been very various, e.g., compare Aristotle , Bichat , Cuvier . The notion of physiological division of labor, introduced in the 1820s by the French physiologist Henri Milne-Edwards , allowed to "compare and study living things as if they were machines created by

21330-414: The rate and depth of breathing in such a way that normality is restored within seconds or minutes. All the blood returning from the body tissues to the right side of the heart flows through the alveolar capillaries before being pumped around the body again. On its passage through the lungs the blood comes into close contact with the alveolar air, separated from it by a very thin diffusion membrane which

21488-435: The respiratory gases in the alveolar air) by the aortic bodies , the carotid bodies , and the blood gas and pH sensor on the anterior surface of the medulla oblongata in the brain. There are also oxygen and carbon dioxide sensors in the lungs, but they primarily determine the diameters of the bronchioles and pulmonary capillaries , and are therefore responsible for directing the flow of air and blood to different parts of

21646-443: The respiratory system is lined with mucous membranes that contain mucosa-associated lymphoid tissue , which produces white blood cells such as lymphocytes . The lungs make a surfactant , a surface-active lipoprotein complex (phospholipoprotein) formed by type II alveolar cells . It floats on the surface of the thin watery layer which lines the insides of the alveoli, reducing the water's surface tension. The surface tension of

21804-416: The respiratory system is the equalizing of the partial pressures of the respiratory gases in the alveolar air with those in the pulmonary capillary blood (Fig. 11). This process occurs by simple diffusion , across a very thin membrane (known as the blood–air barrier ), which forms the walls of the pulmonary alveoli (Fig. 10). It consists of the alveolar epithelial cells , their basement membranes and

21962-421: The rigidity of turtle and tortoise shells, significant expansion and contraction of the chest is difficult. Turtles and tortoises depend on muscle layers attached to their shells, which wrap around their lungs to fill and empty them. Some aquatic turtles can also pump water into a highly vascularised mouth or cloaca to achieve gas-exchange. Crocodiles have a structure similar to the mammalian diaphragm -

22120-427: The same amount of oxygen to the lungs at altitude as at sea level. During inhalation, the air is warmed and saturated with water vapor during its passage through the nose passages and pharynx . Saturated water vapor pressure is dependent only on temperature. At a body core temperature of 37 °C it is 6.3  kPa (47.0 mmHg), irrespective of any other influences, including altitude. Thus at sea level, where

22278-539: The same change in lung volume at sea level results in a 50 kPa difference in pressure between the ambient air and the intrapulmonary air, whereas it result in a difference of only 25 kPa at 5500 m. The driving pressure forcing air into the lungs during inhalation is therefore halved at this altitude. The rate of inflow of air into the lungs during inhalation at sea level is therefore twice that which occurs at 5500 m. However, in reality, inhalation and exhalation occur far more gently and less abruptly than in

22436-479: The same rate as the fall in air pressure with altitude. Therefore, in order to breathe in the same amount of oxygen per minute, the person has to inhale a proportionately greater volume of air per minute at altitude than at sea level. This is achieved by breathing deeper and faster (i.e. hyperpnea ) than at sea level (see below). There is, however, a complication that increases the volume of air that needs to be inhaled per minute ( respiratory minute volume ) to provide

22594-534: The same way. Consider an imaginary organism that is a cube of side-length, L . Its volume increases with the cube ( L ) of its length, but its external surface area increases only with the square ( L ) of its length. This means the external surface rapidly becomes inadequate for the rapidly increasing gas-exchange needs of a larger volume of cytoplasm. Additionally, the thickness of the surface that gases must cross (d x in Fick's law) can also be larger in larger organisms: in

22752-407: The smaller bronchi and bronchioles . In response to low partial pressures of oxygen in the inhaled air these sensors reflexively cause the pulmonary arterioles to constrict. (This is the exact opposite of the corresponding reflex in the tissues, where low arterial partial pressures of O 2 cause arteriolar vasodilation.) At altitude this causes the pulmonary arterial pressure to rise resulting in

22910-460: The sponge and the water is subsequently circulated through the sponge by cells called choanocytes which have hair-like structures that move the water through the sponge. The cnidarians include corals , sea anemones , jellyfish and hydras . These animals are always found in aquatic environments, ranging from fresh water to salt water. They do not have any dedicated respiratory organs ; instead, every cell in their body can absorb oxygen from

23068-439: The sponge's body is therefore exposed to a constant flow of fresh oxygenated water. They can therefore rely on diffusion across their cell membranes to carry out the gas exchange needed for respiration. In organisms that have circulatory systems associated with their specialized gas-exchange surfaces, a great variety of systems are used for the interaction between the two. In a countercurrent flow system, air (or, more usually,

23226-403: The surrounding water, and release waste gases to it. One key disadvantage of this feature is that cnidarians can die in environments where water is stagnant , as they deplete the water of its oxygen supply. Corals often form symbiosis with other organisms, particularly photosynthetic dinoflagellates . In this symbiosis , the coral provides shelter and the other organism provides nutrients to

23384-402: The syrinx, in birds, results in sound. Because of this, gas movement is vital for communication purposes. Biological system These specific systems are widely studied in human anatomy and are also present in many other animals. The notion of system (or apparatus) relies upon the concept of vital or organic function : a system is a set of organs with a definite function. This idea

23542-409: The thoracic cavity from the abdominal cavity. When it contracts, the sheet flattens, (i.e. moves downwards as shown in Fig. 7) increasing the volume of the thoracic cavity in the antero-posterior axis. The contracting diaphragm pushes the abdominal organs downwards. But because the pelvic floor prevents the lowermost abdominal organs from moving in that direction, the pliable abdominal contents cause

23700-576: The tissues via a complex network of tubes. This respiratory system is separated from their circulatory system. Gases enter and leave the body through openings called spiracles , located laterally along the thorax and abdomen . Similar to plants, insects are able to control the opening and closing of these spiracles, but instead of relying on turgor pressure , they rely on muscle contractions . These contractions result in an insect's abdomen being pumped in and out. The spiracles are connected to tubes called tracheae , which branch repeatedly and ramify into

23858-452: The trachea (1.8 cm), these bronchi (1–1.4 cm in diameter) enter the lungs at each hilum , where they branch into narrower secondary bronchi known as lobar bronchi, and these branch into narrower tertiary bronchi known as segmental bronchi. Further divisions of the segmental bronchi (1 to 6 mm in diameter) are known as 4th order, 5th order, and 6th order segmental bronchi, or grouped together as subsegmental bronchi. Compared to

24016-437: The trachea and the bronchi, as well as the larger bronchioles which simply act as air conduits , bringing air to the respiratory bronchioles, alveolar ducts and alveoli (approximately generations 17–23), where gas exchange takes place. Bronchioles are defined as the small airways lacking any cartilaginous support. The first bronchi to branch from the trachea are the right and left main bronchi. Second, only in diameter to

24174-466: The trachea down into the posterior air sacs and into the parabronchi which lead from the posterior air sacs into the lung. The air that enters the lungs joins the air which is already in the lungs, and is drawn forward across the gas exchanger into anterior air sacs. During exhalation, the posterior air sacs force air into the same parabronchi of the lungs, flowing in the same direction as during inhalation, allowing continuous gas exchange irrespective of

24332-486: The traditional immune cells and others to the site of infections. Surfactant immune function is primarily attributed to two proteins: SP-A and SP-D. These proteins can bind to sugars on the surface of pathogens and thereby opsonize them for uptake by phagocytes. It also regulates inflammatory responses and interacts with the adaptive immune response. Surfactant degradation or inactivation may contribute to enhanced susceptibility to lung inflammation and infection. Most of

24490-444: The volume of the lungs were to be instantaneously doubled at the beginning of inhalation, the air pressure inside the lungs would be halved. This happens regardless of altitude. Thus, halving of the sea level air pressure (100 kPa) results in an intrapulmonary air pressure of 50 kPa. Doing the same at 5500 m, where the atmospheric pressure is only 50 kPa, the intrapulmonary air pressure falls to 25 kPa. Therefore,

24648-410: The water as an electron acceptor. Diffusion only takes place with a concentration gradient . Gases will flow from a high concentration to a low concentration. A high oxygen concentration in the alveoli and low oxygen concentration in the capillaries causes oxygen to move into the capillaries. A high carbon dioxide concentration in the capillaries and low carbon dioxide concentration in

24806-428: The water containing dissolved air) is drawn in the opposite direction to the flow of blood in the gas exchanger. A countercurrent system such as this maintains a steep concentration gradient along the length of the gas-exchange surface (see lower diagram in Fig. 2). This is the situation seen in the gills of fish and many other aquatic creatures . The gas-containing environmental water is drawn unidirectionally across

24964-463: Was already present in Antiquity ( Galen , Aristotle ), but the application of the term "system" is more recent. For example, the nervous system was named by Monro (1783), but Rufus of Ephesus (c. 90–120), clearly viewed for the first time the brain, spinal cord, and craniospinal nerves as an anatomical unit, although he wrote little about its function, nor gave a name to this unit. The enumeration of

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