Gas exchange is the biological process by which gases move passively by diffusion across a surface. Typically, this surface is - or contains - 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 their exchange is required. Smaller organisms, such as bacteria, have a high surface-area to volume ratio. In these unicellular organisms, the external boundary of the organism - typically the cell membrane - is usually sufficient for the necessary gas exchange. 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 a small surface-area to volume ratio, specialised structures with convoluted surfaces such as gills, pulmonary alveoli and spongy mesophyll provide the additional area needed for effective gas exchange. These convoluted surfaces may sometimes be internalised into the body of the organism: this is the case for alveoli, which form the inner surface of the mammalian lung; spongy mesophyll, which is found inside the leaves of some kinds of plant; and the gills of those molluscs that have them, which may be found in the mantle cavity.
In aerobic organisms, gas exchange 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 the cell membrane of methanogenic archaea; nitrogen is exchanged in denitrifying and nitrogen fixing bacteria; and hydrogen sulfide and other gases are exchanged in giant tube worms.
- 1 Physical principles of gas-exchange
- 2 Mammals
- 3 Other vertebrates
- 4 Plants
- 5 Invertebrates
- 6 Summary of main gas exchange systems
- 7 Foot note
- 8 See also
- 9 References
Physical principles of gas-exchange
Diffusion and surface area
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 are 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 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:
- J is the flux, the amount of gas diffusing per unit area of membrane per unit time. Note that this is already scaled for the area of the membrane.
- D is the diffusion coefficient, which will differ from gas to gas, and from membrane to membrane, according to the size of the gas molecule in question, and the nature of the membrane itself (particularly its viscosity, temperature and hydrophobicity).
- φ is the concentration of the gas.
- x is the position across the thickness of the membrane.
- dφ/dx is therefore the concentration gradient across the membrane. If the two compartments are individually well-mixed, then this is simplifies to the difference in concentration of the gas between the inside and outside compartments divided by the thickness of the membrane.
- The negative sign indicates that the diffusion is always in the direction that - over time - will destroy the concentration gradient, i.e. the gas moves from high concentration to low concentration until eventually the inside and outside compartments reach equilibrium.
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 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 the amount of gas that can diffuse in a given time. This is because the amount of gas diffusing per unit time (dq/dt) 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 a given time will be in rough proportion to the volume of its cytoplasm. The volume of a unicellular organism is very small, therefore 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 the same way. Consider an imaginary organism that is a cube of side-length, L. Its volume increases with the cube (L3) of its length, but its external surface area increases only with the square (L2) 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 (dx in Fick's Law) can also be larger in larger organisms: in 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 .
Interaction with circulatory systems
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 the external environment. These organs are frequently coupled to gas-distributing circulatory systems, which transport gases to and from the gas-exchanging surface, and help maintain the continuous concentration gradients down which waste gases such as carbon dioxide can be lost, and oxygen can be taken up. Circulatory systems are needed for similar reasons to specialised gas-exchange surfaces: diffusion is too slow to meet the requirements for gas exchange into deeper tissue far from the gas-exchange surface.
Some multicellular organisms such as flatworms (Platyhelminthes) are relatively large but very thin, allowing their outer body surface to act as 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 the sponge's body is therefore exposed to a constant flow of fresh oxygenated water, so can rely on diffusion across its cell membrane to carry out the gas exchange needed for respiration. 
In organisms that have circulatory systems associated with specialised gas-exchange surfaces, there are two main ways in which the system may operate: cocurrent or countercurrent flow.
In a cocurrent flow system, the blood and gas (or the fluid containing the gas) move in the same direction. This means the size of the gradient is variable over the length of the gas-exchange surface, and subsequently the exchange will stop when an equilibrium has been reached. An inefficient concurrent flow is approximately what is seen in the alveoli of mammals: because the air moves tidally into and out of dead-end sacs, it is not possible to align the capillaries of the alveoli consistently anti-parallel to this gas-flow, because the direction of gas-flow changes from inhalation to exhalation, but the direction of blood-flow does not.
In a countercurrent flow system, gas (or the water containing the gas) is drawn in the opposite direction to the flow of blood. This countercurrent maintains a steep concentration gradient along the length of the gas-exchange surface. This is the situation seen in the gills of fish: gas (or the fluid containing the gas) is drawn unidirectionally across the gas-exchange surface, with the blood-flow in the capillaries beneath consistently arranged anti-parallel to the gas-flow for the whole of the breathing cycle.
The major function of the specialised gas exchange system in mammals is the equilibration of the blood gases with those in the alveolar air. The alveolar and pulmonary capillary gases equilibrate across the blood–air barrier, a membrane which forms the walls of the pulmonary alveoli.
The pulmonary alveoli consist of the alveolar epithelial cells, their basement membranes and the endothelial cells of the pulmonary capillaries. This blood gas barrier is extremely thin (in humans, on average, 2.2 μm thick) but extremely strong. This strength comes from the type IV collagen in between the endothelial and epithelial cells. Damage can occur to this barrier at a pressure difference of around 5.3 kPa (40 mmHg). The large surface area of the membrane comes from the folding of the membrane into 300 million alveoli, at a diameter of approximately 75-300 µm. The branching of from the bronchioles in the lungs also contributes to the extremely large surface area (approximately 145 m2) across which gas exchange can occur.
The lungs of an average person can contain between 2.5 and 3 liters of alveolar air. The semi-stagnant volume of air[note 1] that remains in the alveoli after a normal exhalation is termed the functional residual capacity. With each breath only about 350 mL (i.e. less than 15%) of this alveolar air is expelled into the ambient air to be replaced with the same volume of fresh, but moistened, atmospheric air. It is therefore obvious that the composition of the alveolar air (or functional residual capacity) changes very little under normal circumstances: the alveolar partial pressure of oxygen () remains very close to 14 kPa (105 mmHg), and that of carbon dioxide () varies minimally around 5.3 kPa (40 mmHg) throughout the respiratory 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 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 narrow, long tubes (the airways: nose, pharynx, larynx, trachea, bronchi and their branches down to the bronchioles). This anatomy, and 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 blood and body tissues are subsequently exposed to this "portable atmosphere" (the functional residual capacity - not to the outside air.
All the blood returning from the body tissues to the right side of the heart flows through the pulmonary 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 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 14 kPa (105 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 the arterial , and, to a lesser extent, a fall in the arterial , will reflexly cause deeper and faster breathing till the blood gas tensions 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 till blood gas normality is restored.
Since the blood arriving in the pulmonary capillaries has a of, on average, 6 kPa (45 mmHg), while the pressure in the alveolar air is 14 kPa (105 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 pulmonary capillaries has a 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 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 not only by the monitoring of the arterial blood gases (which accurately reflect composition of the alveolar air) by the aortic, 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 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 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 a respiratory acidosis, or a respiratory alkalosis will occur. In the long run these can be compensated by renal adjustments to the H+ and HCO3− 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 CO2 from 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 O2 molecule each the blood is said to be “saturated” with oxygen, and no further increase in the will meaningfully increase the oxygen concentration of the blood. Most of the carbon dioxide in the blood is carried as HCO3− ions in the plasma. However the conversion of dissolved CO2 into HCO3− (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 the red blood cells. The reaction can go in both directions depending on the prevailing . 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 CO2, and carbamino groups) in arterial blood (i.e. after it has equilibrated with 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).
Most fish do not possess lungs, and instead extract oxygen from dissolved water using gills. The saturated concentration of oxygen in water is around 10 mg L−1, whereas the 21% of air that is oxygen corresponds to a concentration of around 250 mg L−1; hence extracting oxygen from water can be more difficult than extracting it from air. The gas exchange organs in fish are their gills. Gills are specialised organs containing filaments, which further divide into lamellae. The lamellae contain capillaries that provide a large surface area and short diffusion distance, as they 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 the lamellae in the opposite direction. This countercurrent maintains a steep concentration gradient. 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 the operculum (gill cover).
Amphibians have three main organs involved in gas exchange: the lungs, the skin, and the gills. The relative importance of these structures differs according to the age and environment of the amphibian. The skin of amphibians and their larvae is 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 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 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 and birds 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 the rigidity of turtle and tortoise shells, significant expansion and contraction 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 - 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 back out of 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 inspiration, fresh air is taken from the trachea down into the posterior air sacs whilst air already in the lungs is drawn forward into anterior air sacs. During expiration, the posterior air sacs force air into the parabronchi of the lungs where gas exchange takes place, whilst the anterior air sacs are emptied into the trachea and exhaled. The unidirectional airflow through the parabronchi exchanges respiratory gases with a crosscurrent blood flow. The partial pressure of O2 () in the parabronchioles declines along their length as O2 diffuses into the blood. The capillaries leaving the exchanger near the entrance of airflow take up more O2 than capillaries leaving near the exit end of the parabronchi. When the contents of all capillaries mix, the final of the mixed arterial blood is higher than that of the exhaled air.
Gas exchange in plants 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 is now taken up and carbon dioxide released.
Plant gas exchange occurs mostly through the leaves. Gases diffuse into and our of the intercellular spaces within the leaf through pores called stomata, which are typically found on the lower surface of the leaf. Gases enter into 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 subsequently 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 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 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 the plant (or part of a plant) in a chamber and measuring changes in the concentration of carbon dioxide 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 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 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 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 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 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.
Unlike the invertebrates groups mentioned so far, insects are usually terrestrial, and exchange gases across a moist surface in direct contact with 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 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 the insect's body. These branches terminate in specialised tracheole cells which provides a thin, moist surface for efficient gas exchange, directly with cells.
Summary of main gas exchange systems
|Surface area||Diffusion distance||Maintaining concentration gradient||Respiratory organs|
|Human||Total alveoli = 70–100 m2||Alveolus and capillary (two cells)||Constant blood flow in capillaries; breathing||Lungs|
|Fish||Many lamellae and filaments per gill||Usually one cell||Countercurrent flow||Gills|
|Insects||Specialised tracheole cell||One cell||Buccal pumping||Spiracles|
|Sponges||Ostia pores||One cell||Water movement||None|
|Flatworms||Flat body shape||Usually one cell||Countercurrent flow||None|
|Cnidarians||Oral arms||Usually one cell||Water movement||None|
|Reptiles||Many lamellae and filaments per gill||Alveolus and capillary (two cells)||Countercurrent flow||Lungs|
|Amphibians||Many lamellae and filaments per gill||Alveolus and capillary (two cells) or one cell||Countercurrent flow||Lungs, skin and gills|
|Plants||High density of stomata; air spaces within leaf||One cell||Constant air flow||Stomata|
- Although the functional residual capacity is described here as a “semi-stagnant” volume of air, this is only true in the sense that a lake of water with a small inlet and outlet seems “stagnant” compared with the rest of river. In the case of the “functional residual capacity” the entire volume of trapped air (or the “lake” in the river analogy) is always thoroughly mixed with the incoming inhaled air. This is brought about by the microscopic subdivisions of the “functional residual capacity” into many millions of smaller, minute air sacs, the alveoli, into which the inhaled air enters turbulently. Under normal circumstances the “functional residual capacity” is far from “stagnant” but always represents a thoroughly stirred large volume of the stored air with a small amount of diluted (with water vapor) fresh inhaled air, after each in-breath.
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