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Gas exchange is a biological process through which different gases are transferred in opposite directions across a specialized respiratory surface. Gases are constantly required by, and produced as a by-product of, cellular and metabolic reactions, so an efficient system for their exchange is extremely important. It is linked with respiration in animals, and both respiration and photosynthesis in plants, bacteria and some protista.

In respiration, oxygen (O
₂) is required to enter cells, while waste carbon dioxide (CO
₂) must be excreted. Respiration, which takes place in the mitochondria has four main steps: glycolysis, pyruvate oxidation, the citric acid cycle and oxidative phosphorylation. During the process of respiration, glucose is broken down into carbon dioxide (CO
₂) and water (H
₂O), with ATP being produced throughout these steps.

Photosynthesis, is the process by which plants, bacteria and some protista use light energy to produce glucose from CO
₂ and O
₂. This conversion is facilitated by the green pigment called chlorophyll, found in the chloroplast organ of these organisms.

The exchange of gases in both respiration and photosynthesis occurs as a result of diffusion down a concentration gradient: gas molecules moving from an area of high concentration to low concentration.

Diffusion

Multicellular

In multicellular organisms, diffusion alone is not efficient, so specialised respiratory systems, such as gills or lungs, are required. This is the case for mammals, fish, invertebrates, amphibians, reptiles and protista, which have evolved circulatory systems. These systems transport gases to and from the respiratory surface and maintain a continuous concentration gradient.

Some multicellular organisms such as flatworms are large but very thin, allowing their outer body surface to act as a gas exchange membrane because the surface area to volume ratio is high. Another example of an organism which uses diffusion as a mechanism of gas exchange is the sponge.

Fick's Law

Diffusion follows Fick’s Law. It is a passive process, meaning that no energy is required.

Factors affecting diffusion

• Gases must first dissolve in a liquid in order to diffuse across a membrane, so all gas exchange systems require a moist environment
• The concentration gradient
• The temperature, as this plays a key role in the kinetics of diffusion
• The distance the gases must diffuse
• The surface area available

Mammals

Main article: Respiratory system § Gas exchange

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.

Exchange membrane

An alveolus (plural: alveoli, from Latin alveus, "little cavity"), is an anatomical structure that has the form of a hollow cavity. Mainly found in the lung, the pulmonary alveoli are spherical outcroppings of the respiratory bronchioles and are the primary sites of gas exchange with the blood.

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.

Alveolar air

The changes in the composition of the alveolar air during a normal breathing cycle at rest. The scale on the left, and the blue line, indicate the partial pressures of carbon dioxide in kPa, while that on the right and the red line, indicate the partial pressures of oxygen, also in kPa (to convert kPa into mm Hg, multiply by 7.5).

The lungs of an average person can contain between 2.5 and 3 liters of alveolar air. The semi-stagnant volume of air 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 ($P_{{\mathrm {O} }_{2}}$) remains very close to 14 kPa (105 mmHg), and that of carbon dioxide ($P_{{\mathrm {CO} }_{2}}$) 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.

A highly diagrammatic illustration of the process of gas exchange in the mammalian lungs, emphasizing the differences between the gas compositions of the ambient air, the alveolar air (light blue) with which the pulmonary capillary blood equilibrates, and the blood gas tensions in the pulmonary arterial (blue blood entering the lung on the left) and venous blood (red blood leaving the lung on the right). All the gas tensions are in kPa. To convert to mm Hg, multiply by 7.5.

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 very special 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.

Pulmonary circulation

A diagrammatic histological cross-section through a portion of lung tissue showing a normally inflated alveolus (at the end of a normal exhalation), and its walls containing the pulmonary capillaries (shown in cross-section). This illustrates how the pulmonary capillary blood is completely surrounded by alveolar air. In a normal human lung all the alveoli together contain about 3 liters of alveolar air. All the pulmonary capillaries contain about 100 ml blood.

A histological cross-section through an alveolar wall showing the layers through which the gases have to move between the blood plasma and the alveolar air. The dark blue objects are the nuclei of the capillary endothelial and alveolar type I epithelial cells (or type 1 pneumocytes). The two red objects labeled "RBC" are red blood cells in the pulmonary capillary blood.

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 $P_{{\mathrm {CO} }_{2}}$, and, to a lesser extent, a fall in the arterial $P_{{\mathrm {O} }_{2}}$, 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 $P_{{\mathrm {O} }_{2}}$ 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 $P_{{\mathrm {CO} }_{2}}$ 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 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 $P_{{\mathrm {CO} }_{2}}$ 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 $P_{{\mathrm {O} }_{2}}$ 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 $P_{{\mathrm {CO} }_{2}}$. 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 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).

Plants

High precision gas exchange measurements reveal important information on plant physiology

Gas exchange in plants is dominated by the roles of carbon dioxide and water vapor. CO
₂ is the only carbon source for autotrophic organisms, making it an essential gas for uptake. Light energy facilitates the chemical reaction of photosynthesis:

CO₂ + 2H₂A + photons → [​CH₂O​] + 2A + H₂O

carbon dioxide + electron donor + light energy → carbohydrate + oxidized electron donor + water

Oxygen, essential for respiration during the night, plays a minor role in plants' gas exchange as it is always present in sufficient amounts.

Leaf structure

A stylised cross-section of a leaf, showing the key plant organs involved in gas exchange, photosynthesis and respiration

Plant gas exchange occurs via the plant leaves, where gases diffuse into the intercellular spaces via pores called stomata.

Each component of the leaf is adapted to facilitate the process of gas exchange:

• Spongy mesophyll layer: The cells are loosely packed, allowing for an increased surface area, and subsequently an increased rate of diffusion.
• Waxy cuticle: This layer defends against water loss
• Upper and lower epidermis: Usually one layer thick, but may be thicker in plants which thrive in hot regions
• Guard cell: Due to the high differences in water potential in the plant versus the surrounding air, water vapor tends to evaporate from plants. The balance between the uptake of CO
  ₂ and loss of water is mediated by guard cells. The turgidity of these cells determine the state of the stomatal opening, based on the osmotic pressure from the outside air.
• Palisade mesophyll: These cells contain the largest number of chloroplasts, containing the pigment required for photosynthesis.
• Stomata: Stomata are located primarily on the lower side of leaves, and undergo a complex regulatory system. As the condition of the stomata unavoidably influences both the CO
  ₂ and water vapor exchanges, plants experience a gas exchange dilemma: gaining enough CO
  ₂ without losing too much water.

Measuring gas exchange

Gas exchange measurements are common tools in plant science. 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 and the intercellular CO
₂ concentration, which reveal important information about the photosynthetic condition of the plants.

Hydrogencarbonate indicator is often used as a method to show the CO
₂ concentration in a solution. In this experiment, a leaf is placed inside a concealed boiling tube, and different levels of light intensity can be studied. The indicator ranges from yellow to magenta depending on the concentration of CO
₂, with yellow being the highest concentration and magenta being the lowest concentration.

Fish

Unlike mammals, fish do not possess lungs, and instead they extract oxygen from dissolved water. Uptake of oxygen is more difficult for fish than mammals, because the concentration of oxygen dissolved in water is around 1%, compared to the 21% in air. This led to the evolution of gills and opercula (otherwise known as the gill cover).

Gills

The gas exchange organs in fish are known as gills. Gills are specialized organs containing filaments and lamellae: the lamellae contain capillaries and 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. Each gill is made up of four bony gill arches that are lined with filaments. These filaments increase the surface area to volume ratio, allowing for efficient diffusion of gases. On these filaments are the lamellae, containing a transport system homologous to that in mammals, and the countercurrent flow involved in this transport system means that they can obtain oxygen at a much faster rate than mammals such as humans.

Concurrent flow

Diagrammatic representation of a concurrent vs countercurrent flow

In a concurrent flow, seen in the pulmonary exchange system, the blood and water move in the same direction meaning the the gradient is variable over the length of the gills, and subsequently the exchange will stop when an equilibrium has been reached.

Countercurrent flow

In the countercurrent flow associated with gills, 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 CO
₂ into the water.

The countercurrent system is advantageous to fish as it allows for maximum oxygen flow in order to maintain the steep concentration gradient. It also decreases the amount of energy required by the fish, as it does not have to continually splash water over its gills to obtain oxygen. The unidirectional flow of the water means that once the oxygen has been absorbed, the water will be passed out via the operculum, increasing efficiency of the system.

Invertebrates

Insects

Photographic representation of spiracles.

Due to the hard and impermeable nature of an insect's cytoskeleton, they have a more specialised gas exchange system, requiring gases to be directly transported to the tissues via a complex network of tubes called tracheae. They have no specialised transport system, and their respiratory system is separated from their circulatory system. Instead, they use openings called spiracles to transport gases. Similar to plants, they are able to control the opening and closing of these spiracles, although instead of relying on turgor pressure, they rely on muscle contractions.

These muscle contractions result in an insects abdomen being pumped in and out, in different directions depending on the type of insect.

The respiratory system in insects is primarily used to deliver O
₂ to respiring tissue and remove waste CO
₂ produced from respiration. The spiracles involved in this system are located laterally along the thorax and abdomen of insects, and lead to a specialised tracheole cell at the end of the complex tube system which provides a thin, moist surface for efficient gas exchange.

Sponges

Diagram representing the body structure of Porifera. The diagram shows the mechanism of water uptake for sponges. Yellow: pinacocytes, red: choanocytes, grey: mesohyl, pale blue: water flow

The phylum Porifera contains some of the simplest metazoans. Sponges are sessile creatures, meaning they are unable to move on their own. They rely on attachment to substrates in order to thrive. They obtain nutrients via the flow of water, and they obtain gases via simple diffusion across their cell membranes. Pores within sponges, called ostia, draw water into them and the water is subsequently circulated around the sponge via cells called choanocytes. These choanocytes have hair-like structures, similar to the villi in human intestines, which move the water across the sponges membrane surface.

Roundworms and flatworms

Cross section of a nematode, highlighting the simplistic nature of a nematode's anatomy.

Roundworms, formally known as nematodes, and flatworms, formally known as platyhelminthes do not have a formal respiratory system, and instead rely on diffusion of CO
₂ and O
₂.

Flatworms are highly simplistic animals, and have only one digestive opening. Contrary to this, roundworms have a complete digestive tract with two openings for the mouth and anus. This simplicity is mirrored in the gas exchange mechanisms for roundworms. The diffusion interface is known as a cuticle , and is a semi-permeable layer of skin on the outermost layer of their bodies. Due to the high concentration of O
₂ in the environment and the low concentration in the body of a roundworm, a high concentration gradient is maintained. This means that roundworms have no need for lungs, or a tracheal system.

O
₂ from the exterior will diffuse through the cuticle, and continue to diffuse to the tissues which require it. Once the oxygen has been absorbed, carbon dioxide will be released and diffuse out via the cuticle.

Cnidarians

Cnidarians make up some of the most beautiful creates on Earth. They are always found in aquatic environments, meaning that their gas exchange properties involve absorbing oxygen from water.

Cnidarians include corals, sea anemones, jellyfish and hydras. These animals are always found in aquatic environments, ranging from fresh water to salt water. All cnidarians contain special stinging cells called cnidocytes, which are often used in defence and for food capture. They have a formal respiratory system for gas exchange, however they do not have any respiratory organs. Instead, every cell in their body is adapted to absorb oxygen from the surrounding water, and allow enable waste products to be expelled. One key disadvantage of this feature is that cnidarians often die in environments where water is stagnant, as they deplete the water of its oxygen supply.

Corals are cnidarians which often form symbiosis with other organisms. In this symbiosis, the coral provides shelter and the other organism provides nutrients to the coral, including oxygen. However, too much oxygen can be toxic to the coral, so they have developed a method of producing antioxidants to control the oxygen concentration.

In amphibians

Gas exchange organs

Amphibians have three main organs involved in gas exchange:

• Lungs
• Gills
• Skin

The lungs are much more primitive than other amniotes, with few internal septa and a larger alveoli leading to less efficient diffusion. In order to increase the efficiency of diffusion, amphibians use a process called buccal pumping.

The dermal region of amphibians is highly vascularised, leading to efficient gas exchange. However, for this gas exchange to maintain its efficiency, the skin must act as a moist diffusion interface.

Amphibians have external gills which are used when the animal lives primarily under water. For example, frogs only have gills at the pre-metamorphosis tadpole stage. The gills are then absorbed into the body during metamorphosis, and the lungs will then take over. In toads, which spend more time on land, they have a larger alveolar surface with more developed lungs.

Buccal pumping

Buccal pumping is a method of ventilation used by amphibians. The particular amphibian moves the lower floor of its mouth in a "pumping" manner which can be observed by the naked eye.

In reptiles

Squamates

All reptiles breathe using lungs. Squamates, otherwise known as lizards and snakes, ventilation occurs by the axial musculature, but this musculature is also used during movement, so some squamates rely on buccal pumping to maintain gas exchange efficiency.

Crocodilians

Crocodiles have a diaphragm similar to a human diaphragm. Their diaphragm differs slightly from that of a human, and is referred to as the "hepatic piston". This particular type of ventilation creates a unidirectional flow of air through the lungs, and is also observed in birds.

Crocodiles have developed a mechanism to override the lack of secondary palate, allowing them to swallow and breathe at the same time. This allows them to protect their brains against damage by prey.

Turtles and tortoises

Sea turtle gasping for air at the water's surface

Due to the rigidity of turtle shells, expansion and contraction is much more difficult in comparison to other amniotes. Turtles and tortoises have subsequently developed other mechanisms to overcome this. They have primarily turned to muscle power for gas exchange. They depend on muscle layers attached to their shells, which wrap around their lungs to fill and empty them.

Turtles have an opening, homologous to an anus, called a cloaca. Through this opening, the turtle excretes, urinates and lays eggs. However, turtles can also use this opening to breathe.

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

See also

• Respiratory system
• Ventilation (physiology)

External links

• http://www.walz.com

Extra reading

Category:Biological processes *