Gas exchange

Diffusion

Diffusion follows Fick’s Law. It is a passive process (no energy is required) affected by factors such as the surface area available, the distance the gas molecules must diffuse across and the concentration gradient.

Gases must first dissolve in a liquid in order to diffuse across a membrane, so all gas exchange systems require a moist environment.

In single-celled organisms, diffusion can occur straight across the cell membrane; as organisms increase in size, so does the distance gases must travel across. (Their surface area-to-volume ratio also decreases.) Diffusion alone is not efficient enough and specialized respiratory systems are required. This is the case with humans and with fish that have evolved circulatory systems: these are able to transport the gases to and from the respiratory surface and maintain a continuous concentration gradient.

In mammals

The major function of the pulmonary gas exchanger 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, consisting 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 is 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). This membrane is folded into about 300 million small air sacs called alveoli (each between 75 and 300 µm in diameter) branching off from the bronchioles in the lungs, thus providing an extremely large surface area (approximately 145 m²) for gas exchange to occur.

The lungs of an average person at rest, and breathing normally contain between about 2.5 and 3 liters of alveolar air. This semi-stagnant volume of air that always remains in the lung 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 O 2 ) remains very close to 14 kPa (105 mmHg), and that of carbon dioxide ( P C O 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.

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 fairly narrow and relatively 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 the animal 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.

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 C O 2 , and, to a lesser extent, a fall in the arterial P 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 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 C O 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 C O 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 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 C O 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).

Varying response

During physical exercise, excess carbon dioxide is produced as a result of increased respiration, and muscles and cells require increased oxygen. The body responds to this change by increasing the breathing rate, maximizing the rate of possible gas exchange.

In plants

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 essential for the conversion of light into sugar during photosynthesis. Due to the high differences in water potential in the plant versus the surrounding air, water vapor tends to evaporate from plants. Gas exchange is mediated through pores (known as stomata and located mainly on the lower side of leaves) that underlie 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.

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.

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

In fish

Fish must extract oxygen dissolved in water, not air, which has led to the evolution of gills and opercula. 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.

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.

Other examples

Insects such as crickets do not have an inner skeleton, so they exchange gases across structures known as trachea and tracheoles: tubes that run directly into the insect's body. Air enters the trachea through spiracles and diffuses into the respiring tissues.

Amphibians are able to use their skin as a respiratory surface. They also have lungs and sometimes gills can be used.