Gas Exchange - Anatomy & Physiology

Introduction

The air in the alveoli is renewed regularly, thanks to the ventilation process. Gas exchange in the lungs takes place between the blood in the capillary network surrounding the alveoli, and the air in the alveoli itself.

All of the blood from the right ventricle flows through the pulmonary artery to the capillary network which surrounds the alveoli. Another set of pulmonary capillaries receive small amounts of arterial blood from the left ventricle, via the bronchial arteries. These capillaries provide oxygen and nutrients to the lung tissue.

Principles of Gaseous Exchange

Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by diffusion. The oxygen must first dissolve before passing through the respiratory epithelium. Gas moves from a region of high partial pressure to a region of low partial pressure, down a partial pressure gradient. Partial pressure is a term used to measure gases. 'P' is the symbol used for this term. The distance between the air within the alveoli, and the blood is approx 0.7micrometers. This distance is decreased during inhalation as the lung distends. This tiny distance allows extremely fast and efficient diffusion.

Oxygen

The PO₂ is always lower in the alveoli compared to the external environment due to the oxygen diffusing across the alveolar wall continuously, and the CO2 entering the alveoli which has the effect of 'diluting' the oxygen, as it is travelling in the opposite direction to the O₂. The PO₂ in the alveoli is still higher than that in the capillaries, so oxygen diffuses into the blood. Once through the alveolar and capillary walls, the oxygen combines with haemoglobin to form oxyhaemoglobin and is transported within the bloodstream.

Carbon Dioxide

Carbon dioxide enters the red blood cell as a waste product from cells. In the red blood cell it reacts with water to form carbonic acid, CA. CA dissociates to bicarbonate ions and hydrogen ions. These diffuse into plasma, where H⁺ are buffered by haemoglobin. Approx 5% of the total body CO₂ dissolves in the plasma, approx 5% of the total body CO₂ is carried as carboxyhaemoglobin on proteins and approx 90% is carried as bicarbonate ions in the plasma. The PCO₂ in the capillaries is higher than that in the alveoli, thus CO₂ diffuses into the alveoli, where it is exhaled.

V-Q Ratio

The adequacy of pulmonary gas exchange relies on the V-Q ratio. The alveoli should receive the ideal amounts of blood and gas for gas exchange. In disease situations, the amount of air delivered may be reduced, the alveolar wall may be thickened or the alveolar surface area may be reduced meaning that less gas is able to diffuse out of the alveolus. Alternatively, blood supply may be impaired so that despite sufficient ventilation, insufficient exchange occurs to support the body.

Species Differences

Terrestrial vertebrates also have the ability to undergo gas exchange within their skin, as well as the lungs. This may account to 2% of the total gas exchange occurring within the body. This is important during its thermoregulatary functions, which involves reduced cutaneous circulation when cold temperatures are experienced.

References

Sjaastad, O.V., Hove, K. and Sand, O. (2004) Physiology of Domestic Animals. Oslo: Scandinavian Veterinary Press.

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