Gas exchange in the lungs, blood and tissues

Gas exchange is the physical process by which gases move passively, meaning that no energy is required to power the transport, by diffusion across a surface.

External respiration is another term for gas exchange.

It describes both the bulk flow of air into and out of the lungs and the transfer of oxygen and carbon dioxide into the bloodstream through diffusion.

Internal respiration, on the other hand, describes the capillary gas exchange in body tissues.

While the flow of air from the external environment happens due to pressure changes in the lungs, the mechanisms of alveolar gas exchange are more complex.

The primary three components of gas exchange are the surface area of the alveolo-capillary membrane, the partial pressure gradients of the gasses, and the matching of ventilation and perfusion.

So, if we were to draw a path for the oxygen molecules entering the body, it would start from the nose or mouth and end up in the lungs, where it reaches the alveoli which are wrapped in an intricate network of tiny blood vessels called pulmonary capillaries.

So, from the alveoli, the gas molecules will go into the blood in the capillaries.

Carbon dioxide follows the same path, but in the opposite direction, moving from the blood in the capillaries to the air in the alveoli and then getting exhaled.

Now, the important role in this process belongs to the alveolo–capillary membrane where the layer of alveolar cells lining the alveoli meets the endothelial cells that make up the pulmonary capillary, and is where gas exchange happens.

With that in mind, let’s just say that when it comes to the surface area of the alveolo-capillary membrane, bigger is better because a respiratory membrane with a large surface area has more gas to diffuse across it in a given period of time leading to a more efficient gas exchange.

With emphysema, for example, which is a condition where the alveoli are gradually destroyed, the total surface area that allows gas exchange is reduced.