internal respiration

Internal respiration is the delivery at the far end of the supply line — where the blood finally hands oxygen to the cells that need it and picks up the carbon dioxide they want to be rid of. If external respiration is loading the truck at the lungs, internal respiration is the doorstep drop-off out in the tissues.

More precisely, it is the exchange of gases between the blood in the systemic capillaries and the surrounding tissue cells. Oxygen diffuses out of the blood, where its partial pressure is higher, into the cells that are consuming it; carbon dioxide, produced by the cells' metabolism, diffuses the opposite way into the blood to be carried back to the lungs.

Internal respiration completes the loop that external respiration begins. It is easy to overlook in pulmonology, because the lungs themselves are healthy in this step, yet tissues can still be starved of oxygen if the blood does not arrive, does not carry enough oxygen, or the cells cannot use it — problems that often sit at the border between lung, heart and blood disorders.