“Respiration is a complicated biological process and can be described by a great number of variables. When pulmonologists assess lung function, one of the first variables they measure is tidal volume (TV), or the volume of air displaced between inspiration and expiration when a person is breathing passively. Another important variable is a person’s functional residual capacity (FRC), which is the volume of air left in the lungs at the end of passive expiration. FRC can be broken down into two component volumes: expiratory reserve volume (ERV) and residual volume (RV). ERV is the volume of air displaced between passive expiration and a maximal-exertion expiration, and RV is the air remaining in the lungs even after a maximal-exertion expiration.

Equally important as air flow to proper lung functioning is blood flow, as pulmonary circulation is responsible for gas exchange with the outside environment. Deoxygenated blood is pumped from the right ventricle through the pulmonary artery, eventually travelling through alveolar capillaries. It is in the alveolar capillaries that carbon dioxide and oxygen are exchanged. After gas exchange has occurred, the newly-oxygenated blood travels back to the heart via the pulmonary veins.

The level of resistance to blood flow in pulmonary vessels has important physiological consequences. Pulmonary Vascular Resistance (PVR) through these vessels is given by the equation:

In this equation, input pressure is the blood pressure in the pulmonary arteries in mmHg, output pressure is the blood pressure in the pulmonary veins in mmHg, and cardiac output is the amount of blood leaving the heart in L/min. PVR is often given in the units of mmHg•min/L.”
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