The primary function of the respiratory system is to provide for the gas exchange between external environment and the body as a whole; this includes the processes responsible for the uptake of O₂ and excretion of CO₂ in the lungs. The respiratory system also plays an important role in the regulation of the acid-base balance during exercise and metabolic acidosis.

The respiratory system consists of the lungs, respiratory airways, leading into (and out) of the lungs and a pump (chest wall and respiratory muscles) that ventilates the lungs. The lungs are paired organs located within the thoracic cavity. The right and the left lung are separated by the heart and other mediastinal structures. The right lung is separated into three lobes and the left into two lobes. Each lung is enclosed in a protective, double-layered serous membrane called pleura. The respiratory system is divided into two functional zones:

1. Conducting zone
   For transporting air from and to the exterior. While passing through the conducting zone the inhaled air is filtrated, moistered and warmed.
2. Respiratory zone
   This is the region of the lung where the gas exchange occurs; it includes the respiratory bronchioles (which contain small clusters of alveoli), alveolar ducts and alveoli. The respiratory zone also contains the transitional zone.

Ambient air is brought into and exchanged with the air in the lungs through the process of pulmonary ventilation. The primary function of the pulmonary ventilation is to maintain a constant concentration (partial pressure) of O₂ and CO₂ in the alveolar air. At rest, a normal human breaths 12-15 times a minute. Five hundred millilitres of air per breath, or 6-8 L·min^-1 air is inspired and expired. After passing through the nasal or oral cavities, pharynx, larynx and trachea, the inspired air passes down the right and left primary bronchi, which are the two larger air passages of the lungs. These bronchi further branch into numerous smaller bronchi and bronchioles conducting the air to the alveoli, which are the basic functional component of the lungs.

The gas exchange occurs in the alveoli, which are surrounded by pulmonary capillaries. The exchange of O₂ and CO₂ between the alveolar air and pulmonary capillary blood occurs by simple diffusion. Gas molecules diffuse from alveoli to the blood or vice versa across the thin alveolocapillary membrane made up of the pulmonary epithelium, the capillary endothelium and their basement membranes. The total area of the alveolocapillary membrane in both lungs is about 70 m^2. The diffusion rates depend on the partial pressure differences of the gas across the alveolocapillary membrane and its area. Since O₂ partial pressure in the alveoli is greater than that in the venous blood entering the pulmonary capillaries, O₂ moves from the lungs into the blood. Since partial pressure of CO₂ in the blood is greater than those in the alveoli, CO₂ moves from the blood into the lungs and is expired. Thus the deoxygenated (venous) blood ejected from the right heart ventricle into the pulmonary vascular circuit, passes via the pulmonary artery to the pulmonary capillary bed, where it is oxygenated and returned to the left atrium of the heart via the pulmonary veins. At rest 250 mL of O₂ enters the body per minute and 200 mL of CO₂ is excreted.