The pulmonary circuit takes deoxygenated blood, containing little oxygen and large amounts of carbon dioxide, from the heart to the lungs. At the lungs, carbon dioxide moves from the blood into the air in the lungs and oxygen from within the lungs moves into the blood. While this is commonly called gas exchange, these flows are relatively independent of each other. Then the pulmonary circuit takes oxygenated blood, high in oxygen and low in carbon dioxide, back to the heart.
The pulmonary circuit is one of the two circuits of the cardiovascular system, the other being the systemic circuit that takes oxygenated blood from the heart to the rest of the body and returns deoxygenated blood. The heart has four chambers, the two upper chambers are called atria and they receive blood from veins; the two lower chambers are called ventricles and they pump the blood back out into arteries. The right atrium receives deoxygenated blood from the systemic circuit and pushes it into the right ventricle. The right ventricle pumps it into the pulmonary artery, the start of the pulmonary circuit.
The pulmonary artery divides after leaving the heart into the right and left pulmonary arteries; all of these are referred to as elastic arteries, able to expand as the heart pumps blood into them and then contract to keep that blood flowing on. The right and left pulmonary arteries go towards the right and left lungs respectively, but do not reach them. Each lung is made up of lobes, so the pulmonary arteries divide into lobar arteries going towards each lobe, these then divide into smaller arteries down to arterioles that feed into the capillary beds.
The capillary beds are where everything happens, not only in the lungs but in all organs and tissues of the body. They are a network of the smallest blood vessels, spreading between the arterioles (small arteries) that deliver blood to them and the venules (small veins) that take it away. Each capillary bed or network spreads throughout the tissue that it services, so that all the cells within the body are close to a capillary. The wall of capillaries is only one cell thick, enabling molecules within the blood stream inside the capillary to pass through into the interstitial fluid outside the capillary and vice versa. The interstitial fluid not only surrounds the capillary, but the cells of the tissue as well.
The lobes of the lungs are composed of small nodules called alveoli that contain air. Each alveoli has a closely associated capillary. The amount of carbon dioxide is higher in the capillary than in the alveoli, as air has only 38 molecules of carbon dioxide per 1 million molecules; through a process called diffusion, these amounts attempt to equalize. So that the amount of carbon dioxide in the blood in the capillary becomes the same as in the gas contained in the alveoli.
Some of the carbon dioxide is dissolved in the blood plasma, some is attached to hemoglobin proteins within erythrocytes (red blood cells) and some has reacted with water (H2O) to form carbonic anhydrase attached to the erythrocytes. As the dissolved carbon dioxide passes through the blood vessel wall, that attached to the hemoglobin detaches and that contained in the carbonic anhydrase splits into carbon dioxide and water, making it available to pass through into the air within the alveoli too.
The same principle applies to the flow of oxygen from within the alveoli to the blood stream. Air is approximately 20 to 21 percent oxygen. The oxygen level within the deoxygenated blood entering the lungs is considerably lower. Some oxygen dissolves in the blood plasma, but most attaches to the hemoglobin proteins within the erythrocytes once the carbon dioxide that was attached to them has detached.
The above is the primary purpose of the pulmonary circuit, however, the lungs also play a role in the body's renin-angiotensin-aldosterone system (RAAS), a part of the endocrine (hormonal) system that regulates blood pressure and fluid balance. The lungs produce an enzyme called angiotensin-converting enzyme (ACE) that is central to RAAS. When blood pressure is low or there is a loss in blood volume, perhaps from a wound, the kidneys produce a hormone called renin. Renin cleaves (splits) an organic molecule called angiotensinogen produced by the liver. This produces a molecule called angiotensin I that travels within the blood system. When it reaches the lungs, ACE converts it to angiotensin II, which then triggers blood vessel constriction and the production of aldosterone by the adrenal glands, which work to maintain blood pressure. It is complex, but it means that the active substance, angiotensin II, effectively spreads out from the heart onwards.
The capillary web connects to venules, the smallest of veins, which merge to form larger and larger veins. Finally the pulmonary veins empty into the left atrium of the heart, ending the pulmonary circuit. The left atrium pushes the blood into the left ventricle, which then pumps it into the systemic circuit to supply the body with its needed oxygen.