18.7F: Blood Flow in the Lungs

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Pulmonary circulation in the lungs is responsible for removing carbon dioxide from and replacing oxygen in deoxygenated blood.

Learning Objectives

Outline the path of pulmonary circulation: blood flow in the lungs

Key Points

Oxygen-depleted blood is pumped away from the heart via the pulmonary arteries.
The lung’s capillaries line small sacs in the lungs called the alveoli. The alveoli are the site of oxygen and carbon dioxide exchange in the lungs.
Air is inhaled through the nose or the mouth and fills the lungs. Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction. The air, along with the diffused carbon dioxide, is then exhaled.
The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle.

Key Terms

Alveoli: A small air sac in the lungs where oxygen and carbon dioxide are exchanged with blood.
pulmonary vein: The vein that returns blood to the left atrium of the heart after it has circulated through the lungs.
pulmonary artery: The artery that connects the right ventricle of the heart to the lungs.

The pulmonary circulatory system is the portion of the cardiovascular system in which oxygen-depleted blood is pumped away from the heart to the lungs via the pulmonary artery. Oxygenated blood is then returned to the heart via the pulmonary vein.

From the right ventricle of the heart, blood is pumped through the pulmonary semilunar valve into the left and right pulmonary arteries (one for each lung) and travels through the lungs. The pulmonary arteries carry deoxygenated blood to the lungs, where they release carbon dioxide and pick up oxygen during respiration.

This diagram of an alveolus indicates the capillary beds, connective tissue, alveolar sacs, alveolar duct, mucous gland, mucosal lining, pulmonary vein, pulmonary artery, and atrium.

An Alveolus: The alveoli are the site of oxygen and carbon dioxide exchange in the lungs.

The pulmonary arteries divide into thin-walled capillaries closely associated with the alveoli, small air sacs in the lungs where gas exchange occurs. Air is inhaled through the nose or the mouth and fills the lungs. Oxygen passively flows from the air inside the alveoli into the blood in the alveolar capillaries, while carbon dioxide passively flows in the opposite direction. The air, along with the diffused carbon dioxide, is then exhaled.

The oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left atrium of the heart, completing the pulmonary cycle. This blood is pumped through the bicuspid valve into the left ventricle, then distributed to the body through the systemic circulation before returning to the right atrium.