# 5.6: Perfusion

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Recall from Chapter 4 that perfusion is the passage of blood through the vessels of body structures. Perfusion of the lungs proceeds as follows.

The right atrium receives blood from systemic veins from all parts of the body except the lungs. This blood has little oxygen because the oxygen was removed and used as the blood flowed through capillaries and past body cells. Therefore, this blood is called deoxygenated blood. It also has a high concentration of CO2, which diffused into the blood from the body cells (Figure 5.10(a), Figure 5.10(b), Figure 5.10(c)).

Deoxygenated blood from the right atrium flows into the right ventricle, which then pumps it through the pulmonary arteries to the lungs. As these arteries enter and pass through the lungs, they branch into smaller vessels until they enter the thin‑walled pulmonary capillaries. These capillaries carry the blood close to the walls of the alveoli. This allows gases to diffuse between the blood in the capillaries and the air in the alveoli. The blood then enters pulmonary veins, which carry blood to the left atrium. Since this blood has a high concentration of O2, it is called oxygenated blood. It also has a low concentration of CO2. This blood will pass from the left atrium into the left ventricle, which will pump it through the systemic arteries to all parts of the body except the lungs.

Like the rate of ventilation, the rate of perfusion must vary as a person's rate of activity, and therefore the need for gas exchange, varies. Blood flow to an area of the body can be changed by altering the cardiac output and changing the diameter of the arteries delivering blood to body structures.

## Age Changes in Perfusion

There are essentially no age changes that affect the pulmonary arteries and veins. Furthermore, aging does not change cardiac output.

The reason for the minimal change in pulmonary vessels compared with other vessels in the body may be that blood pressure in the pulmonary vessels is much lower than that in the systemic vessels. When diseases such as emphysema cause a rise in pulmonary artery pressure, these arteries undergo changes that resemble atherosclerosis.

Though pulmonary arteries and veins remain largely unchanged by aging, the pulmonary capillaries decrease in number and accumulate some fibrous material. Whether these are true age changes or are due to the effects of air pollution is uncertain.

Normally, the reduction in perfusion due to changes in pulmonary vessels is slight. The effect on reducing gas exchange does not become apparent until the respiratory system is called on to deliver the maximum rate of gas exchange. Even then, this causes only a small reduction in maximum gas exchange.

However, heart disease and certain types of pneumonia and emphysema can reduce perfusion of the lungs. Such reductions decrease the rate of gas exchange and therefore decrease the ability of the respiratory system to maintain homeostasis of O2, CO2, and acid/base balance. These effects are often noticed as the sensation of being out of breath and being fatigued when one engages in vigorous physical activity.

This page titled 5.6: Perfusion is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by Augustine G. DiGiovanna via source content that was edited to the style and standards of the LibreTexts platform; a detailed edit history is available upon request.