Question

From Guyton and Hall Textbook of Medical Physiology:

Every time a person inspires, the pressure in the thoracic cavity becomes more negative than usual, causing the blood vessels in the chest to expand. This reduces the quantity of blood returning to the left side of the heart and thereby momentarily decreases the cardiac output and arterial pressure.

Why does vasodilation increase blood flow in the capillaries, but decrease it here?

As I understand it, vasodilation results in decreased resistance. According to Ohm's law, decreased resistance should increase blood flow.

Answer 1

Vasodilation increases blood flow in the capillaries due to reduced resistance in these smaller vessels. However, in the thoracic cavity, during inspiration, the expansion of blood vessels is a compensatory mechanism to decrease the quantity of blood returning to the left side of the heart, momentarily decreasing cardiac output and arterial pressure.

Step-by-step explanation:

Vasodilation refers to the widening or relaxation of blood vessels, which typically leads to increased blood flow. In capillaries, vasodilation reduces resistance, allowing more blood to flow through these smaller vessels, facilitating efficient nutrient and oxygen exchange with tissues.

On the other hand, in the thoracic cavity during inspiration, the negative pressure created expands the blood vessels as a compensatory response. This expansion is a temporary mechanism to decrease the amount of blood returning to the left side of the heart. By doing so, the heart can handle the increased venous return during inspiration without causing an excessive increase in cardiac output and arterial pressure.

In summary, while vasodilation generally increases blood flow in capillaries by reducing resistance, the expansion of blood vessels in the thoracic cavity during inspiration serves a specific purpose in regulating cardiac output and arterial pressure.