What causes a decrease in venous return toward the heart?

A decrease in venous return toward the heart is primarily influenced by changes in intrathoracic pressure, which occurs during expiration. When a person exhales, the pressure in the thoracic cavity increases, which decreases the gradient that facilitates the return of blood from the veins back to the heart. This increased pressure opposes the flow of blood in the thoracic veins, thereby reducing venous return.

In contrast, inhalation creates a negative pressure in the thoracic cavity, which enhances venous return by promoting blood flow toward the heart as the large veins expand and blood is drawn in. Additionally, increased heart rate and physical activity typically work to enhance venous return due to associated mechanisms like muscle contractions and the pumping effect they have on veins. Thus, while expiration leads to a decrease in venous return due to increased intrathoracic pressure, conditions such as inhalation, increased heart rate, and physical activity generally support or enhance venous return.