Compare bulk flow and diffusion. Explain how bulk flow and diffusion interact to promote gas exchange between air and blood and between blood and tissues.

Bulk flow and diffusion are two processes that work together to promote gas exchange between air and blood, as well as between blood and tissues. Bulk flow transports gases like oxygen and carbon dioxide within blood vessels, while diffusion enables the movement of these gases between blood and tissues or air. In the lungs, oxygen diffuses from the alveoli into pulmonary capillaries (external respiration) while carbon dioxide diffuses in the opposite direction. In the tissues, oxygen diffuses from the capillaries into the cells (internal respiration), and carbon dioxide produced by cellular metabolism diffuses from the cells into the capillaries. The interaction between bulk flow and diffusion is essential for maintaining adequate oxygen and carbon dioxide levels in the body, with both processes adapting to various physiological states, like increased activity during exercise.