How does convection move air and blood in the respiratory system?

In the context of the human respiratory system, convection—also referred to as mass flow—is the primary mechanism responsible for the bulk movement of air and blood between the environment and the body's tissues.1

Air Movement

Convection moves air into and out of the lungs through a process driven by pressure differences. When the muscles of the chest and abdomen expand the lungs, the internal pressure falls below atmospheric pressure, causing air to flow into the lungs (inspiration). Conversely, when these muscles relax, the lungs contract, increasing the internal pressure and forcing air out (expiration). This cyclic movement ensures that fresh air reaches the conducting airways, though only a portion of this air ultimately reaches the alveoli for gas exchange.213

Blood Movement

Convection moves blood between the lungs and the rest of the body through the circulatory system. The heart acts as a pump, driving the continuous flow of blood through the pulmonary circulation. This process transports oxygen-poor blood to the lungs for gas exchange and returns oxygen-rich blood to the heart for distribution to the tissues.14