Circulation and Gas Exchange
In medicine, the circulation is the movement of blood throughout the body. The blood is pumped by the heart into the aorta, where it travels to the body's major organs, and returns to the heart by way of the veins. The heart is a muscular organ in the chest cavity that pumps blood using the four chambers of the heart. The right side of the heart receives deoxygenated blood from the veins, passes it to the lungs and then the left side of the heart where it is oxygenated. The blood then returns to the right side of the heart, where it is propelled through the pulmonary artery to the lungs, where it is oxygenated, then returned to the left side of the heart, through the pulmonary veins, where it is drained into the vena cava, and returned to the heart, completing a complete cycle. When the heart is functioning normally, the blood flows in a continuous cycle, and this is called the circulatory system. After the blood has been oxygenated, it returns to the heart via the pulmonary arteries, where it is pumped into the right ventricle. The right ventricle pumps the blood into the pulmonary trunk, which carries the blood to the lungs, where it is oxygenated. The blood then returns to the left ventricle, where it is pumped into the aorta for distribution to the rest of the body. The blood in the aorta is carried by the arteries to small arteries, arterioles, and capillaries. The arterioles open into the venules, which lead to the veins. The blood is finally drained by the veins into the right atrium of the heart, where it is pumped into the superior and inferior vena cava. From the inferior, the blood flows into the right atrium, where it is pumped into the right ventricle. The blood then flows into the pulmonary trunk, which carries it to the lungs, where it is oxygenated. The blood is then returned to the left atrium, where it is pumped into the left ventricle. The blood flows into the aortic arch, and from there into the systemic circulation, which carries it to the rest of the body. The systemic circulation is sometimes referred to as the whole circulation. Gas exchange is an exchange of gases between a gas-containing fluid (blood) and a gas-free fluid (lungs). Lungs are the organs that exchange gases between the alveoli and the blood. The alveoli are the tiny, microscopic air sacs where gas exchange occurs. The alveoli are surrounded by a thin membrane known as the pleura. The lungs can exchange gases by diffusion, convection, or combination. Diffusion occurs when gases cross from one side of a membrane to the other. Convection is the movement of gases across a membrane due to the transfer of heat. Combination occurs when gases are exchanged by blood vessels, which are a type of a gas exchanger. Gas exchange occurs through the walls of the alveoli. Near the alveolar surface, the membrane is thin and allows small molecules to diffuse into the alveolar space. Further down, the membrane becomes thicker and allows larger molecules to pass. The layer of white blood cells in the blood acts as the barrier between the alveolar space and the blood.
