![]() ![]() ![]() Carbon dioxide diffuses from the blood to the alveoli, where it is expelled through exhalation. Hemoglobin molecules in red blood cells release the carbon dioxide picked up from body tissues and become saturated with oxygen. Oxygen diffuses across the thin endothelium of the alveoli sacs into the blood within the surrounding capillaries. Alveoli are the respiratory surfaces of the lungs. Arterioles direct blood flow to the capillaries surrounding lung alveoli. In the lungs, pulmonary arteries form smaller blood vessels called arterioles. As the heart circulates blood, oxygen-depleted blood returning to the heart is pumped to the lungs. Oxygen is obtained as a result of respiratory system activity. Oxygen and carbon dioxide are transported through the body via the cardiovascular system. The process by which organisms exchange gases between their body cells and the environment is called respiration. Gas exchange is the primary function of red blood cells. Red blood cells flowing over the alveoli pick up oxygen, which is then carried to other parts of the body. Red blood cell degradation and erythropoiesis typically occur at the same rate to ensure homeostasis in red blood cell circulation.Īlveoli in the human lung. These organs and tissues contain white blood cells called macrophages that engulf and digest damaged or dying blood cells. When they become old or damaged, the vast majority of red blood cells are removed from circulation by the spleen, liver, and lymph nodes. Due to their lack of a nucleus and other organelles, adult red blood cells can not undergo mitosis to divide or generate new cell structures. Adults have around 25 trillion red blood cells in circulation at any given time. Red blood cells circulate on average for about four months. As a result, red blood cell production decreases. When the kidneys sense the increase in oxygen levels in the blood, they slow the release of erythropoietin. As more red blood cells enter blood circulation, oxygen levels in the blood and tissues increase. Erythropoietin stimulates the production of red blood cells by red bone marrow. When the kidneys detect low oxygen levels, they produce and release a hormone called erythropoietin. Low oxygen levels can occur for various reasons including blood loss, presence in high altitude, exercise, bone marrow damage, and low hemoglobin levels. ![]() New red blood cell production, also called erythropoiesis, is triggered by low levels of oxygen in the blood. Red blood cells are derived from stem cells in red bone marrow. Bone marrow is where blood cell production takes place. Bone marrow, scanning electron micrograph (SEM). ![]()
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