02 Transport and Gas Exchange

MEDICAL ANIMATION TRANSCRIPT: The respiratory system regulates oxygen and carbon dioxide levels within the blood. Respiration includes ventilation, gas exchange between the air, blood, and tissues within the body, and the use of oxygen for metabolism. Inhalation allows oxygen to enter the body, pulling air into the nose and mouth, lungs, and into the air sacs called alveoli, where gas exchange takes place. Alveoli move freely when air is inhaled and exhaled. Capillaries are small blood vessels that line the walls of the alveoli. During gas exchange, oxygen enters and carbon dioxide exits the bloodstream via the alveolar-capillary membrane. Once oxygen molecules move from the alveoli into the capillaries, they dissolve into the plasma and enter the red blood cell or erythrocyte. Erythrocytes contain millions of soluble proteins called hemoglobin. Hemoglobin contains four subunits each capable of binding one molecule of oxygen. Once one molecule of oxygen binds to one of the subunits, the other sites bind oxygen more readily. Dissolved and bound oxygen flows through the arterial bloodstream to capillaries within tissues. Upon arrival, carbon dioxide loading of the erythrocyte promotes oxygen unloading. Oxygen metabolism within cells produces carbon dioxide gas as a metabolic waste. Carbon dioxide exits the cells and tissues and is converted into bicarbonate within the erythrocytes. Converting carbon dioxide to bicarbonate releases hydrogen ions that decrease oxygen affinity for hemoglobin, freeing the oxygen to be delivered to tissue cells. After delivering oxygen to the tissues, the carbon dioxide-rich blood returns to the lungs through the venous circulation and then to the pulmonary artery. Inside each erythrocyte, the bicarbonate conversion is reversed, recreating carbon dioxide, which diffuses across the erythrocyte into the alveoli and lungs and is excreted out of the body.