This is simply the exchange of oxygen and carbon dioxide at two locations. One is between the body cells and blood in systemic capillaries; the other is between the lungs and blood in pulmonary capillaries.
Oxygen (O2) is brought to capillaries surrounding body cells by systemic arterioles. The concentration of dissolved oxygen is expressed as its partial pressure (PO2); this value is 104 mm Hg at this point. In the thin-walled capillaries oxygen diffuses out of the plasma along its concentration gradient (block arrow). When the blood leaves the capillaries and enters the systemic venuoles the oxygen level has dropped to 40 mm Hg.
Carbon dioxide (CO2) is a waste product of cellular respiration. Accordingly, it's concentration is highest in the fluid surrounding body cells. Dissolved carbon dioxide diffuses along its concentration gradient (block arrow) into the capillaires. When this blood enters the systemic venuoles its partial pressure (PCO2) has risen from 40 to 45 mm Hg.
Blood entering and leaving systemic capillaries differs in its acidity (i.e., H+ level) as evidenced by the pH values. Blood entering these capillaries has a pH of 7.41 while that leaving these capillaries has a pH of 7.37 (these values will vary but pH will always be higher in the systemic arterioles.) Remember, the relationship between hydrogen ion concentration and pH is inverse; a high pH indicates less hydrogen ions. There are fewer hydrogen ions in systemic arteriolar blood because there is less carbon dioxide there.
Carbon dioxide picked up from the body cells (PCO2) enters the pulmonary capillaires from pulmonary arterioles. It diffuses along its concentration gradient (block arrow) into the lungs. Simultaneously, oxygen diffuses along its concentration gradient (block arrow) from the lungs and into the blood.
Blood leaving the lungs in the pulmonary venuole has more oxygen (PO2 = 104 mm Hg) than when it entered the pulmonary capillary (PO2 = 40 mm Hg). Also, the carbon dioxide level has dropped from a PCO2 of 45 to 40 mm Hg. Because there is now less carbon dioxide in the blood the hydrogen ion concentration is also lower as seen by a pH change from 7.37 to 7.41.
Last Updated: 7/18/2005