Gas exchange - partial pressure, locations, external and internal respiration

▪ In a mixture, each individual gas exerts a pressure that is proportional to the concentration of that gas within the mixture. • This part of the total pressure is called a "partial pressure". • A gas moves along the part of the pressure gradient determined by its own concentration. • Gas molecules diffuse from regions of higher pressure (higher concentration) to regions of lower pressure (lower concentration). • At the lungs, gas exchange occurs between the alveolar and blood compartments. This is external respiration. • At the systemic tissue cells, gas exchange occurs between the blood and systemic cell compartments. This is internal respiration. • The gas exchange between the alveolar spaces in the lungs and the blood in pulmonary capillaries is called external respiration. • Ventilation brings air, rich in oxygen, into the alveolar spaces in the lung. • Air in the alveolar space has a: • High PO2 • Low PCO2 • The blood in the pulmonary capillary compartment (entering lungs) has a: • Low PO2 • High PCO2 • Diffusion of gases is dependent on the partial pressure of the gases. • O2 moves from the alveolar compartment to the capillary compartment. • CO2 moves from the capillary compartment to the alveolar compartment. • The gas exchange between the systemic blood capillaries and tissue cells is called internal respiration. • Blood, high in oxygen and low in carbon dioxide, circulates past tissue cells. • The blood near the tissues has a: • High PO2 • Low PCO2 • Each cell in the tissue compartment has a: •Low PO2 ▪ High PCO2 • Diffusion of gases is dependent on the partial pressure of the gases. • O2 moves from the capillary blood compartment to the cell compartment. ▪ CO2 moves from the cell compartment to the blood in the capillary compartment. ▪ During external respiration, oxygen will diffuse from the alveoli into the pulmonary capillaries ▪ CO2 moves in the opposite direction ▪ During internal respiration, oxygen will diffuse from the systemic capillaries into the tissue ▪ CO2 moves in the opposite direction

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