Diffusion of oxygen into blood and carbon dioxide into alveoli (external respiration) is the diffusion of oxygen from air in the alveoli of lungs to blood in the pulmonary capillaries and diffusion of carbon dioxide in the opposite direction.
Transport of Oxygen and Carbon Dioxide:
Transport of oxygen and carbon dioxide in the blood and body fluids to and from the cells. 98.5 per cent oxygen bound to haemoglobin in the RBC and 1.5 per cent of oxygen dissolved in the blood plasma is transported to the body cells. Whereas carbon dioxide from the tissues is diffused into the b1,ood and transported to the lungs in the form of bicarbonate ions (78 per cent), as carbamino compounds (13 per cent) and a small percentage (9 per oent) dissolved in blood plasma. From the lungs carbon dioxide is removed in the exhalation.
This occurs in tissues throughout the body the exchange of oxygen and carbon dioxide between the systemic capillaries and tissue cells is called the internal respiration. Partial pressure of oxygen (PO2) in the systemic capillaries is higher (100 mtnHg) than the PO2 in tissues cells in the tissue cells (40 mmHg) at rest. Due to the pressure difference, oxygen diffises out of the capillaries into the tissue cells and blood PO2 drops to 40 rnm Hg. Tissues are constantly using oxygen and producing carbon dioxide. Partial Pressure of carbon dioxide (PCO2) of cells is higher (45 mrnHg) than that of systemic capillary blood (40 mmHg). As a result carbon dioxide diffise fiom tissue cells into the systemic capillaries until the PCO2 increases to 45 mm Hg.