6.5 Transport of Gases

. Transport of Gases . . Transport of Oxygen Molecular oxygen is carried in blood in two ways bound to haemoglobin within the red blood cells and dissolved in plasma. Oxygen is poorly soluble in water, so only % of the oxygen is transported in the dissolved form. % of ­oxygen binds with haemoglobin in a reversible manner to form oxyhaemoglobin (HbO ). The rate at which haemoglobin binds with O is regulated by the partial pressure of O . Each haemoglobin carries maximum of four molecules of            3DUWLDOSUHVVXUHRIR[\JHQ PP+J 3HUFHQWDJHVDWXUDWLRQRIKDHPRJORELQZLWKR[\JHQ Figure . Oxygen dissociation curve oxygen. In the alveoli high pO , low pCO , low temperature and less H + concentration, favours the formation of oxyhaemoglobin, whereas in the tissues low pO2, high pCO , high H + and high temperature favours the dissociation of oxygen from oxyhaemoglobin. A sigmoid curve (S–shaped) is obtained when percentage saturation of haemoglobin with oxygen is plotted against pO . This curve is called oxygen­haemoglobin dissociation curve (Figure . ). This S–shaped curve has a steep slope for pO values between and 50mmHg and then flattens ­between and mm Hg. Under normal physiological conditions, every 100mL of oxygenated blood can deliver about 5mL of O to the ­tissues. . . Transport of Carbon – dioxide Blood transports CO from the tissue cells to the lungs in three ways Breathing through nose is healthier than through mouth– Why? XI Std Biology-Zoology Chapter- i. Dissolved in plasma About – % of CO is transported in a dissolved form in the plasma. ii. Bound to haemoglobin About – % of dissolved CO is bound and carried in the RBCs as carbaminohaemoglobin (Hb CO ) CO + Hb Hb CO iii. As bicarbonate ions in plasma about % of CO is transported as bicarbonate ions This is influenced by pCO and the degree of haemoglobin oxygenation. RBCs contain a high concentration of the enzyme, carbonic anhydrase, whereas small amounts of carbonic ­anhydrase is present in the plasma. At the tissues the pCO is high due to catabolism and diffuses into the blood to form HCO – and H + ions. When CO diffuses into the RBCs, it combines with water forming carbonic acid (H CO ) catalyzed by carbonic anhydrase. Carbonic acid is unstable and dissociates into hydrogen and bicarbonate ions. Carbonic anhydrase facilitates the reaction in both directions. CO + H O carbonic anhydrase H CO H CO carbonic anhydrase HCO – + H + The HCO – moves quickly from the RBCs into the plasma, where it is carried to the lungs. At the alveolar site where pCO is low, the reaction is reversed leading to the formation of CO and water. Thus CO trapped as HCO – at the tissue level is transported to the alveoli and released out as CO . Every 100mL of deoxygenated blood delivers 4mL of CO to the alveoli for elimination.