(ii) Exchange of gases: (a) Exchange of gases in lungs: It is | .

(ii) Exchange of gases:

(a) Exchange of gases in lungs: It is also called external respiration. In this gaseous exchange oxygen passes from alveoli to pulmonary capillary blood and CO2. Comes to alveoli from pulmonary capillary.

(2) Release of CO2 by the blood: The PCO2 (partial pressure of carbon dioxide) of blood reaching the alveolar capillaries is higher than the PCO2 of alveolar air. Therefore, carbon dioxide diffuses from the blood of alveolar capillaries into the alveolar air.

(b) Exchange of gases in tissues: In the tissues, exchange of gases occurs between the blood and the tissue cells. This exchange occurs via tissue fluid that bathes the tissue cells. The blood reaching the tissue capillaries has PO2 higher than that in the tissue cells and PCO2 lower than that in the tissue cells.

(iii) Transport of gases: Blood carries O2 from respiratory organs to the tissue cells for oxidation and CO2 from tissue cells to respiratory organs for elimination. Blood should be slightly alkaline to help the transport of O2 and CO2 properly.