(i) Why is respiratory pigment needed in multicellular organisms with large body size ? (ii) Give reasons for the following : (a) Rings of cartilage are present in the throat. (b) Lungs always contain a residual volume of air. (c) The diaphragm flattens and ribs are lifted up when we breathe in. (d) Walls of alveoli contain an extensive network of blood vessels.

(i) Respiratory pigment is needed in multicellular organisms with large body size because it helps to transport oxygen from the lungs to the body tissues and carbon dioxide from the body tissues to the lungs. (ii) (a) Rings of cartilage are present in the throat to keep the trachea open and allow for the passage of air. (b) Lungs always contain a residual volume of air because the lungs are never completely empty of air, even after exhalation. (c) The diaphragm flattens and ribs are lifted up when we breathe in because the diaphragm contracts and moves downward, increasing the volume of the thoracic cavity and allowing air to enter the lungs. (d) Walls of alveoli contain an extensive network of blood vessels because the alveoli are the site of gas exchange between the lungs and the bloodstream, and the blood vessels allow for the exchange of oxygen and carbon dioxide.