(i) Why is respiratory pigment needed in multicellular organisms with large body sizes? (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) Multicellular organisms have large body sizes, so diffusion alone isn’t enough. Respiratory pigments like hemoglobin help in transporting oxygen efficiently to all body parts.

(ii) Reasons:

1. Cartilage rings in throat: Prevent trachea from collapsing during breathing.
2. Residual air in lungs: Maintains continuous gas exchange even between breaths.
3. Diaphragm flattens and ribs lift: Increases chest cavity volume, drawing air into lungs (inhalation).
4. Alveoli wall with blood vessels: Enables efficient gas exchange between air and blood.