(a) Why is respiratory pigment needed in multicellular organisms with large body size?

(b) Give reasons for the following :

1. Rings of cartilage are present in the throat.
2. Lungs always contain a residual volume of air.
3. The diaphragm flattens and ribs are lifted up when we breathe in.
4. Walls of alveoli contain an extensive network of blood vessels.

(a) In large multicellular organisms, all cells are not in direct contact with the external environment, so simple diffusion is not enough to supply oxygen to every cell. A respiratory pigment like haemoglobin is therefore needed to:

• pick up oxygen in the lungs,
• carry it through blood to body tissues,
• and help transport CO₂ back to the lungs.

(b)

1. The rings of cartilage in the throat are present to keep the trachea (windpipe) open and prevent it from collapsing when there is less air pressure during breathing.
2. The lungs always contain a residual volume of air so that there is continuous exchange of gases (oxygen and carbon dioxide) even between two breaths, preventing the lungs from collapsing completely.
3. During inhalation, the diaphragm flattens and the ribs move upward and outward, which increases the volume of the chest cavity and reduces air pressure inside the lungs. This allows air to rush in from outside to fill the lungs.
4. The walls of alveoli contain an extensive network of blood capillaries to provide a large surface area for the exchange of gases. Oxygen from the alveolar air diffuses into the blood, and carbon dioxide from the blood diffuses into the alveoli to be exhaled.