(i) In large multicellular organisms, diffusion alone cannot supply oxygen to all cells. Respiratory pigments (e.g., haemoglobin) carry oxygen from the lungs to oxygen-deficient tissues and release it there.
(ii)
(a) Rings of cartilage are present in the throat to ensure that the air-passage does not collapse, keeping it open at all times for continuous airflow.
(b) Lungs always contain a residual volume of air so that there is sufficient time for oxygen to be absorbed and for carbon dioxide to be released, ensuring continuous gas exchange.
(c) When we breathe in, lifting the ribs and flattening the diaphragm increases the chest cavity volume, causing air to be sucked into the lungs to fill the expanded alveoli.
(d) The extensive network of blood vessels in alveolar walls facilitates efficient exchange of gases — oxygen is absorbed into the blood and carbon dioxide is released into the alveoli.
Source: Chapter 5 — Life Processes, Section 5.3 Respiration
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