Breathing and Exchange of Gases — Class 11 Biology

This chapter describes how animals, especially humans, take in oxygen and remove carbon dioxide. Mechanisms vary with habitat: lower invertebrates use diffusion, earthworms use moist skin, insects use tracheal tubes, aquatic arthropods and molluscs use gills, and terrestrial vertebrates use lungs. The human respiratory system consists of external nostrils, nasal chamber, pharynx, larynx (the sound box with the epiglottis), trachea, bronchi, bronchioles and alveoli, the thin vascularised sacs where gas exchange occurs; the lungs lie in an airtight thoracic chamber bounded by ribs, sternum and the diaphragm. Breathing has two stages: inspiration, when contraction of the diaphragm and external intercostal muscles increases thoracic and pulmonary volume and lowers intra-pulmonary pressure to draw air in, and expiration, when their relaxation reverses this. Respiratory volumes and capacities such as tidal volume, vital capacity and residual volume are measured with a spirometer. Exchange of oxygen and carbon dioxide at alveoli and tissues occurs by diffusion along partial-pressure gradients. Oxygen is carried mainly as oxyhaemoglobin and described by the sigmoid oxygen dissociation curve, while most carbon dioxide is transported as bicarbonate with the help of carbonic anhydrase. The respiratory rhythm is regulated by centres in the medulla and pons, and disorders include asthma and emphysema.