Respiration and gaseous exchange
What this domain tests
The equations and sites of aerobic and anaerobic respiration, alveolar structure, the breathing mechanism, lung volumes, and the distinction between breathing and respiration. Data questions comparing inhaled and exhaled air, and questions on the effects of smoking, are frequent.
Core rules
- Aerobic respiration: glucose + oxygen gives carbon dioxide + water + energy (ATP), mainly in mitochondria. Anaerobic respiration in muscle gives lactic acid and a small amount of energy, building an oxygen debt. In yeast it gives ethanol and carbon dioxide.
- Alveoli number about 300 million, giving a huge surface area; walls are one cell thick, moist, and surrounded by capillaries. Steep concentration gradients are maintained by breathing and by blood flow.
- Inhalation: the diaphragm contracts and flattens, external intercostal muscles contract lifting the ribs, thorax volume increases, pressure falls and air enters. Exhalation is largely passive.
- Gases move by diffusion. Oxygen combines with haemoglobin to form oxyhaemoglobin; most carbon dioxide travels as hydrogencarbonate in plasma.
- The trachea is held open by C-shaped cartilage rings and lined with ciliated epithelium and mucus that trap dust.
- Tidal volume is air moved in one normal breath; vital capacity is the maximum exhaled after a maximum inhalation; residual volume cannot be exhaled.
Worked example
Explain why exhaled air still contains about 16 per cent oxygen. Not all inspired air reaches the alveoli; some fills the dead space of the trachea and bronchi. Even in alveolar air, oxygen is not fully removed because haemoglobin becomes saturated and blood flow limits further uptake, and each breath mixes with residual air. So exhaled air has less oxygen and more carbon dioxide and water vapour than inhaled air, but oxygen never falls to zero. Correct reasoning must mention dead space and mixing, not just that oxygen is used up.
Traps that cost marks
Treating breathing and respiration as the same process. Claiming anaerobic respiration in humans produces carbon dioxide. Stating that alveoli have cilia or mucus. Writing that the diaphragm relaxes in order to inhale.