Glossary: Respiratory System in Humans and Animals

This chapter's terms in context

Chapter 8 covers five structural terms of the human respiratory system, trachea, bronchus, alveolus, diaphragm and the process of gaseous exchange, that together explain how oxygen enters the bloodstream and carbon dioxide leaves it. Air travels from the nose or mouth down the trachea, a tube supported by C-shaped rings of cartilage that keep it permanently open, lined with ciliated epithelium and mucus-secreting cells that trap dust and microorganisms before they reach the lungs. The trachea then splits into two bronchi, one leading to each lung, and each bronchus branches repeatedly into progressively narrower bronchioles, ending in clusters of alveoli, the tiny air sacs where gaseous exchange actually takes place.

Gaseous exchange is the diffusion of oxygen from air in the alveolus into the blood in the surrounding capillaries, and the simultaneous diffusion of carbon dioxide from that blood into the alveolar air to be exhaled, both movements happening down their own concentration gradients with no energy required from the cell. The alveolus is structured precisely to make this diffusion efficient: its wall is only one cell thick, keeping the diffusion distance short; it is surrounded by a dense network of capillaries, also one cell thick, to keep the diffusion distance short on the blood side as well; the human lungs contain hundreds of millions of alveoli, giving an enormous total surface area for exchange; and alveoli are kept moist so that oxygen can dissolve before diffusing across the membrane. A frequent exam error is naming only one or two of these four adaptations when a question asks for several, since each one increases the rate of diffusion for a distinct and separately markable reason.

The diaphragm is a dome-shaped sheet of muscle separating the thoracic cavity from the abdominal cavity, and it drives the mechanical process of breathing by changing the volume, and therefore the pressure, inside the thoracic cavity. During inhalation, the diaphragm contracts and flattens while the external intercostal muscles contract to pull the ribcage up and out, increasing thoracic volume and decreasing pressure inside the lungs below atmospheric pressure, so air is drawn in. During exhalation, the diaphragm relaxes and domes upward again while the ribcage moves down and in, decreasing thoracic volume and forcing air out. Students sometimes describe breathing purely in terms of the lungs 'expanding and contracting' without linking that change to the diaphragm's and ribcage's specific muscular actions, which is the mechanism SPM structured questions actually reward.

Distinguishing breathing, the mechanical, muscular process of moving air in and out of the lungs, from gaseous exchange, the chemical process of diffusion between alveolar air and blood, is one of the clearest conceptual separations examiners test in this chapter, since the two processes are connected but are not the same event and are marked separately in structured questions.

Paper 2 questions on this chapter often ask a candidate to explain why a person's breathing rate increases during exercise, and the strongest answers link the whole chain of reasoning together: increased muscle activity increases the rate of aerobic respiration, which increases carbon dioxide production, which is detected by receptors that increase the rate and depth of breathing to remove the excess carbon dioxide and take in more oxygen. A common scoring gap is stopping the explanation at 'the body needs more oxygen' without completing the chain back to the underlying stimulus, carbon dioxide concentration, that actually triggers the change. Practising this full explanatory chain, from muscle activity, to respiration rate, to carbon dioxide level, to breathing rate, prepares a candidate for the extended-response version of this question that appears across several past exam series.