Biology

Respiration

6,880 words, 10 sections

This chapter is the human respiratory system and the movement of gases through it: the airway air travels down, the mechanics that move it, the volumes the lungs hold, the membrane across which oxygen and carbon dioxide change places, the blood that carries them, and the disorders that follow when the whole arrangement is damaged. It is one of the most examined units in Biology 12. The papers hold 174 distinct questions on it across 78 official papers between 2000 and 2026, drawn from six boards, and the practice bank carries 973 live questions. The mean item difficulty is 1.37 on the bank scale, the same figure as the rest of the subject's easier questions, and Biology is 81 of the 180 marks in the paper. The weight sits in two topics, Respiratory System Anatomy and Ventilation with 88 distinct questions and Gas Exchange and Transport with 76, while Respiratory Health and Smoking has only 10. The syllabus gives three outcomes, on the parts of the system, on gas exchange in the lungs, and on the effect of smoking, and all three are covered below; where the outcome and the papers disagree on emphasis, the papers win.

Air runs nasal cavity, pharynx, larynx, trachea, primary bronchi, secondary bronchi, tertiary bronchi, bronchioles, alveoli, and an ordering question is among the most frequently asked in the chapter. Each stage does one job, and knowing the job is what lets you place a structure the papers have not named. The nasal cavity is lined with ciliated epithelium and carries goblet cells that secrete mucus; the mucus traps particles and the cilia drive them towards the pharynx, so the airway is filtered and cleared of dust. The same cavity warms air, using a venous plexus of capillaries lying directly underneath the mucous membrane. Inside the nose air becomes filtered, moist and warm. The pharynx is the common passage for food and air. The larynx is the voice box, holding the vocal cords, which are two thin-edged fibrous bands; its primary function is voice production and nothing else.

Below that the airway changes character. The epiglottis is the flap that keeps food out of the respiratory passage during swallowing. The trachea lies ventral to the oesophagus, which is how the two tubes share the neck without crossing, and its cartilaginous rings do three jobs at once: they support the tube, they keep the passage open, and they prevent it collapsing. The bronchi are built of cartilage and smooth muscle, the cartilage in irregular hyaline plates that stop the airway closing and the smooth muscle regulating its diameter. The bronchioles sit at the end of the bronchi and terminate in the alveoli, and they are the one part of this airway with no cartilage at all: their patency is held open by smooth muscle and elastic fibres, which is precisely why they are the place where an asthma attack does its damage.

The lungs are a pair of slightly unequal organs in the intrathoracic cavity. The left lung has two lobes, superior and inferior; the right has three, superior, medial and inferior. Each lung is encased in the pleura, a double-layered thin membranous tissue that secretes a slippery secretion and so lets the lung expand and contract without friction. The space between the two lungs is the mediastinum, and the hilum is the point at which the airway and vessels enter. The lungs are not, and this is worth stating because the papers state it, the respiratory organ of all organisms.

Common trap

"Bronchi" was written as a wrong option 7 times, the most repeated distractor in the chapter, and it is wrong almost always because of cartilage. It sounds right because the bronchi obviously belong to the airway and because they are the structure students picture when they think of branching lung tubes. What separates the two is a single fact: the bronchi have cartilage in their walls and the bronchioles have none. So when the question asks which structure has no cartilage, and offers larynx, trachea, bronchioles and bronchi, the answer is bronchioles; and when it asks which of the following is incorrect about the lungs, offering intrathoracic cavity, slightly unequal in size, encased in pleura and respiratory organ of all organisms, the answer is the fourth, not the bronchi at all.

Worked example

The question asks which of larynx, trachea, bronchioles and bronchi contains no cartilage. Take each in turn. The larynx is built on cartilage, which is why it can be damaged by a blow to the throat and why it holds the cords open for speech. The trachea is the clearest case of all: it is ringed by cartilage, and those rings are what keep the tube open. The bronchi carry irregular hyaline cartilage plates, which is exactly why the papers describe them as made of cartilage and smooth muscle. That eliminates three of the four. The bronchioles are the terminal branches, and once the cartilage runs out the airway is held open only by smooth muscle tone and elastic fibres. Bronchioles are the answer, and the step students get wrong is answering bronchi because the word sounds smaller and more peripheral, when the direction of the cartilage gradient is what decides it, and cartilage decreases as the airway branches until it disappears.

8 more sections in this chapter

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Introduction

1. The airway: the route air actually takesAir runs nasal cavity, pharynx, larynx, trachea, primary bronchi, secondary bronchi, tertiary bronchi, bronchioles, alveoli, and an ordering question is among t