Biology

Digestion

6,080 words, 10 sections

Unit 14 of the Biology syllabus is the chapter that turns a meal into molecules the body can actually use, and it follows the whole route: the alimentary canal in order, the glands that empty into it, what each enzyme hydrolyses, where each macronutrient is absorbed, and the disorders that follow when the machinery fails. Across the official papers linked to this chapter it has been asked 274 distinct times between 2000 and 2026, by all six examining boards, and Biology as a whole carries 81 of the 180 marks in the paper. The weight is heavily lopsided: Digestive System Anatomy and Secretions accounts for 199 distinct questions between 2001 and 2026, Diet, Nutrition and Metabolic Disorders 40 between 2000 and 2026, and Absorption and Nutrient Utilization 35 between 2001 and 2026. The mean item difficulty is 1.37, so the questions are narrow rather than hard, and nearly all of them ask you to name one structure, one cell, one enzyme or one hormone; the practice bank holds 815 active items on this chapter. A student who has it can walk a bolus from the mouth to the rectum, say which secretion arrives at each station, trace glucose, amino acid and chylomicron to the vessel that carries each away, and account for an ulcer without guessing.

The digestive system of man is a long coiled tube extending from the mouth to the anus, and the parts are listed in the direction of passage of food. The sequence is the oral or buccal cavity, the oesophagus, the stomach, the small intestine (duodenum, jejunum and ileum), and the large intestine (ascending colon, transverse colon, descending colon, caecum and rectum). Some accounts carry the sigmoid colon and, more usefully, the dimensions: the jejunum is the middle region of the small intestine at about $2.5$ metres, the ileum is the last region at about $3.6$ metres, and the large intestine is shorter at about $1.5$ metres but much wider, about $6.5$ cm across. Three sites of digestion are named and no more: the oral cavity, the stomach and the small intestine. That number is worth holding on to, because papers keep asking where something happens first.

The mouth does four jobs before any enzyme is involved. It selects food, using taste, smell and sight, with the tongue's taste buds carrying most of the work. It grinds, by mastication with the molar teeth, and the reason is mechanical as much as culinary: small pieces pass down a narrow oesophagus, and small pieces present more surface for an enzyme to attack. It lubricates, with saliva. And it digests, which is the one job that surprises candidates. The human jaw carries 20 deciduous teeth in childhood and 32 permanent teeth in the adult, and papers ask you to pick an incisor off a labelled lower-jaw diagram. The appendix hangs off the caecum, the first part of the large intestine, and a sphincter at the end of the ileum, the ileocolic, meters residue into the large intestine.

Swallowing has three phases, and the order is examinable. The oral phase is voluntary. Once the food reaches the back of the mouth, the pharyngeal phase takes over and is the one that stops food and liquid being aspirated into the lungs: the tongue moves up and back against the roof of the mouth, the soft palate rises and closes the nasal opening, and the epiglottis, a flap of cartilage, swings into a near horizontal position over the glottis while the larynx rises beneath the back of the tongue. The oesophageal phase is then involuntary peristalsis carrying the bolus down. At the junction with the stomach sits the cardiac sphincter, a ring of muscle that closes the entrance against backflow and opens when a peristaltic wave reaches it.

Worked example

A paper gives three numbered phases of swallowing, 1 pharyngeal, 2 oral, 3 oesophageal, and asks for the sequence. The principle is that the voluntary phase always comes first and the involuntary phases follow in anatomical order, so the order is oral, then pharyngeal, then oesophageal, giving $2 \rightarrow 1 \rightarrow 3$ . Step one: the oral phase is under conscious control, so nothing involuntary can precede it. Step two: the pharyngeal phase must precede the oesophageal phase, because it is the phase in which the airway is sealed and the bolus is handed over to the gut; the oesophageal phase is simply transport. Step three: write $2, 1, 3$ and check it against the option list. The step students get wrong is treating this as a random arrangement and hunting for a pattern; the second phase is not necessarily the second step, because the pharyngeal phase is short and violent while the oesophageal phase is long, and it is the pharyngeal one that has to come early.

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Introduction

1. The alimentary canal in orderThe digestive system of man is a long coiled tube extending from the mouth to the anus, and the parts are listed in the direction of passage of food. The sequen