Digestive System

The digestive system carries 2 of the 40 Zoology questions. The paper asks it in two ways: a name-and-number question (which cells secrete HCl, which enzyme needs enterokinase, the pH of pancreatic juice, the dental formula) and a route question (where is a nutrient absorbed, and into which vessel does it go). Almost every mark sits in the enzyme table, the gland table and the absorption routes. The traps are few but they are asked every year: bile has no enzymes, pepsin works only in acid and trypsin only in alkali, and most absorption happens in the ileum, not the stomach.

5.1 The alimentary canal and the digestive glands

The alimentary canal is a single muscular tube about 8–9 m long, open at both ends, from mouth to anus. Its digestive glands — salivary, gastric, intestinal, liver and pancreas — pour their secretions into it. The liver, pancreas and salivary glands are exocrine here because their ducts open into the canal.

The plan of the canal

Every region of the canal is built on the same four coats, from the lumen outwards:

Coat	Tissue	Job
Mucosa (mucous membrane)	epithelium + lamina propria (loose connective tissue with capillaries and lymphatics) + muscularis mucosae	secretion, absorption, a first line of defence
Submucosa	loose connective tissue, blood and lymph vessels, Meissner's plexus	carries away absorbed food; supplies the mucosa
Muscularis externa	inner circular and outer longitudinal smooth muscle, with Auerbach's (myenteric) plexus between them	peristalsis and segmentation
Serosa / adventitia	mesothelium + connective tissue	reduces friction; suspends the gut

The two nerve plexuses together form the enteric nervous system, the "little brain" of the gut; it can run peristalsis on its own, but the parasympathetic (vagus) speeds it up and the sympathetic slows it down.

The mouth (buccal cavity)

• Lined by stratified squamous non-keratinised epithelium, which resists the abrasion of food.
• Teeth: an adult has 32 permanent teeth; the dental formula is 2·1·2·3 / 2·1·2·3 = 16 + 16 (incisors, canine, premolars, molars per half jaw). The deciduous (milk) set has 20 teeth, formula 2·1·0·2 / 2·1·0·2. Teeth crush and grind food (mastication), giving the surface area that chemical digestion needs.
• Tongue: skeletal muscle covered with a mucous membrane; it mixes food, helps swallowing, and carries the taste buds.
• Saliva moistens the food, dissolves sapid substances so that they can reach the taste buds, and begins starch digestion. The ball of chewed food is the bolus.

The salivary glands are three pairs, and the paper asks which is which:

Gland	Position	Duct	Secretion
Parotid (largest)	in front of the ear, below the zygomatic arch	Stensen's duct opens beside the upper second molar	serous — watery, amylase-rich
Submandibular	under the lower jaw	Wharton's duct opens under the tongue	mixed (serous + mucous); most of the resting saliva
Sublingual (smallest)	floor of the mouth	ducts of Rivinus	mainly mucous

Saliva is 1–1.5 L a day, pH 6.8 (range 6.2–7.4), about 99.5 % water. It contains salivary amylase (ptyalin), maltase in small amount, the glycoprotein mucin (which becomes mucus), lysozyme (kills bacteria), immunoglobulin A, and the enzyme lingual lipase from the serous glands of the tongue. Salivary amylase converts starch → maltose (plus dextrins) and works best at pH 6.8; it is stopped by the acid of the stomach.

The pharynx and the oesophagus

The pharynx is the shared passage for food and air; the epiglottis closes the glottis during swallowing so that the bolus enters the oesophagus and not the trachea.

The oesophagus is a muscular tube about 25 cm long passing through the diaphragm at the cardiac sphincter. It is lined by stratified squamous non-keratinised epithelium, has no digestive glands of its own and secretes no enzymes. Food moves down by peristalsis — a wave of contraction behind the bolus — helped by gravity; the whole trip takes about 5–8 seconds. The cardiac sphincter stops the acid contents of the stomach from running back; when it is incompetent the result is gastro-oesophageal reflux (heartburn).

The stomach

The stomach is a J-shaped muscular bag holding 1.5–2 L. Its regions are the cardia (around the opening), the fundus (the dome above the opening), the body and the pyloric antrum, which ends at the pyloric sphincter.

The mucosa is folded into gastric pits lined by simple columnar epithelium made of several cell types:

Cell	Secretion	Function
Surface mucous cells	mucus + bicarbonate	a 1 mm gel layer that keeps HCl off the wall
Parietal (oxyntic) cells	HCl (pH 0.9–1.5 as secreted) and intrinsic factor	acid; intrinsic factor is needed to absorb vitamin B₁₂
Chief (zymogenic) cells	pepsinogen, rennin (infants), gastric lipase	protein and milk digestion
G cells (pyloric antrum)	gastrin	hormone — stimulates acid secretion
Enterochromaffin-like cells	histamine	amplifies acid secretion
D cells	somatostatin	inhibits acid secretion

Gastric juice is 2–3 L a day at pH 1.5–2.5. It contains HCl (about 0.5 %), pepsinogen, rennin in infants, gastric lipase, mucus and intrinsic factor.

HCl is not an enzyme. It does five jobs:

1. Converts pepsinogen → pepsin (removes part of the molecule).
2. Gives pepsin its acid optimum (pH 1.5–2).
3. Kills most bacteria swallowed with food — a chemical barrier.
4. Denatures dietary protein, unfolding it so that enzymes can reach the peptide bonds.
5. Stimulates the release of secretin and CCK when the acid chyme reaches the duodenum, and helps absorb a little calcium and iron.

Pepsin is the only proteolytic enzyme of the stomach. Secreted as inactive pepsinogen, it is switched on by HCl and then activates more pepsinogen (autocatalysis). It breaks proteins down to peptones and polypeptides — never to amino acids. Rennin (chymosin) is present in the stomach of infants; it curdles milk by converting casein → calcium paracaseinate, so that the milk stays long enough to be digested. Adults have little rennin and curdle milk with pepsin instead.

The stomach produces chyme — a soupy, acidic mixture — and passes it into the duodenum in squirts through the pyloric sphincter. Water, alcohol, some salts and a few drugs (aspirin) are absorbed here, but almost no food.

The small intestine

The small intestine is about 6–7 m long and is the organ of digestion and absorption. It has three parts:

Part	Length	Special features
Duodenum	about 25 cm, C-shaped	receives the bile duct and the pancreatic duct at the hepatopancreatic ampulla; Brunner's glands in the submucosa secrete alkaline mucus
Jejunum	about 2.5 m	longest villi, most digestion and absorption per cm
Ileum	about 3.5 m	Peyer's patches (aggregated lymphoid follicles); the main site of absorption

Its wall multiplies the absorptive surface three times over: circular folds (plicae circulares), then villi about 1 mm long, then microvilli (the brush border) on every columnar cell. The total area becomes about 200 m², roughly a tennis court.

The epithelium is simple columnar with goblet cells (mucus) and, at the base of the glands, Paneth cells (antibacterial lysozyme). Between the villi lie the crypts of Lieberkühn, simple tubular glands that supply new epithelial cells every 3–5 days. Each villus holds a blind-ended lymph vessel (the lacteal) in its centre, a network of blood capillaries, and smooth muscle that makes it sway.

Intestinal juice (succus entericus) is about 2 L a day, pH 7.8–8.2. It is watery and alkaline, with mucus and the brush-border enzymes. The enzymes are not really poured into the lumen — they sit in the membrane of the microvilli, so that the last step of digestion happens as the nutrient touches the cell that will absorb it.

The large intestine

The large intestine is about 1.5 m long and about 6 cm wide. It runs caecum → ascending colon → transverse colon → descending colon → sigmoid colon → rectum → anal canal. Its epithelium is simple columnar with very numerous goblet cells; there are no villi and no digestive enzymes.

• Caecum: a blind pouch below the ileocaecal junction, with the ileocaecal valve that stops backflow. In man it is small (about 6 cm) and its worm-like extension, the vermiform appendix (8–10 cm), is a vestigial organ — in herbivores the caecum is huge and digests cellulose.
• Colon: absorbs water and mineral salts (about 1.5 L of water a day) and compacts the residue into faeces. The resident bacteria ferment leftovers and synthesise vitamin K and some B vitamins (biotin, folate, B₁₂), which are absorbed here.
• Rectum: stores faeces until defaecation; the anal canal is lined by stratified squamous epithelium and guarded by two sphincters, the internal (involuntary, smooth muscle) and the external (voluntary, skeletal muscle).

The liver and the gall bladder

The liver is the largest gland of the body — about 1.5 kg, under the diaphragm on the right. It is built of about a million lobules, each with a central vein and plates of hepatocytes separated by sinusoids lined with Kupffer cells. It has a double blood supply: the hepatic artery (oxygen) and the hepatic portal vein (nutrients from the gut).

Its functions, in the order the paper asks them:

Function	Detail
Bile secretion	0.5–1 L a day; bile salts emulsify fat
Carbohydrate metabolism	stores glycogen (glycogenesis), breaks it down (glycogenolysis), makes glucose from amino acids and fat (gluconeogenesis)
Protein metabolism	deamination of amino acids, urea formation (ornithine cycle), synthesis of albumin, globulin, prothrombin, fibrinogen
Fat metabolism	oxidation of fat, formation of lipoproteins, cholesterol synthesis
Storage	glycogen, vitamin A, D, E, K and B₁₂, iron (as ferritin)
Detoxification	converts ammonia to urea, destroys drugs and alcohol, conjugates bilirubin
Defence	Kupffer cells phagocytose bacteria and old red cells
Haemopoiesis	in the fetus; stores iron for it later

Bile is a golden-green, alkaline (pH 7.6–8.6) fluid, 0.5–1 L a day, with no enzymes at all. Its composition:

Component	Amount / note	Role
Bile salts (sodium glycocholate, taurocholate)	the important part	emulsify fat; help absorb fatty acids and vitamins A, D, E, K
Bile pigments — bilirubin (yellow) and biliverdin (green)	from the breakdown of haemoglobin in the spleen	excretion only; give faeces their colour
Cholesterol	may crystallise into gallstones	excreted
Lecithin, water, bicarbonate		solvent, neutraliser

The gall bladder is a pear-shaped sac under the liver. It stores bile and concentrates it 5–10 times by absorbing water. Cholecystokinin (CCK) from the duodenum makes its smooth muscle contract and the sphincter of Oddi relax, so bile squirts into the duodenum when fatty food arrives.

The pancreas

The pancreas is a soft, lobulated gland lying behind the stomach, with its head in the curve of the duodenum and its tail touching the spleen. It is heterocrine — both exocrine and endocrine:

• Exocrine part (about 99 %): acini of serous cells drain into ducts that join to form the main pancreatic duct (duct of Wirsung), which opens with the bile duct at the hepatopancreatic ampulla.
• Endocrine part: the islets of Langerhans — β cells (insulin), α cells (glucagon), δ cells (somatostatin), PP cells.

Pancreatic juice is 1–1.5 L a day, pH 8.0–8.3, and is the most powerful digestive secretion. It contains:

Enzyme	Action
Pancreatic amylase	starch → maltose
Trypsin (from trypsinogen)	proteins and peptones → peptides
Chymotrypsin (from chymotrypsinogen)	proteins → peptides
Carboxypeptidase (from procarboxypeptidase)	peptides → amino acids, from the C-terminal end
Pancreatic lipase (steapsin)	emulsified fats → fatty acids + glycerol
Ribonuclease and deoxyribonuclease	RNA and DNA → nucleotides
Bicarbonate	neutralises the acid chyme; brings the duodenal pH to about 8

The three proteases are secreted as inactive zymogens. Trypsinogen is switched on by enterokinase; trypsin then switches on chymotrypsinogen and procarboxypeptidase. This chain is a safety device: if the enzymes were made active inside the pancreas they would digest it — the cause of acute pancreatitis.

The glands and their secretions — one table

Gland	Secretion	Volume / day	pH	Enzymes	Hormone that drives it
Salivary (parotid, submandibular, sublingual)	saliva	1–1.5 L	6.8	salivary amylase, maltase, lingual lipase	parasympathetic reflex (sight, smell, taste of food)
Gastric (parietal, chief, G cells)	gastric juice	2–3 L	1.5–2.5	pepsin, rennin, gastric lipase	gastrin
Liver	bile	0.5–1 L	7.6–8.6	none	secretin (volume), CCK (gall bladder contraction)
Pancreas	pancreatic juice	1–1.5 L	8.0–8.3	amylase, trypsin, chymotrypsin, carboxypeptidase, lipase, nucleases	secretin (bicarbonate), CCK (enzymes)
Intestinal (crypts, Brunner's)	succus entericus	about 2 L	7.8–8.2	maltase, sucrase, lactase, peptidases, enterokinase, intestinal lipase	local and vagal reflexes

The enzyme table

Learn this table by rows: the paper rarely asks more than one cell of it, but it can ask any cell.

Enzyme	Source	Substrate → product	Site	Optimum pH
Salivary amylase (ptyalin)	salivary glands	starch → maltose + dextrin	mouth	6.8
Gastric lipase	chief cells	butterfat → fatty acids + glycerol	stomach	5–6
Pepsin	chief cells (as pepsinogen)	proteins → peptones, polypeptides	stomach	1.5–2
Rennin (chymosin)	chief cells (infants)	casein → calcium paracaseinate	stomach	about 4
Pancreatic amylase	pancreas	starch → maltose	duodenum	7.1–8.2
Trypsin	pancreas (as trypsinogen)	proteins, peptones → peptides	duodenum	7.5–8.5
Chymotrypsin	pancreas	proteins → peptides	duodenum	7.5–8
Carboxypeptidase	pancreas	peptides → amino acids	duodenum	7.5
Pancreatic lipase	pancreas	emulsified fat → fatty acids + glycerol	duodenum	7–8
Nucleases	pancreas	DNA, RNA → nucleotides	duodenum	7.5
Enterokinase	intestinal mucosa	trypsinogen → trypsin	duodenum	7.5
Maltase	intestinal brush border	maltose → 2 glucose	jejunum, ileum	6.5–7
Sucrase (invertase)	intestinal brush border	sucrose → glucose + fructose	jejunum, ileum	6.5
Lactase	intestinal brush border	lactose → glucose + galactose	jejunum, ileum	6
Peptidases (aminopeptidase, dipeptidase)	intestinal brush border	peptides → amino acids	jejunum, ileum	7.5–8
Intestinal lipase	intestinal mucosa	fats → fatty acids + glycerol	ileum	7.5–8

1Mouth (oral cavity)2Palate3Uvula4Tongue5Teeth6Salivary glands (the three pairs are numbered 7–9)7Sublingual gland8Submandibular gland9Parotid gland10Pharynx11Oesophagus12Liver13Gall bladder14Common bile duct15Stomach16Pancreas17Pancreatic duct18Small intestine (19–21 are its three parts)19Duodenum20Jejunum21Ileum22Appendix23Large intestine (24–29 are its parts)24Transverse colon25Ascending colon26Caecum27Descending colon28Sigmoid colon29Rectum30Anus

5.2 Physiology of digestion and absorption

Digestion is the breakdown of large, insoluble food molecules into small, soluble ones that can cross the gut wall. It is mechanical (cutting, grinding, churning, emulsifying) and chemical (hydrolysis by enzymes). Absorption is the passage of the products through the mucosa into blood or lymph. Assimilation is their use by the cells.

Mechanical digestion

Process	Where	What it does
Mastication (chewing)	mouth	breaks food up, mixes it with saliva, increases surface area
Swallowing (deglutition)	pharynx	the bolus is pushed into the oesophagus; the epiglottis guards the airway
Peristalsis	all of the canal	a wave of contraction behind the bolus moves it forward; it works even against gravity
Segmentation	small intestine	local rings of contraction chop and mix chyme with the enzymes; it does not move food along
Churning	stomach	the muscular wall kneads chyme and pushes it through the pylorus
Emulsification	duodenum	bile salts break fat into droplets of about 1 μm

Chemical digestion of carbohydrates

Step	Enzyme	Site	Product
starch → maltose + dextrin	salivary amylase	mouth	maltose, dextrin
starch → maltose	pancreatic amylase	duodenum	maltose
maltose → 2 glucose	maltase	brush border	glucose
sucrose → glucose + fructose	sucrase	brush border	glucose, fructose
lactose → glucose + galactose	lactase	brush border	glucose, galactose

Cellulose is not digested at all: man has no cellulase, so it passes through as roughage (dietary fibre) — which is not useless, because it gives the colon something to work on and keeps the faeces soft.

Chemical digestion of proteins

Step	Enzyme	Site	Product
protein → peptones, polypeptides	pepsin	stomach	peptones
peptones → peptides	trypsin, chymotrypsin	duodenum	peptides
peptides → amino acids (from the C-terminal end)	carboxypeptidase	duodenum	amino acids
peptides → amino acids (from the N-terminal end)	aminopeptidase	brush border	amino acids
dipeptides → amino acids	dipeptidase	brush border	amino acids

Protein digestion therefore needs three pH environments in turn: acid in the stomach, alkaline in the duodenum, and neutral at the brush border. This is why a person whose pancreatic duct is blocked loses most protein digestion, and why the faeces of such a patient contain undigested muscle fibres.

Chemical digestion of fats

Fats are not digested in the mouth and hardly at all in the stomach (gastric lipase acts only on already-emulsified butterfat, at pH 5–6). The sequence is:

1. Emulsification by bile salts in the duodenum — one large drop becomes many small ones, so the surface for lipase increases perhaps a thousandfold.
2. Hydrolysis by pancreatic lipase: triglyceride → fatty acids + monoglycerides (and some glycerol).
3. Micelle formation: bile salts gather round the fatty acids and monoglycerides and make water-soluble micelles about 5 nm across, which carry them to the brush border.
4. Absorption by simple diffusion into the epithelial cell.

Chemical digestion of nucleic acids

Pancreatic ribonuclease and deoxyribonuclease split RNA and DNA into nucleotides; intestinal nucleosidases and phosphatases complete the job to pentose sugar, phosphate and nitrogenous bases.

The parts played by HCl, mucus, bile and enterokinase

Substance	Secreted by	Part played	What it is NOT
HCl	parietal cells	activates pepsinogen, gives pepsin its acid pH, kills bacteria, denatures protein, triggers secretin/CCK	not an enzyme; it digests nothing itself
Mucus	mucous cells of the whole canal	a viscous glycoprotein gel that lubricates and, with bicarbonate, keeps a pH of about 7 at the cell surface; protects against acid, pepsin and mechanical injury	not a digestive juice
Bile	liver, stored in gall bladder	emulsifies fat; forms micelles; helps absorb vitamins A, D, E, K; neutralises acid chyme; excretes bilirubin and cholesterol	no enzymes
Enterokinase (enteropeptidase)	duodenal mucosa	converts trypsinogen → trypsin, the master switch of pancreatic proteolysis	not a digestive enzyme of food; it activates an enzyme
Bicarbonate	pancreas, Brunner's glands, bile	neutralises acid chyme, raises duodenal pH to about 8	not an enzyme

The stomach wall survives its own acid by a mucosal barrier: a thick mucus layer, bicarbonate trapped in it, tight junctions between the surface cells so that acid cannot leak between them, and a brisk blood flow that carries away any H⁺ that gets through. Prostaglandins maintain this barrier — which is why aspirin and other NSAIDs, which block prostaglandin synthesis, cause ulcers.

The hormones of digestion

Hormone	Secreted by	Stimulus	Action
Gastrin	G cells of the pyloric antrum	food in the stomach — especially protein and distension; vagal stimulation	stimulates parietal cells to secrete HCl and chief cells to secrete pepsinogen; thickens the gastric mucosa
Secretin	S cells of the duodenum	acid chyme (pH below about 4.5) entering the duodenum	stimulates the pancreas to pour bicarbonate-rich juice; stimulates the liver to secrete bile; slows gastric emptying
CCK (cholecystokinin)	I cells of the duodenum and jejunum	fatty acids and amino acids in the duodenum	gall bladder contraction and relaxation of the sphincter of Oddi; pancreatic enzyme secretion; slows gastric emptying; causes a feeling of fullness
Gastric inhibitory peptide (GIP)	duodenal K cells	glucose, fat	inhibits gastric secretion and motility; stimulates insulin
Motilin, somatostatin	gut	—	motility; inhibition

The order to remember: acid → secretin → bicarbonate; fat → CCK → bile and enzymes; food in the stomach → gastrin → acid.

Absorption

Absorption happens mostly in the small intestine — about 90 % of it, and most of that in the ileum, whose villi are numerous and whose transit is slow. The duodenum absorbs iron and calcium; the jejunum absorbs most of the monosaccharides and amino acids; the ileum absorbs vitamin B₁₂ (with intrinsic factor), bile salts and the rest; the colon absorbs water, sodium and chloride.

Nutrient	Absorbed as	Site	Mechanism	Route
Carbohydrates	monosaccharides — glucose, galactose, fructose	jejunum, ileum	glucose and galactose by secondary active transport with Na⁺ (SGLT-1); fructose by facilitated diffusion (GLUT-5)	blood capillary → hepatic portal vein → liver
Proteins	amino acids, di- and tripeptides	jejunum, ileum	Na⁺-dependent co-transport (active)	blood capillary → hepatic portal vein → liver
Fats	fatty acids and monoglycerides (as micelles)	jejunum, ileum	simple diffusion into the epithelial cell, where they are re-esterified to triglyceride and packed with protein into chylomicrons	lacteal → lymph → thoracic duct → left subclavian vein
Water	water	small intestine (about 8 L a day), colon (1.5 L)	osmosis, following salts	blood capillary
Salts	Na⁺, Cl⁻, K⁺, HCO₃⁻	mostly ileum, some colon	active transport and exchange	blood capillary
Iron	Fe²⁺	duodenum	active transport, helped by vitamin C	blood → transferrin
Calcium	Ca²⁺	duodenum	active transport, requires vitamin D (calcitriol)	blood
Vitamins A, D, E, K	with the fat	ileum	in micelles	lacteal
Vitamin B₁₂	with intrinsic factor	ileum	receptor-mediated endocytosis	blood
Bile salts	—	ileum	active transport (enterohepatic circulation)	portal vein → liver

The mechanism of absorption is worth one more line, because it explains the "against the gradient" questions: the epithelial cell keeps its internal Na⁺ low by pumping Na⁺ out into the blood with the Na⁺/K⁺ ATPase of its basolateral membrane. Na⁺ therefore keeps entering the cell from the lumen, and it drags glucose or an amino acid in with it. The energy is spent on the pump, not on the entry step — that is what "secondary active transport" means.

The hepatic portal route

Every capillary in a villus drains into veins that join to form the superior mesenteric vein, which unites with the splenic vein to form the hepatic portal vein. This vein enters the liver and breaks up again into sinusoids, so the liver sees the absorbed glucose and amino acids before any other organ — the first-pass effect. The liver stores or converts what is needed and releases the rest into the hepatic veins → inferior vena cava → heart.

Fats take a different road. A chylomicron is far too large to enter a blood capillary, so it enters the lacteal, is carried through lymph nodes to the thoracic duct, and joins the blood at the left subclavian vein — bypassing the liver at first.

Applied notes: ulcer, jaundice and appendicitis

Peptic ulcer. An ulcer is an open sore where acid and pepsin have eaten through the mucosa. It occurs in the stomach (gastric ulcer) or, more often, the first part of the duodenum (duodenal ulcer). For decades the cause was thought to be stress and excess acid; in 1982 Barry Marshall and Robin Warren showed that most ulcers are caused by the bacterium Helicobacter pylori, which lives in the mucus layer, splits urea with its urease and neutralises acid around itself, and damages the barrier. The 2005 Nobel Prize followed. Other causes are NSAIDs (aspirin, ibuprofen), smoking, alcohol and severe stress. Symptoms: burning epigastric pain, worse when hungry in duodenal ulcer and after food in gastric ulcer; complications are bleeding, perforation and obstruction. Treatment: acid suppression (proton-pump inhibitors), antibiotics against H. pylori, and stopping NSAIDs. The ulcer is not caused by spicy food.

Jaundice. Yellowing of the skin, sclera and mucous membranes when plasma bilirubin rises above about 1 mg/dL (normal 0.3–1.0 mg/dL). Bilirubin is the yellow pigment made when haemoglobin is broken down in the spleen; the liver conjugates it and passes it into bile, and the gut bacteria turn it into stercobilin, which colours the faeces brown.

Type	Cause	Plasma bilirubin	Urine	Faeces
Pre-hepatic (haemolytic)	too much haemoglobin broken down — malaria, incompatible transfusion, haemolytic anaemia of the newborn	unconjugated rises	no bilirubin, excess urobilinogen	normal or dark
Hepatic (hepatocellular)	liver cells damaged — viral hepatitis, alcohol, cirrhosis	both fractions rise	bilirubin present	pale
Post-hepatic (obstructive)	bile duct blocked — gallstone, cancer of the head of the pancreas	conjugated rises	dark, bilirubin present	pale, clay-coloured

The obstructive type is the one to recognise: dark urine, pale stools, yellow skin and itch. In the newborn, physiological jaundice appears in the first week because the immature liver conjugates bilirubin slowly; it is treated with blue light (phototherapy).

Appendicitis. Inflammation of the vermiform appendix, usually because its lumen is blocked by a faecolith or lymphoid swelling. The classic story is pain that begins around the umbilicus and settles in the right iliac fossa at McBurney's point, with nausea, low fever and tenderness with guarding. The danger is perforation and peritonitis, so the treatment is appendicectomy. The appendix has no digestive function in man; it is a vestige of the large caecum of herbivores, though it does contain lymphoid tissue.

Worked examples

Formula and fact sheet

Lengths, volumes and numbers

Fact	Value
Alimentary canal	8–9 m
Oesophagus	25 cm
Stomach capacity	1.5–2 L
Small intestine	6–7 m (duodenum 25 cm, jejunum 2.5 m, ileum 3.5 m)
Large intestine	1.5 m
Appendix	8–10 cm, attached to the caecum
Absorptive area of small intestine	about 200 m²
Permanent teeth	32 (2·1·2·3 / 2·1·2·3)
Deciduous teeth	20 (2·1·0·2 / 2·1·0·2)
Liver mass	about 1.5 kg — largest gland
Saliva	1–1.5 L/day, pH 6.8
Gastric juice	2–3 L/day, pH 1.5–2.5
Bile	0.5–1 L/day, pH 7.6–8.6, no enzymes
Pancreatic juice	1–1.5 L/day, pH 8.0–8.3
Intestinal juice	about 2 L/day, pH 7.8–8.2
Normal plasma bilirubin	0.3–1.0 mg/dL; jaundice above about 1 mg/dL

Names to place

Item	Name
Ducts	parotid — Stensen's; submandibular — Wharton's; sublingual — Rivinus; pancreas — Wirsung
Cells	HCl and intrinsic factor — parietal (oxyntic); pepsinogen — chief; gastrin — G cells; secretin — S cells; CCK — I cells
Sphincters	cardiac, pyloric, ileocaecal, sphincter of Oddi, internal and external anal
Glands	Brunner's in the duodenal submucosa; crypts of Lieberkühn in the intestine; Peyer's patches in the ileum
Discoverers	H. pylori and peptic ulcer — Marshall and Warren (1982)

The enzymes and their pH optima

Enzyme	Optimum pH	One-line job
Salivary amylase	6.8	starch → maltose in the mouth
Pepsin	1.5–2	protein → peptones in the stomach
Rennin	about 4	milk casein curdling in infants
Pancreatic amylase	7.1–8.2	starch → maltose
Trypsin	7.5–8.5	peptides, alkaline only
Chymotrypsin, carboxypeptidase	7.5	peptides → amino acids
Pancreatic lipase	7–8	fat → fatty acids + glycerol
Enterokinase	7.5	trypsinogen → trypsin
Maltase, sucrase, lactase	6–7	disaccharides → monosaccharides
Peptidases	7.5–8	peptides → amino acids

Absorption in one line each

• Monosaccharides and amino acids → blood capillary → hepatic portal vein → liver.
• Fats → micelles → epithelial cell → chylomicrons → lacteal → thoracic duct → left subclavian vein.
• Vitamin B₁₂ needs intrinsic factor and is absorbed in the ileum.
• Iron and calcium are absorbed in the duodenum; calcium needs vitamin D.
• Water and salts are absorbed in the colon.

Hormones

Hormone	Source	Stimulus	Effect
Gastrin	G cells, pyloric antrum	protein, distension, vagus	HCl + pepsinogen
Secretin	S cells, duodenum	acid chyme	pancreatic bicarbonate
CCK	I cells, duodenum	fat, amino acids	gall bladder contraction, pancreatic enzymes

Traps

1. Bile has no enzymes — it emulsifies.
2. Pepsin only in acid; trypsin only in alkali.
3. Most absorption is in the ileum, not the stomach.
4. HCl is not an enzyme.
5. Fat goes to the lacteal, not the blood capillary.
6. Enterokinase activates trypsinogen, not pepsinogen.
7. Secretin → bicarbonate; CCK → bile and enzymes.
8. The colon has no villi and no enzymes.

Chapter summary

• The canal is a 8–9 m tube of four coats (mucosa, submucosa, muscularis, serosa); the enteric plexus runs peristalsis, modified by vagus (up) and sympathetic (down).
• The mouth has 32 permanent teeth, the bolus is made there, and salivary amylase (pH 6.8) starts starch digestion; the three gland pairs are parotid (Stensen's), submandibular (Wharton's) and sublingual (Rivinus).
• The stomach secretes 2–3 L of juice at pH 1.5–2.5: parietal cells give HCl and intrinsic factor, chief cells pepsinogen, rennin and lipase, G cells gastrin. HCl activates pepsinogen, kills bacteria and denatures protein; pepsin works only at pH 1.5–2 and stops at peptones.
• The small intestine (duodenum 25 cm, jejunum 2.5 m, ileum 3.5 m) receives bile and pancreatic juice, and its folds, villi and microvilli raise the absorptive area to about 200 m².
• The liver (1.5 kg, largest gland) makes 0.5–1 L of bile a day at pH 7.6–8.6 with no enzymes; bile salts emulsify fat. The gall bladder stores and concentrates bile and contracts under CCK.
• Pancreatic juice (1–1.5 L, pH 8.0–8.3) supplies amylase, trypsin, chymotrypsin, carboxypeptidase, lipase, nucleases and bicarbonate. Enterokinase converts trypsinogen to trypsin, which activates the other two zymogens.
• Hormones: gastrin (food → acid), secretin (acid → bicarbonate), CCK (fat → bile + enzymes).
• Absorption: monosaccharides and amino acids into blood capillaries → hepatic portal vein → liver; fats as chylomicrons into lacteals → thoracic duct; B₁₂ in the ileum with intrinsic factor; water and salts in the colon.
• Applied: peptic ulcer is mostly caused by H. pylori; obstructive jaundice gives dark urine and pale stools; appendicitis gives right iliac fossa pain at McBurney's point.

Practice questions

Set A — Recall (24 questions)

1. The pH of gastric juice is abouta6.8b7.8–8.2c1.5–2.5d8.0–8.3
2. Hydrochloric acid of the gastric juice is secreted byachief cellsbparietal cellscgoblet cellsdG cells
3. The enzyme that converts trypsinogen into trypsin isaenterokinasebpepsincrennindcarboxypeptidase
4. Bile is stored and concentrated in thealiverbduodenumcpancreasdgall bladder
5. Bile helps the digestion of fat byahydrolysing triglyceridesbemulsifying fat into tiny dropletscconverting fat into glyceroldactivating pepsin
6. The dental formula of an adult man isa2·1·0·2 / 2·1·0·2b3·1·4·2 / 3·1·4·2c2·1·2·3 / 2·1·2·3d2·2·2·3 / 2·2·2·3
7. The largest gland of the human body is thealiverbpancreascthyroiddparotid
8. Intrinsic factor, needed for the absorption of vitamin B₁₂, comes fromachief cellsbPaneth cellscBrunner's glandsdparietal cells
9. Digested fat is absorbed from the villus into theablood capillariesbhepatic portal veinclactealsdcolonic crypts
10. The duodenal hormone released by acid chyme isagastrinbsecretincinsulindcholecystokinin
11. Gastrin is secreted by theaS cells of the duodenumbI cells of the jejunumcacinar cells of the pancreasdG cells of the pyloric antrum
12. Cholecystokinin causesacontraction of the gall bladderbsecretion of bicarbonate by the pancreascsecretion of HCl by the stomachdclosure of the ileocaecal valve
13. The milk-curdling enzyme of the infant's stomach isapepsinbtrypsincrennindlactase
14. Most of the absorption of digested food occurs in theastomachbileumcduodenumdcolon
15. The optimum pH for pepsin is abouta2b5c7d8
16. Bile pigments are formed from the breakdown ofacholesterolbbile saltsclecithindhaemoglobin
17. The number of teeth in the deciduous set of man isa16b20c28d32
18. The vitamin synthesised by the bacteria of the human colon isavitamin Abvitamin Ccvitamin Kdvitamin D
19. Peyer's patches are characteristic of theaileumbjejunumcduodenumdcolon
20. Brunner's glands lie in theamucosa of the stomachbcrypts of the coloncpancreasdsubmucosa of the duodenum
21. The volume of gastric juice secreted in 24 hours is abouta0.5–1 Lb2–3 Lc1–1.5 Ld5–6 L
22. The duct of Wharton is the duct of theaparotid glandbsublingual glandcsubmandibular glanddpancreas
23. Which of the following is NOT a function of hydrochloric acid?adigestion of protein into amino acidsbactivation of pepsinogenckilling of swallowed bacteriaddenaturation of dietary protein
24. The vermiform appendix is attached to theacolonbrectumcileumdcaecum

Answers and explanations — Set A

1. (c) — a fed stomach is at pH 1.5–2.5. 6.8 is saliva, 7.8–8.2 intestinal juice and 8.0–8.3 pancreatic juice.

2. (b) — parietal (oxyntic) cells secrete HCl and intrinsic factor. Chief cells make pepsinogen, G cells gastrin.

3. (a) — enterokinase (enteropeptidase) of the duodenal mucosa is the only activator of trypsinogen; trypsin then activates the other zymogens.

4. (d) — the gall bladder stores bile and concentrates it 5–10 times; the liver makes it.

5. (b) — bile salts lower surface tension and break fat into droplets, raising the area for lipase. Bile has no lipase, so (a) is wrong.

6. (c) — 2 incisors, 1 canine, 2 premolars, 3 molars per half jaw, 16 + 16 = 32. Option (a) is the deciduous formula.

7. (a) — the liver, about 1.5 kg; the pancreas is a mixed gland but smaller.

8. (d) — parietal cells. Without intrinsic factor, B₁₂ is not absorbed in the ileum and pernicious anaemia follows.

9. (c) — fatty acids are rebuilt into triglycerides and packed as chylomicrons, which are too large for a capillary and enter the lacteal.

10. (b) — secretin, from S cells, when duodenal pH falls below about 4.5; it drives bicarbonate secretion.

11. (d) — G cells of the pyloric antrum. S cells make secretin, I cells cholecystokinin.

12. (a) — CCK contracts the gall bladder and relaxes the sphincter of Oddi; bicarbonate is secretin's job.

13. (c) — rennin (chymosin) converts casein to calcium paracaseinate. Adults have little of it.

14. (b) — the ileum, the longest part of the small intestine, with the greatest number of villi.

15. (a) — pepsin is a strongly acid protease, optimum pH 1.5–2; it is inactive at neutrality.

16. (d) — haemoglobin is broken down to haem and globin; haem gives biliverdin and bilirubin.

17. (b) — 20 milk teeth (2·1·0·2 per half jaw); the permanent set has 32.

18. (c) — colonic bacteria make vitamin K (and some B vitamins), which is absorbed in the colon.

19. (a) — aggregated lymphoid follicles in the ileum. They are part of the gut's immune defence.

20. (d) — Brunner's glands sit in the submucosa of the duodenum and pour alkaline mucus into it.

21. (b) — 2–3 L a day at pH 1.5–2.5; 1–1.5 L is saliva or pancreatic juice.

22. (c) — Wharton's duct drains the submandibular gland; Stensen's is the parotid, Rivinus the sublingual.

23. (a) — HCl does not split peptide bonds; pepsin does that, and only as far as peptones.

24. (d) — the appendix opens from the caecum, below the ileocaecal junction.

Set B — Understanding (13 questions)

1. Hydrochloric acid must be present before pepsin can digest protein because HClasupplies the energy for hydrolysisbconverts pepsinogen to pepsin and keeps the pH at pepsin's optimumcemulsifies the proteindactivates enterokinase
2. A patient with a blocked pancreatic duct passes fatty stools. The best explanation is thatapancreatic lipase does not reach the intestinebthe stomach stops making pepsincthe liver stops making biledthe villi stop absorbing glucose
3. The stomach wall is not digested by its own juice becauseapepsin is secreted in its active formbthe wall contains no proteincsaliva neutralises the acidda mucus-bicarbonate layer and tight junctions protect the cells
4. Bile contains no enzymes, yet fat digestion fails when the bile duct is blocked. This is becauseabile contains a hidden lipasebbile digests fat directlycbile salts emulsify fat and form micelles that lipase needsdbile neutralises pepsin
5. Glucose uptake by the intestinal cell is called secondary active transport becauseathe Na⁺ gradient, maintained by the Na⁺/K⁺ pump, drives itbATP is split at the brush border for every glucosecglucose simply diffuses down its gradientdthe lacteal pulls glucose in
6. Secretin secretion stops soon after acid chyme is neutralised becauseathe pancreas becomes exhaustedbgastrin switches it offcthe stimulus, a duodenal pH below about 4.5, has gonedbile destroys secretin
7. The hepatic portal vein matters because itacarries oxygen to the liverbcarries absorbed nutrients from the gut to the liver firstcdrains bile from the liverdcarries fat to the thoracic duct
8. In obstructive jaundice the faeces are pale becauseabile salts are not absorbedbthe liver stops making bile pigmentscfat is not digesteddno bilirubin reaches the gut, so no stercobilin forms
9. After a total gastrectomy a patient needs vitamin B₁₂ injections becauseathe ileum has been removedbpepsin is needed to release B₁₂cintrinsic factor is no longer secreteddcolonic bacteria are lost
10. In lactose intolerance, bloating and diarrhoea occur becauseacolonic bacteria ferment the lactose and hold water osmoticallyblactose is absorbed too quicklyclactase digests lactose inside the colondlactose destroys the villi
11. The duodenum needs pancreatic bicarbonate in order toaactivate pepsinbemulsify fatckill bacteriadraise the pH so that pancreatic enzymes can work
12. Pepsin is inactive at pH 7 becauseawater destroys itbits active site loses shape and the molecule is denaturedcit needs enterokinasedit acts only on fat
13. The villus is the unit of absorption because it hasaa thin epithelium, a capillary net and a lacteal close to the surfaceball the digestive enzymes of the gutcPeyer's patches inside itdstratified squamous epithelium

Answers and explanations — Set B

1. (b) — HCl removes part of the pepsinogen molecule to make pepsin and holds the pH at 1.5–2, where pepsin works. It supplies no energy.

2. (a) — lipase, amylase and the proteases are all in pancreatic juice; without them fat passes out undigested (steatorrhoea).

3. (d) — the mucosal barrier: thick mucus with trapped bicarbonate, tight junctions, and good blood flow. Prostaglandins maintain it, which is why NSAIDs cause ulcers.

4. (c) — emulsification and micelle formation are physical, not chemical, but without them lipase has almost no surface to act on.

5. (a) — the cell spends ATP on the basolateral Na⁺/K⁺ pump, not on the entry step; the Na⁺ gradient does the work.

6. (c) — S cells respond to H⁺; once bicarbonate has raised the pH the stimulus disappears. This is negative feedback.

7. (b) — first-pass delivery: the liver stores, converts or releases what the gut absorbed before the rest of the body sees it.

8. (d) — stercobilin, from bilirubin acted on by gut bacteria, is the brown pigment. No bile in the gut, no colour.

9. (c) — parietal cells are lost with the stomach, so B₁₂ cannot be absorbed in the ileum even if it is eaten.

10. (a) — unabsorbed lactose is a substrate for colonic bacteria; the gas and the osmotic load cause the symptoms.

11. (d) — pancreatic enzymes have optima near pH 8; the acid chyme would otherwise inactivate them.

12. (b) — pepsin is adapted to acid; at neutral pH its shape and charge change and it stops working.

13. (a) — short diffusion distance plus two exit routes, one for water-soluble and one for fat-soluble products.

Set C — Application (9 questions)

1. A person has 2.0 L of gastric juice at pH 2.0. The amount of HCl present isa0.002 molb0.20 molc0.020 mold2.0 mol
2. Taking the same juice as in the previous question, the mass of HCl is abouta0.37 gb0.73 gc1.46 gd7.3 g
3. A meal contains 250 g carbohydrate, 70 g protein and 60 g fat. Taking 4, 4 and 9 kcal g⁻¹, its energy isa1820 kcalb1120 kcalc2120 kcald860 kcal
4. A cylinder of radius 1.5 cm and length 5.0 m has folds that multiply its area 450 times. The absorptive area is abouta0.47 m²b4.7 m²c47 m²d210 m²
5. A man has burning epigastric pain, and a urea breath test is positive. The most likely cause isatoo much spicy foodbHelicobacter pylorica low-acid stomachdvitamin B₁₂ deficiency
6. A 20-year-old has pain that began near the umbilicus and is now in the right iliac fossa, with fever and tenderness at McBurney's point. The diagnosis isagastric ulcerbgallstonescviral hepatitisdacute appendicitis
7. A patient has yellow sclera, dark urine and clay-coloured stools. The cause isaexcessive haemolysisbviral hepatitiscobstruction of the bile ductdiron deficiency
8. A child with cystic fibrosis has bulky fatty stools. The step most affected isapancreatic enzymes failing to reach the duodenumbfailure of bile salt synthesiscloss of gastrindloss of intrinsic factor
9. A diet contains 100 g of protein. Taking protein as 16 % nitrogen, the nitrogen intake isa6.25 gb16 gc100 gd1.6 g

Answers and explanations — Set C

1. (c) — pH 2 means [H⁺] = 10⁻² mol L⁻¹, and HCl is fully dissociated, so n = 0.01 × 2.0 = 0.020 mol.

2. (b) — m = nM = 0.020 × 36.5 = 0.73 g of HCl. Option (c) doubles the amount by mistake.

3. (a) — 250 × 4 = 1000, 70 × 4 = 280, 60 × 9 = 540; total 1820 kcal (about 7.6 MJ).

4. (d) — plain area 2πrL = 2π(0.015)(5) = 0.47 m²; × 450 = 211 m², the textbook figure of about 200 m².

5. (b) — the urea breath test detects H. pylori urease; the bacterium, not spice, causes most peptic ulcers.

6. (d) — migration of pain from the umbilicus to the right iliac fossa with localised tenderness is the classic picture; perforation is the danger.

7. (c) — dark urine with pale stools means conjugated bilirubin is in the blood but cannot reach the gut: obstructive jaundice.

8. (a) — thick secretions block the pancreatic ducts, so lipase and the other enzymes never arrive; the fat is not hydrolysed.

9. (b) — 16 % of 100 g = 16 g N. Option (a) is the reverse calculation (100/16), the commonest slip.