Chapter Review

Nutrition

Human Digestive System

Five Processes of Digestion

The digestive system performs five sequential processes: ingestion, digestion (both mechanical and chemical), absorption, assimilation, and egestion. The alimentary canal runs from the oral cavity to the anus, with three major associated digestive glands — salivary glands, liver, and pancreas.

Key Points

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    Ingestion — intake of food through the oral cavity
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    Digestion — breakdown of complex molecules by enzymes (chemical) and mastication/churning (mechanical)
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    Absorption — uptake of diffusible molecules into blood or lymph, primarily in the ileum
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    Assimilation — cellular utilisation of absorbed nutrients for energy or building material
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    Egestion — elimination of undigested waste through the anus
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    Three digestion sites: oral cavity, stomach, and small intestine

Oral Cavity and Salivary Digestion

The oral cavity performs selection, mastication, lubrication, and initial digestion. Three pairs of salivary glands secrete saliva containing water, mucus, sodium bicarbonate (pH ≈ 8), and the enzyme amylase (ptyalin) that breaks starch into maltose.

Key Points

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    Three salivary gland pairs: parotid (near ears), submandibular (behind jaws), sublingual (below tongue)
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    Salivary amylase digests starch and glycogen into maltose — not disaccharides
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    Fresh saliva is alkaline (pH ≈ 8), shifting to pH 6 as CO₂ escapes
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    Bolus = softened, lubricated, partly digested food mass rolled by the tongue for swallowing
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    Teeth grind food to increase surface area for enzyme action

Swallowing and Peristalsis

Swallowing begins voluntarily but becomes a reflex once food reaches the pharynx. The epiglottis covers the glottis to direct food into the esophagus. Peristalsis — wave-like muscle contractions — propels food through the entire alimentary canal and works against gravity.

Key Points

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    Epiglottis deflects bolus away from the glottis (windpipe) into the esophagus
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    Soft palate closes the nasal opening during swallowing
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    Peristalsis: contraction behind bolus + relaxation ahead of bolus
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    Peristalsis is gravity-independent — works even upside down
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    Anti-peristalsis reverses the direction and causes vomiting
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    Hunger pangs are intensified peristaltic contractions from low blood glucose

Stomach and Gastric Digestion

The stomach stores food and partially digests proteins using gastric juice secreted by three cell types in the gastric glands. HCl activates pepsinogen to pepsin, which hydrolyses proteins into peptones and polypeptides. The churning muscular walls convert food into chyme.

Key Points

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    Cardiac sphincter (top) prevents reflux; pyloric sphincter (bottom) controls chyme release into duodenum
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    Mucous cells → mucus (protects stomach lining from self-digestion)
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    Parietal (oxyntic) cells → HCl (pH 2–3; kills microbes; activates pepsinogen)
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    Zymogen (chief) cells → pepsinogen (inactive precursor of pepsin)
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    Pepsin partially digests proteins into peptones and polypeptides — not amino acids
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    Gastrin: hormone released by protein-rich food; stimulates gastric juice secretion via blood
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    Mucus layer prevents ulcers; if it breaks down, gastric enzymes erode the stomach wall

Pancreas and Duodenal Digestion

When acidic chyme enters the duodenum, it triggers secretin release, which stimulates the pancreas to secrete pancreatic juice. Pancreatic enzymes — amylase, lipase, and trypsin — digest all three major food types. Trypsin is secreted as inactive trypsinogen, activated by enterokinase on the duodenal lining.

Key Points

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    Secretin: hormone from duodenal mucosa; stimulates pancreatic secretion and inhibits gastric secretion
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    Pancreatic amylase → starch into maltose
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    Lipase → fats into fatty acids + glycerol
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    Trypsinogen → (activated by enterokinase) → trypsin → proteins into peptones + polypeptides
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    Sodium bicarbonate in pancreatic juice neutralises acidic chyme and provides alkaline pH for enzymes
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    Small intestine regions: duodenum (20–25 cm), jejunum (~2.4 m), ileum (three-fifths of total length)

Liver and Bile

The liver secretes bile — a green fluid containing bile pigments (from haemoglobin breakdown) and bile salts. Bile has no enzymes; bile salts physically emulsify fats into small droplets to increase surface area for lipase action. Bile may be stored in the gall bladder before release into the duodenum.

Key Points

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    Bile contains NO digestive enzymes — its role is physical (emulsification) and excretory (bile pigments)
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    Bile pigments: formed from haemoglobin breakdown; accumulation in blood causes jaundice
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    Bile salts: emulsify large fat globules into small droplets for lipase efficiency
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    Gall bladder stores and concentrates bile; gall stones (cholesterol precipitates) can block bile release
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    Liver is vulnerable to rupture (large, fixed, fragile), hepatitis, alcohol damage, and cancer

Complete Enzyme Digestion Cascade

Food escaping the duodenum is fully digested in the jejunum and ileum by intestinal juice enzymes. Aminopeptidase breaks polypeptides into dipeptides, erepsin splits dipeptides into amino acids, maltase converts maltose to glucose, and lactase breaks lactose into glucose and galactose.

Key Points

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    Complete digestion pathway for proteins: proteins → peptones/polypeptides (pepsin) → dipeptides (aminopeptidase) → amino acids (erepsin)
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    Complete digestion pathway for carbohydrates: starch → maltose (amylase) → glucose (maltase)
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    Lactase deficiency causes lactose intolerance — intestinal gas and diarrhoea from milk products
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    Erepsin acts on dipeptides only, not directly on polypeptides
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    Final absorbable products: glucose, amino acids, fatty acids, glycerol

Absorption in the Ileum

Nearly all absorption occurs in the ileum, whose internal surface is amplified by circular folds, villi, and microvilli. Simple sugars and amino acids enter blood capillaries directly, while most fats recombine inside epithelial cells and enter lacteals as lipoprotein droplets before reaching the blood via the thoracic duct.

Key Points

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    Three structural adaptations maximise surface area: circular folds, villi, microvilli (brush border)
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    Each villus contains blood capillaries (for sugars/amino acids) and a lacteal (for fats)
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    Sugars and amino acids absorbed into blood capillaries via diffusion or active transport
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    Most fatty acids + glycerol recombine into fats inside epithelial cells → enter lacteals as lipoprotein droplets
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    Lipoproteins reach bloodstream via the thoracic lymphatic duct
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    Villus epithelial cells are continuously shed and replaced from intestinal crypt cell division

Large Intestine and Egestion

The large intestine (caecum, colon, rectum) absorbs water and salts from undigested material, concentrating it into faeces. Useful bacteria synthesise vitamin K. Faeces are stored in the rectum and expelled through the anus, controlled by internal (involuntary) and external (voluntary) anal sphincters.

Key Points

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    Large intestine absorbs water and salts — no significant digestion occurs here
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    Intestinal bacteria synthesise vitamin K (prolonged antibiotics can cause deficiency)
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    Rectum stores faeces; defecation reflex triggered by rectal filling
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    Internal sphincter: smooth muscle, involuntary; external sphincter: striped muscle, voluntary
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    Disorders: diarrhoea (excessive water loss), constipation (excessive water absorption), appendicitis, piles

Formulas

Salivary Amylase Reaction

Salivary amylase digests starch into maltose in the oral cavity at pH 6-8

Pepsin Reaction

Pepsin partially digests proteins into peptones and polypeptides in the stomach at pH 2-3

Trypsinogen Activation and Protein Digestion

Trypsinogen is activated by enterokinase in the duodenum to trypsin, which digests proteins

Complete Protein Digestion Pathway

Polypeptides are broken to dipeptides by aminopeptidase, then to amino acids by erepsin

Fat Digestion and Absorption

Fats are emulsified by bile, then hydrolysed by lipase; most recombine and enter lacteals