Digestion
Digestion breaks down ingested food into small, absorbable molecules. The PMDC MDCAT 2026 syllabus expects you to trace food through the human alimentary canal, identify the enzymes secreted at each stage, describe peristalsis and villi, and recognise common digestive disorders. Expect 3-4 MCQs per paper.
Human Digestive System
The human digestive system has two parts: the alimentary canal (mouth → pharynx → oesophagus → stomach → small intestine → large intestine → rectum → anus) and the accessory glands (salivary glands, liver, pancreas, gall bladder).
Five processes of digestion
- Ingestion — taking food into the mouth.
- Digestion — mechanical (chewing, churning) and chemical (enzymatic) breakdown.
- Absorption — passage of digested products into blood/lymph.
- Assimilation — use of absorbed nutrients in cells.
- Egestion — removal of undigested residue (defecation).
Key definitions
- Peristalsis
- Wave-like, involuntary contractions of circular and longitudinal smooth muscle that push food along the gut. Driven by the enteric nervous system.
- Bolus
- Soft mass of chewed food mixed with saliva, ready to be swallowed.
- Chyme
- Acidic, semi-fluid mixture of partly digested food that leaves the stomach for the small intestine.
- Villi & microvilli
- Finger-like projections (villi, ~1 mm) of the small intestine lining, each covered by epithelial cells with microvilli ("brush border"), greatly increasing absorptive surface area to ~250 m2.
- Enzyme
- Biological catalyst — usually a protein — that speeds up specific hydrolysis reactions without being consumed.
Three pairs of salivary glands (parotid, submandibular, sublingual) secrete ~1.5 L of saliva per day. Saliva contains:
- Salivary amylase (ptyalin) — hydrolyses starch → maltose. Optimum pH ~6.8.
- Mucus — lubricates the bolus.
- Lysozyme — antibacterial.
Teeth perform mastication (mechanical breakdown). The tongue rolls the bolus and pushes it through the pharynx; the epiglottis closes the trachea during swallowing.
A muscular J-shaped sac (capacity ~1-2 L). Three muscle layers (longitudinal, circular, oblique) churn food into chyme.
Gastric juice contains:
- HCl — secreted by parietal (oxyntic) cells; gives pH ~1.5-2; activates pepsinogen and kills bacteria.
- Pepsinogen → Pepsin — secreted by chief (peptic) cells; digests proteins to peptides. Optimum pH ~2.
- Mucus — secreted by goblet cells; protects the stomach lining from acid and pepsin.
- Rennin — in infants; clots milk protein casein.
- Intrinsic factor — needed to absorb vitamin B12 in the ileum.
- Gastric lipase — minor fat digestion.
The pyloric sphincter regulates passage of chyme into the duodenum.
~6-7 m long. Three regions: duodenum, jejunum, ileum. Receives bile from the gall bladder and pancreatic juice from the pancreas through the hepatopancreatic duct. Optimum pH ~7-8 (alkaline).
Bile (from liver, stored in gall bladder): no enzymes; contains bile salts that emulsify fats into tiny droplets, increasing the surface area for lipase action. Bile pigments (bilirubin) give faeces their colour.
Pancreatic juice contains:
- Pancreatic amylase — starch → maltose.
- Trypsinogen → Trypsin (activated by enterokinase); chymotrypsinogen → chymotrypsin — protein → peptides.
- Pancreatic lipase — triglycerides → fatty acids + monoglycerides.
- Nucleases — nucleic acids → nucleotides.
- Sodium bicarbonate — neutralises gastric acid.
Intestinal juice (succus entericus) from glands in the intestinal wall completes digestion: maltase, sucrase, lactase, peptidases (erepsin), enterokinase, intestinal lipase.
Each villus has:
- Surface columnar epithelium with microvilli (brush border).
- A central lacteal (lymph capillary) for fat absorption.
- A network of blood capillaries for sugars and amino acids.
Glucose and amino acids enter blood capillaries and travel via the hepatic portal vein to the liver. Fatty acids and monoglycerides reform inside epithelial cells as triglycerides, are packaged into chylomicrons, and enter the lacteal → lymph → bloodstream.
~1.5 m long. Caecum (with appendix) → ascending, transverse, descending, sigmoid colon → rectum → anus. Functions:
- Absorbs water, mineral salts, and some vitamins.
- Houses gut microbiota that synthesise vitamin K, B12, and biotin.
- Stores faeces in the rectum until egestion.
Digestive enzymes — the master reference
| Enzyme | Source | Site of action | Optimum pH | Substrate → Product |
|---|---|---|---|---|
| Salivary amylase (ptyalin) | Salivary glands | Mouth | ~6.8 (slightly acidic) | Starch → maltose |
| Pepsin (from pepsinogen) | Chief cells (stomach) | Stomach | ~2 (strongly acidic) | Protein → peptides |
| Rennin | Stomach (infants) | Stomach | ~3 | Casein → paracasein (clots milk) |
| Gastric lipase | Stomach | Stomach | ~5 | Triglycerides → fatty acids + glycerol (minor) |
| Pancreatic amylase | Pancreas | Duodenum | ~7–8 | Starch → maltose |
| Trypsin (from trypsinogen) | Pancreas | Small intestine | ~8 | Protein / peptide → smaller peptides |
| Chymotrypsin | Pancreas | Small intestine | ~8 | Protein / peptide → smaller peptides |
| Pancreatic lipase | Pancreas | Small intestine | ~8 | Triglycerides → fatty acids + monoglycerides |
| Nucleases (DNase, RNase) | Pancreas | Small intestine | ~8 | DNA / RNA → nucleotides |
| Enterokinase | Intestinal wall | Duodenum | ~8 | Trypsinogen → trypsin (activator) |
| Maltase | Intestinal wall | Small intestine | ~7 | Maltose → 2 glucose |
| Sucrase | Intestinal wall | Small intestine | ~7 | Sucrose → glucose + fructose |
| Lactase | Intestinal wall | Small intestine | ~7 | Lactose → glucose + galactose |
| Peptidases (erepsin) | Intestinal wall | Small intestine | ~7 | Peptides → amino acids |
| Bile salts (NOT an enzyme) | Liver, stored in gall bladder | Small intestine | ~8 | Emulsify fats (no chemical bond broken) |
Common digestive disorders
- Peptic ulcer — erosion of stomach/duodenal lining; often due to Helicobacter pylori.
- Diarrhoea — rapid loss of watery stools; major cause of child mortality in Pakistan.
- Constipation — infrequent, hard stools; low fibre / dehydration.
- Gallstones — cholesterol or bilirubin stones in gall bladder.
- Dental caries — bacterial decay of tooth enamel.
- Lactose intolerance — deficiency of lactase → bloating, diarrhoea after milk.
- Jaundice — yellow skin/sclera due to elevated bilirubin.
- Appendicitis — inflammation of the appendix; surgical emergency.
- Hepatitis — viral inflammation of the liver (A, B, C).
Worked MCQs
Five MCQs that capture the high-yield testing patterns for this chapter.
Q1. The enzyme in saliva that begins starch digestion is:
Salivary amylase, also called ptyalin, hydrolyses starch into maltose at slightly alkaline mouth pH (~6.8). It is denatured by stomach acid, so most starch digestion is finished later by pancreatic amylase.
Q2. Pepsin is most active at which pH?
Pepsin works in the strongly acidic stomach environment (~pH 1.5-2). It is secreted as inactive pepsinogen by chief cells and activated by HCl. At neutral or alkaline pH, pepsin is rapidly inactivated.
Q3. Bile salts assist fat digestion by:
Bile salts are amphipathic — they break large fat globules into many tiny droplets (emulsification), increasing surface area for pancreatic lipase. Bile itself contains no digestive enzymes.
Q4. Most absorption of digested nutrients takes place in the:
The small intestine has villi and microvilli giving an enormous absorptive surface (~250 m²). Glucose and amino acids enter blood capillaries; fatty acids enter lacteals. The large intestine mainly absorbs water and salts.
Q5. The enzyme that activates trypsinogen to trypsin in the duodenum is:
Enterokinase (enteropeptidase), secreted by intestinal mucosa, converts inactive trypsinogen into active trypsin. Trypsin then activates more trypsinogen and the other pancreatic zymogens (chymotrypsinogen, procarboxypeptidase) — a cascade.
Quick Recap
- Mouth: salivary amylase → starch to maltose; mastication; bolus formed.
- Stomach: HCl + pepsin (protein digestion at pH ~2); rennin in infants.
- Liver / gall bladder: bile (no enzyme) emulsifies fats.
- Pancreas: amylase, trypsin, chymotrypsin, lipase, nucleases + NaHCO3.
- Small intestine: maltase, sucrase, lactase, peptidases, lipase, enterokinase. Absorption via villi (lacteal + capillaries).
- Large intestine: water absorption, gut microbiota, faeces formation.
- Peristalsis = wave-like smooth-muscle contractions that move food along.
- Disorders: peptic ulcer, diarrhoea, constipation, gallstones, dental caries, lactose intolerance, jaundice.