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PY4.1-12 | Gastro-intestinal Physiology — Part 3

Digestion and Absorption of Nutrients (PY4.7)

Digestion is the breakdown of large molecules; absorption is the transfer from lumen to blood or lymph. Most absorption occurs in the jejunum (the first 2 metres of small intestine after the duodenum).

Figure: Digestion and Absorption of Nutrients (PY4.7)

Structural adaptations that maximise absorption:
• Circular folds (plicae circulares) → 3× increase in surface area
• Villi (1mm tall) → 10× increase
• Microvilli (brush border) → 20× increase
→ Total absorptive area: approximately 200 m² (the size of a tennis court)

Carbohydrate digestion and absorption:
• Salivary amylase (mouth) + pancreatic amylase (duodenum) → oligosaccharides, maltose
• Brush border disaccharidases → monosaccharides (glucose, galactose, fructose)
• Absorption: Glucose and galactose by SGLT-1 (Na⁺-dependent secondary active transport); Fructose by GLUT-5 (facilitated diffusion); all exit via GLUT-2 into portal blood.

Protein digestion and absorption:
• Pepsin (stomach, pH 2) → polypeptides
• Trypsin, chymotrypsin, carboxypeptidase (duodenum) → dipeptides, tripeptides, amino acids
• Brush border peptidases → single amino acids
• Absorption: Active transport (coupled to Na⁺) into enterocytes → portal blood

Fat digestion and absorption (most complex):
• Gastric lipase (stomach) — minor, hydrophilic fats
• Bile salts emulsify fat → small droplets (increase surface area for lipase)
• Pancreatic lipase (+ co-lipase) → monoglycerides + fatty acids
• Monoglycerides + fatty acids enter micelles (bile salt transport vehicles) → diffuse across brush border
• Inside enterocyte: reassembled into triglycerides → packaged into chylomicrons (lipoproteins)
• Chylomicrons exit via lacteals (lymphatic capillaries in villi) → thoracic duct → systemic blood
• Long-chain fatty acids go via lymph; short-chain fatty acids (from butter, coconut oil) go directly into portal blood.

Vitamin and mineral absorption:
• Iron: As Fe²⁺ in duodenum (HCl from stomach converts Fe³⁺→Fe²⁺). Absorbed by DMT-1 transporter. Vitamin C enhances; phytates inhibit.
• Calcium: Active transport in duodenum, facilitated by Vitamin D (calcitriol upregulates calbindin).
• Vitamin B12: Bound to intrinsic factor → absorbed in terminal ileum by cubilin receptors.
• Fat-soluble vitamins (A, D, E, K): Absorbed with fats via micelles.

GIT Movements — Regulation and Significance (PY4.8)

The gut wall contains smooth muscle that generates different types of movements — each precisely matched to its digestive function.

Figure: GIT Movements — Regulation and Significance (PY4.8)

Types of GIT movements:

1. Peristalsis — propulsive movement
• A wave of contraction preceded by relaxation (receptive relaxation).
• The "law of the intestine" (Bayliss & Starling): contraction above, relaxation below the bolus.
• Mediated by Auerbach's plexus; enhanced by parasympathetic (vagus), inhibited by sympathetic.

2. Segmentation — mixing movement
• Alternating rings of contraction and relaxation — churns chyme, mixes it with digestive juices, increases contact with mucosa.
• Most active in the duodenum and jejunum.
• No net propulsion — purely local mixing.

3. Mass peristalsis — occurs in the colon, 1–3 times/day, propels faecal matter into rectum. Triggered by the gastrocolic reflex (food in stomach → strong colonic contractions — which is why you often feel the urge to defecate after a meal).

4. Migrating Myoelectric Complex (MMC) — "housekeeper" contractions during fasting, every 90 minutes, sweep food residue and bacteria from the small intestine into the colon. Regulated by motilin. Disruption → small intestinal bacterial overgrowth (SIBO).

Swallowing (deglutition): Three phases:
1. Oral phase (voluntary): Tongue pushes bolus to posterior pharynx.
2. Pharyngeal phase (reflex): Soft palate closes nasopharynx; larynx elevates, epiglottis covers glottis; upper oesophageal sphincter opens. Breathing pauses.
3. Oesophageal phase (reflex): Primary peristalsis propels bolus; lower oesophageal sphincter (LOS) relaxes.

Defecation reflex:
• Faeces in rectum → rectal wall distension → stretch receptors → afferent signals to sacral spinal cord (S2–S4) → parasympathetic efferents → contraction of sigmoid colon and rectum + internal anal sphincter relaxes (involuntary, smooth muscle).
• External anal sphincter (voluntary, striated muscle — under conscious control via pudendal nerve) → when socially appropriate: relax → defecation.
• Valsalva manoeuvre (raised intra-abdominal pressure) assists.

Role of dietary fibre:
• Insoluble fibre (cellulose, wheat bran): Adds bulk to stool, speeds transit, reduces constipation and colon cancer risk.
• Soluble fibre (pectin, guar gum, oats): Forms a gel, slows glucose absorption (lowers GI index), binds bile acids (lowers LDL cholesterol).
• WHO recommendation: ≥25g/day. Typical Indian rural diet: adequate (traditional whole grains, vegetables). Urban Indian diet: increasingly deficient.

Liver and Gallbladder — Structure, Functions, Secretion (PY4.9)

The liver (weighing ~1.5 kg in adults) is the largest gland in the body and performs >500 metabolic functions. Its digestive role is the secretion of bile.

Figure: Liver and Gallbladder — Structure, Functions, Secretion (PY4.9)

Functional unit: the hepatic lobule — hexagonal structure with a central vein and portal triads (hepatic artery, portal vein, bile duct) at the corners. Blood flows from portal triads → hepatic sinusoids → central vein. Bile flows in the opposite direction: hepatocytes → bile canaliculi → bile ducts → hepatic duct.

Composition of bile:
• Water (97%)
• Bile salts (primary: cholic and chenodeoxycholic acid, secondary: deoxycholic and lithocholic acid) — essential for fat emulsification
• Bilirubin — breakdown product of haem; conjugated in liver (water-soluble) → excreted in bile
• Cholesterol, phospholipids, mucus
• Bile is not an enzyme — it is an emulsifier. It does not digest fats chemically; it increases the surface area for pancreatic lipase.

Enterohepatic circulation: Bile salts are reabsorbed in the terminal ileum (95%), returned to liver via portal blood, and re-secreted. Total bile salt pool recycles 6–10 times per day. Bile acid sequestrants (cholestyramine) interrupt this cycle → lower LDL.

Gallbladder: Stores and concentrates bile (10×) between meals. Cholecystokinin (CCK) → contracts gallbladder + relaxes sphincter of Oddi → bile released into duodenum. The gallbladder cannot synthesise bile — it only stores it.

Functions of the liver (beyond bile):
• Metabolism: glycogen synthesis/breakdown, gluconeogenesis, lipid synthesis, protein synthesis (albumin, clotting factors, CRP)
• Detoxification: drug metabolism (CYP450 enzymes), ammonia → urea (urea cycle)
• Storage: glycogen, fat-soluble vitamins (A, D, B12), iron (as ferritin)
• Immunological: Kupffer cells (macrophages lining sinusoids) phagocytose gut bacteria from portal blood

Liver Function Tests (LFTs):

Test	Measures	Elevated in
Serum bilirubin	Bilirubin conjugation and secretion	Hepatitis, obstruction
ALT (SGPT)	Hepatocellular damage	Hepatitis (most specific)
AST (SGOT)	Hepatocellular + muscle damage	Hepatitis, MI
ALP	Biliary obstruction, bone	Cholestasis, bone disease
GGT	Biliary/alcohol	Alcohol abuse, biliary disease
Serum albumin	Synthetic function	Chronic liver disease
PT/INR	Clotting factor synthesis	Liver failure