Part 6: Gastrointestinal Physiology & Absorption

GI Motility

The gastrointestinal tract is essentially a 9-metre muscular tube that must move food in one direction (mouth → anus) while simultaneously mixing it with digestive secretions for maximal enzyme contact. This requires exquisitely coordinated muscular activity — too fast and nutrients aren't absorbed; too slow and bacterial overgrowth, bloating, and constipation result.

Smooth Muscle Physiology

GI smooth muscle is arranged in two layers: an inner circular layer (contraction narrows the lumen) and an outer longitudinal layer (contraction shortens the tube). Between them lies the myenteric (Auerbach's) plexus.

GI smooth muscle displays electrical slow waves — rhythmic depolarisation-repolarisation cycles generated by interstitial cells of Cajal (ICC), the pacemaker cells of the gut. Slow waves set the maximum possible contraction frequency but don't always produce contractions:

Peristalsis & Segmentation

Two fundamental motor patterns dominate GI motility:

A third pattern exists during fasting — the Migrating Motor Complex (MMC):

• Cyclical pattern every ~90 minutes during fasting
• Phase I: Quiescence (45–60 min)
• Phase II: Irregular contractions (30 min)
• Phase III: Intense, rhythmic sweeping contractions ("housekeeper waves") that clear residual food, bacteria, and debris from the small intestine — controlled by motilin
• Abolished by eating (replaced by segmentation)

Enteric Nervous System

The ENS contains ~500 million neurons (as many as the spinal cord) and can operate the entire GI tract independently of the brain — earning it the name "the second brain." It has two major plexuses:

Secretions

The GI tract secretes approximately 7–9 litres of fluid per day — the vast majority of which is reabsorbed. These secretions provide the enzymes, acid, bicarbonate, and bile necessary for chemical digestion.

Saliva

Salivary glands (parotid, submandibular, sublingual) produce 1–1.5 L/day of saliva — a hypotonic fluid rich in enzymes and immunological defences:

• Salivary α-amylase (ptyalin): Begins starch digestion (cleaves α-1,4 bonds, inactivated at gastric pH <4)
• Lingual lipase: Begins fat digestion (important in neonates; active at low pH, continues working in stomach)
• Mucins: Lubrication for swallowing
• IgA: Immune defence against oral pathogens
• Bicarbonate: Buffers acid; protects tooth enamel (pH ~7)

Gastric Acid & Enzymes

The stomach secretes 2–3 L/day of gastric juice with a pH as low as 1–2. The key cell types:

Pancreatic Enzymes

The exocrine pancreas secretes ~1.5 L/day of enzyme-rich, bicarbonate-rich fluid into the duodenum:

• Acinar cells produce digestive enzymes: trypsinogen, chymotrypsinogen, proelastase, procarboxypeptidase (all released as zymogens to prevent autodigestion), pancreatic lipase, colipase, pancreatic amylase, and nucleases
• Ductal cells secrete HCO₃⁻ (via CFTR and Cl⁻/HCO₃⁻ exchangers) to neutralise gastric acid in the duodenum — pH must rise to ~7 for pancreatic enzymes to work optimally
• Activation cascade: Enterokinase (brush border enzyme on duodenal enterocytes) activates trypsinogen → trypsin → trypsin then activates all other zymogens

Bile Production

The liver produces ~500–1,000 mL of bile/day. Bile is concentrated and stored in the gallbladder between meals, then released postprandially in response to CCK.

Enterohepatic Circulation: ~95% of bile salts are reabsorbed in the terminal ileum via the ASBT (apical sodium-dependent bile acid transporter), returned to the liver via portal blood, and re-secreted. This cycle occurs 6–8 times per day. Disruption (e.g., ileal resection) → bile salt wasting → fat malabsorption and reduced cholesterol elimination.

Digestion

Digestion breaks macronutrients into absorbable units: monosaccharides, amino acids/dipeptides/tripeptides, and fatty acids/monoglycerides. Each macronutrient class follows a specific enzymatic pathway.

Carbohydrates

Humans eat ~300 g/day of carbohydrates: starch (~60%), sucrose (~30%), and lactose (~10%). Only monosaccharides (glucose, galactose, fructose) can be absorbed.

Proteins

Daily protein intake: ~70–100 g, plus ~30–40 g of endogenous protein from desquamated epithelial cells and enzymes. Digestion begins in the stomach and completes at the brush border.

Lipids

Fat digestion is the most complex because lipids are hydrophobic — they must be emulsified (broken into tiny droplets) before water-soluble lipase can access them.

1. Emulsification: Bile salts + lecithin break large fat globules into small droplets (~1 μm), increasing surface area ~10,000-fold
2. Enzymatic hydrolysis: Pancreatic lipase (with colipase anchoring it to the droplet surface) cleaves triglycerides → 2-monoglyceride + 2 free fatty acids
3. Micelle formation: Bile salts + lipid products form mixed micelles that ferry lipids through the unstirred water layer to the enterocyte brush border
4. Absorption: Fatty acids and monoglycerides diffuse across the apical membrane → inside the cell, they're re-esterified to triglycerides, packaged with apolipoproteins → chylomicrons → exocytosed into lacteals (lymphatics)

Absorption

The small intestine is the primary absorptive organ, with a surface area of approximately 200 m² (a tennis court!) thanks to three amplification structures: circular folds (plicae circulares × 3), villi (× 10), and microvilli (× 20). Each enterocyte has ~3,000 microvilli forming the "brush border."

Small Intestine Transporters

Micelle Formation & Lipid Uptake

The critical micelle concentration (CMC) of bile salts must be reached for effective fat absorption. Below CMC, lipid digestion products remain in large aggregates that cannot penetrate the unstirred water layer.

• Short-chain fatty acids (C < 12): Water-soluble → absorbed directly into portal blood without micelles
• Long-chain fatty acids (C ≥ 12): Require micelles → absorbed → re-esterified → packaged into chylomicrons → lacteals
• Medium-chain triglycerides (MCTs): Partially water-soluble → absorbed without bile salts → portal blood directly. Used therapeutically in fat malabsorption (e.g., pancreatic insufficiency, short bowel syndrome)

Vitamin Absorption

Water & Electrolyte Absorption

The GI tract handles ~9 L of fluid daily (2 L ingested + 7 L secreted). The small intestine absorbs ~7.5 L, the colon ~1.4 L, leaving only ~100–200 mL in stool.

• Small intestine: Water follows solute absorption (osmotic gradient created by Na⁺ and nutrient absorption). Na⁺ absorbed via SGLT1 co-transport, Na⁺/H⁺ exchangers, and ENaC
• Colon: Absorbs Na⁺ via ENaC (aldosterone-regulated, same as in the kidney collecting duct); secretes K⁺; absorbs water against a greater osmotic gradient
• Cl⁻ secretion: Crypt cells secrete Cl⁻ via CFTR channels → Na⁺ and water follow paracellularly → this mechanism drives secretory diarrhoea when over-activated

Gut Regulation

GI function is orchestrated by three overlapping control systems: hormones (endocrine and paracrine), nerves (intrinsic ENS + extrinsic autonomic), and local factors (pH, distension, nutrient content). This creates a remarkably adaptive system that adjusts secretion and motility in real time based on meal composition.

Hormones (Gastrin, CCK, Secretin)

Neural Control

The GI tract receives extrinsic innervation from both divisions of the autonomic nervous system:

• Parasympathetic (Vagus nerve, S2-S4): Generally excitatory — ↑ motility, ↑ secretion, relaxes sphincters (except lower oesophageal sphincter, which it contracts). The vagus innervates everything from oesophagus to the splenic flexure of the colon
• Sympathetic (T5-L2 splanchnic nerves): Generally inhibitory — ↓ motility, ↓ secretion, contracts sphincters, ↓ blood flow ("fight or flight" diverts blood away from the gut)

Gut-Brain Axis

The gut-brain axis is a bidirectional communication network linking the central nervous system, the ENS, the gut microbiome, and the immune system:

• Vagal afferents: 80% of vagal fibres are sensory, transmitting information about distension, nutrients, pH, and microbial metabolites to the brainstem (NTS)
• Serotonin (5-HT): 95% of the body's serotonin is produced by enterochromaffin cells in the gut — it activates vagal afferents and modulates motility, secretion, and visceral sensation
• Microbiome signalling: Gut bacteria produce short-chain fatty acids (butyrate, propionate, acetate), neurotransmitters (GABA, dopamine), and influence the hypothalamic-pituitary-adrenal axis
• Clinical relevance: Irritable bowel syndrome (IBS) is now understood as a disorder of the gut-brain axis, explaining why stress worsens symptoms and antidepressants can improve them

Advanced Topics

Microbiome Physiology

The human gut harbours approximately 10¹³–10¹⁴ microorganisms (roughly equal to the number of human cells), with the highest density in the colon (~10¹¹ per mL of content). Key physiological roles:

Malabsorption Syndromes

Hepatic Metabolic Integration

The liver receives all absorbed nutrients (except long-chain fats) via the portal vein — establishing the "first-pass" metabolic processing:

• Carbohydrate metabolism: Glycogenesis (postprandial), glycogenolysis, and gluconeogenesis (fasting) — the liver is the body's glucose buffer
• Protein metabolism: Deamination of amino acids, urea synthesis (urea cycle), synthesis of plasma proteins (albumin, clotting factors, lipoproteins)
• Lipid metabolism: β-oxidation of fatty acids, ketogenesis (during fasting/starvation), VLDL synthesis, cholesterol metabolism
• Detoxification: Phase I (CYP450 oxidation) and Phase II (conjugation with glucuronic acid, sulphate, glutathione) → water-soluble metabolites excreted in bile or urine

import numpy as np
import matplotlib.pyplot as plt

# Simulate gastric emptying curves for different meal compositions
time_min = np.linspace(0, 240, 100)  # 4 hours

# Exponential decay model: V(t) = V0 * exp(-k * t)
v0 = 100  # Starting volume (% of meal)
k_liquid = 0.03       # Fast emptying
k_carb = 0.015        # Moderate
k_protein = 0.010     # Slower
k_fat = 0.006         # Slowest (CCK-mediated)

plt.figure(figsize=(10, 6))
plt.plot(time_min, v0 * np.exp(-k_liquid * time_min), 'b-', linewidth=2, label='Liquid (water)')
plt.plot(time_min, v0 * np.exp(-k_carb * time_min), 'g-', linewidth=2, label='Carbohydrate meal')
plt.plot(time_min, v0 * np.exp(-k_protein * time_min), 'm-', linewidth=2, label='Protein meal')
plt.plot(time_min, v0 * np.exp(-k_fat * time_min), 'r-', linewidth=2, label='Fat-rich meal (CCK delay)')

plt.xlabel('Time (minutes)')
plt.ylabel('Gastric Content Remaining (%)')
plt.title('Gastric Emptying Rates by Meal Composition')
plt.legend()
plt.grid(True, alpha=0.3)
plt.annotate('Fat triggers CCK → slows emptying', xy=(120, 48), fontsize=9,
             arrowprops=dict(arrowstyle='->', color='red'), xytext=(150, 70))
plt.tight_layout()
plt.show()

Interactive Tool

Use this GI Motility & Secretion Analyser to document gastrointestinal function parameters and generate a comprehensive assessment report. Download as Word, Excel, or PDF.

Conclusion & Next Steps

In this article, we explored the gastrointestinal system as an integrated organ of digestion and absorption. From the slow waves of ICC pacemaker cells orchestrating smooth muscle contraction, through the carefully sequenced secretory cascade (saliva → gastric acid → pancreatic enzymes → bile), to the elegant transport mechanisms that move nutrients across the 200 m² absorptive surface of the small intestine — every component is precisely regulated by hormones and the enteric nervous system.

We also examined clinical correlates: from cholera's hijacking of CFTR channels to the autoimmune destruction in coeliac disease, and from the incretin effect revolutionising diabetes treatment to the emerging recognition of the microbiome as a physiological organ in its own right.

Next, we ascend to the master regulatory system that coordinates metabolism across all organs — the endocrine system, where hormones from the hypothalamus, pituitary, thyroid, adrenals, and pancreas integrate the body's metabolic responses.