The Lower Gastrointestinal Tract: Structure & Function in Nutrient Absorption

 The lower gastrointestinal (GIT) tract plays a critical role in nutrient absorption, fluid balance, and waste excretion. It includes the small intestine (jejunum, ileum), large intestine (colon), rectum, and anus—each with unique structural and functional adaptations that optimise digestion and absorption.

Let’s explore the detailed mechanisms at work throughout this system.

1. Small Intestine: The Primary Site of Nutrient Absorption

The jejunum and ileum are specialised for efficient absorption with features that maximise surface area and transport capacity:

•  Villi & Microvilli – Dramatically increase absorptive surface area (~200m²).
•  Tight Junctions – Regulate selective permeability to nutrients and ions.
•  Enterocytes – Specialised epithelial cells that transport nutrients via carrier-mediated, facilitated, and active transport mechanisms.

Carbohydrate Absorption: Transporters & Enzymatic Breakdown

Carbohydrates must be broken down into monosaccharides before absorption.

🔹 Step 1: Enzymatic Hydrolysis –

Pancreatic amylase breaks down starch into maltose, maltotriose, and α-limit dextrins. Brush border enzymes (maltase, sucrase, lactase) hydrolyse disaccharides into glucose, galactose, and fructose.

🔹 Step 2: Monosaccharide Absorption –

• Glucose & Galactose: Na⁺-dependent secondary active transport via SGLT-1 (Sodium-Glucose Linked Transporter-1).
• Fructose: Facilitated diffusion via GLUT-5 (passive transport).
• Basolateral exit: GLUT-2 moves monosaccharides into circulation.

Protein Absorption: Peptide Transporters

Proteins are hydrolysed into small peptides and amino acids before absorption.

🔹 Step 1: Digestion –

• Pepsin initiates breakdown in the stomach.
• Pancreatic proteases (trypsin, chymotrypsin, carboxypeptidases) continue digestion in the small intestine.

🔹 Step 2: Absorption –

• Amino acids: Na⁺-dependent transporters use secondary active transport.
• Small peptides: Transported via PEPT1, which couples peptide absorption to H⁺ symport, enhancing uptake efficiency.

Lipid Absorption: Micelles & Chylomicrons

Lipids present unique challenges in absorption due to their hydrophobic nature.

🔹 Step 1: Emulsification –

Bile salts disperse lipids into micelles, increasing surface area for enzymatic digestion.

🔹 Step 2: Enzymatic Digestion –

Pancreatic lipase hydrolyses triglycerides into monoglycerides & free fatty acids.

🔹 Step 3: Absorption –

Fatty acids passively diffuse across enterocyte membranes. Inside, they are re-esterified into triglycerides and packed into chylomicrons, which enter lymphatic circulation.

2. Large Intestine: Water Balance & Microbial Fermentation

While the colon absorbs fewer nutrients than the small intestine, it plays a major role in:

• Water & Electrolyte Homeostasis – Recovers ~90% of intestinal fluid via tight junctions & aquaporins.
• Microbial Fermentation – Gut bacteria ferment non-digestible carbohydrates, producing short-chain fatty acids (SCFAs) that serve as an energy source for colonocytes.
• Gut Barrier Maintenance – Protects against pathogens via mucus secretion, tight junction integrity, and antimicrobial peptide production.

Short-Chain Fatty Acid (SCFA) Metabolism

SCFAs (acetate, propionate, butyrate) are produced via bacterial fermentation of fibre.

•  Acetate – Systemic energy source, metabolised in muscle and liver.
•  Propionate – Acts as a gluconeogenic precursor in the liver.
•  Butyrate – Main energy source for colonocytes, supporting gut integrity and anti-inflammatory effects via histone deacetylase inhibition.

3. Rectum & Anus: Waste Elimination

The rectum stores faecal matter until expulsion via defecation reflex, coordinated by:

• Internal anal sphincter (involuntary control via enteric nervous system).
• External anal sphincter (voluntary control via pudendal nerve).
• Peristaltic Waves – Propagate stool from the sigmoid colon to the rectum.

Neurological Control of Defecation

🚀 Parasympathetic activation via pelvic splanchnic nerves stimulates rectal contraction.

🚀 Somatic control via pudendal nerve allows voluntary relaxation of the external anal sphincter.

Clinical Considerations: Disorders Affecting Absorption & Function

• Coeliac Disease – Autoimmune destruction of intestinal villi, impairing nutrient absorption.
• Crohn’s Disease – Inflammatory condition affecting ileum, leading to fat malabsorption & bile acid diarrhoea.
• Ulcerative Colitis – Chronic inflammatory disease limited to the colon, causing mucosal ulceration, crypt abscesses, and bloody diarrhoea due to impaired epithelial barrier function.
•  Irritable Bowel Syndrome (IBS) – Abnormal colonic motility affecting stool consistency & gas transit.
•  Diarrhoeal Disorders – Excessive fluid loss due to altered electrolyte transport, e.g., cholera toxin activation of CFTR increases chloride secretion, leading to watery diarrhoea.

Final Thought:

The lower GIT is highly specialised for efficient nutrient absorption, microbial fermentation, and waste elimination, ensuring metabolic homeostasis and gut health. Understanding these processes is critical for diagnosing and managing digestive disorders.