Absorption and Gastrointestinal Regulation

Detailed Notes

19.1 The Small Intestine: Primary Site of Absorption

The small intestine is the principal organ of nutrient absorption, with a surface area of approximately 200–250 m² — roughly the size of a tennis court — achieved through three levels of structural specialization:

1. Plicae circulares (circular folds): Permanent, deep folds of the mucosa and submucosa that slow chyme transit and increase surface area approximately 3-fold.
2. Villi: Finger-like projections of the mucosa (~0.5–1.0 mm tall), each containing a capillary network and a central lacteal (lymphatic capillary). Villi increase surface area approximately 10-fold.
3. Microvilli: Microscopic projections of the apical plasma membrane on each absorptive enterocyte, forming the brush border. They increase surface area approximately 20-fold and contain brush border enzymes (disaccharidases, peptidases) that complete terminal digestion at the absorptive surface.

Regional Specialization of Absorption:

19.2 Absorption of Carbohydrates

Dietary carbohydrates are absorbed almost exclusively as monosaccharides — primarily glucose, galactose, and fructose — after luminal and brush border digestion breaks down starch, glycogen, and disaccharides.

19.2.1 Transport Mechanisms

Glucose and Galactose: SGLT1 (Na⁺-Cotransport)

• The sodium-glucose linked transporter 1 (SGLT1) on the apical (luminal) membrane couples the downhill movement of Na⁺ (driven by the low intracellular Na⁺ concentration maintained by the Na⁺/K⁺-ATPase pump on the basolateral membrane) to the uphill transport of glucose or galactose into the enterocyte.
• This is secondary active transport: the energy comes indirectly from ATP via the Na⁺ gradient, not directly from ATP hydrolysis by SGLT1 itself.
• Each SGLT1 transports 2 Na⁺ ions along with 1 glucose (or galactose) molecule.

Fructose: GLUT5 (Facilitated Diffusion)

• Fructose enters the enterocyte via GLUT5, a facilitative transporter on the apical membrane that moves fructose down its concentration gradient without coupling to Na⁺.
• This is passive (facilitated) transport — no direct or indirect energy expenditure.

19.2.2 Exit from the Enterocyte: GLUT2

All three monosaccharides — glucose, galactose, and fructose — exit the enterocyte across the basolateral membrane via GLUT2, another facilitative glucose transporter. From there, they enter the capillary blood of the villus and travel via the hepatic portal vein to the liver, where galactose and fructose are largely converted to glucose.

Table 19.1 — Carbohydrate Absorption: Transporters and Mechanisms

19.3 Absorption of Proteins

Dietary proteins are absorbed primarily as amino acids, dipeptides, and tripeptides after gastric pepsin and pancreatic proteases (trypsin, chymotrypsin, carboxypeptidase) have broken them down in the lumen.

19.3.1 Transport Mechanisms

Amino Acids: Na⁺-Dependent and Na⁺-Independent Cotransporters

• Multiple specific transporters on the apical membrane handle different classes of amino acids (neutral, basic, acidic, imino). Most operate via Na⁺-dependent secondary active transport, coupling amino acid uptake to the Na⁺ gradient.
• Some amino acids use Na⁺-independent facilitated transporters.

Dipeptides and Tripeptides: PepT1 (H⁺-Cotransport)

• The peptide transporter 1 (PepT1) on the apical membrane couples the downhill movement of H⁺ (maintained by the Na⁺/H⁺ exchanger on the apical membrane, which itself is driven by the Na⁺ gradient) to the uptake of dipeptides and tripeptides.
• This is a H⁺-coupled secondary active transport system and is quantitatively the major route for protein absorption — more di/tripeptides are absorbed via PepT1 than free amino acids via amino acid transporters.

19.3.2 Intracellular Processing and Exit

Once inside the enterocyte:

1. Dipeptides and tripeptides are hydrolyzed to free amino acids by intracellular peptidases (cytosolic aminopeptidases).
2. Free amino acids exit the enterocyte across the basolateral membrane via multiple facilitative amino acid transporters.
3. Amino acids enter the capillary blood of the villus and travel via the hepatic portal vein to the liver.

Table 19.2 — Protein Absorption: Transporters and Mechanisms

19.4 Absorption of Lipids

Lipid absorption is fundamentally different from carbohydrate and protein absorption because lipids are hydrophobic and must be handled through a series of emulsification, micellar transport, and repackaging steps.

19.4.1 Luminal Phase: Micelle Formation

1. In the duodenum, bile salts emulsify large lipid droplets into smaller droplets, increasing the surface area for pancreatic lipase.
2. Pancreatic lipase hydrolyzes triglycerides into monoglycerides and free fatty acids (FFAs).
3. These lipid digestion products, along with cholesterol, phospholipids, and fat-soluble vitamins, aggregate with bile salts to form mixed micelles — tiny (~4–8 nm) water-soluble discs with hydrophobic interiors and hydrophilic exteriors.
4. Micelles shuttle the lipid products through the unstirred water layer adjacent to the brush border. At the enterocyte surface, monoglycerides and FFAs diffuse passively across the apical membrane (they are lipid-soluble and can cross the phospholipid bilayer directly).

19.4.2 Intracellular Phase: Chylomicron Formation

Inside the enterocyte:

1. Monoglycerides and FFAs are transported to the smooth endoplasmic reticulum (SER), where they are re-esterified into triglycerides.
2. Triglycerides are then packaged with cholesterol, phospholipids, and apolipoproteins (especially apoB-48) into chylomicrons — large (75–1,200 nm diameter) lipoprotein particles synthesized in the Golgi apparatus.

19.4.3 Exit: Lacteals and the Lymphatic Route

Unlike carbohydrates and proteins, chylomicrons are too large (~75–1,200 nm) to enter blood capillaries. Instead, they are exocytosed from the basolateral side of the enterocyte and enter the lacteals — the central lymphatic capillaries within each villus. From there, chylomicrons travel through the lymphatic system:

• Intestinal lacteals → mesenteric lymphatics → cisterna chyli → thoracic duct → left subclavian vein (at the junction with the left internal jugular vein) → systemic venous circulation.

This lymphatic route bypasses the hepatic portal system, so ingested lipids enter the systemic circulation directly — unlike carbohydrates and amino acids, which pass through the liver first.

Table 19.3 — Summary of Lipid Absorption

19.5 Absorption of Water

Water absorption in the small intestine is entirely passive — it moves by osmosis, following the osmotic gradient established by the active transport of solutes (especially Na⁺, but also glucose, amino acids, and other electrolytes) from the lumen into the intestinal epithelial cells and then into the intercellular spaces and capillary blood.

• As solutes are actively transported out of the lumen, the luminal osmolarity decreases and the paracellular/intercellular osmolarity increases. Water follows via both the paracellular pathway (between tight junctions) and the transcellular pathway (through aquaporin water channels in the cell membrane).
• Of the approximately 9 liters of fluid entering the small intestine daily (2 L ingested + 7 L from GI secretions), roughly 8 liters are absorbed in the small intestine. The remaining ~1 liter passes into the large intestine, where most of it is also absorbed, leaving only ~100–200 mL excreted in feces.

19.6 Absorption of Electrolytes

19.6.1 Sodium (Na⁺)

Sodium is the most important ion for driving intestinal absorption because its active transport creates the electrochemical gradient that powers the uptake of many other substances (glucose, galactose, amino acids, di/tripeptides, HCO₃⁻). Na⁺ is absorbed by several mechanisms:

• Na⁺-glucose and Na⁺-amino acid cotransport (SGLT1, various amino acid transporters): as described above, Na⁺ enters the enterocyte coupled with these nutrients.
• Na⁺/H⁺ antiporter (NHE3): Na⁺ enters the enterocyte in exchange for H⁺ being pumped out into the lumen. The H⁺ then combines with luminal HCO₃⁻ to form CO₂ and H₂O (or aids in PepT1 function).
• Epithelial Na⁺ channels (ENaC) in the colon: electrogenic Na⁺ absorption driven by the electrochemical gradient; especially important in the large intestine under aldosterone regulation.

On the basolateral side, Na⁺/K⁺-ATPase pumps Na⁺ out of the enterocyte into the interstitial fluid, maintaining the low intracellular Na⁺ concentration essential for all Na⁺-coupled apical transport.

19.6.2 Potassium (K⁺)

Potassium is absorbed primarily by passive diffusion via the paracellular pathway, driven by the electrochemical gradient (K⁺ concentration is higher inside the cell, but water absorption creates a solvent drag effect). In the colon, K⁺ can also be secreted under aldosterone control when Na⁺ is being actively reabsorbed via ENaC.

19.6.3 Chloride (Cl⁻)

Chloride absorption occurs by:

• Passive paracellular diffusion: driven by the transepithelial electrical gradient created by Na⁺ absorption (Cl⁻ follows Na⁺ to maintain electroneutrality).
• Cl⁻/HCO₃⁻ antiport: Cl⁻ enters in exchange for HCO₃⁻ leaving the cell, especially in the ileum and colon.

19.6.4 Calcium (Ca²⁺)

Calcium absorption is actively regulated and occurs primarily in the duodenum:

• Apical entry: Ca²⁺ enters the enterocyte through TRPV6 calcium channels (facilitating passive diffusion down the electrochemical gradient).
• Intracellular: Ca²⁺ binds to calbindin-D9k, a vitamin D-dependent calcium-binding protein that shuttles Ca²⁺ across the cytoplasm without allowing free Ca²⁺ to rise to toxic levels.
• Basolateral exit: Ca²⁺ is pumped out against its gradient by the Ca²⁺-ATPase (PMCA1b) and Na⁺/Ca²⁺ exchanger (NCX1).

The active form of vitamin D — 1,25-dihydroxyvitamin D₃ (calcitriol) — upregulates the expression of TRPV6, calbindin-D9k, and Ca²⁺-ATPase, thereby increasing calcium absorption. This is crucial: without adequate vitamin D, dietary calcium cannot be efficiently absorbed.

19.6.5 Iron (Fe²⁺)

Iron absorption occurs primarily in the duodenum and is tightly regulated because the body has no active excretory pathway for excess iron.

Heme vs. Non-Heme Iron:

• Heme iron (from hemoglobin and myoglobin in meat): Absorbed intact by heme carrier protein 1 (HCP1) on the apical membrane. Inside the enterocyte, heme oxygenase releases Fe²⁺ from the heme ring.
• Non-heme iron (Fe³⁺ from plant sources): Must first be reduced to Fe²⁺ by duodenal cytochrome B (DcytB) on the apical membrane. Fe²⁺ then enters via the divalent metal transporter 1 (DMT1).

Intracellular Fate:

• If body iron stores are adequate, Fe²⁺ binds to ferritin within the enterocyte and is stored. Since enterocytes are sloughed off every 3–5 days, the iron is lost in feces — this is the body's main mechanism for preventing iron overload.
• If body iron stores are low, Fe²⁺ is exported across the basolateral membrane by ferroportin, oxidized back to Fe³⁺ by hephaestin (a copper-dependent ferroxidase), and bound to transferrin for transport in the blood.

Table 19.4 — Comparison of Heme and Non-Heme Iron Absorption

19.7 Absorption of Vitamins

19.7.1 Fat-Soluble Vitamins (A, D, E, K)

Fat-soluble vitamins are absorbed along with dietary lipids:

• They are incorporated into mixed micelles in the intestinal lumen alongside monoglycerides, FFAs, and cholesterol.
• They diffuse passively across the enterocyte apical membrane with the micellar lipid products.
• They are packaged into chylomicrons and exported via the lacteals into the lymphatic system.
• This means that any condition impairing fat digestion or absorption — pancreatic insufficiency, bile salt deficiency (cholestasis, ileal resection), or mucosal disease — will also impair the absorption of fat-soluble vitamins, potentially leading to deficiencies (e.g., night blindness from vitamin A deficiency, osteomalacia from vitamin D deficiency, coagulopathy from vitamin K deficiency).

19.7.2 Water-Soluble Vitamins (B-Complex and Vitamin C)

Most water-soluble vitamins are absorbed in the jejunum by:

• Passive diffusion (at high concentrations) for most B vitamins and vitamin C.
• Carrier-mediated transport (at low, physiological concentrations) using specific transporters. For example, vitamin C (ascorbic acid) is absorbed via SVCT1 (sodium-dependent vitamin C transporter 1).

19.7.3 Vitamin B₁₂ (Cobalamin) — A Special Case

Vitamin B₁₂ absorption is the most complex of any vitamin and occurs exclusively in the ileum:

1. In the stomach, dietary B₁₂ is released from food proteins by pepsin and HCl.
2. Free B₁₂ binds to haptocorrin (R-protein) in the stomach.
3. In the duodenum, pancreatic proteases degrade haptocorrin, releasing B₁₂.
4. B₁₂ then binds to intrinsic factor (IF), a glycoprotein secreted by the parietal cells of the gastric mucosa.
5. The B₁₂-IF complex is resistant to proteolytic digestion and travels to the terminal ileum, where it binds to specific cubilin receptors on the apical membrane of ileal enterocytes.
6. The entire B₁₂-IF complex is internalized by receptor-mediated endocytosis. Inside the enterocyte, IF is degraded and B₁₂ is released, bound to transcobalamin II, and exported into the portal blood.

Clinical Relevance — Pernicious Anemia: In pernicious anemia, autoimmune destruction of gastric parietal cells leads to loss of intrinsic factor production. Without IF, vitamin B₁₂ cannot be absorbed regardless of dietary intake, leading to megaloblastic anemia and neurological degeneration. Treatment requires intramuscular B₁₂ injections (bypassing the GI tract) or very high-dose oral supplementation.

19.8 Absorption in the Large Intestine

The large intestine absorbs far fewer nutrients than the small intestine but plays a critical role in water and electrolyte recovery and in salvaging products of bacterial metabolism.

19.9 Hormonal Regulation of Gastrointestinal Function

The GI tract is the body's largest endocrine organ, containing specialized enteroendocrine cells scattered throughout the mucosa that secrete hormones regulating motility, secretion, and satiety. The four major GI hormones are discussed below.

19.9.1 Gastrin

| Stimuli for release | 1. Gastric distension (stretch of the stomach wall)

1. Partially digested proteins (peptides) and amino acids in the lumen
2. Vagal stimulation → release of gastrin-releasing peptide (GRP) from enteric neurons → stimulates G cells
3. Alcohol and caffeine (moderate stimulants) | | Inhibition of release | Somatostatin (produced by D cells in the stomach); pH < 2.0 in the antrum (negative feedback — when gastric acid reaches very low pH, gastrin secretion is suppressed) | | Primary actions | 1. Stimulates parietal cells to secrete HCl (directly and via histamine release from ECL cells)
4. Stimulates chief cells to secrete pepsinogen
5. Increases gastric motility (mixing and emptying)
6. Trophic effect: stimulates growth of the gastric mucosa (increases parietal cell and ECL cell numbers)
7. Relaxes the pyloric sphincter |

19.9.2 Secretin

| Primary actions | 1. Stimulates pancreatic duct cells and biliary duct cells to secrete HCO₃⁻ (bicarbonate)-rich fluid → neutralizes acidic chyme, creating an optimal pH (~7–8) for pancreatic enzymes

1. Inhibits gastric acid secretion (antagonizes the effects of gastrin on parietal cells)
2. Inhibits gastric motility and emptying (slowing the delivery of acidic chyme into the duodenum)
3. Inhibits gastrin release from G cells | | Net effect | "The bicarbonate hormone" — secretin's primary mission is to neutralize acid and protect the duodenal mucosa. |

19.9.3 Cholecystokinin (CCK)

| Stimuli for release | 1. Fatty acids and monoglycerides (the most potent stimuli) in the duodenum

1. Amino acids and small peptides in the duodenum | | Primary actions | 1. Gallbladder contraction → bile is ejected into the duodenum for fat emulsification
2. Relaxation of the sphincter of Oddi (hepatopancreatic sphincter) → allows bile and pancreatic enzymes to enter the duodenum
3. Stimulates pancreatic acinar cells to secrete digestive enzymes (amylase, lipase, proteases)
4. Inhibits gastric emptying → slows the delivery of chyme to allow adequate time for fat digestion
5. Satiety signal: CCK acts on the brain (via vagal afferents and directly at the hypothalamus) to promote a feeling of fullness and reduce food intake | | Net effect | "The fat-digesting hormone" — CCK coordinates the gallbladder, pancreas, and stomach to optimize fat digestion. |

19.9.4 Gastric Inhibitory Peptide (GIP) / Glucose-Dependent Insulinotropic Peptide

| Stimuli for release | 1. Glucose in the duodenal lumen (primary stimulus)

1. Fatty acids in the duodenal lumen | | Primary actions | 1. Incretin effect: GIP stimulates the pancreatic β cells to release insulin — but only when blood glucose is elevated (hence "glucose-dependent"). This accounts for the observation that oral glucose elicits a greater insulin response than intravenous glucose.
2. Inhibits gastric acid secretion (the function that gave it its original name, "gastric inhibitory peptide")
3. Inhibits gastric motility and emptying | | Net effect | "The feed-forward insulin trigger" — GIP prepares the body for incoming glucose by priming insulin release. |

Table 19.5 — Summary of Major GI Hormones

19.10 Neural Regulation of GI Function

The GI tract is regulated by extrinsic (autonomic) and intrinsic (enteric) neural systems. The enteric nervous system (ENS) — often called the "second brain" — contains approximately 100–500 million neurons organized into two major plexuses:

• Myenteric (Auerbach's) plexus: between the circular and longitudinal muscle layers; primarily controls GI motility (peristalsis, segmentation).
• Submucosal (Meissner's) plexus: in the submucosa; primarily controls secretion and local blood flow.

The ENS can function independently of the CNS, but is modulated by parasympathetic (generally excitatory — increases motility and secretion) and sympathetic (generally inhibitory — decreases motility and secretion, constricts sphincters) input.

GI regulation is also divided into three functional phases based on where the stimulus originates:

19.10.1 Cephalic Phase

Stimulus: Sight, smell, taste, and even thought of food — before food enters the stomach.

Pathway: Sensory input → cerebral cortex → hypothalamus → dorsal motor nucleus of the vagus → vagus nerve (CN X) → enteric neurons.

Effects:

• Vagal stimulation of parietal cells → HCl secretion.
• Vagal stimulation of chief cells → pepsinogen secretion.
• Vagal stimulation (via GRP) of G cells → gastrin release → further amplifies HCl secretion.
• This phase accounts for approximately 20–30% of gastric acid secretion during a meal.

19.10.2 Gastric Phase

Stimulus: Food entering the stomach → gastric distension and presence of peptides/amino acids in the lumen.

Pathway:

1. Vago-vagal reflex (long reflex): Distension activates stretch receptors in the stomach wall → vagal afferents → medulla → vagal efferents → increased secretion and motility.
2. Local (short) reflexes: Distension activates enteric sensory neurons → direct activation of secretory and smooth muscle cells within the ENS.
3. Hormonal: Peptides and distension stimulate gastrin release from G cells → gastrin enters the bloodstream and stimulates parietal cells.

Effects:

• Increased HCl, pepsinogen, and gastrin secretion.
• Increased gastric motility (mixing waves, peristalsis).
• This phase accounts for approximately 60–70% of gastric acid secretion.

19.10.3 Intestinal Phase

Stimulus: Chyme entering the duodenum.

Initial (excitatory) subphase: Duodenal distension initially briefly stimulates gastric secretion and motility via enteric reflexes — this is minor.

Dominant (inhibitory) subphase — The Enterogastric Reflex: As chyme accumulates, three factors inhibit gastric activity:

1. Duodenal distension → activates enteric sensory neurons → inhibits gastric motility and secretion (enterogastric reflex via both short ENS and long vagal pathways).
2. Acidity (pH < ~4.5) in the duodenum → triggers secretin release → inhibits gastric secretion and motility.
3. Hyperosmolarity and fatty acids in the duodenum → triggers CCK and GIP release → inhibit gastric emptying and secretion.

Effects:

• Decreased gastric motility → slowed gastric emptying.
• Decreased gastric secretion.
• Pyloric sphincter constriction.
• This carefully regulated braking mechanism prevents the duodenum from being overwhelmed by acidic, hyperosmotic chyme and ensures adequate time for digestion and absorption.

Table 19.6 — Summary of Neural Regulation Phases

19.11 Clinical Correlations

19.11.1 Malabsorption Syndromes

Malabsorption refers to impaired absorption of nutrients and can result from defects at any stage of digestion or absorption:

19.11.2 Pernicious Anemia

• Cause: Autoimmune destruction of gastric parietal cells → loss of intrinsic factor (IF) production.
• Consequence: Vitamin B₁₂ cannot be absorbed in the terminal ileum → megaloblastic anemia (impaired DNA synthesis in RBC precursors) and subacute combined degeneration of the spinal cord (neurological symptoms from demyelination).
• Key lab findings: Low serum B₁₂, elevated homocysteine and methylmalonic acid, macrocytic (↑ MCV) anemia.
• Treatment: Intramuscular vitamin B₁₂ injections (lifelong) — oral B₁₂ cannot compensate because IF is absent.

19.11.3 Dumping Syndrome

• Cause: Rapid gastric emptying of hyperosmolar chyme into the small intestine, most commonly after gastrectomy or gastric bypass surgery (loss of pyloric sphincter control).
• Early dumping (30–60 minutes post-meal): Hyperosmolar chyme draws water into the intestinal lumen by osmosis → abdominal cramps, nausea, diarrhea, vasomotor symptoms (tachycardia, flushing, hypotension from fluid shifts).
• Late dumping (1–3 hours post-meal): Rapid absorption of glucose triggers excessive insulin release (exaggerated GIP-mediated incretin response) → reactive hypoglycemia (weakness, sweating, confusion, dizziness).
• Management: Small, frequent meals; avoid simple sugars; separate liquids from solids during meals.

19.11.4 Gastrinoma (Zollinger-Ellison Syndrome)

• Cause: A gastrin-secreting tumor (gastrinoma), usually in the duodenum or pancreas, that produces excessive amounts of gastrin autonomously (unregulated by normal negative feedback).
• Consequence: Massive, unregulated gastric acid hypersecretion → severe, recurrent peptic ulcers (often in atypical locations such as the distal duodenum or jejunum), diarrhea (acid inactivates pancreatic lipase → steatorrhea; acid damages intestinal mucosa).
• Diagnosis: Elevated fasting serum gastrin levels; secretin stimulation test (paradoxical rise in gastrin after secretin administration, whereas normal G cells are suppressed).
• Association: ~25% of gastrinomas occur as part of Multiple Endocrine Neoplasia type 1 (MEN1).