11.2 Large Intestine, GI Motility & Enteric Neuroendocrine Control

Where Section 11.1 Left Off

Section 11.1 followed a meal from the mouth through the small intestine, where essentially all enzymatic digestion and nutrient absorption occur. What arrives at the ileocecal valve is a dilute slurry of water, electrolytes, indigestible fiber, sloughed epithelium and bile pigments. This section covers what happens to it, and — more importantly for passages — how the whole tract is coordinated.

The Large Intestine

Gross Structure

Chyme passes the ileocecal valve into the cecum, then travels through the ascending, transverse, descending and sigmoid colon to the rectum and anal canal. Three structural features distinguish colon from small intestine:

• Taeniae coli — three longitudinal smooth muscle bands that are shorter than the colon itself, gathering the wall into pouches.
• Haustra — the resulting sacculations, which shift position as the taeniae contract.
• Epiploic appendages — fat-filled peritoneal tags.

The mucosa has no villi. Instead it is simple columnar epithelium dense with goblet cells, because the colon's job is absorption of water and lubrication of increasingly solid contents, not surface area for nutrient uptake.

Absorption and the Water Budget

Roughly $9\text{ L}$ of fluid enters the gut lumen daily — about $2\text{ L}$ ingested and $7\text{ L}$ secreted as saliva, gastric juice, bile, pancreatic juice and intestinal secretions. The small intestine reclaims most of it; the colon absorbs about 1.5 L, leaving roughly $100\text{--}200\text{ mL}$ in feces.

Colonic absorption is driven by active $\text{Na}^+$ transport across the apical membrane (stimulated by aldosterone, exactly as in the nephron of Section 10.6), with $\text{Cl}^-$ following electrically and water following osmotically. $\text{K}^+$ and $\text{HCO}_3^-$ are secreted, which is why severe diarrhea produces hypokalemia and metabolic acidosis.

Bacterial Flora

The colon houses on the order of $10^{14}$ bacteria, mostly obligate anaerobes (see 7.6). They are not passengers:

• Fermentation of undigested carbohydrate and fiber yields short-chain fatty acids — butyrate, propionate, acetate — which colonocytes oxidize as their preferred fuel, plus the gases that make up flatus.
• Vitamin synthesis: gut bacteria produce vitamin K and several B vitamins, including biotin, that the host absorbs. This is why prolonged broad-spectrum antibiotic therapy can prolong bleeding time by depleting vitamin K-dependent clotting factors (see 10.2).
• Colonization resistance: a stable flora competitively excludes pathogens; disruption permits Clostridioides difficile overgrowth.

Rectum, Sphincters and Defecation

The rectum stores feces. Two sphincters guard the anus: the internal anal sphincter is smooth muscle under involuntary autonomic control, and the external anal sphincter is skeletal muscle under voluntary somatic control. Rectal distension triggers the defecation reflex — a spinal reflex that relaxes the internal sphincter and contracts the rectum — while cortical input decides whether the external sphincter relaxes. The pairing of an involuntary inner and voluntary outer sphincter is a structural motif worth recognizing; the urinary bladder is built the same way.

Gastrointestinal Motility

Smooth muscle throughout the tract is electrically coupled by gap junctions and paced by interstitial cells of Cajal, which generate slow waves. Slow waves set the maximum contraction frequency; whether a slow wave actually triggers contraction depends on neural and hormonal input.

The Enteric Nervous System

The gut contains its own nervous system of roughly 100 million neurons — sometimes called the "second brain" — organized in two plexuses within the wall.

Both are capable of complete reflex arcs — sensory neuron, interneuron, motor neuron — entirely within the gut wall, so a segment of intestine removed from the body still exhibits peristalsis. The autonomic nervous system (see 9.2) modulates rather than creates this activity:

• Parasympathetic (vagus, CN X; pelvic splanchnics distally): rest-and-digest — increases motility and secretion, relaxes sphincters.
• Sympathetic: fight-or-flight — decreases motility and secretion, contracts sphincters, and shunts blood away from the splanchnic bed.

Hirschsprung disease illustrates the plexuses' importance: failure of neural crest cells (see 11.7) to colonize a distal colonic segment leaves it aganglionic, permanently contracted and unable to propagate peristalsis, so the bowel proximal to it dilates massively.

Endocrine Control: The Major GI Hormones

Two logic checks passages exploit. First, secretin is the acid hormone and CCK is the fat-and-protein hormone — mixing them up reverses the entire pancreatic response. Second, the incretin effect explains why oral glucose triggers more insulin release than an identical intravenous glucose load: GIP and GLP-1 anticipate the absorbed glucose before it arrives.

Hepatic Integration

The liver, introduced in 11.1 as the source of bile, also completes the digestive story metabolically. Everything absorbed by the small and large intestine except long-chain fat drains into the hepatic portal vein and passes through the liver before reaching the systemic circulation. There the hepatocyte performs the first-pass functions the AAMC lists explicitly: buffering absorbed glucose into glycogen and releasing it on demand (see 5.4), and detoxification of ammonia via the urea cycle (see 5.3) and of xenobiotics via cytochrome P450 oxidation and conjugation in the smooth endoplasmic reticulum (see 8.1).