GI Secretion, Digestion, and Absorption

1. Salivary and Gastric Secretions

The salivary glands secrete a watery, enzyme-containing fluid under autonomic control: parasympathetic stimulation gives the large-volume watery secretion, sympathetic stimulation a smaller, mucus-rich secretion. Saliva contains salivary α-amylase (begins starch digestion) and lingual lipase (begins fat digestion). In the stomach, parietal cells secrete HCl (denatures proteins, activates pepsinogen, kills microbes); chief cells secrete pepsinogen, converted by HCl to active pepsin; G cells release gastrin to drive acid secretion; surface mucous cells secrete the mucus-bicarbonate barrier — an adherent gel trapping bicarbonate that neutralizes acid at the epithelial surface, protecting the stomach from autodigestion.

2. Pancreatic and Biliary Secretions

Pancreatic acinar cells secrete digestive enzymes (proteases as inactive zymogens — trypsinogen, chymotrypsinogen, procarboxypeptidase — plus amylase, lipase, nucleases); duct cells secrete a bicarbonate-rich fluid that neutralizes duodenal acid. CCK (in response to fat and amino acids) drives enzyme-rich secretion; secretin (in response to acid) drives bicarbonate-rich secretion. The liver continuously synthesizes bile, stored and concentrated in the gallbladder; its key components are bile salts, which emulsify fat and, with phospholipids, form micelles that solubilize lipid products. Most bile salts are reabsorbed in the terminal ileum and returned to the liver — the enterohepatic circulation — so a small pool is reused daily.

3. Digestion and Absorption of Macronutrients

Carbohydrates are broken down by salivary and pancreatic amylase to disaccharides, then by brush-border enzymes (maltase, sucrase, lactase) to monosaccharides, absorbed via sodium-coupled transport (SGLT1) and facilitated diffusion (GLUT2/GLUT5). Proteins are digested to peptides by pepsin and pancreatic proteases, then to amino acids/dipeptides by brush-border peptidases, absorbed via sodium- and H+-coupled transporters. Lipids are emulsified by bile salts, hydrolyzed by pancreatic lipase to fatty acids and monoglycerides, packaged into micelles, absorbed, re-esterified, and exported in chylomicrons via lymphatics.

4. Fluid, Electrolyte, and Vitamin Absorption

About 9 L of fluid enters the GI tract daily (~2 L ingested, ~7 L secretions); nearly all is reabsorbed, mostly in the small intestine, sodium actively absorbed and water following osmotically. The colon reclaims the remainder (absorbing Na+ and water, secreting K+ and bicarbonate). Fat-soluble vitamins (A, D, E, K) follow the lipid pathway; water-soluble vitamins use various transporters. Vitamin B12 requires intrinsic factor, a glycoprotein from gastric parietal cells; the complex is absorbed by receptors in the terminal ileum.

How it works

1. Salivary amylase and lingual lipase begin digestion in the mouth.
2. The stomach secretes acid and pepsinogen; the mucus-bicarbonate barrier protects the wall.
3. Duodenal chyme triggers secretin (→ bicarbonate) and CCK (→ enzymes and bile).
4. Bile salts emulsify fats and form micelles; pancreatic enzymes finish macronutrient digestion.
5. Brush-border enzymes complete carbohydrate and protein breakdown; monomers are absorbed.
6. Lipids are repackaged as chylomicrons and exported via lymph; bile salts recycle enterohepatically.
7. Water, electrolytes, and vitamins (B12 via intrinsic factor) are absorbed.

Common confusions

Memory aids

"S-G-P-B = Some Good Pancakes, Butter" for the secretions — Saliva, Gastric juice, Pancreatic juice, Bile. Stomach cells: "Chief = Pep, Parietal = Acid & IF." Lipid pathway: "Every Fat Must Move Chylomicrons" — Emulsify, Form micelles, Move across, Chylomicrons out.

Quick review

Topic Recap

Salivary, gastric, pancreatic, and biliary secretions work together: saliva starts starch and fat digestion; the stomach supplies acid, pepsinogen, gastrin, and a mucus-bicarbonate barrier; the pancreas supplies enzymes (CCK) and bicarbonate (secretin); bile salts emulsify fat and form micelles, recycled enterohepatically. Carbohydrates and proteins are digested to monomers and absorbed via transporters into portal blood; lipids travel as chylomicrons through lymph. Water, electrolytes, and vitamins — including B12, requiring intrinsic factor in the terminal ileum — complete absorption.

Knowledge Check

1. Which gastric cells secrete HCl and intrinsic factor?
2. What is the functional difference between CCK and secretin?
3. Why are bile salts essential for fat absorption?
4. Where and how is vitamin B12 absorbed?
5. By what route do absorbed dietary fats leave the intestine, and why?

Answers and Rationales

1. Parietal cells. Chief cells secrete pepsinogen, not acid.
2. CCK stimulates enzyme-rich pancreatic secretion and gallbladder contraction (fat/amino acids); secretin stimulates bicarbonate-rich secretion (acid).
3. They emulsify fat to increase lipase surface area and form micelles that carry lipid products to the brush border.
4. In the terminal ileum, as a complex with intrinsic factor (from gastric parietal cells) bound by specific ileal receptors.
5. Via lymph as chylomicrons; these triglyceride-rich particles are too large for blood capillaries and reach the blood through the thoracic duct.