11.1 Digestive System & Absorption

GI Tract Architecture & Gastric Secretions

The gastrointestinal (GI) tract is a continuous lumen extending from the oral cavity to the anus, specialized for mechanical processing, chemical hydrolysis, and nutrient uptake. Structurally, the alimentary canal exhibits a conserved four-layer tissue organization: the innermost mucosa (epithelium, lamina propria, muscularis mucosae), the submucosa (vascular network and Meissner's submucosal plexus controlling secretions), the muscularis externa (inner circular and outer longitudinal smooth muscle layers with Auerbach's myenteric plexus controlling peristalsis), and the outer serosa or adventitia.

Oral Cavity and Esophageal Processing

Digestion initiates in the oral cavity via mechanical mastication and chemical salivary breakdown. Salivary amylase (ptyalin) cleaves internal $\alpha$-1,4 glycosidic bonds in starch, yielding maltose, maltotriose, and $\alpha$-dextrins. Concurrently, Ebner's glands on the tongue secrete lingual lipase, a acid-tolerant enzyme that cleaves short- and medium-chain triacylglycerols. Lingual lipase remains active in the stomach due to its acidic pH optimum ($pH\ 3.5-6.0$). Peristaltic contractions transport the swallowed bolus down the esophagus through the lower esophageal sphincter (cardiac sphincter) into the stomach.

Gastric Gland Cell Types and Physiological Functions

The stomach serves as a temporary storage vessel, mechanical churner, and acidic digestive chamber. The gastric mucosa contains deep invaginations called gastric pits that lead into tubular gastric glands housing four principal secretory cell lineages:

1. Parietal (Oxyntic) Cells: Located predominantly in the gastric body and fundus, parietal cells secrete hydrochloric acid (HCl) and Intrinsic Factor. Parietal cells utilize an apical primary active transport pump, the $H^+/K^+$ ATPase (proton pump), to transport hydrogen ions against a million-fold concentration gradient ($pH\ 1.5-2.0$) into the lumen in exchange for potassium ions. Intracellular carbonic anhydrase catalyzes $CO_2 + H_2O \rightarrow H_2CO_3 \rightarrow H^+ + HCO_3^-$. Bicarbonate ($HCO_3^-$) exits the basolateral membrane into systemic capillaries via a $Cl^-/HCO_3^-$ anion exchanger, generating the postprandial alkaline tide. Intrinsic Factor is a glycoprotein mandatory for Vitamin B12 (cobalamin) protection and subsequent receptor-mediated endocytosis in the terminal ileum; absence of Intrinsic Factor results in pernicious (macrocytic) anemia.
2. Chief (Zymogenic) Cells: Secretes pepsinogen, an inactive zymogen precursor. Exposure to luminal $HCl$ ($pH < 2.0$) induces a conformational change in pepsinogen, causing auto-catalytic cleavage of a 44-amino-acid N-terminal masking peptide to form active pepsin. Pepsin is an endopeptidase with an optimal $pH\ 1.5-2.0$ that specifically cleaves peptide bonds adjacent to aromatic amino acids (Phenylalanine, Tryptophan, Tyrosine).
3. G Cells: Neuroendocrine cells concentrated in the gastric antrum that synthesize and release the peptide hormone gastrin into the bloodstream. Gastrin secretion is stimulated by stomach distension, vagal stimulation (mediated by Gastrin-Releasing Peptide, GRP), and partially digested peptides (especially Phe and Trp). Gastrin binds $CCK_2$ receptors on parietal cells and Enterochromaffin-Like (ECL) cells (which secrete histamine), stimulating robust $HCl$ secretion.
4. Mucous (Foveolar & Neck) Cells: Secrete a thick, viscous, bicarbonate-rich mucin gel layer ($100-200\ \mu\text{m}$) that coats the stomach lining. This mucosal barrier establishes a steep pH gradient ($pH\ 1.5-2.0$ at the luminal surface vs. $pH\ 7.0$ at the epithelial cell surface), protecting the gastric mucosa from self-digestion and mechanical abrasion.

Small Intestines, Pancreatic Cascades & Hepatobiliary Function

The small intestine is divided into three regions: the duodenum (primary site of chemical digestion), jejunum (primary site of nutrient absorption), and ileum (site of Vitamin B12 and bile salt reabsorption). When acidic chyme ($pH\ 1.5-2.0$) enters the duodenum through the pyloric sphincter, it triggers coordinated endocrine responses from pancreatic and hepatobiliary systems.

Pancreatic Duct and Acinar Secretions

The exocrine pancreas consists of acinar cells (producing digestive enzymes) and ductal cells (producing aqueous bicarbonate solution):

• Pancreatic Duct Cells: Acidic chyme ($pH < 4.5$) entering the duodenum triggers S cells to release secretin into the circulation. Secretin binds $G_s$-coupled receptors on pancreatic duct cells, upregulating $cAMP$ and opening apical $CFTR$ chloride channels. Chloride efflux drives apical $Cl^-/HCO_3^-$ exchangers, flooding the duodenal lumen with sodium bicarbonate ($NaHCO_3$). This neutralizes gastric acid to $pH\ 7.8-8.0$, inactivating pepsin and establishing the optimal pH for pancreatic enzymes.
• Pancreatic Acinar Cells: Chyme containing fatty acids and amino acids triggers duodenal I cells to release cholecystokinin (CCK). CCK acts via $G_q$-coupled $CCK_1$ receptors on acinar cells to induce exocytosis of digestive zymogens: trypsinogen, chymotrypsinogen, procarboxypeptidases A and B, and proelastase, as well as active pancreatic amylase, pancreatic lipase (requiring colipase), and nucleases (DNase, RNase).

The Enteropeptidase Activation Cascade

To prevent autopenetration and pancreatitis, pancreatic proteases are synthesized and stored as inactive zymogens. Upon reaching the duodenal brush border, the membrane-bound serine protease enteropeptidase (enterokinase) specifically cleaves an N-terminal hexapeptide from trypsinogen, producing active trypsin. Active trypsin then acts as the master activator, cleaving and activating all other pancreatic zymogens: $\text{Trypsinogen} \xrightarrow{\text{Enteropeptidase}} \text{Trypsin}$ $\text{Trypsinogen} \xrightarrow{\text{Trypsin (Autocatalysis)}} \text{Trypsin}$ $\text{Chymotrypsinogen} \xrightarrow{\text{Trypsin}} \text{Chymotrypsin}$ $\text{Procarboxypeptidase} \xrightarrow{\text{Trypsin}} \text{Carboxypeptidase}$ $\text{Proelastase} \xrightarrow{\text{Trypsin}} \text{Elastase}$

Hepatobiliary Function: Bile Synthesis and Micelle Formation

The liver synthesizes primary bile acids (cholic acid and chenodeoxycholic acid) from cholesterol via the rate-limiting enzyme cholesterol $7\alpha$-hydroxylase. Bile acids are conjugated with glycine or taurine to form amphipathic bile salts (e.g., glycocholate, taurocholate). Bile is secreted by hepatocytes into bile canaliculi and stored in the gallbladder.

Upon postprandial CCK stimulation, the gallbladder contracts while the Sphincter of Oddi relaxes, discharging concentrated bile into the duodenal lumen. Bile salts possess a hydrophobic steroid nucleus and a hydrophilic polar head group. Bile salts surround large lipid droplets, breaking them down into microscopic droplets (mechanical emulsification). Bile salts, unesterified cholesterol, and phospholipids spontaneously assemble into mixed micelles ($4-7\ \text{nm}$ diameter). Micelles solubilize hydrophobic lipid hydrolysis products (free fatty acids, 2-monoacylglycerols, lysophospholipids, fat-soluble vitamins A, D, E, K), ferrying them across the aqueous unstirred layer to the brush border apical membrane of enterocytes.

Molecular Mechanisms of Nutrient Absorption

The intestinal epithelium maximizes surface area via circular folds (plicae circulares), villi, and apical microvilli forming the brush border (total surface area $\sim 300\ \text{m}^2$).

Monosaccharide Absorption

Carbohydrates must be broken down into monosaccharides (glucose, galactose, fructose) prior to enterocyte uptake:

• Glucose and Galactose: Transported across the apical membrane against their concentration gradients via SGLT1 (Sodium-Glucose Linked Transporter 1). SGLT1 is a secondary active symporter that co-transports $2\ Na^+$ ions alongside 1 glucose or galactose molecule. The favorable electrochemical $Na^+$ gradient is continuously maintained by basolateral $Na^+/K^+$ ATPase ($3\ Na^+$ out, $2\ K^+$ in, utilizing ATP).
• Fructose: Transported across the apical membrane via GLUT5 down its concentration gradient via facilitated diffusion.
• Basolateral Exit: All three monosaccharides exit the enterocyte basolateral membrane via GLUT2 facilitated diffusion into the lamina propria capillaries, draining into the hepatic portal vein directly to the liver.

Amino Acid and Peptide Absorption

Proteins hydrolyzed by stomach pepsin and pancreatic proteases are reduced to free amino acids, dipeptides, and tripeptides:

• Free Amino Acids: Cross the apical membrane via multiple distinct $Na^+$-dependent secondary active cotransporters classified by amino acid charge (acidic, basic, neutral).
• Di- and Tripeptides: Co-transported across the apical membrane via PepT1 (Peptide Transporter 1), a secondary active symporter driven by a $H^+$ gradient (established by apical $NHE3\ Na^+/H^+$ exchangers). Inside the enterocyte, intracellular cytosolic peptidases rapidly hydrolyze peptides into free amino acids.
• Basolateral Exit: Free amino acids exit the basolateral membrane via $Na^+$-independent facilitated diffusion transporters into intestinal capillaries, entering the hepatic portal circulation.

Lipid Absorption and Chylomicron Assembly

Unlike water-soluble nutrients, dietary lipids bypass the hepatic portal system:

1. Apical Uptake: Free fatty acids (FFAs) and 2-monoacylglycerols (2-MAGs) dissociate from luminal micelles and diffuse across the enterocyte apical membrane (or enter via fatty acid transport proteins like CD36).
2. Re-esterification: In the smooth endoplasmic reticulum (SER), FFAs and 2-MAGs are re-synthesized into triacylglycerols (TAGs), cholesteryl esters, and phospholipids.
3. Chylomicron Packaging: In the Golgi apparatus, TAGs and lipids are packaged with specific apolipoproteins—principally Apolipoprotein B-48 (ApoB-48)—forming spherical lipoprotein particles called chylomicrons ($100-500\ \text{nm}$ diameter).
4. Exocytosis into Lacteals: Chylomicrons undergo exocytosis across the basolateral membrane. Because chylomicrons are too large to penetrate the continuous basement membrane of intestinal capillaries, they enter lacteals (blind-ended lymphatic capillaries). Lymphatic vessels coalesce into the thoracic duct, which empties directly into the venous bloodstream at the junction of the left subclavian and internal jugular veins.