Human Digestive System: Gastric Secretions, Small Intestine, and Large Intestine

Gastric Juice Secretion and Hormonal Regulation

• Daily production volume: The human stomach produces approximately $2\text{--}3\,\text{litres}$ of gastric juice each day.
• Regulatory hormones: Secretion of gastric juice is biochemically regulated by gastrin, secretin, and gastric inhibitory polypeptide (serotonin).
• Sensory stimulation: The sight, smell, or taste of food activates neural pathways that stimulate gastric secretion.
• Mucosal neural activation: Physical contact and neuronal stimulation of the gastric mucosa trigger endocrine cells to release gastrin.
• Gastrin stimulation and mechanical motility:
  • Gastrin is secreted directly into the bloodstream in response to gastric distention and the presence of partially digested peptides and amino acids within the stomach cavity.
  • Gastrin enhances the power of gastric peristaltic waves, driving chyme forcefully toward the pylorus.
  • Gastrin promotes the relaxation of the pyloric sphincter, permitting small increments of acidic chyme to enter the duodenum.
• Gastric inhibition: Serotonin (acting as a gastric inhibitory polypeptide) suppresses the secretion of gastric acid.
• Infant gastric enzymes: Infant stomachs uniquely secrete rennin (chymosin), an enzyme that converts soluble milk protein caseinogen into insoluble casein, which is subsequently cleaved and digested by pepsin.

Small Intestine Overview and Duodenal Physiology

• Anatomical dimensions: The small intestine is approximately $6\,\text{m}$ in total length and comprises three sequential segments: the duodenum, jejunum, and ileum.
• Primary role: The small intestine constitutes the principal organ of food digestion and nutrient assimilation.
• Duodenum dimensions and digestive landmarks: The duodenum is the initial segment of the small intestine, measuring approximately $25\,\text{cm}$ in length, and serves as the region where fatty acid digestion both initiates and concludes.
• Exocrine inputs: The duodenal lumen receives combined exocrine secretions from the pancreas and the liver (via the gall bladder).
• Pancreatic juice entry and neutralization:
  • Pancreatic juice enters the duodenum via the pancreatic duct.
  • Pancreatic juice maintains an alkaline $\text{pH } 8$, neutralizing the strong hydrochloric acid and deactivating the acidic digestive enzymes delivered from the stomach.
• Cholecystokinin ($\text{CCK}$) mechanism:
  • The presence of free fatty acids in the duodenal lumen stimulates local endocrine cells to synthesize and release cholecystokinin ($\text{CCK}$).
  • $\text{CCK}$ triggers contraction of smooth muscles within the gall bladder wall, discharging concentrated bile through the common bile duct into the duodenum.
• Secretin mechanism:
  • The arrival of acidic chyme into the duodenal lumen prompts endocrine cells to release secretin.
  • Secretin travels systemically through the blood circulation to the liver, where it induces hepatocytes and bile ducts to secrete bicarbonate ions into bile, neutralizing gastric acids and creating an alkaline medium.

Pancreatic Juice Enzymes

• Pancreatic amylase: Hydrolyses residual undigested starches into the disaccharide maltose.
• Pancreatic lipase: Catalyses the hydrolysis of dietary fats and oils into free fatty acids and glycerol.
• Trypsinogen activation cascade:
  • The pancreas releases trypsinogen as an inactive zymogen.
  • Enterokinase, an enzyme secreted by intestinal mucosal glands, cleaves inactive trypsinogen to generate active trypsin.
  • Active trypsin digests dietary proteins into smaller polypeptides.
• Chymotrypsinogen activation: Active trypsin cleaves the inactive pancreatic precursor chymotrypsinogen into its active proteolytic form, chymotrypsin.

Bile Juice Composition and Lipid Emulsification

• Synthesis and storage: Bile is synthesized continuously by the liver and stored and concentrated within the gall bladder.
• Composition of bile:
  • Bile contains water, bile salts, and bile pigments, but lacks digestive enzymes entirely.
  • Bile salts: Composed principally of sodium taurocholate and sodium glycocholate.
  • Bile pigments: Composed of yellow-brown bilirubin and greenish biliverdin, which represent excretory breakdown products of hemoglobin.
• Emulsification process:
  • Bile salts act as biological detergents that break large, insoluble fat globules into minute droplets, forming an emulsion.
  • Emulsification exponentially multiplies the surface area of lipids, enabling pancreatic lipase to efficiently hydrolyse fats and oils into absorbable fatty acids and glycerol.

Jejunum Enzymatic Digestion and Chyle Formation

• Dimensions: The jejunum represents the middle segment of the small intestine, measuring approximately $2.5\,\text{m}$ in length.
• Completion of enzymatic digestion: The jejunum represents the final digestive region where the chemical hydrolysis of proteins, carbohydrates, and lipids is completed.
• Brush-border and luminal enzymes:
  • Maltase: Hydrolyses maltose into glucose.
  • Sucrase: Cleaves sucrose into simple monosaccharides.
  • Lactase: Cleaves lactose into glucose and galactose.
  • Peptidase: Hydrolyses oligopeptides and small peptides into individual amino acids.
  • Erepsin: A protein-cleaving enzyme complex that hydrolyses remaining polypeptides into amino acids.
  • Nucleotidase: Breaks down nucleotides into nucleosides and inorganic phosphate.
• Resistance of cellulose: Human gastrointestinal secretions contain no cellulolytic enzymes; hence, cellulose digestion does not occur.
• Formation of chyle: Through continuous enzymatic digestion and alkaline buffering, acidic chyme is converted into a thin, milky, watery emulsion known as chyle.

Ileum Architecture and Nutrient Absorption Pathways

• Dimensions and role: The ileum is the terminal and longest division of the small intestine, measuring approximately $3.6\,\text{m}$ in length, and functions as the specialized site for the absorption of digested nutrients.
• Surface area amplifying structures:
  • Plicae circulares: Deep, permanent circular mucosal folds that dramatically increase internal absorptive surface area.
  • Villi: Thousands of microscopic, finger-like mucosal projections extending from the intestinal wall.
  • Microvilli: Submicroscopic cellular projections extending from the simple columnar epithelial border, establishing a massive surface area that renders the small intestine the most extensive absorptive surface of the alimentary tract.
• Histology of a single intestinal villus:
  • Lined with a single layer of simple columnar epithelial cells.
  • Encloses an internal core containing an extensive blood capillary network (arteriole, capillary bed, venule) and a central lymphatic capillary termed a lacteal.
• Absorption of water-soluble nutrients: Free amino acids and monosaccharides are actively or passively absorbed across the simple columnar epithelium into the blood capillary network via active transport or diffusion.
• Absorption and packaging of lipids:
  • Hydrophobic fatty acids and glycerol diffuse into the simple columnar epithelial cells.
  • Within the enterocytes, fatty acids and glycerol re-esterify into neutral fats.
  • Re-formed fats are combined with cholesterol and wrapped in protein envelopes to assemble microscopic spherical droplets termed chylomicrons.
• Lymphatic transport of lipids:
  • Chylomicrons exit epithelial cells and enter the central lacteal.
  • Chylomicron-rich lymph flows through the lymphatic vessels into the thoracic duct, which empties directly into the left subclavian vein of the cardiovascular system.
• Hepatic portal transport: The venous capillaries draining all intestinal villi converge to form the hepatic portal vein, delivering absorbed water-soluble nutrients straight to the liver for metabolic processing, regulation, distribution, and storage.

Large Intestine Morphology and Systemic Functions

• Structural measurements: The large intestine measures approximately $1.5\,\text{m}$ in length and $6.5\,\text{cm}$ in width.
• Mucosal secretions: The mucosal lining contains numerous mucus-secreting goblet cells and glands, providing surface lubrication to facilitate the transit and expulsion of solid digestive waste.
• Primary physiological roles:
  • Reabsorption of residual water and unabsorbed fluid nutrients.
  • Reabsorption of essential electrolytes, specifically sodium ($\text{Na}^+$) and chloride ($\text{Cl}^-$).
  • Microbial synthesis of essential metabolic vitamins.
  • Compaction of indigestible chyme residue into solid faeces.
  • Elimination of metabolic and digestive waste via egestion.
• Structural divisions: Anatomically demarcated into three consecutive sections: caecum, colon, and rectum.

Caecum and Vermiform Appendix

• Caecum localization: Positioned within the lower right quadrant of the abdominal cavity.
• Ileocecal valve: A muscular sphincter valve situated at the ileocecal junction that regulates the unidirectional flow of undigested waste from the terminal ileum into the caecum.
• Caecal absorption: Serves as an active site for the reabsorption of water and dissolved salts.
• Vermiform appendix:
  • A narrow, blind-ended diverticulum measuring approximately $18\,\text{cm}$ in length attached to the inferior border of the caecum.
  • Functions predominantly as a vestigial organ with no significant digestive capacity.

Colon Divisions and Symbiotic Microflora

• Four anatomical divisions:
  • Ascending colon: Passes superiorly along the right side of the abdominal cavity.
  • Transverse colon: Crosses horizontally across the upper abdomen.
  • Descending colon: Travels inferiorly along the left abdominal wall.
  • Sigmoid colon: An S-shaped terminal segment leading into the pelvic cavity.
• Colonic absorption: Specializes in consolidating luminal waste by reabsorbing residual water, salts, and free vitamins.
• Symbiotic bacterial flora:
  • The lumen of the colon harbors vast populations of commensal and symbiotic bacteria, notably Lactobacillus.
  • Bacterial metabolic activity synthesizes vital micronutrients, including thiamin (vitamin $\text{B}_1$), niacin, biotin (vitamin $\text{H}$), and vitamin $\text{K}$.
  • Synthesized vitamins are absorbed across the colonic mucosa into the bloodstream, where they support cellular metabolism and normal central nervous system ($\text{CNS}$) function.

Rectum and the Mechanics of Defecation

• Rectal anatomy: A straight, highly muscular tube commencing at the termination of the sigmoid colon and terminating at the anal canal, which is bounded by internal and external anal sphincter valves.
• Adult defecation reflex:
  • Accumulation and compaction of faeces within the rectal chamber causes wall distention.
  • Rectal stretch receptors stimulate autonomic defecation reflexes.
  • Coordinated reflex activity triggers voluntary and involuntary relaxation of the external anal sphincter, permitting faeces to be evacuated from the body through egestion.
• Infant defecation mechanics: In infants, defecation reflexes trigger immediate, automatic emptying of the lower bowel due to the neurological absence of voluntary corticospinal control over the external anal sphincter.