Small Intestine

Anatomy and Regional Functionalization of the Small Intestine

• General Overview: The Small Intestine (SI) is a crucial segment of the Gastrointestinal Tract (GIT) situated between the stomach and the large intestine.

• Anatomical Divisions: The small intestine is partitioned into three distinct sections, listed in order of flow:

  • Duodenum: The initial section after the stomach, separated by the pyloric sphincter. It is the primary site for the mixing of pancreatic digestive enzymes and bile with ingested food. It is also responsible for the absorption of nutrients, including specific focus on iron and calcium. Additionally, it releases endocrine hormones such as secretin and cholecystokinin ($CCK$).

  • Jejunum: The middle section specializing in both digestion and absorption. Collectively, the duodenum and jejunum (the first $25\,\%$ of the small intestine) handle the digestion and absorption of most chyme entering from the stomach.

  • Ileum: The final section which continues digestion and absorption. It has the specialized function of absorbing bile acids and Vitamin $B_{12}$.

• Major Physiological Functions: The SI coordinates the digestion and absorption of proteins, fats, carbohydrates, electrolytes, water, and minerals ($iron$), along with vitamins.

• Sphincter Control: The pyloric sphincter regulates the passage of chyme from the stomach into the duodenum.

Structural Adaptations for Maximized Surface Area

• Levels of Folding: To optimize absorption, the small intestine employs multiple levels of hierarchical folding to exponentially increase the luminal surface area:

  • Folds of Kerckring: Also known as circular folds, these represent the entire layer of the intestinal wall folded upon itself.

  • Villi: Finger-like protrusions of tissue that cover the circular folds and extend into the lumen. Each villus contains a specialized lymphatic vessel known as a lacteal and blood capillaries.

  • Microvilli: Even smaller projections found on the surface membranes of the individual epithelial cells covering the villi. This layer is collectively referred to as the brush border membrane.

  • Crypt Regions: These are invaginations or projections in the opposite direction (into the wall) of the villi.

Cellular Dynamics and Differentiation within the Crypts

• Stem Cells: Located within the crypts, these cells divide and differentiate into four distinct types of epithelial cells:

  • Absorptive Cells (Enterocytes): The primary cell responsible for nutrients uptake. They are characterized by microvilli on their apical (luminal) surface, forming the brush border.

  • Goblet Cells: Responsible for the secretion of mucus, which provides lubrication for food passage and protects the intestinal wall from acid damage.

  • Enteroendocrine Cells: Hormone-producing cells found in the epithelium. Examples include $I$ cells (which produce $CCK$) and $S$ cells (which produce secretin).

  • Paneth Cells: Specialized cells that secrete antibacterial peptides, serving as a defensive mechanism to protect the GIT from bacterial invasion.

Carbohydrate Digestion and Monosaccharide Absorption

• The Brush Border Enzyme Concept: A brush border enzyme is an enzyme anchored directly to the small projections (microvilli) of epithelial cells. They possess catalytic activity in the lumen and are vital for breaking down polymers into absorbable units before transport.

  • Example: Enterokinase, which is responsible for activating the pancreatic proenzyme trypsinogen into its active form, trypsin.

• Digestion of Starch and Polysaccharides:

  • Starch (consisting of amylose and amylopectin) is first attacked by salivary and pancreatic amylases.

  • Amylase breaks starch into maltose, maltotriose, and $\alpha$-limit dextrins. These products cannot be absorbed directly and must be further reduced.

  • Brush Border Enzymes for Starch Producst:

    • Maltose and maltotriose are digested by maltase, sucrase, or alpha-dextrinase into glucose.

    • $\alpha$-limit dextrins are specifically broken down by alpha-dextrinase into glucose.

• Digestion of Disaccharides:

  • Sucrose: Broken down by sucrase into glucose and fructose.

  • Lactose: Broken down by lactase into glucose and galactose.

• Transport Mechanisms (Apical and Basolateral):

  • Only monosaccharides (glucose, galactose, and fructose) can be absorbed by the intestinal tract.

  • Glucose and Galactose: These share a transport pathway. They enter the enterocyte from the lumen via the $Na^+$-dependent glucose transporter ($SGLT$). This is a secondary active transport mechanism that utilizes the $Na^+$ gradient generated by the $Na^+ / K^+$ ATPase pump. They exit the cell across the basolateral membrane via a facilitated glucose transporter ($GLUT$).

  • Fructose: Moves across the apical membrane via the facilitated carrier $GLUT5$ and exits the basolateral surface through $GLUT2$.

Protein Digestion and Amino Acid Absorption

• Stages of Protein Breakdown:

  • Stomach: Digestion begins with pepsinogen, which is released by chief cells and activated into pepsin by the acidic pH of the stomach lumen.

  • Small Intestine (Lumen): Pancreatic proteases take over. Major proteases include trypsin and chymotrypsin, which cleave internal peptide bonds.

  • Exopeptidases: Carboxypeptidase (a pancreatic protease) and aminopeptidase (a brush border enzyme) cleave individual amino acids from the carboxyl and amino terminals of peptides, respectively.

• Absorption Pathways:

  • Free Amino Acids: Absorbed via secondary active transport coupled to $Na^+$ through various specific transport pathways.

  • Short Peptides (Dipeptides and Tripeptides): These can be absorbed via secondary active transport coupled to $H^+$ instead of $Na^+$.

  • Intracellular Processing: Once inside the enterocyte, intracellular peptidases further break down small peptides into individual amino acids.

• Circulatory Entry: Amino acids leave the basolateral surface of the enterocyte via facilitated diffusion to enter the interstitial space and the blood circulation.

Lipid Digestion and Transport Mechanisms

• Emulsification: Large fat droplets (mostly triglycerides) must be mechanically disrupted and stabilized by bile acids and phospholipids. This prevents re-aggregation and increases the surface area for water-soluble pancreatic lipase.

• Action of Pancreatic Lipase: Lipase breaks down triglycerides into monoglycerides and free fatty acids.

• Micelles: These are spherical arrangements of lipid molecules in aqueous solution, composed of fatty acids, monoglycerides, and bile salts. They exist in a dynamic state, constantly breaking down to release free lipids that can diffuse across the epithelium.

• Resynthesis and Chylomicrons:

  • Once absorbed, fatty acids and monoglycerides are resynthesized into triglycerides within the Endoplasmic Reticulum ($ER$) to maintain a favorable diffusion gradient from the lumen.

  • Within the Golgi, these triglycerides aggregate into droplets coated with amphipathic proteins, known as chylomicrons.

  • Chylomicrons also contain phospholipids, fat-soluble vitamins, and cholesterol.

• Lymphatic Absorption: Chylomicrons are secreted via exocytosis across the basolateral membrane into lacteals (lymphatic vessels) rather than capillaries, as lacteals are leakier. They eventually reach the systemic circulation through the thoracic duct.

• Tissue Uptake: Lipoprotein lipase, located on the endothelial cells of blood vessels, breaks down the triglycerides within chylomicrons back into monoglycerides and fatty acids for tissue use.

Iron Absorption and Homostasis

• Form of Absorption: The GIT absorbs divalent iron ($Fe^{2+}$ or ferrous iron) via active transport across the apical membrane.

• Intracellular Storage (Ferritin): Once inside the enterocyte, iron binds to the protein ferritin, creating a storage complex. If iron stores are high, ferritin levels increase to trap iron in the cell, which is eventually lost when the cell sloughs off into the feces.

• Blood Transport (Transferrin): Iron that is not bound to ferritin is released into the blood and transported bound to the plasma protein transferrin.

• Regulatory Mechanism:

  • High Iron Stores: Ferritin production increases $\rightarrow$ less iron enters the blood.

  • Depleted Iron Stores: Ferritin production decreases $\rightarrow$ more iron is released into the blood.

• Toxicities and Deficiencies:

  • Toxicity: Resulting from genetic defects, excessive supplementation (common in adult males or post-menopausal women), or poisoning in children. Once in the body, there is no formal excretion mechanism for iron.

  • Iron Deficiency Anemia: Characterized by a reduced number or size of red blood cells. Symptoms include fatigue, light-headedness, and headaches.

Water and Electrolyte Absorption and Secretion

• Critical Importance: Fluid management ensures contact between food and enzymes, aids nutrient diffusion, and prevents epithelial damage during transit.

• Absorption (Villi): Predominantly occurs in the villi and depends on sodium gradients.

  • The $Na^+ / K^+$ ATPase pump establishes a low intracellular $Na^+$ concentration.

  • $Na^+$ uptake (via transporters like $SGLT$) creates an osmotic gradient.

  • $Cl^-$ (negative charge) follows the positive $Na^+$ gradient.

  • Water follows paracellularly through tight junctions via osmosis.

• Secretion (Crypts): Occurs in the crypts and depends on Chloride gradients.

  • NKCC1: This secondary active transporter moves $Na^+$, $K^+$, and $2Cl^-$ into the enterocyte, utilizing the $Na^+$ gradient to accumulate intracellular $Cl^-$.

  • CFTR: A chloride channel on the brush border. When stimulated by $cAMP$, it opens, allowing $Cl^-$ to move into the lumen. $Na^+$ and water follow this gradient into the lumen.

Small Intestine Motility

• During Digestion (Segmentation): The most common motion. It involves localized contractions that mix food with enzymes and mechanically break it down.

  • Frequency: Set by the Basic Electrical Rhythm ($BER$) from pacemaker cells. It is approximately $12\,\text{contractions/min}$ in the duodenum and $9\,\text{contractions/min}$ in the ileum.

  • Net Movement: Results in very slow migration toward the large intestine.

• After Absorption (Migrating Myoelectric Complex - MMC): Peristaltic waves that sweep undigested material toward the large intestine and prevent bacterial overgrowth.

  • Process: Waves start in the lower stomach and travel about $2\,\text{feet}$ before dying out, followed by a new overlapping wave. The full process takes roughly $2\,\text{hours}$.

  • Regulation: Initiated by the hormone motilin. Eating inhibits motilin release to allow segmentation to resume.

Pathophysiology of Digestion

• Lactose Intolerance: Caused by the loss of lactase expression post-weaning.

  • Undigested lactose creates an osmotic gradient that retains water in the gut (diarrhea).

  • Bacteria in the large intestine digest the lactose, producing gas, distension, and pain.

  • Treatment involves consuming lactase pills or lactose-free milk (pre-treated with lactase).

• Cholera: Caused by the bacterium Vibrio cholerae.

  • Mechanism: The bacteria produce a toxin that increases $cAMP$ production. This keeps the $CFTR$ chloride channel open constantly.

  • Impact: Massive loss of $Cl^-$, $Na^+$, and water, leading to up to $20\,\text{litres}$ of stool per day (normal is $0.1\,\text{litres}$).

  • Treatment: Clean water containing salts and glucose to utilize $Na^+$-coupled transport to encourage reabsorption.

The Large Intestine (LI)

• Anatomy: Characterized by a larger diameter than the SI but a shorter length. It includes the cecum, appendix, the colon (ascending, transverse, descending, sigmoid), the rectum, and the anus.

• Ileocecal Valve: A sphincter between the ileum and cecum that prevents the backflow of bacteria from the LI into the SI.

• Physiological Functional Domains:

  • Colon: Re-absorption of water and products of bacterial metabolism (e.g., fatty acids); serves as a reservoir for waste.

  • Rectum: Storage for feces.

  • Anus: Defecation control via two sphincters: Internal (smooth muscle/involuntary) and External (skeletal muscle/voluntary).

• Structure and Cell Types: Unlike the SI, the LI has no villi, but it does contain crypts and stem cells.

  • Cell types: Includes Absorptive cells (enterocytes similar to SI but lacking brush border enzymes), abundant Goblet cells, and endocrine cells.

  • Other: Gallstone management includes manual removal or drugs to dissolve the stones.