Gastrointestinal System - Small and Large Intestine, Liver, Gallbladder, Pancreas

Small Intestine

Location and Gross Anatomy

• Located in the abdominal cavity.
• Approximately 6 meters long with a 2.5 cm diameter.
• Receives secretions from the gallbladder, liver, and pancreas.
• Receives chyme from the stomach via the pyloric sphincter.
• Three portions:
  • Duodenum: The first portion, receiving ducts from the gallbladder, liver, and pancreas.
  • Jejunum: The second portion, extending from the duodenum to the ileum; it is intraperitoneal and supported by the mesentery.
  • Ileum: The last portion, extending from the jejunum to the cecum, featuring the ileocecal valve; it is also intraperitoneal and supported by mesentery.
• Function:
  • Secretes intestinal juice (enzymes, water, and mucus).
  • Major site of digestion and absorption.
• Nerve supply: parasympathetic (vagus) and sympathetic (thoracic nerves via the superior mesenteric and celiac plexus).

Histology

• The wall consists of four tunics:
  • Tunica Mucosa: Modified to increase surface area for nutrient absorption.
    • Plicae circulares: Permanent macroscopic folds in the mucosa and submucosa that force chyme to spiral through the lumen, promoting mixing, increasing time, and space for absorption.
    • Villi: Fingerlike projections of the mucosa into the intestinal lumen.
      • Epithelium: Absorptive simple columnar epithelium with tight cellular junctions; secretory enteroendocrine cells and abundant goblet cells present.
      • Lamina propria: Contains a capillary bed and a lacteal.
    • Microvilli: Project from the apical surfaces of the simple columnar cells of the villi, collectively called the brush border; intestinal enzymes are loosely attached, hence brush border enzymes.
    • Intestinal crypts: Depressions in the mucosa that secrete intestinal juice (enzymes, mucus).
    • Goblet cells: Secrete mucus.
    • Enteroendocrine cells: Produce enterogastrones.
  • Tunica Submucosa:
    • Includes lymphoid tissue (Peyer’s Patches) and mucus-secreting glands (Brunner’s Glands).
    • The secreted mucus is thick and alkaline, protecting the intestinal wall from gastric juices.
  • Tunica Muscularis: Outer longitudinal and inner circular layers.
  • Tunica Serosa: Typical serosa (visceral peritoneum).

Intestinal Juice

• Composition:
  • Mostly water with mucus.
  • pH ranges from 7.4-7.8.
• Brush Border Enzymes:
  • Disaccharidases (maltase, sucrase, lactase, etc.) which break disaccharides into component monosaccharides.
  • Peptidases/proteases which complete peptide digestion.
  • Enterokinase (activates pancreatic trypsinogen).
  • Lipases for digestion of lipids.
  • Nucleases for DNA/RNA digestion.
• Control of Secretion:
  • Volume ranges from 1-2 liters per day.
  • The rate of secretion is controlled by long and short reflexes.
  • Parasympathetic NS (Vagal) efferents.
  • Secretin stimulation.
  • Distention of the small intestine.
  • Presence of hypertonic chyme; high acidity.
  • Tactile/chemical irritation.

Role in Digestion

• Chemical Activity:
  • Chyme entering the small intestine contains partially digested proteins and carbohydrates, and mostly undigested lipids and nucleic acids.
  • The remainder of digestion occurs in the small intestine, using pancreatic juice enzymes, bile, and some intestinal enzymes (brush border).
  • Disaccharidases:
    • Sucrase: $sucrose \rightarrow glucose + fructose$
    • Lactase: $lactose \rightarrow glucose + galactose$
    • Maltase: $maltose \rightarrow glucose + glucose$
  • Peptidases: Digest peptides into amino acids.
  • Lipase: Digests lipids into fatty acids and glycerol.
• Mechanical Activity:
  • Segmentation movements mix chyme with enzymes; initiated by pacemaker cells in the longitudinal smooth muscle layers.
  • Intensity increases with distention of the small intestine, hypertonic or acidic chyme, or chyme with high amino acid or peptide content, the gastrocolic reflex, or parasympathetic NS stimulation.
  • Intensity decreases with sympathetic NS stimulation.
  • Peristalsis moves chyme towards the large intestine.
  • Gastrin increases the motility of the ileum and relaxes the ileocecal valve.

The Liver

Location

• Located in the right upper quadrant of the abdominal cavity, tucked up against the inferior surface of the diaphragm.

Microscopic Anatomy

• Functional units: lobules.
• Components of a lobule:
  • Hepatocytes: Form plates (cords) that surround bile canaliculi; used for storage, biotransformation, and the synthesis of plasma proteins.
  • Sinusoids: Lined by phagocytes; purify the blood.

Functions

• Endocrine Functions:
  • In response to growth hormones, secrete IGFs (insulin-like growth factors) which promote growth in tissues by stimulating protein synthesis and mitosis.
  • Contributes to the activation of vitamin D.
  • Forms triiodothyronine from thyroxin.
  • Secretes angiotensinogen which is acted on by renin to form angiotensin I.
  • Metabolizes steroid hormones for excretion.
• Clotting Functions:
  • Produces many of the plasma clotting factors, including prothrombin and fibrinogen.
  • Produces bile salts needed for the absorption of vitamin K; vitamin K is needed for the synthesis of clotting factors.
• Plasma Proteins:
  • Synthesizes and secretes albumin, binding proteins for steroid hormones, trace elements, and lipoproteins.
• Digestive Functions:
  • Synthesizes and secretes bile salts which are necessary for the digestion and absorption of fats – doesn’t make digestive enzymes!
• Organic Metabolism:
  • Converts plasma glucose to glycogen and lipids (triacylglycerols).
  • Converts plasma amino acids to glucose (gluconeogenesis).
  • Produces glucose from glycogen and other sources.
  • Converts fatty acids to ketones during fasting.
  • Produces urea, the major endproduct of amino acid catabolism, and releases it into the blood.
• Cholesterol Metabolism:
  • Synthesizes cholesterol and releases it into the blood.
  • Secretes plasma cholesterol into bile.
  • Converts plasma cholesterol into amino acids.
• Excretory and Degradation Functions:
  • Secretes bilirubin and many other bile pigments into the bile.
  • Transforms many endogenous and foreign molecules.
  • Destroys aged, damaged, or defective erythrocytes.
  • Excretes via the bile many endogenous and foreign organic molecules as well as trace elements.

Bile

Composition

• Alkaline solution in water.
• Other components:
  • Bicarbonate
  • Bile salts
  • Bile pigments (mostly bilirubin)
  • Cholesterol
  • Neutral fats
  • Phospholipids
  • Electrolytes
  • Lecithin

Function

• The digestive function of bile salts is to emulsify fats.

Control of Secretion

• 500 to 800 ml average daily volume.
• Production is constant throughout the day but will be increased if fatty chyme is present.

Gall Bladder

Location

• Located in a fossa on the ventral surface of the right lobe of the liver.

Structure

• A pouch with walls made of smooth muscle and lined by a mucus membrane.

Function

• Stores and concentrates bile:
  • $\text{Na}^+$ actively transported.
  • $\text{H}_2\text{O}$ follows.

Control of Emptying

• The intestinal hormone cholecystokinin (CCK) stimulates contraction of the gall bladder and relaxation of the hepatopancreatic sphincter when fatty chyme is present in the duodenum.
• Vagal efferents also stimulate gall bladder contraction and sphincter relaxation.

Pancreas

Location

• Deep to the greater curvature of the stomach; mostly retroperitoneal.

Structure

• Glandular organ shaped like a comma.
• Exocrine secretions are carried to the duodenum by one (or more) ducts.
• Islets of Langerhans:
  • Alpha cells produce glucagon.
  • Beta cells produce insulin.
• Acinar Cells: Produce enzymes (trypsinogen, chymotrypsinogen, procarboxypeptidase, lipase, amylase, RNAase, DNAase).
• Epithelial cells in pancreatic ducts secrete bicarbonate.

Pancreatic Juice

• Composition:
  • Alkaline (pH = 8).
  • Consists of water, bicarbonate ions, and enzymes.
    • Trypsinogen, chymotrypsinogen, and procarboxypeptidase digest proteins.
      • Activation of these enzymes occurs in the intestinal lumen.
      • Intestinal enterokinase converts trypsinogen to trypsin, then:
        • Trypsin converts chymotrypsinogen to chymotrypsin.
        • Trypsin converts procarboxypeptidase to carboxypeptidase.
    • Lipase: Digests lipids.
    • Amylase: Digests complex carbohydrates.
    • Ribonuclease: Digests RNA.
    • Deoxyribonuclease: Digests DNA.
• Control of Secretion:
  • Average daily volume is 1200-1500 ml.
  • Vagal efferents stimulate secretion.
  • Secretin stimulates the production of watery, bicarbonate-rich pancreatic juice.
  • Cholecystokinin stimulates the production of enzyme-rich pancreatic juice.

Large Intestine

Functions

• Temporarily stores unabsorbed material.
• Bacterial actions:
  • Produce vitamins (B complex and K).
  • Ferment carbohydrates and produce gas and acid.
  • Convert amino acids to indole, skatole, mercaptans.
  • Convert bile pigments to urobilogen.
• Absorb water from chyme.
• Absorb $\text{Na}^+$ and $\text{Cl}^-$ into the blood.
• Secrete bicarbonate ion, $\text{K}^+$.
• Feces formation.

Movements

• Haustral contractions:
  • Haustra are created by slow segmentations which help to mix the contents of the large intestine and aid in water reabsorption.
  • Are somewhat propulsive.
• Mass Movements:
  • Long, slow, powerful contractions that occur 3-4 times/day.
  • Facilitated by tenia coli.
  • Move feces towards the rectum.
  • Occur as part of the gastrocolic reflex.

Defecation

• Stimulus: distention of the rectal wall.
• Receptors: stretch receptors in the rectal wall.
• Coordinating center: sacral spinal cord.
• Effectors: muscle layers of the lower colon and rectum; contraction elevates pressure on feces; the internal anal sphincter relaxes to allow feces to move into the anal canal.