Gastrointestinal System - Small and Large Intestine, Liver, Gall Bladder, and Pancreas

Small Intestine

Location, Functions, and Gross Anatomy (Objective 20)

• Location: Abdominal cavity.
• Length: 6 meters long; 2.5 cm diameter.
• Function:
  • Receives secretions from the gallbladder, liver, and pancreas.
  • Receives chyme from the stomach via the pyloric sphincter.
  • Secretes intestinal juice (enzymes, water, mucus).
  • Major site of digestion and absorption.
• Three Portions:
  • Duodenum:
    • First portion.
    • Receives ducts from the gallbladder, liver, and pancreas.
  • Jejunum:
    • Second portion.
    • Extends from the duodenum to the ileum.
    • Intraperitoneal and supported by mesentery.
  • Ileum:
    • Last portion.
    • Extends from the jejunum to the cecum.
    • Note the ileocecal valve.
    • Intraperitoneal and supported by mesentery.
• Nerve Supply:
  • Parasympathetic (vagus nerve).
  • Sympathetic (thoracic nerves via the superior mesenteric and celiac plexus).

Histology (Objective 21)

• Wall Structure: Four tunics
  • Tunica Mucosa:
    • Modified to increase surface area for nutrient absorption.
    • Modifications:
      • Plicae Circulares: Permanent macroscopic folds in the mucosa and submucosa.
        • Forces chyme to spiral through the lumen.
        • Promotes mixing.
        • Increases time and space for absorption.
      • Villi: Fingerlike projections of the mucosa into the intestinal lumen.
        • Epithelium: Absorptive simple columnar epithelium with tight cellular junctions.
        • Cell Types:
          • Secretory enteroendocrine cells.
          • Abundant goblet cells.
        • 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 (brush border enzymes) are loosely attached.
      • Intestinal Crypts: Depressions in the mucosa.
        • Cells secrete intestinal juice (enzymes, mucus).
        • Goblet cells: Secrete mucous.
        • Enteroendocrine cells: Produce enterogastrones.
  • Tunica Submucosa:
    • Includes lymphoid tissue (Peyer’s Patches).
    • Mucus-secreting glands (Brunner’s Glands).
    • 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 and Control (Objective 22)

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

Liver

Location (Objective 23)

• Right upper quadrant of the abdominal cavity.
• Tucked up against the inferior surface of the diaphragm.

Microscopic Anatomy

• Functional units are called lobules.
• Components of a Lobule:
  • Hepatocytes:
    • Form plates (cords) that surround bile canaliculi.
    • Used for storage, biotransformation, and synthesis of plasma proteins.
  • Sinusoids:
    • Lined by phagocytes.
    • Purify the blood.

Functions of the Liver

• Hundreds of functions; key examples include:
  • Endocrine Functions:
    • Secrete IGFs (insulin-like growth factors) in response to growth hormones, promoting tissue growth 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 plasma clotting factors, including prothrombin and fibrinogen.
    • Produces bile salts needed for vitamin K absorption; vitamin K is needed for clotting factor synthesis.
  • Plasma Proteins:
    • Synthesizes and secretes albumin, binding proteins for steroid hormones, trace elements, and lipoproteins.
  • Digestive Functions:
    • Synthesizes and secretes bile salts necessary for digestion and absorption of fats (but does not 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 end product 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 and Control of Secretion (Objective 24)

• 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:
  • Average daily volume: 500 to 800 ml.
  • Production is constant throughout the day, but increases with fatty chyme.

Gall Bladder

Location (Objective 25)

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

Structure

• Pouch with walls made of smooth muscle and lined by mucous membrane.

Function

• Store and concentrate bile.
  • $Na^+$ actively transported.
  • $H_2O$ 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 (Objective 26)

• 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.

Cells

• 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 (Objective 27)

• Composition:
  • Alkaline ($pH = 8$).
  • Water, bicarbonate ions, and enzymes.
    • Trypsinogen, chymotrypsinogen, and procarboxypeptidase digest proteins.
      • Activation occurs in the intestinal lumen:
        • Intestinal enterokinase converts trypsinogen to trypsin, which then activates chymotrypsinogen to chymotrypsin.
    • 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.

Small Intestine – Role in Digestion (Objective 28)

Chemical Activity

• Chyme entering the small intestine has partially digested proteins and carbohydrates, and mostly undigested lipid and nucleic acids.
• The remainder of digestion takes place in the small intestine, using pancreatic juice enzymes, bile, and some intestinal enzymes (brush border).
• Disaccharidases:
  • Sucrase: Sucrose yields glucose + fructose.
  • Lactase: Lactose yields glucose + galactose.
  • Maltase.
• Peptidases:
  • Peptides yields amino acids.
• Lipase:
  • Lipids yields fatty acid + 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 with 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.

Large Intestine

Functions (Objective 29-30)

• 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, mercaptens.
  • Convert bile pigments to urobilogen.
• Absorb water from chyme.
• Absorb $Na^+$ and $Cl^-$ into the blood.
• Secrete bicarbonate ion, $K^+$.
• 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.
  • Occurs as part of the gastrocolic reflex.

Defecation (Objective 31)

• Stimulus: Distention of the rectal wall.
• Receptors: Stretch receptors in the rectal wall.
• Coordinating Center: Sacral spinal cord.
• Effectors: Muscle layers of lower colon and rectum.
  • Contraction elevates pressure on feces.
  • Internal anal sphincter relaxes to allow feces to move into the anal canal.