Digestive Processes 2 - 2.27.25

Digestive Processes Overview

• Overview of digestive enzymes and their roles in digestion.

  • Amylase: Breaks down carbohydrates.

  • Sucrase-isomaltase: Digests specific sugars.

  • Maltase: Converts maltose into glucose.

  • Lactase: Digests lactose.

  • Pepsin: Begins protein digestion in the stomach.

  • Trypsin: Continues protein digestion in the small intestine.

  • Peptidase: Further digests peptides into amino acids.

  • Lipase: Breaks down fats.

Intestinal Motility

Segmentation

• Characteristics:

  • Stationary constrictions form and reform.

  • Mixes chyme with bile and digestive juices.

  • Brings chyme into contact with the intestinal mucosa.

  • Transitions to peristalsis after most nutrients are absorbed.

Peristalsis

• Gradual movement toward the colon:

  • Triggered by hormones in the duodenum.

  • Moves chyme towards the colon over approximately 2 hours.

  • Refilling of the stomach suppresses peristalsis and reactivates segmentation.

Digestion and Absorption of Carbohydrates

• Enterocytes can only absorb monosaccharides.

  • Larger sugar molecules must be broken down chemically.

  • Begins with salivary amylase:

    • Optimal pH of 6.8 to 7.0.

    • Inactivated by stomach acid.

  • Continued by pancreatic amylase.

  • Contact digestion produces monosaccharides like glucose, which pass through enterocytes via facilitated diffusion.

Lactose Intolerance

• Lactase production typically ceases after weaning in most mammals.

• Those unable to digest lactose become intolerant:

  • Undigested lactose leads to increased osmolarity in the intestine.

  • Results in water retention and diarrhea.

  • Bacterial fermentation causes gas, bloating, and cramps.

  • Yogurt and cheese can often be consumed as they are pre-digested.

Digestion and Absorption of Proteins

• Begins in the stomach with pepsin:

  • Digesting the first 10-15% of proteins.

  • Optimal pH of 1.5 to 3.5, inactivated by pancreatic juice.

• Pancreatic enzymes take over in the duodenum.

• Brush border enzymes break down peptides into free amino acids for absorption by enterocytes.

Digestion and Absorption of Lipids

Mechanical Digestion

• Gastric and intestinal motility create emulsification droplets.

• Bile coats droplets to prevent recombination.

Chemical Digestion

• Begins in the mouth via lingual lipase.

• Continues with gastric and pancreatic lipase,

  • Producing monoglycerides and free fatty acids.

Micelles

• Formed from bile molecules arranged around a lipid center:

  • Carry lipids from chyme to enterocytes.

  • Lipids are absorbed via lacteal lymph capillaries as chyle, which is fat-heavy intestinal lymph that returns to the bloodstream.

Digestion and Absorption of Electrolytes

• Electrolytes diffuse freely following a concentration gradient:

  • Calcium absorption is facilitated by parathyroid hormone in response to hypocalcemia.

  • Iron absorption is mediated to prevent toxicity:

    • Hepcidin prevents overabsorption by blocking export channels in enterocytes.

    • Excess iron binds to ferritin in enterocytes and is excreted as feces.

Digestion and Absorption of Vitamins

• Fat-soluble vitamins: A, D, E, and K are absorbed with fats.

• Water-soluble vitamins: B complex and C are primarily absorbed by diffusion:

  • Vitamin B12 requires intrinsic factor for absorption.

Consolidation in the Large Intestine

• Takes approximately 36-48 hours to process food:

  • Absorbs additional water and electrolytes.

  • Feces composition: 75% water, 25% solids (undigested fiber, bacterial cells, etc.).

Defecation

At Rest

• Nerve Activation:

  • Hypogastric nerve: Relaxes the rectum, closes the internal anal sphincter (sympathetic).

  • Pudendal nerve: Closes the external sphincter (somatic motor).

Defecation Process

1. Rectum contracts.

2. Internal sphincter relaxes.

3. External sphincter relaxes (must occur simultaneously).

Reflexive Defecation Controls

• Controlled by two reflexes stimulated by rectum stretching:

  • Intrinsic reflex: Activates peristalsis in descending and sigmoid colon.

  • Spinal reflex: Carries stretch stimulus to sacral spinal cord for parasympathetic stimulation, enhancing peristalsis and relaxing the internal anal sphincter.

Voluntary Defecation

• Initiated by rectal stretch:

  • Parasympathetic reflex relaxes the internal anal sphincter.

  • Involves voluntary relaxation of skeletal muscle at the external anal sphincter (pudendal nerve).

• Voluntary suppression can occur, temporarily ceasing intestinal contractions until another mass movement occurs.