Physiology of Digestion and Absorption

Physiology of Digestion and Absorption

  • Definition of Digestion

    • The process of breaking down complex nutrients into simple molecules.

  • Definition of Absorption

    • The process of transporting those simple molecules across the intestinal epithelium into the bloodstream.

  • Relationship Between Digestion and Absorption

    • Both processes are necessary for the assimilation of nutrients into the body.

    • Absorption cannot occur if food is not digested.

    • The process of digestion is fruitless if the digested nutrients cannot be absorbed.

Structure of the Small Intestine

  • Mucosal Surface Features

    • Large surface area due to the structure of the small intestine mucosa.

    • Epithelial cells have leaky junctions that enhance absorption of digested material, electrolytes, and water.

  • Surface Convolutions

    1. Plicae Circulares

    • Large folds of mucosa that add to the intestinal surface area; presence varies among animal species.

    1. Villi

    • Fingerlike epithelial projections covering the mucosal surface; present in all species and increase the intestinal surface area by 10-14 times compared to flat surfaces.

    1. Brush Border

    • Composed of submicroscopic microvilli that further enlarge the surface area; located on the villi.

  • Crypts of Lieberkühn

    • Gland-like structures at the base of the villi, covered with continuous epithelium.

Enterocytes and Their Functions

  • Definition of Enterocytes

    • The epithelial cells covering the villi and crypts in the intestinal mucosa.

  • Cell Membrane Composition

    • Apical Membrane:

      • Faces the lumen; contains microvilli and glycoproteins synthesized by enterocytes.

      • Glycoproteins are essential for digestive and absorptive functions; area is known as the glycocalyx.

    • Basolateral Membrane:

      • Does not face the gut lumen; plays a crucial role in absorption as it facilitates the exit of nutrients absorbed through the apical membrane into the bloodstream.

Tight Junctions and Cell Attachments

  • Structure of Tight Junctions

    • Form a narrow band of attachment between adjacent enterocytes at the apical end, separating the apical and basolateral membranes.

  • Permeability

    • Allows permeation of water and small electrolytes, especially in the duodenum and jejunum.

    • Not permeable to organic molecules.

    • The tightness can be regulated by neurohumoral substances.

  • Extracellular Fluid (ECF)

    • Lateral space created by the basolateral membrane filled with ECF, separated from the intestinal lumen by tight junctions and from blood by the basement membrane of capillaries.

    • Both tight junctions and capillary endothelium are permeable to water and small molecules.

Goblet Cells and the Protective Layer

  • Function of Goblet Cells

    • Secrete mucus which forms a layer over the mucosa.

  • Components of the Mucus Layer

    • Blends into the glycocalyx at the brush border surface, forming a viscous coating that traps particles near the apical membrane.

  • Unstirred Water Layer

    • A layer of fluid near the intestinal surface that flows slowly compared to the central lumen, contributing to a barrier through which nutrients must pass to enter enterocytes.

Process of Digestion

  • Overview of Digestion

    • Physical and chemical breakdown of food particles into subunits for absorption.

  • Physical Breakdown

    • Increases the surface area of food particles, facilitating exposure to digestive enzymes.

    • Grinding action in the distal stomach is aided by pepsin and hydrochloric acid, assisting in breaking connective tissues of animal-origin food.

  • Chemical Digestion

    • Achieved through hydrolysis, the splitting of chemical bonds by water insertion, leading to simpler molecules.

    • Types of bonds cleaved:

    • Glycosidic linkages in carbohydrates.

    • Peptide bonds in proteins.

    • Ester bonds in fats.

    • Phosphodiester bonds in nucleic acids.

Enzymes of Digestion

  • Categories of Digestive Enzymes

    1. Enzymes acting in the gut lumen (luminal phase).

    2. Enzymes acting on the membrane surface of epithelial cells (membranous phase).

  • Luminal Phase of Digestion

    • Involves enzymes secreted by major GI glands (salivary, gastric, and pancreas).

    • Results in incomplete hydrolysis, forming short-chain polymers.

  • Membranous Phase of Digestion

    • Enzymes bound to the surface of the small intestine complete digestion into absorbable monomers.

Carbohydrate Digestion

  • Sources of Dietary Carbohydrates

    • Primarily derived from plants as sugars and starches.

  • Luminal Phase of Carbohydrate Digestion

    • Only applicable to starches; sugars are digested in the membranous phase.

    • The main enzyme involved is alpha-amylase, secreted by the pancreas and salivary glands in some species.

  • Complete Digestion of Polysaccharides

    • All polysaccharides are converted to monosaccharides before absorption.

    • Specific membranous-phase enzymes are designated based on their substrate type.

Protein Digestion

  • Importance of Proteins

    • Serve as essential sources of amino acids in animal diets.

  • Luminal Phase Digestion of Proteins

    • Carried out by enzymes secreted as inactive zymogens from gastric glands and pancreas, activated in the stomach or intestinal lumen.

  • Membranous Phase Digestion

    • Peptides are further digested to individual amino acids through enzymatic action in the microenvironment of unstirred water layers and intestinal mucus.

  • Complete Digestion Outcomes

    • Final digestion occurs on the enterocyte surface and within the cells. Peptides are broken down into:

    • Tripeptides

    • Dipeptides

    • Free amino acids

    • All are absorbed by enterocytes; intracellular peptidases convert them into amino acids for transport into the bloodstream.

Non-Digestible Components

  • Role of Fibers in Herbivorous Diets

    • Provide important energy sources but cannot be digested by hydrolysis.

Absorption Process

  • Definition of Absorption

    • Movement of products of digestion across the intestinal mucosa into the vascular system for distribution.

  • Diffusion and Chemical Gradients

    • Molecules naturally flow to areas of lower concentration, and charged particles move toward opposite charges.

    • Charged ions (especially cations) and most organic nutrient molecules do not freely penetrate the GI epithelium.

  • Transport Mechanisms

    • Required for ions and organic molecules illustrated by:

      • Active transport

      • Secondary active transport

      • Tertiary active transport

      • Passive transport

    • Transport mechanisms are polarized within enterocytes; specific mechanisms exist on either the apical or basolateral membranes.

Neonatal Digestion

  • Immunological Protection in Livestock

    • Young livestock (horses, cattle, sheep, swine) do not receive mother's antibodies through the placenta and must acquire them from colostrum.

  • Alterations in Neonatal Digestive Tract

    • 1. Acid secretion from the stomach is delayed for several days after birth.

    • 2. Delayed development of pancreatic function avoids acid and trypsin digestion of proteins.

    • 3. Special intestinal epithelium present at birth allows for protein absorption intact.

  • Gut Closure

    • The special absorption-capable epithelium disappears after 24 hours post-birth, ending the protein-absorptive function, known as gut closure.