git 2

Gastrointestinal Tract Physiology Overview

  • Understanding the gastrointestinal tract (GIT) is crucial for recognizing its structure, functions, and the processes of digestion and absorption.

General Organization/Functional Anatomy of the GIT

  • Major Components: Mouth, esophagus, stomach, small intestine, large intestine, rectum, anus.

  • Accessory Organs: Liver, pancreas, gallbladder, salivary glands.

Smooth Muscle Function

  • Types of Muscle: Smooth muscle allows peristalsis, which moves food through the digestive tract.

  • Functionality: Smooth muscle contractions are involuntary and controlled by the autonomic nervous system and hormones.

GIT Motility

  • Types of Movement: Peristalsis (propels contents forward) and segmentation (mixes contents).

  • Significance: Essential for efficient digestion and absorption of nutrients.

GIT Secretions and Hormones

  • Secretions: Mucus, enzymes, and hormones that facilitate digestion.

  • Hormones: Gastrin, CCK, secretin, GIP, and others that regulate digestive activities.

Digestion and Absorption of Food Substances

  • Stages: Involves chemical breakdown of food and absorption of nutrients by intestinal cells.

  • Processes: Involves enzymatic actions and transport mechanisms for nutrient uptake.

Liver and its Functions

  • Metabolism: Responsible for carbohydrate, lipid, and protein metabolism.

  • Detoxification: Processes drugs and toxins, synthesizes proteins, and produces bile.

  • Bile Production: Vital for fat digestion and waste excretion.

Nutrition and Metabolism

  • Nutrient Utilization: Involves breakdown and conversion processes to keep cellular functions active.

  • Regulatory Roles: Insulin, glucagon, and other hormones play crucial roles in maintaining metabolic balance.

GIT Secretions & Hormones Details

  • Digestive Enzymes:

    • Salivary Glands: Salivary α-amylase (works on starch).

    • Stomach: Pepsins (from pepsinogens) act on proteins.

    • Pancreas: Produces trypsin, chymotrypsin, and others for further digestion in the small intestine.

    • Bile: Emulsifies fats and aids in lipid digestion.

  • Mucus: Protects the lining of the GIT, lubricates food material, and offers a barrier against pathogens.

1 Salivary Secretion

  • Components: Contains serous (ptyalin) and mucus (mucin) type secretions.

    •Mechanism: Involves primary secretion from acini followed by modification in ducts.

•sodium ions are actively reabsorbed.

• potassium ions are actively secreted.

•chloride ions to be reabsorbed passively.

• And bicarbonate ion secreted.

Gradient-

Sodium conc (mEq/L) 145-15

• Potassium conc (mEq/L) 4.5-30

• Bicarbonate ions (mEq/L) 28 - 70

Notes;

  • Increased rate of production: saliva rich in sodium is produced (copious) I.e less time for sodium to be secreted

    •saliva rich in potassium is produced (sticky) I.e more time for potassium to be secreted

  • Regulation: Controlled by nervous stimuli (PNS) (e.g., sight, taste) and local reflexes.

2 Gastric Acid Secretion

  • Types of Cells: Oxyntic cells (HCl), Chief cells (pepsinogen), and G cells (gastrin).

  • Mechanism: Acid secretion involves ion transport mechanisms and interaction with histamine.

1. Cl- is actively transported from the cytoplasm of the parietal cell into the lumen of the canaliculus

• 2. Na+ are actively transported out of the canaliculus into the cytoplasm of the parietal cell

• 3. a negative potential of -40 to -70 millivolts is created in the canaliculus

• 4. the negative potential causes diffusion of small quantities of Na+ and K+ back into the lumen Mechanism (cont’d)

• 5. water inside the oxyntic cell dissociates into H+ and OH- ((Water dissociation occurs in the presence of carbonic anhydrate)

• 6. H+ are actively secreted into the lumen in exchange for K+ using H+-K+ ATPase

• 7. Na+ are reabsorbed back into the ECF using Na+K+ATPase.

• 8. water moves downhill from the ECF through the cell to the lumen

• 9. CO 2 in the cell combines with OH- in the presence of CA to form HCO3- which is reabsorbed back into the ECF

• Final secretion is thus:

• HCl and water, conc= 150 to 160 mEq/L, KCl= 15 mEq/L

Summary:

1) Cl ion is returned back to extracellular fluid

2) hydrogen and Cl ions are released into lumen

3) Na-K pump provides required energy

4) Water diffuses from ECF into lumen

5) H and Cl provides HCL

Phases of Gastric Secretion

  • Cephalic Phase: Triggered by thought/sight of food; involves neural signals.

  • Gastric Phase: Initiated by food entry; involves neural and hormonal mechanisms.

  • Intestinal Phase: Inhibitory signals when chyme enters small intestine; regulates gastric emptying.

Pancreatic Secretion

  • Types of Secretions: Exocrine (digestive enzymes) and endocrine (insulin/glucagon).

  • Regulation: Secretin (bicarbonate) and CCK (enzymes) are stimulated by chyme presence.

Bile Secretion and Function

  • Production: Liver produces bile continuously; gallbladder stores and concentrates it.

  • Role in Digestion: Bile acids emulsify fats; aids fat absorption through micelle formation.

GIT Hormones

  • Gastrin: Stimulates gastric secretion and motility; regulated by peptides/amino acids.

  • CCK: Stimulates enzyme secretion from pancreas and gallbladder contraction.

  • Secretin: Increases bicarbonate secretion and reduces gastric acid secretion.

  • GIP: Inhibits gastric activity and stimulates insulin release.

Summary of GIT Physiology

  • GIT mechanics are vital for digestion efficiency, nutrient absorption, and overall metabolic homeostasis.

  • Proper understanding of the interplay between hormones, enzymes, and structural organization is fundamental for comprehensive knowledge of gastrointestinal physiology.