Gastrointestinal Tract Overview

Gastrointestinal Tract

• The gastrointestinal (GI) tract is responsible for digestion and absorption.
• Digestion converts food into a sterile, neutral, isotonic substance (small sugars, amino acids, etc.).
• Excretion is the removal of waste products.
• Motility involves smooth muscle contractions to move ingested food from mouth to anus.
• Absorption is the uptake of nutrients from the GI tract into the bloodstream.

Digestive System Adaptations

• The GI tract has structural variations that relate to functional processes.
• Fluid balance in the gut is crucial for digestion and absorption.
• Hormones and the enteric nervous system regulate GI activity.

Objectives

• Outline the function and general structure of the digestive tract.
• Identify regional structural variations and their functional connections.
• Describe fluid balance within the gut.
• Explain the role of hormonal and enteric nervous systems.

Organs of the GI Tract

• Mouth
• Esophagus
• Liver
• Stomach
• Gall bladder
• Pancreas
• Small intestine
• Large intestine
• Appendix
• Rectum

Function of the GIT System

• Digestion: Breaks down food physically and chemically.
• The non-specific phase physically reduces large particles.
• The specific phase uses enzymes to break down food molecules.
• Chyme formation: semiliquid, sterile mixture of ingested food due to stomach acid.
• Motility: Muscle contractions mix/move food.
• Absorption: Nutrients and water transferred to the bloodstream.

Initial Physical Disruption

• Food is ingested, and initial chemical changes occur, creating chyme.
• Chyme is disinfected and diluted by chemical reactions.
• Nutrients and electrolytes are absorbed.
• Water and electrolytes are absorbed.
• Feces are produced for elimination.

Absorption

• Absorption: Specific uptake of nutrients, water, and electrolytes (active or passive).
• The small intestine has a high surface area (villi & microvilli) for efficient absorption.
• Most water is absorbed in the small intestine.
• Water and electrolytes are absorbed in the large intestine.
• Remaining water and electrolytes are reabsorbed, faeces are produced.

Excretion

• Faeces are propelled into the rectum.
• The urge to defecate occurs when the sphincters relax.
• Defecation is the expulsion of faeces.
• Diarrhoea and vomiting are major causes of dehydration due to loss of water before reabsorption.

Balance of Gut

• Detailed accounting of water and electrolytes in the GI tract during digestion and absorption process.

Small Intestine

• Absorbs nutrients and electrolytes.
• Fluid passes slowly through the small intestine.
• Epithelial cells actively and passively absorb molecules.
• Absorption is often coupled with sodium (Na+) transport.
• Nutrients are transported to the liver via the hepatic portal circulation.
• The majority of water is absorbed in the small intestine.

Large Intestine

• Final absorption of water (0.15L)
• Slow transit time for further water and electrolyte absorption.
• Faeces are formed and collected in the descending and sigmoid colon.

Rectum

• Faeces are periodically propelled into the rectum.
• Defecation occurs after sphincter relaxation.

Enteric Nervous System

• A subdivision of the autonomic nervous system.
• Controls GI motility.
• Two nerve plexuses in the gut wall (short reflex pathway).
• Parasympathetic control primarily affects GI activity.

Hormonal Activations

• Peptide hormones are released by endocrine cells in the GI tract walls.
• Some hormones act locally (paracrine action).
• Other hormones affect distant parts of the gut (endocrine action).
• Gastrin and secretin groups are important examples.