Overview of the GI Tract

• Mouth:

  • Begins mechanical breakdown (chewing) and chemical digestion (saliva contains amylase).

• Esophagus:

  • Muscular tube transporting food from mouth to stomach via coordinated contractions (peristalsis).

• Stomach:

  • Secretes acid and enzymes (e.g., pepsin); churns food into chyme.

  • Acts as a temporary reservoir and starts protein digestion.

• Small Intestine:

  • Major site for digestion and nutrient absorption.

  • Divided into: duodenum, jejunum, ileum.

  • Receives bile (for fat emulsification) and pancreatic enzymes.

• Large Intestine (Colon):

  • Absorbs water and electrolytes; forms and stores faeces.

  • Houses beneficial gut microbiota.

• Rectum:

  • Temporary storage of faeces before elimination.

• Anus:

  • Controlled by internal (involuntary) and external (voluntary) sphincters; allows defecation.

Function of Accessory Organs

• Liver:

  • Produces bile (aids fat digestion), processes nutrients, detoxifies harmful substances, stores glycogen, and synthesises plasma proteins.

• Pancreas:

  • Exocrine function: releases digestive enzymes (amylase, lipase, proteases) into the duodenum.

  • Endocrine function: secretes insulin and glucagon into the bloodstream.

• Gallbladder:

  • Stores and concentrates bile; releases it into the duodenum in response to fatty meals.

Digestive Enzymes and Bile Secretion

“SALP” — Salivary, Acid, Liver, Pancreas (sources of major digestive juices)

The Role of Gut Motility and Smooth Muscle Contraction

• Motility:

  • Smooth muscle contraction moves food through the GI tract (peristalsis) and mixes contents (segmentation).

  • Essential for digestion, nutrient absorption, and waste elimination.

  • Disorders (e.g., ileus, achalasia) cause impaired digestion and symptoms like constipation or vomiting.

Enteric Nervous System (ENS) and Brain-Gut Connection

• Enteric Nervous System:

  • Intrinsic network of neurons (“the gut brain”) within the GI tract wall.

  • Regulates local motility, secretion, blood flow, and responses to stretch.

  • Functions independently, but communicates with the central nervous system (CNS) via the vagus nerve and sympathetic nerves.

• Brain-Gut Connection:

  • CNS influences GI function via the autonomic nervous system (e.g., stress can alter motility).

  • ENS involved in “gut feelings” and disorders such as irritable bowel syndrome (IBS).

  • Gut microbiota also play a role in signalling between the gut and brain.

References

• British National Formulary, “Gastro-intestinal system overview”

• NHS, “How your digestive system works”

• NICE, “Irritable bowel syndrome in adults: diagnosis and management”