Digestive System

• Functions:

  • Ingestion: Taking food into the digestive tract

  • Digestion: Mechanical - Chewing, mixing, and churning food. Chemical - Catabolic breakdown of food.

  • Absorption: Taking up the soluble digestion products into the body’s cells.

  • Defecation: Elimination of indigestible material.

• Upper GI tract:

  • Mouth: Chemical and Mechanical digestion. Consists of the Tongue, Salivary glands, and Teeth.

  • Pharynx and Oesophagus: A tube extending behind the nasal and oral cavities until the voice box and the esophagus. The esophagus is a long fibromuscular tube stretched from the pharynx to the stomach made of longitudinal and circular muscles that transports material to the stomach by the peristalsis.

  • Stomach: Muscular sac located between the esophagus and the duodenum. The main parts include the cardiac notch, fundus, body, and pyloric part.

• Lower GI tract:

  • Small Intestine: Extends from the pylorus to the ileocecal valve,  composed of the duodenum, jejunum, and ileum. The primary function is the absorption of vitamins and nutrients including electrolytes, iron, carbohydrates, proteins, and fats.

  • Large Intestine: Terminal Ileum > Ascending colon > Traverse colon > Descending colon > Sigmoid colon > Rectum. The main function is water absorption and the production and absorption of vitamins.

• Accessory Organs:

  • Liver: The largest internal organ, its main use is the storage of glycogen and secretion of bile.

  • Gallbladder: Pear-shaped, attached to the liver, serving as a bile storage facility. When a large meal is consumed, nerve and chemical signals cause the gallbladder to contract which causes the release of bile into the digestive system. The biliary tree comprises of: The common hepatic duct, the cystic duct, and the common bile duct.

  • Pancreas: Functions include the production of hormones and enzymes that help digest carbohydrates, proteins, and fats.

1.14 General Microanatomy of GI tract

• Mucosa:

  • Epithelium: Covers the inner surface of the digestive tract, starts as stratified squamous epithelial in the esophagus, and changes to the simple columnar epithelium in the stomach. In the intestine, it remains columnar but acquires microvilli to increase the surface area for absorption. Lies on the basement membrane.

  • Lamina Propria: Thin layer of loose connective tissue.

  • Muscularis Mucosae: A relatively thin layer of smooth muscle.

• Submucosa:

  • Submucosal plexus: A set of large blood vessels, lymphatics, and neurons.

  • Submucosal glands: Layer of dense connective tissue.

• Muscularis:

  • Myentric Plexus: A layer of smooth muscles.

  • Plexus Of Auerbach: Lies between circular and longitudinal layers of smooth muscle and contracts to produce peristalsis.

• Serosa/Adventita

1.15 Genitourinary system

• Upper Urinary Tract:

  • Kidney: Production of urine and osmoregulation

  • Ureters: Extend from each kidney towards the bladder

• Lower Urinary Tract:

  • Bladder: A large muscle bag that can inflate and deflate by the presence or absence of urine.

  • Urethra: Transports urine from the bladder to outside the body.

1.16 Kidneys

• Overview: Nephrons act as functional units of the kidney and are the place where filtration and urine production occurs. They mostly lie within the renal cortex.

• Structure: Renal cortex surrounds the kidney and extends inwards at points to form renal columns within the renal medulla that divides it into triangular-shaped tissues known as renal pyramids.

• Renal Medullas: Collect urine and transfers them into the minor calyces and then the major calyces. These drain into the widened areas of the kidney known as the renal pelvis, the renal pelvis leaves the kidney at the hilum and thins to continue as the ureter.

• Urethra: In males usually surrounded by the prostate before passing through the pelvic floor whilst in females it passes directly through the pelvic floor.Has two associated muscles:

  • Internal Sphincter: Keeps bladder neck and urethra free of urine until pressure builds in the bladder, Involuntary smooth muscle

  • External Sphincter: Controls release of urine from the urethra. Voluntary skeletal muscle.