GIT anatomy - major organs

Gastrointestinal System - Digestive Tract

• The digestive tract is a series of organs that process food, from ingestion to elimination.
• It includes the oral cavity, pharynx, esophagus, stomach, small intestine, large intestine, and accessory organs like teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
• Surface anatomy is divided into 9 regions: right hypochondrium, epigastric, left hypochondrium, right lumbar, umbilical, left lumbar, right iliac, hypogastric, and left iliac.

Oral Cavity

• The roof is formed by the hard and soft palate.
• The floor is covered by mucosa supported by geniohyoid and mylohyoid muscles.
• The tongue rests on the floor.
• The lateral walls are supported by pads of fat and the buccinator muscle.
• Functions include taste sensation, grinding of food through teeth, tongue, and palatal surfaces, lubrication with mucus and saliva, and limited digestion of carbohydrates and lipids.
• Major components are teeth, tongue, and salivary glands (parotid, sublingual, and submandibular).

Pharynx

• Connects the oral cavity to the esophagus.
• Serves as a common passageway for food, liquids, and air.
• Subdivisions: nasopharynx, oropharynx, and laryngopharynx.
• Oropharynx and laryngopharynx are involved in food transport to the esophagus, and have stratified squamous epithelium similar to the oral cavity.

Esophagus

• A muscular tube connecting the pharynx to the stomach.
• Descends through the thoracic cavity.
• Collapses unless food is moving through it.
• Located posterior to the trachea and anterior to the vertebral column.
• The opening between the esophagus and the stomach is controlled by the esophageal sphincter (also cardiac sphincter).

Stomach

• Inner surface has folds called rugae.
• The stomach wall has four layers: mucosa, submucosa, muscularis externa, and serosa.
• The muscularis externa has three layers of muscles: oblique, circular, and longitudinal.
• Functions: mechanical digestion of food into chyme, enzymatic digestion of proteins by pepsin (activated in acid), neutralizing bacteria with HCl (from parietal cells), absorption of alcohol, sugar, salt, water, and drugs, and storing food for up to four hours.
• Blood supply comes from branches of the celiac trunk: left gastric, right gastric, and left gastromental.

Small Intestine

• Longest part of the gastrointestinal tract (about 6.7 meters).
• The major site of enzymatic digestion and nutrient absorption.
• Enzymes (mostly from the pancreas) are involved in the digestion.
• Absorption process takes about 3-6 hours.
• Divided into three parts: duodenum, jejunum, and ileum.
• Duodenum is the shortest and receives bile from the liver and gallbladder.
• Jejunum is about 2.5 meters long.
• Ileum is the longest (about 3.5 meters) part.
• Anatomical modifications for absorption include plica circularis (circular folds), villi (finger-like projections), and microvilli (fine hair-like projections).
• Presence of Brunner's glands in the duodenum submucosa, intestinal crypts, villi, and numerous goblet cells are important structural features.

Large Intestine

• Absorbs water and electrolytes from undigested food.
• Compacts and stores fecal matter until elimination.
• Divided into cecum, ascending colon, transverse colon, descending colon, sigmoid colon, rectum, and anal canal.
• Has haustra (pouches), appendices epiploicae (small fatty appendages), and taenia coli (bands of smooth muscle).
• Lined with simple columnar epithelium and numerous goblet cells.

Appendix

• An extension from the cecum (first part of the large intestine).
• Function is unknown.
• Receives blood supply from the appendicular artery and vein.
• The removal of the appendix is an appendicectomy, done in the case of inflammation (Appendicitis).

Rectum & Anal Canal

• The rectum is similar to the colon but has a continuous coat of longitudinal muscle, no taenia coli, and limited peritoneum.
• Peritoneum covers the front and sides of the upper one-third and front of the middle third of the rectum.
• As it moves to the anal canal, it transitions to stratified squamous epithelium.

Blood Supply

• Different sections of the GI tract (foregut, midgut, hindgut) have different branches of the arterial system that supply them. Specific arteries are associated with each segment of the digestive tract.