Editing Anatomy and Physiology (section)

== Gastrointestinal System ==

The tract runs from mouth to anus: mouth, pharynx, esophagus, stomach, small intestine (duodenum, jejunum, ileum), large intestine, rectum, anus. Accessory organs are the salivary glands, liver, gallbladder, and pancreas.

* '''Stomach''': secretes hydrochloric acid, pepsinogen, and intrinsic factor. Intrinsic factor is required for vitamin B12 absorption in the ileum, which is why gastric bypass, gastrectomy, and pernicious anemia cause B12 deficiency.
* '''Small intestine''': the primary site of nutrient absorption, with villi and microvilli providing enormous surface area. The duodenum receives bile and pancreatic enzymes.
* '''Large intestine''': water and electrolyte reabsorption and formation of stool. This is why diarrhea causes dehydration and metabolic acidosis from bicarbonate loss.
* '''Liver''': protein synthesis (albumin, clotting factors), drug and hormone metabolism, bile production, glycogen storage and gluconeogenesis, and conversion of ammonia to urea. Failure of ammonia conversion produces hepatic encephalopathy.
* '''Gallbladder''': stores and concentrates bile, released in response to cholecystokinin when fat enters the duodenum. Bile emulsifies fat and enables absorption of fat-soluble vitamins A, D, E, and K.
* '''Pancreas''': exocrine secretion of amylase, lipase, protease, and bicarbonate; endocrine secretion of insulin and glucagon.

Peristalsis moves contents forward and is stimulated by the parasympathetic nervous system. Opioids, anticholinergics, and immobility slow it; hence constipation risk.

'''Nursing application:'''
* Prolonged vomiting or nasogastric suction removes hydrochloric acid and produces '''metabolic alkalosis''' with hypokalemia; prolonged diarrhea removes bicarbonate and produces '''metabolic acidosis'''.
* Liver failure explains bleeding (clotting factor loss), edema and ascites (low albumin), jaundice (bilirubin), and confusion (ammonia) as one connected picture.
* Bowel sounds return in a predictable order after abdominal surgery: small intestine first, then stomach, then colon. Absent bowel sounds with distention and vomiting suggest ileus or obstruction.
* Fat-soluble vitamin deficiency follows any condition that blocks bile flow, so watch for bleeding from vitamin K deficiency in biliary obstruction.