Gastrointestinal System Review

Primary Roles: Ingestion, digestion, absorption, and excretion.
Saliva Functions: Moistens mouth, softens food, initiates digestion, protects teeth, and fights bacteria via immunoglobulin A.
Composition: Water, potassium, chloride, bicarb, and hydrogen.
Glands: - Parotid glands: Located below/front of ear; secrete amylase to break down carbohydrates. - Sublingual glands: Located beneath the tongue. - Submandibular glands: Located beneath the floor of the mouth.
Control: Regulated by the autonomic nervous system (parasympathetic "rest and digest").

Pharynx Structure: Divided into oropharynx and nasopharynx. Connects the oral/nasal cavities to the esophagus and larynx.
Airway Protection: Uvula and soft palate close the nasopharynx; epiglottis covers the larynx during swallowing.
Stages of Swallowing: 1. Buccal phase (voluntary). 2. Pharyngeal phase (involuntary). 3. Esophageal stage (involuntary).
Esophagus: - Peristalsis: Wave-like propulsion. Primary (swallowing-initiated) or secondary (distention-initiated). - Sphincters: Upper esophageal sphincter (voluntary; prevents air entry) and lower esophageal sphincter (involuntary; prevents gastric reflux).

Function: Converts food into chyme (semi-fluid pulp). Can hold up to $1.5\,L$ for $2$ to $4$ hours.
Gastric Glands and Cells: - Mucus Cells: Secrete alkaline mucus to neutralize acid and protect the stomach wall. - Parietal Cells: Produce intrinsic factor (required for B12 absorption) and HCL. - Chief Cells: Secrete pepsinogen (inactive form of pepsin). - Enteroendocrine Cells: Secrete gastrin to stimulate parietal and chief cells.
Hydrochloric Acid (HCL): - Maintains pH between $1.5$ and $3.5$ . - Denatures proteins and kills bacteria. - Regulated by acetylcholine, histamine, and gastrin.
Pharmacology: Histamine receptor blockers (famotidine, ranitidine) and PPIs (panto, lansoprazole) decrease acid production.

Structure: Extends $7\,m$ ; consists of the duodenum, jejunum, and ileum.
Surface Area: Villi, microvilli (intestinal brush border), and circular folds increase absorptive surface $600$ times.
Absorption: Nutrients enter capillaries or lacteals (lymphatic vessels) and travel to the liver via the portal system.
Blood Supply: Main supply from the mesenteric artery (highly susceptible to ischemia). Provides collateral circulation.
Duodenum: Shortest part; location of Brunner glands (alkaline mucus) and the ampulla of vater.
Ligament of Treitz: Suspensory muscle marking the division between the duodenum and jejunum; distinguishes upper from lower GI bleeds.
Jejunum and Ileum: Jejunum handles most chemical/nutrient absorption; ileum handles vitamin absorption and ends at the ileocecal valve.

Liver: Largest internal organ located in the RUQ. Functions include metabolism, glucose regulation, toxin filtration, and bile production.
Liver Blood Supply: Receives $\frac{1}{3}$ of CO from the hepatic artery and portal vein ( $75\%$ of liver blood supply).
Gallbladder: Stores and concentrates bile made of cholesterol and bile salts.
Pancreas: Exocrine gland releasing digestive enzymes into the ampulla of vater via the main pancreatic duct.

Large Intestine Regions: Cecum, colon (ascending, transverse, descending, sigmoid), and rectum.
Functions: Absorbs water and electrolytes; bacterial flora synthesize vitamins.
Defecation: Stretch receptors in the rectum trigger the reflex.
Phases of Digestion: 1. Cephalic: Triggered by sight, smell, or thought of food; involves the pons and medulla oblongata. 2. Gastric: Triggered by food entering the stomach. 3. Intestinal: Triggered by chyme entering the duodenum.