Digestive System: Functions and Organs

• The digestive system consists of two main parts: the gastrointestinal tract and accessory organs.
• The gastrointestinal tract includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
• Accessory organs include teeth, tongue, salivary glands, liver, gall bladder, and pancreas.

Six Basic Activities of the Digestive System

• Ingestion occurs through eating.
• Secretion involves the release of water, acids, buffers, and enzymes.
• Mixing and propulsion, also known as motility, move substances along the digestive tract.
• Digestion is both mechanical, involving grinding and mixing, and chemical, breaking down large molecules into smaller ones via hydrolysis.
• Absorption occurs when epithelial cells take up digested substances and pass them to the blood and lymph.
• Defecation involves the elimination of feces.

Mechanical vs. Chemical Digestion

• Mechanical digestion physically breaks down food particles into smaller pieces through crushing, mashing, churning, and mixing, while also adding water to liquify the mixture.
• Chemical digestion uses enzymes to break the chemical bonds of large food molecules into smaller molecules.

GI Tract - 4 Layers

• The mucosa is the innermost layer, made of epithelial cells for secretion, the lamina propria with blood vessels and lymphoid tissue (MALT), and the muscularis mucosae that creates folds.
• The submucosa is a connective tissue layer containing the submucosa plexus (plexus of Meissner) of the enteric nervous system, controlling vasoconstriction and secretion.
• The Muscularis layer contains smooth muscle, and the myenteric plexus (plexus of Auerbach) of the enteric nervous system which controls contraction and motility.
• The serosa is the outermost layer made of connective and epithelial tissue.
• The visceral peritoneum covers organs, and the parietal peritoneum lines the abdominal cavity, both holding everything in place and creating a cavity.

Control of Digestive Activity via the Parasympathetic Nervous System

• The cephalic phase is triggered by thinking, smelling, seeing, or tasting food, stimulating the "juices to flow".
• The gastric phase involves neural responses from stretch and chemoreceptors monitoring pH, and hormonal responses like acetylcholine stimulating gastrin secretion.
• Gastrin stimulates gastric juice secretion, strengthens the esophageal sphincter contraction, increases motility, and relaxes the pyloric and ileocecal sphincters.
• The intestinal phase occurs when chyme enters the duodenum, slowing gastric emptying.
• Secretin decreases gastric secretions.
• Cholecystokinin (CCK) inhibits gastric emptying, increases contraction of the pyloric sphincter and gall bladder, and somatostatin decreases gastric activity.

Mouth and Saliva

• Saliva is 99.5% water.
• Saliva contains ions like sodium, potassium, chloride, bicarbonate, and phosphate.
• Saliva consists of dissolved gasses, urea, uric acid, and mucus.
• Saliva contains bacteria killers like immunoglobulin A and lysozyme.
• Digestive enzymes in saliva include salivary amylase and lingual lipase.
• Mechanical digestion in the mouth involves mastication.
• Chemical digestion starts with salivary amylase breaking down starch into maltose, maltotriose, and a-dextrins, and lingual lipase breaking down triglycerides into monoglycerides and fatty acids.

Stomach

• Epithelial cells in the stomach form gastric glands within gastric pits.
• Exocrine cells in the stomach consist of mucous neck cells which secrete mucous, chief cells which secrete pepsinogen and gastric lipase and parietal cells which secrete HCl and intrinsic factor (for Vit B12 absorption).
• Enteroendocrine cells, specifically G cells, secrete gastrin.
• Peristaltic movements mix stomach contents.
• The stomach produces chyme.
• Mechanical digestion in the stomach involves churning and mashing.
• Chemical digestion in the stomach consists of pepsin, secreted as pepsinogen and activated by acid, breaking specific peptide bonds in proteins into peptide fragments.
• Gastric lipase splits short chain triglycerides from milkfat.

Pancreas

• Exocrine cells (99%) in acini produce pancreatic juice.
• Pancreatic juice contains water, salts, and sodium bicarbonate (pH 7.1-8.2).
• Enzymes found in pancreatic juice include pancreatic amylase, trypsin, chymotrypsin, carboxypeptidase, elastase, pancreatic lipase, ribonuclease, and deoxyribonuclease.
• Secretin stimulates the flow of pancreatic juice, and CCK stimulates secretion of pancreatic enzymes.

Liver and Gall Bladder

• The liver receives blood from the hepatic artery and the hepatic portal vein.
• The portal triad consists of the hepatic artery, hepatic portal vein, and bile duct.
• Bile consists of water, bile acids, bile salts, cholesterol, lecithin, bile pigments, and ions, with a pH of 7.6-8.6.
• The main function of bile is the emulsification of lipids.

Nine Main Functions of the Liver

• Carbohydrate metabolism
• Lipid metabolism
• Protein metabolism
• Processing of drugs and hormones
• Activation of Vitamin D
• Excretion of bilirubin
• Synthesis of bile salts
• Storage
• Phagocytosis

Small Intestine

• The small intestine consists of three parts: the duodenum, jejunum, and ileum.
• Most absorption occurs in the duodenum.
• The small intestine contains villi and intestinal glands.
• Lacteals are lymphatic vessels within the villi.
• Secretions in the small intestine include sucrase, maltase, lactase, peptidases, and lipase.

Absorption in the Small Intestine:

• Glucose enters absorptive cells via secondary active transport, exits via facilitated diffusion, and enters blood capillaries.
• Amino acids and peptides enter absorptive cells via active or secondary active transport, exit via diffusion, and enter blood capillaries.
• Short chain fatty acids enter absorptive cells via diffusion, exit via diffusion, and enter blood capillaries.
• Longer chain fatty acids (from micelles) enter absorptive cells via diffusion and are formed into chylomicrons that exit via exocytosis and enter lymphatic lacteals.

Large Intestine

• The large intestine finishes the process of absorption, especially water.
• Vitamin K and some B vitamins are produced by bacteria in the large intestine.
• Activity in the large intestine is governed by various reflexes, including the gastroileal reflex, haustral churning, peristalsis, mass peristalsis, and the gastrocolic reflex.
• Diarrhea is a condition characterized by frequent, watery bowel movements.
• Constipation is a condition characterized by infrequent or difficult bowel movements.

Absorptive State (Right After Eating)

• Glucose is oxidized.
• Glucose is stored as glycogen and triglycerides.
• Protein is used for building new tissue.
• Protein is converted to glucose, fatty acids, and ketones.
• Fat is primarily stored as triglycerides, with some being used.

Blood Lipids ® Lipoproteins

• Chylomicrons package fat from the digestive tract and include triglycerides (85%), cholesterol (5%), phospholipids (8%), and proteins (2%), and transported to the liver via lymphatic system
• Very Low Density Lipoproteins (VLDL) are formed by the liver and include triglycerides (50-60%), cholesterol (15-20%), phospholipids (15-18%), and proteins (10%), transporting fats to tissues.
• Low Density Lipoproteins (LDL) are transformed from VLDLs consisting of triglycerides (8%), cholesterol (50%), phospholipids (20%), and proteins (22%), transporting cholesterol to cells, including endothelial cells of blood vessels.
• High Density Lipoproteins (HDL) are formed by the liver, containing triglycerides (3%), cholesterol (17%), phospholipids (30%), and proteins (50%), transporting cholesterol from tissues back to the liver.

Post-Absorptive State (A While After Eating)

• New glucose is made.
• Fat is oxidized.
• Amino acids are oxidized.
• Ketones are oxidized.