The Digestive System

• The GI tract includes the mouth, esophagus, stomach, small and large intestines, terminating in the anus.
• Salivary glands, liver, gallbladder, and pancreas aid digestion by secreting juices into the GI tract.
• The GI tract is lined with simple columnar epithelium, specialized for absorption, often with apical cilia or microvilli.
• Digestion begins in the mouth. Saliva functions to moisten food, dissolve food molecules, ease swallowing, begin polysaccharide digestion, and protect the oral cavity.
• The stomach stores food temporarily, chemically and mechanically digests food using acids, enzymes, and movements, regulates chyme release, and secretes intrinsic factor for vitamin B12 absorption.
• The four layers of the stomach are the mucosa, submucosa, muscularis externa, and the serosa.
• • The stomach is lined by gastric mucosa with parietal cells (secreting HCl), chief cells (producing pepsinogen), and cells creating a protective mucosal barrier.
• Gastric juice contains hydrochloric acid (HCl) from parietal cells, which denatures proteins and activates pepsinogen to form pepsin.
• Intrinsic factor binds to vitamin B12 and allows it to be absorbed in the small intestine.
• Pepsin breaks down proteins into peptides.
• Pepsin breaks down peptides into amino acids.
• Stomach distension and the presence of food stimulate the release of ACh, gastrin, and histamine, leading to increased acid production.
• Chyme is a semi-digested acidic fluid. It enters the small intestine through the pyloric sphincter, which regulates its release based on stomach content and pH levels.
• The liver is involved in digestion, metabolism, detoxification, protein synthesis, and storage of vitamins and minerals. It processes absorbed substances and produces bile.
• Bile is a digestive fluid produced by hepatocytes in the liver, stored in the gallbladder, and secreted into the duodenum. It emulsifies fats and is mostly reabsorbed in the ileum.
• Pancreatic juices contain enzymes (amylase, lipase, proteases like trypsin and chymotrypsin), water, and bicarbonate ions, which aid in digestion and neutralize stomach acid.
• The small intestine has a lining of villi and microvilli that increase surface area for absorption, a rich blood supply for nutrient transport, and a single layer epithelium to minimize diffusion distance.
• During mastication three pairs of glands, the parotid, submandibular and sublingual, secrete saliva.
• The innermost layer of the stomach, the mucosa, is covered by epithelial tissue and is mainly comprised of gastric glands that secrete gastric juices.
• Chyme enters the duodenum through the pyloric sphincter.
• The small intestine comprises the duodenum, jejunum and ileum.
• The small intestine is the main site for the digestion of food and the absorption of the products of this digestion.
• The presence of chyme in the stomach distends it and causes contractions and opening of the sphincter.
• The rate at which the stomach empties depends on the volume in the stomach and the fall in the pH of the chyme, both leading to an increase in emptying.
• Chyme that enters the duodenum is acidic, hypertonic and partly digested.
• The lining of the small intestine is folded into many small, finger-like projections called villi
• The small intestine is composed of four main tissue layers, which are (from outside to centre):

  • Serosa – a protective outer covering composed of a layer of cells reinforced by fibrous connective tissue
  • Muscle layer – outer layer of longitudinal muscle (peristalsis) and inner layer of circular muscle (segmentation).
  • Submucosa – composed of connective tissue separating the muscle layer from the innermost mucosa
  • Mucosa – a highly folded inner layer which absorbs material through its surface epithelium from the intestinal lumen.
• The surface of the villi is covered with a layer of epithelial cells which, in turn, have many small projections called microvilli (collectively called the brush border) that project towards the lumen of the intestine.
• Each villus contains a single, blind-ended lymphatic vessel, called a lacteal, and a capillary network. Most nutrients are absorbed into the bloodstream via these vessels.
• The venous drainage from the small intestine, large intestine, pancreas and from some parts of the stomach passes via the hepatic portal vein into the liver; here, it passes through a second capillary bed to be further processed before returning to the circulation.
• The major contributor to this osmotic gradient is Na+/K+ ATPase is located on the basolateral membrane, and hydrolysis of ATP to ADP leads to the expulsion of 3 Na+ ions from the cell in exchange for 2 K+ ions against the concentration gradients, leading to a low concentration of Na+ and a high concentration of K+ within the cells.
• Carbohydrates are absorbed mostly in the form of monosaccharides
• The monosaccharides are transported across the epithelium into the bloodstream by a cotransporter.
• The polypeptides produced in the stomach are broken down into oligopeptides in the small intestine by proteases secreted by the pancreas: trypsin and chymotrypsin.
• Fat digestion occurs almost entirely in the small intestine.
• The pancreatic enzyme, lipase breaks fat down into monoglycerides and free fatty acids.
• The free fatty acids and monoglycerides form tiny particles with the bile acids, called micelles.
• The outer region of the micelle is hydrophilic, whereas the inner core is hydrophobic.
• The fat-soluble vitamins, A, D, E and K, follow the pathways for fat absorption (absorbed by diffusion or via a membrane transport protein).
• The exocrine pancreas secretes a digestive fluid called pancreatic juice.