The Human Digestive System Mechanical & Chemical Digestion A Tour of the Mammalian Digestive System The Oral Cavity Presence of food triggers a nervous reflex that causes salivary glands to deliver saliva. Even before food is actually in the mouth, salivation may occur in anticipation because of learned associations between eating and the time of day, cooking, odors, or other stimuli. …continued More than 1 L of saliva is secreted into the oral cavity each day. Saliva contains the following: mucin: glycoprotein that lubricates food buffers: prevents tooth decay salivary amylase: digestive enzyme that breaks down starch and glycogen into smaller polysaccharides and disaccharides The Pharynx The swallowing reflex is triggered when a bolus of food reaches the pharynx … continued The esophageal sphincter (a muscle that is usually contracted) relaxes, allowing the esophagus to open and the bolus of food to enter the esophagus. The larynx moves upward and tips the epiglottis over the glottis, preventing food from entering the trachea. After the food has entered the esophagus, the larynx moves downward and opens the breathing passage The Esophagus Conducts food from the pharynx down to the stomach. Peristalsis squeezes a bolus along the narrow esophagus. Salivary amylase continues to break down starch and glycogen as the bolus passes through the esophagus. Peristalsis Waves of muscular contraction move the bolus down the esophagus to the stomach. The Stomach Located on left side of the abdominal cavity, just below the diaphragm. This organ has very elastic walls and accordion-like folds which allow it to stretch to accommodate about 2 L of food and fluid. Anatomy of the Stomach Secretion of Gastric Juices The epithelium that lines the lumen of the stomach secretes gastric juices - a digestive fluid that mixes with the food. Contains a high concentration of hydrochloric acid (HCl) - pH ~ 2 Function of acid is to help break down meat and plant material aswell as to kill most bacteria that are swallowed with the food. … continued Gastric Juice also contains pepsin, an enzyme that breaks down protein. Pepsin works well in acidic environments. The precursor to pepsin is pepsinogen. HCl in gastric juice converts pepsinogen to pepsin. … continued The mucosa that lines the stomach consists of simple columnar epithelium organized into gastric glands. Gastric pits lead into the gastric glands which have three types of secretory cells. Mucous Cells Chief Cells Parietal Cells Control of Gastric Secretion Controlled by a combination of nervous impulses and hormones. When we see, smell, or taste food, impulses from the brain to the stomach initiate the secretion of gastric juices. Substances in the food itself stimulate the walls of the stomach to release a hormone called gastrin. … continued The production of gastrin further stimulates the secretion of gastric juices (example of positive feedback). If the pH of the stomach contents becomes too low, the acid will inhibit the release of gastrin thus decreasing the secretion of gastric juices. 3 L of gastric juices are secreted every day. Chyme Every 20 seconds, the stomach contents are mixed by the churning action of the smooth muscle. As a result of mixing and enzyme action is a nutrient broth called acid chyme. Rate of Release from Stomach Much of the time, the stomach is closed off at either end. The cardiac orifice normally only dilates when a bolus is driven through by peristalsis. The occasional backflow of acid chyme from the stomach into the lower end of the esophagus causes heartburn. Anatomy Review … continued The pyloric sphincter helps regulate the passage of the chyme into the small intestine. A squirt at a time, it takes about 2 - 6 hours for the stomach to empty its contents following a meal. The Small Intestine Although some digestion takes place in the mouth and stomach, most enzymatic digestion of food occurs in the small intestine. Most absorption also takes place in the S.I. Name is based on diameter not length. Approx. 6 metres in length in humans. The Liver Performs many important functions including the production of bile. Bile is a mixture of substances that is stored in the gall bladder until needed. Bile contains no digestive enzymes, but does contain salts which aid in the digestion of and absorption of lipids. The Pancreas Produces several digestive enzymes. Carbohydrases - break down carbohydrates (eg. Pancreatic amylase and maltase) Proteases - break down proteins (eg. Trypsin, Chymotrypsin, and Carboxypeptidase) … continued Lipases - break down lipids (eg. Pancreatic lipase) Nucleases - break down nucleic acids (into nucleotides which are then broken down into bases, sugars and phosphates. Summary: Digestive Juices … Specifics The Duodenum First segment of small intestine. Approx. 25 cm in length. Receives digestive juices from the pancreas, liver, gall bladder, and gland cells of the intestinal wall itself. Hormone Regulation At least 4 regulatory hormones help ensure that digestive secretions are present only when needed. Gastrin (already discussed) Secretin (stimulates pancreas to release buffer) Cholecystokinin or CCK (stimulates pancrease to release enzymes and gall bladder to release bile) Enterogastrone (inhibits peristalsis and acid secretion of stomach) Jejenum & Ileum Remaining portions of small intestine. Function mainly in the absorption of nutrients. Absorption Large folds of epithelium line the small intestine. Villi project outward from the folds. Each villus has a small lymphatic vessel called a lacteal and a network of blood capillaries surrounded by a layer of epithelial cells Microvilli extend into the intestinal lumen to create a “brush border”. Intestinal Wall … continued Nutrients are absorbed across the epithelium and then across the unicellular wall of the capillaries or lacteals. In some cases the transport is passive (down concentration gradient). In some cases the transport is active (ATP used). … continued Sugars and amino acids are carried away from the intestine by the bloodstream. After glycerol and fatty acids are absorbed by epithelial cells, they are recombined to form fat again. The fats are then mixed with cholesterol and coated with special proteins, forming chylomicrons. (HDL or LDL). Chylomicrons are then transported into lacteals. Fate of Macromolecules The capillaries and veins that drain nutrients away from the villi converge into a single circulatory channel, the hepatic portal vessel. Liver has the ability to interconvert various organic molecules. Once the liver makes changes, blood flows to the heart to be pumped around the body. Hepatic Portal System Large Intestine a.k.a. colon. Major function is to reabsorb water. About 7 L of fluid are secreted into the lumen of the digestive tract each day. Most water reabsorption occurs along with nutrient absorption in the S.I. The colon finishes the job by reclaiming most of the water that remains in the lumen. Together, the S.I. & colon reclaim 90%.