The Digestive System GR 15 B How Digestion Happens

Digestion in the mouth Salivary amylase – Digestive enzyme that speeds if the chemical activity to breakdown carbohydrates Ptyalin converts starches to simple sugars

Digestion in the stomach Gastric Glands – Located in the lining of the stomach wall Functions – To secrete gastric juice – To move food toward the pyloric spinchter through peristalsis – Both physical and chemical digestion

Digestion in the stomach Gastric juice: comprised of hydrochloric acid (HCl), pepsinogen, and mucus; about 1500 mls produced daily by gastric glands Functions of gastric secretions: Pepsin breaks down protein; HCl breaks down connective tissue, kills pathogens, and activates pepsinogen to produce pepsin – Acid in the stomach is VERY STRONG (usually having a pH of 1.5-2) Mucus forms a thick shield that protects the stomach from the acid it contains

Digestion in the stomach Activity controlled by parasympathetic nervous system, particularly vagus nerve Vagus nerve stimulation increases motility and secretory rates of gastric glands

Three phases of gastric juice production Cephalic phase: sensory stimulation (sight or smell of food) stimulates parasympathetic nerves via medulla oblongata, stimulating release of gastrin which travels through blood stream and reaches stomach, stimulating gastric gland activity

Digestion in the stomach Gastric phase: 2/3 of gastric juices secreted as food enters stomach and distends walls – Distended walls activate neural pathways that cause the gastric glands to produce even more gastric fluid

Digestion in the stomach Intestinal phase: food passes through the pyloric sphincter to enter the duodenum Duodenum distends and senses the acidity, causing intestinal hormones to be released, slowing gastric gland secretions; Continues to be inhibited until chyme leaves duodenum

Rate of movement of chyme If too slowly, rate of nutrient digestion and absorption decreased and may allow acidity of chyme to cause erosions of stomach lining If too quickly, food particles may not be sufficiently mixed with gastric juices leading to insufficient digestion; chyme not given time to neutralize can cause erosion of intestinal lining

Small Intestine Digestive enzymes needed to complete chemical digestion and two hormones are secreted to stimulate the pancreas and gallbladder Duodenum – Pancreatic juice contains enzymes and sodium bicarbonate functions to neutralize the chyme – Gallbladder provides bile contains acids, mucus, fluids, and 2 pigments functions to emulsify fat

Small Intestine Movement of chyme through the small intestine occurs by – Segmentation mixes the chyme and digestive juices – Peristalsis wave-like action that moves the undigested food to the small intestine

Digestive enzymes in the small intestine

Absorption of nutrients Occurs within the wall of the small intestine Villi contains network of capillaries and lymphatic capillary called lacteals Capillaries absorb and transport sugars (result of carbohydrate digestion) and amino acids (result of protein digestion) to liver for further processing before being sent throughout body

Absorption of nutrients Glycerol and fatty acids (obtained from fat digestion) absorbed by villi, converted into lipoprotein, where it goes directly into lymphatic system for distribution throughout body

Digestion in the large intestine Very little nutrient absorption takes place in the large intestine. Water and electrolytes are reabsorbed by the cecum and ascending colon Peristalsis continues in large intestine, but at slower rate

Digestion in the large intestine As slower intermittent waves move fecal matter toward rectum, water is removed, turning it from watery soup to semi-solid mass As rectum fills with feces, defecation reflex occurs, which causes rectal muscles to contract and anal sphincter to relax If fecal matter moves through too rapidly, not enough water is removed and diarrhea occurs Conversely, if fecal matter remains too long in large intestine, too much water is removed and constipation occurs

Digestion in the large intestine Bacteria in bowel play two important roles: – Help break down indigestible materials – Produce B complex vitamins and most of vitamin K needed for proper blood clotting – Produce gas

OrganDigestive Activity Substance Digested Required Digestive Secretions Mouth, or oral cavity  Chews food and mixes it with saliva  Forms food into bolus and swallows  Sense of taste Starch Salivary amylase and ptyalin Esophagus  Moves bolus to the stomach through peristalsis Not applicableNone Stomach  Stores food  Churns food while mixing it with digestive juices Proteins Hydrochloric acid, pepsin Small intestine  Secretes enzymes  Receives secretions from the pancreas and liver, neutralizes the acidity in chyme  Absorbs nutrients into the bloodstream and lymphatic system Carbohydrates, fats, nucleic acid, proteins Intestinal and pancreatic enzymes, bile from the liver Large intestine  Creates and absorbs fat-soluble vitamins  Reabsorbs water  Forms and eliminates feces Vitamins B12 and K None

Questions _______ is the muscle action that mixes chyme with digestive juices, while ______ is the muscular action that moves food through the digestive system. The exocrine portion of this organ secretes buffers needed to neutralize the acidity of chyme and also secretes the several digestive enzymes: _________. _____ is the mechanical breaking up of fat into smaller particles that can more readily be acted upon by digestive enzymes.