Notes for Chapter 3 - Digestion

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University of Florida
Food Science and Human Nutrition
HUN 2201

HUN 2201 – Ch 3 digestion, absorption, metabolism • Food becomes us • Atoms form molecules o Organic molecules contain carbon bonded to hydrogen • Cells for tissues and organs, which cooperate as organ systems • The Digestive System o Organs of the digestive system  Gastrointestinal tract from mouth to anus  Mouth, pharynx, esophagus, stomach, small intestine, large intestine, anus  Inside of the tube is the lumen  Food within the lumen is technically not yet absorbed inside the body  Transit time: food in GI tract 24-72 hours • Affected by composition of the diet, physical activity, emotions, medications, illnesses  The wall of the GI tract contains 4 layers of tissue • 1. Mucosa lines the lumen, protects and absorbs end products of digestion. Cells have short life span and are in direct contact with digestive secretions. When these cells die, they are sloughed off into the lumen or eliminated in feces. Reproduce rapidly and have high nutrient requirements; one of the first parts of the body affected by nutrient deficiencies • 2. Surrounding mucosa is layer of connective tissue containing nerves and blood vessels. Provides support, delivers nutrients to the mucosa, provides nerve signals that control secretions and muscle contractions. • 3. Connective tissue surrounded by layers of smooth muscle; contraction propels food through digestive tract • 4. External layer made up of connective tissue and supports/protects o Digestive secretions  Mucus moistens, lubricates, and protects  Enzymes catalyze reactions and accelerate breakdown of nutrients o How GI activity is regulated  Nerve signals regulate activity; cause muscle contractions that churn, mix, and propel food and stimulate or inhibit digestive secretions and communicate with brain  Activity in the tract is also regulated by hormones released into the bloodstream; prepare gut for arrival of food • Digestion and absorption o The mouth  Mechanical breakdown and chemical digestion begins  Presence of food stimulates flow of saliva, dissolves molecules which are carried to the taste buds  Salivary amylase breaks carbs and prevents against tooth decay because it contains lysozyme, an antibacterial enzyme  Tongue. Chewing helps break apart fiber, which traps nutrients in some foods o Pharynx  Shared by digestive and respiratory tract; once we voluntarily start swallowing, muscular contractions proceed involuntarily  Tissue flap called epiglottis ensures that bolus passes to stomach, not the lungs o Esophagus  Tube that passes through diaphragm, which connects pharynx and stomach  Peristalsis in 4-8 seconds  To move from esophagus to stomach, food must pass through a sphincter, a valve/muscle that encircles tube of digestive tract  The GI/cardiac sphincter relaxes before peristaltic wave reaches. Prevents food from moving back out of stomach, but when it does, heartburn occurs o Stomach  Expanded portion of GI tract that serves as temporary storage place for food. Bolus is mixed with chime…very little absorption occurs here  Stomach walls are thick and have strong muscles; 2 layers. Lining contains gastric pits and glands covered by cells that secrete substances into the stomach, called gastric juice. Others secrete hormones into blood.  Gastric juice is a mixture of water, mucus, HCL, and pepsinogen. Parietal ells produce intrinsic factor, needed for absorption of vitamin B12.  Pepsinogen, produced by chief cells, is inactive form of protein-digesting enzyme pepsin, which breaks proteins into shorter chains called polypeptides. • Pepsin is secreted in an inactive form and activated by HCL when it enters the stomach. This acidic environment stops function of salivary amylase, stopping digestion of starch and starting digestion of protein.  How stomach activity is regulated • How much your stomach churns, how much gastric juice is released, and how fast material empties out of the stomach are regulated by signals from nerves and hormones • Signals originate from brain, stomach, and small intestine • Food entering the stomach stimulates release of gastric secretions and increase in motility by stretching local nerves, sending signals to brain, and promoting secretion of the hormone gastrin, which triggers release of gastric juices • As chime moves out of stomach, it passes through pyloric sphincter, regulating rate at which food moves out of stomach. Chyme entering small intestine triggers hormonal and nervous signals that can decrease stomach motility and secretions and slow stomach emptying o Normally 2-6 hours but depends on composition of the meal o High fat meal stays in stomach the longest because fat entering the small intestine causes release of hormones that slow GI motility. Carbs leave the fastest o Exercise and emotions also affect gastric emptying… sadness and fear slow, aggression inreases o Exercise will delay stomach emptying because body directs its resources to the muscles o The small intestine  Main site of digestion of food and absorption of nutrients. 20 ft long tube in 3 segments; first 12 inches are duodenum, then jejunum, then ileum  Structure and function • SA increased by length, large circular folds of walls, villi projections on inner surface, and mucosal ells on surface of villi covered with microvilli, aka brush border • Within each villus are a blood and lymph vessel, called a lacteal, only 1 cell layer away from nutrients in intestinal lumen o Nutrients must cross this cell layer to reach the bloodstream or lymphatic system  Motility and secretions • Segmentation enhances absorption by repeatedly moving chime over surface of intestinal mucosa • Cells of small intestine produce some digestive enzyme as well as a watery mucus-containing intestinal juice that aids in absorption but also requires secretions from pancreas and gallbladder o Pancreatic juice contains bicarbonate ions which neutralize chyme, allowing pancreatic and intestinal enzymes to function o Pancreatic amylase continues to break starch into sugars o Pancreatic proteases break down protein o Pancreatic lipases break down fat o Intestinal digestive enzymes break these into smallest units [amino acids and single sugars] which are passed into the blood and delivered to the liver • Gallbladder stores and secretes bile, produced in the liver o Bile secreted into small intestine emulsifies fat, helping lipases access and digest it o Then, the bile and digested fats form small droplets and are absorbed by mucosal cells, where they are incorporated into transport particles and absorbed into lymph before passing through blood • Release of bile and pancreatic juice into small intestine controlled by 2 hormones secreted by mucosal lining of duodenum o Secretin signals pancreas to secrete bicarbonate rich pancreatic juice and liver to secrete bile into gallbladder o Cholecystokinin, CCK, signals pancreas to secrete digestive enzymes and causes gallbladder to contract and release bile into duodenum  How nutrients are absorbed • Small intestine is primary site for absorption. Nutrients must pass from lumen of GI tract into mucosal cells lining the tract into blood/lymph • Several mechanisms involved o Diffusion[fat soluble substances] and osmosis o Active transport allows glucose and amino acids to be absorbed even when they are present in higher concentrations inside mucosal cells o The large intestine  Materials not absorbed in the small intestine pass through the ileocecal valve, which prevents material from re-entering the small intestine  5 feet and divided into the colon and rectum  although most absorption occurs in the small intestine, water and some vitamins and minerals are also absorbed in the colon  peristalsis is slower, may spend 24 hours • slow movement favors growth of bacteria, aka intestinal microflora • microflora act on unabsorbed portion of food, such as fiber, producing nutrients that can be absorbed into body • right mix of bacteria important for immune function, proper growth and development of colon cells, and optimal transit time • probiotics, beneficial bacteria, can promote healthy microflora • prebiotics, substances that serve as a food supply for beneficial bacteria, are found in onions, bananas, garlic • material not absorbed by the colon is feces; mixture of undigested unabsorbed matter, dead cells, secretions from GI tract, water, bacteria • amount of water in feces affected by fiber and fluid intake…fiber retains water, so when adequate fiber and fluid are consumed, feces have a high water content and are easily passed. When not enough fiber is consumed, feces are hard, dry, and difficult to eliminate • Digestion and health o Immune functions of the GI tract  When antigen is present, phagocytes are first line of defense. Phagocytes also release chemicals that signal lymphocytes to join the fight…may be specific or nonspecific, produce antibodies o The immune response and food allergies [celiac and gluten]  Triggers immune response that damages or destroys villi of the small intestine • Causes and consequences of digestive problems o Heartburn and GERD  Heartburn: common, occurs when acidic stomach contents leak back into esophagus…aka gastrointestinal reflux…GERD is disease if it occurs more than 2 times per week  If left untreated, can lead to peptic ulcers and cancers  Can be helped by eating small meals and avoiding spicy, fatty, or acidic foods. Medications also neutralize stomach acid or block its production, or can add calcium to diet  However, by decreasing acidity, they can also reduce absorption of nutrients that require acid to aid in their absorption  Heartburn and GERD are sometimes
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