3: Chapter Physiology of Digestion
Functional Organization of the Digestive System 4 processes that get nutrients into the body Motility Process of moving food through the gut and breaking it up into small particles Gut has two types of muscles 1. Circular bands of muscles ringing the GI tract
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Digestive system and its accessory organs Breaks down complex foods into simple components and absorbs these nutrients into the body 2. Longitudinal muscles running along it Muscles have 2 kinds of contractions 1. Peristalsis Rhythmic contraction of circular muscles surrounding the gut in such a way as to move food forward Pushes food and digestive juices (chyme) Bolus Mass of food in the gut 2. Segmentation contraction of muscles surrounding the gut in such a way as to mix it with GI secretions Mixes food with digestive secretions and breaks up the bolus more Factors that affect gut motility Nerves transmit signals to the gut
Stress can cause diarrhea Caffeine Increase motility Pregnancy Women experience constipation because the smooth muscles that cause peristalsis are relaxed Laxatives Stimulate peristalsis When person stops using them though, they develop rebound constipation Sphincters Rings of muscles that act as valves, blocking or restricting flow Keeps strong stomach acid out of the esophagus (connects the mouth and stomach) Gastric Reflux (heartburn) When sphincter fails to do its job Pyloric Sphincter Valve controlling the flow of chime from the stomach to the small intestine Ileocecal Valve Valve separating the ileum from the colon Anal Sphincter Retains wastes in the rectum (final organ of the digestive tract) until we are ready to defecate Secretion Synthesis and release of substances by cells in response to stimuli
Moisten and lubricate foods to help it pass smoothly through the gut Hydrochloric acid Kills some of the bacteria that enter the gut with food so the immune system has less to fight against Denatures protein Changes it shape and opens it up to digestion by enzymes Necessary for digestion because proteins are long chains of amino acids that aren't accessible for digestion Enzymes Do the work of digestion Each digestive enzyme works on a specific nutrient Protease Enzyme responsible for protein digestion Various proteases break down proteins Help the work of digestive enzymes Hydrochloric acid Bile works as an emulsifier Secretion in the liver that emulsifies dietary lipid Helps to break drops of fat into smaller droplets and keep them suspended in chyme Emulsifiers break fat into smaller particles which provides a much greater surface area for lipase to work so it can digest fat more quickly Digestion Process of breaking down large nutrient molecules into smaller molecules so the body can absorb them Gut cannot absorb large molecules
Starches have to broken down into monosaccharide sugars Proteins are broken down into amino acids Triglycerides (fats) are broken down into fatty acids and monoglycerides Break down foods mechanically and chemically Chewing and Churning (muscular contractions of the gut, particularly in the stomach, that grind food and mix it with secretions) Soften and dissolve the food and expose more surface area to digestive enzymes Chewing makes digestion quicker Absorption Moves nutrients from the lumen (interior of the gut) into the mucosa (cells of the gut lining) Can be passive (do not require energy) or active (require energy) Passive diffusion Absorption of nutrients that does not require energy Molecules move from an area of higher concentration to an area of lower concentration through a member permeable to the substance Driven by concentration gradient Change in concentration of a substance over a distance Diffusion continues until concentrations are the same on both sides of the membrane Use this to absorb alcohol Facilitated Diffusion Movement of molecules across a membrane in which a carrier protein is required to open a channel through the membrane for the substance enter to the cell Concentration gradient also drives this process Requires no energy
Use this to absorb fructose Active transport Moves nutrients against a concentration gradient From a low concentration in the gut to a high concentration in the cell Requires energy Intestinal cell works to absorb the nutrient Endocytosis Portion of the cell membrane surrounds and engulfs a large nutrient molecule Infant's gut uses this to take in mother's antibodies from breastmilk Need this since their own immune systems are immature Water soluble nutrients pass through the mucosal cells into the bloodstream Blood is waterbased so cannot carry fatsoluble nutrients Fats pass into the lymphatic system Food's Path Through the Gut Mouth Chewing starts the mechanical process of digestion Releases the first digestive secretions (saliva) which lubricates the food and makes it easier to swallow Saliva has amylase Enzyme that breaks down starch Breaks the long chains of simple sugar molecules in the starch into shorter pieces
The shorter the chain, the sweeter the taste Saliva also has lipase Enzyme responsible for lipid digestion Esophagus Connects mouth to stomach Peristalsis begins Stomach Secretes powerful powerful hydrochloric acid that denatures protein and kills bacteria Secretes proteases that begin the breakdown of protein Layer of mucus protects the stomach from acid and protease When mucus barrier fails, acid burns the stomach lining and you get an ulcer Lower esophageal sphincter separates the esophagus and the stomach Prevents stomach acid contents from flowing back up the esophagus (sphincter fails in gastric reflux) Esophagus has no mucus barrier to protect it so the acid causes pain (heartburn) Holds food and slowly releases it into the small intestine Small intestine can only handle small amounts of food at a time Factors that affect how quickly the stomach empties into small intestine High fat meals slow stomach emptying Liquids empty quickly Liquids have a lot of calories but don't stay in the stomach long enough to make you feel full Pyloric sphincter regulates the flow of food into the small intestine
Small Intestine Where most chemical breakdown of food and most nutrient absorption happens Tube that consists of 3 segments Duodenum Initial segment Jejunum Middle segment Ileum Terminal segment Peristalsis keeps food moving through the intestine and segmentation continues to mix it Liver makes bile and stores it in the gallbladder After a meal, gallbladder releases bile into small intestine where it emulsifies fats to make them ready for digestion Pancreas secretes lipase, amylase and protease to digest lipid, starch and protein Also secrets sodium bicarbonate to neutralize stomach acid Bile and enzymes enter the small intestine through the common bile duct Intestinal mucosa produces enzymes to break down simple sugars Villi absorb the nutrients Fingers lining the surface of the intestinal lumen Responsible for nutrient absorption Greatly increase surface area of the intestine Sometimes called the brush border of the intestine
Villi are covered in microvilli which increase the surface area even more Large Intestine Colon Cleanup area Absorbs some nutrients but main job is to reabsorb water Large amounts of water are secreted in the stomach and small intestine to make the chyme liquid Body reabsorbs and recycles this water in the colon Keeps water in the colon to make feces soft enough for excretion Don't reabsorb enough = diarrhea (water is coming out of you) Absorb too much = constipation (keeping water in) Hosts a large number of bacteria Half the weight of feces is bacteria Fiber affects the movement of food through the colon Insoluble fibers absorb water and stimulate peristalsis which makes food move through the gut more quickly and shortens the transit time Amount of time food spends in the gut Rectum Feces stay here until you eliminate them By the time feces leave the body, it contains very few nutrients
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