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Kin110_Chapter 3 Digestion.docx

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Biomedical Physio & Kines
Course Code
BPK 110
Gina Whitaker

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Kin110 Chapter 3 Digestion: From Meals to Molecules Atoms (the smallest unit of elements) form molecules (groups of two or more atoms of the same or different element bonded together), which form cells (the smallest unit of life), which form tissues (collection of similar cells that together carry out a specific function), which form organs (discrete structures composed of more than one tissue that performs a specialized function), which form organ systems (group of cooperative organs), which form the entire organism The Digestive System-the organ system that is primarily responsible for the digestion and for the absorption of nutrients into the body Made up of the gastrointestinal (GI) tract (mouth to anus) and accessory organs (salivary glands, pancreas, liver, gallbladder) The inside tube of the GI tract is the lumen. There are four tissue layers that make up the wall of the GI tract External layer-provides support and protection Contraction layers of smooth muscles (which we do not have voluntary control)-helps mix food, break it into small particles, and propel it through the digestive tract Connective tissue-contains nerves and blood vessels that provides support, nourishes the mucosa, and sends nerve signals that control secretions and muscle contractions The mucosa which lines the GI tract is a type of epithelial tissue. Nutrients must pass through before they can reach the blood or lymph Digestion Functions Ingestion-take in food Transport-moving food down the esophagus and into the stomach Secretion-lining food with various enzymes that help in the process of breaking down food Digestion-breaking down food into components small enough to be absorbed into the body Absorption-the process of taking substances from the GI tract into the interior of the body Elimination-getting rid of any excess body waste Transit time through the GI Tract is typically 24-72 hours. -varies with diet, physical activity, emotional state, health status and medication use Overview of Digestion Physical movement Peristalsis-coordinated muscular components that move material through the GI tract Segmentation-rhythmic local contractions of the intestine that mix food with digestive juices and speed absorption by repeatedly moving the food mass over the intestinal wall Chemical breakdown Enzymes-a protein molecule that accelerates the rate of specific chemical reactions without itself being changed Other secretions Mucus-a viscous fluid secreted by glands in the digestive tract and other parts of the body -lubricates, moistens, and protects cells from harsh environments Ions-an atom or a group of atoms that carries an electrical charge (e.g. bicarbonate) Hormones-a chemical messenger that is produced in one location in the body, released into the blood, and travels to other locations where it elicits responses Digestive Enzymes-proteins that catalyze reactions that are responsible for breaking down food into smaller units that can be absorbed Digestion The Mouth Mechanical and chemical digestion begins Saliva is secreted from salivary glands after the sight, smell, or presence of food. It contains lubricants, enzymes, and other substances (It also helps protect against tooth decay because it washes away food particles and contains substances that inhabit the growth of bacteria that cause tooth decay) Enzyme salivary amylase digests starch into maltose (sweet tasting) Teeth break down food particles. This makes food easier to swallow and increase the surface area in contact with digestive juices. Chewing also breaks up fiber, which traps nutrients. If fiber is not broken up, some of the nutrients in the food cannot be absorbed Tongue helps mix food and aids chewing by constantly repositioning food between the teeth Taste buds detect food chemicals The Pharynx-the part of the GI tract that is responsible for swallowing. (also part of the respiratory tract The bolus (a ball of chewed food mixed with saliva) forces the epiglottis to cover the passageway to the lungs Epiglottis-a piece of elastic connective that covers the opening to the lungs during swallowing Sometimes eating too quickly or talking while eating interferes with the movement of the epiglottis and food passes into an upper air passageway. This food can usually be dislodged with a cough, but if it becomes stuck and causes choking, it may need to be forced out by the means of the Heimlich maneuver The epiglottis returns to its original position once bolus has passed; airway to the lungs reopen The Esophagus-connects the pharynx with the stomach Sphincter-a muscular valve that helps control the follow of materials in the GI tract -prevents food from the stomach back into the esophagus -circular muscles contract, pushing the bolus down -longitudinal muscles contract, shortening the passageway ahead of the bolus -the rhythmic contraction of peristalsis propels food down the esophagus -when a wave of peristaltic contraction reaches the stomach, it causes the sphincter to relax, allowing the bolus to enter the stomach Food that enters the stomach Protein-large folded polypeptides Fat-triglycerides, cholesterol, phospholipids, long chain fatty acids -90% of fat calories are from triglycerides Carbohydrates Simple -monosaccharides: glucose, fructose, galactose (does not need to be digested, can be absorbed right away) -disaccharides: sucrose, maltose, lactose Complex -polysaccharides: starches, fibers The Stomach-temporary storage for food The stomach has three layers of smooth muscle-longitudinal, horizontal and diagonal. This allows for optimal churning and mixing The pyloric sphincter at the bottom of the stomach controls how long food stays in the stomach The stomach lining produces gastric juice containing Water Pepsinogen-inactive protease secreted by chief cells of the gastric glands which is activated into pepsin -optimally active at pH 1.8 > 3.5 -pepsin begins protein digestion -results in a mixture of intact protein, polypeptides and some free amino acids Protein + water + pepsin → x + y Hydrochloric acid kills microorganisms, unfolds proteins, activates pepsin Mucus protects stomach lining from pepsin and hydrochloric acid Digestion in the Stomach-The Macronutrients Protein -low pH denatures proteins -pepsin digests up to 20% of protein Fats (minor-10% digested in stomach) -gastric mixing and churning forms an emulsion of lipids and gastric lipase -gastric lipase hydrolyses triglycerides into free fatty acids and monoglycerides (minor) Carbohydrate -no digestion in the stomach (salivary amylase is inactivated at low pH) Little absorption occurs here. Water, alcohol, aspirin, acetaminophen (Tylenol) are absorbed Chyme, a mixture of partially digested food and stomach secretions leaves the stomach to enter the small intestine through a sphincter -stomach emptying is regulated by signals from the small intestines and is affected by meal size and composition Regulation of Stomach Secretions and Motility 1) Nerve/Hormonal Control-thought, smell, sight, taste of food stimulates gastric response 2) Food entering the stomach-stimulates hormone release which increases gastrin 3) Food entering the small intestine decreases gastrin Small Intestine Main site for digestion and absorption Chyme moved by peri
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