Nutrition Science and Everyday Application - beta v 0.1

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THE DIGESTIVE SYSTEM 147 powerful peristaltic contractions help mash, pulverize, and churn it into chyme. Chyme is a semiliquid mass of partially digested food along with gastric juices secreted by cells in the stomach. These gastric juices contain hydrochloric acid, which lowers the pH of the chyme in the stomach. This acidic environment kills many bacteria or other germs that may have been present in the food, and it causes the three-dimensional structure of dietary proteins to unfold. Gastric juices also contain the enzyme pepsin, which begins the chemical breakdown of proteins in the peanut butter and bread. Gastric lipase continues the breakdown of fat from the peanut butter. Small Intestine Chyme released from the stomach enters the small intestine, where most digestion and absorption occurs. The small intestine is divided into three parts, all part of one continuous tube: the duodenum, the jejunum, and the ileum. Once the chyme enters the duodenum (the first segment of the small intestine), the pancreas and gallbladder are stimulated to release juices that aid in digestion. The pancreas (located behind the stomach) produces and secretes pancreatic juices which consist mostly of water, but also contain bicarbonate that neutralizes the acidity of the stomach-derived chyme and enzymes that further break down proteins, carbohydrates, and lipids. The small intestine’s absorptive cells also synthesize digestive enzymes that aid in the breakdown of sugars and proteins. The gallbladder (a small sac located behind the liver) stores, concentrates, and secretes a fluid called bile that helps to digest fats. Bile is made in the liver and stored in the gallbladder. Bile is an emulsifier; it acts similar to a detergent (that would remove grease from a frying pan) by breaking large fat droplets into smaller fat droplets so they can mix with the watery digestive juices. Peristalsis and segmentation control the movement and mixing of chyme through the small intestine. As in the esophagus and stomach, peristalsis consists of circular waves of smooth muscle contractions that propel food forward. Segmentation helps to mix food with digestive juices and facilitates absorption. Nutrient absorption takes place mainly in the latter part of the small intestine, the ileum. The small intestine is perfectly structured for maximizing nutrient absorption. Its surface area is greater than 200 square meters—about the size of a tennis court! The large surface area is due to the multiple levels of folding, villi, and microvilli that cover the internal tissue of the small intestine. Villi are tiny finger-like projections that are covered with enterocytes or absorptive cells. The absorptive cell membrane is made of even smaller projections, called microvilli (Figure 3.12). These microvilli are referred to collectively as the brush border since their appearance resembles the bristles on a brush.
148 ALICE CALLAHAN, PHD, HEATHER LEONARD, MED, RDN, AND TAMBERLY POWELL, MS, RDN Figure 3.12. Histology of the small intestine. (a) The absorptive surface of the small intestine is vastly enlarged by the presence of circular folds, villi, and microvilli. (b) Micrograph of the circular folds. (c) Micrograph of the villi. (d) Electron micrograph of the microvilli. Digested nutrients are absorbed into either capillaries or lymphatic vessels contained within each villus. Amino acids (from protein digestion), small fatty acids (from triglyceride digestion), sugars (from carbohydrate digestion), water-soluble vitamins, and minerals are transported from the intestinal cells into the bloodstream through capillaries. The larger fatty acids, fat-soluble vitamins, and other lipids (that are packaged in lipid transport particles) are transported first through lymphatic vessels and then eventually meet up with the blood. Water-soluble nutrients that enter the bloodstream are transported directly to the liver where the liver processes, stores, or releases these nutrients to other body cells.
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Nutrition: Science and Everyday App
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Nutrition: Science and Everyday App
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UNIT 3 - MOLECULES OF LIFE: PHOTOSY
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UNIT 9 - VITAMINS AND MINERALS PART
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2 ALICE CALLAHAN, PHD, HEATHER LEON
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About the Authors ALICE CALLAHAN, P
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Updates Made to OER Date Unit Page
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Introduction to Designing a Healthy
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Classification of Nutrients Food is
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Introduction to Nutrition Science a
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The Scientific Method Similar to th
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Types of Research Studies and How T
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198 ALICE CALLAHAN, PHD, HEATHER LE
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Sugar: Food Sources, Health Implica
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UNIT 5- LIPIDS 246
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The Functions of Fats Fats serve us
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Lipid Types and Structures Lipids a
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Digestion and Absorption of Lipids
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UNIT 6- PROTEIN 304
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Protein Digestion and Absorption Wh
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Protein Food Choices and Sustainabi
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Introduction to Energy Balance On D
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Energy Balance: Energy In, Energy O
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Indicators of Health: BMI, Body Com
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Best Practices For Weight Managemen
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UNIT 8 - VITAMINS AND MINERALS PART
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Dietary Supplements By now you know
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Vitamins and Minerals Involved In F
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Vitamins Important for Vision Visio
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Introduction to Vitamins and Minera
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Introduction to Bone Health The hum
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Calcium: Critical for Bones and Thr
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Vitamin D: Important to Bone Health
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Vitamins and Minerals Involved in B
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Introduction to Nutrition and Physi
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UNIT 11 — NUTRITION THROUGHOUT TH
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Nutrition in Early Infancy Infancy
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Raising Healthy Eaters “Raising a
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