Digestion Absorption and Transport Chapter 3 Prepared by

Digestion, Absorption, and Transport Chapter 3 Prepared by L. Nicholle Clark DTR, MS

Digestion • The body’s process of breaking down foods into nutrients in preparation for absorption. • Anatomy of the Digestive Tract • Gastrointestinal (GI) Tract- a flexible muscular tube that extends from the mouth, through the esophagus, stomach, small intestine, large intestine, and rectum to the anus. The body surrounds the GI tract. Figure 3 -1.

Anatomy of the Digestive Tract • Lumen- The enter space within the GI tract. Substances must penetrates the GI tract’s wall to enter the body proper. • Mouth-Digestion begins here –four basic tastesweet, salty, sour, bitter. • Pharynx – short tube that is shared by both the digestive system and the respiratory system. Food passes through, and the epiglottis closes off your air passages so you do not choke.

Anatomy of the Digestive Tract • Bolus-after a mouth full of food has been swallowed it is called a bolus. • Esophageal sphincter- the esophagus has a sphincter muscle at each end. At the top the bolus slides down through the sphincter to the stomach. The lower sphincter closes so the bolus proceeds forward and does not slip back into the esophagus.

Anatomy of the Digestive Tract • The stomach adds liquids and grinds the food in to a semi-liquid mass called chyme. • Phloric sphincter- opens and the chyme passes through to the small intestine. • Small intestine (appro 10 ft)- has three segmentsthe duodenum, the jejunum, and the ileum. • Ileocecal valve- opens and the remaining contents enter the colon.

Anatomy of the Digestive Tract • Large Intestine(lg, colon)- water is removed from the contents as it passes out of the lg into the rectum. Strong muscles of the rectum and the anal canal hold back waste until it is time to defecate. • The muscular Action of Digestion • Peristalsis- the relaxing and contracting of the circular and longitudinal muscles of the GI tract that work opposite each other to push the intestinal contents along.

The Muscular Action of Digestion • Stomach Action- has a third layer of diagonal muscles that work to force the chyme downward. • Segmentation-The rhythmic contractions of the intestines that mix the chyme and promote close contact with the digestive juices & the absorbing cells of the intestinal walls.

The Muscular Action of Digestion • Sphincter Contractions-muscles periodically open and close to allow contents to move along at a controlled pace. • The Secretions of Digestion- The breakdown of food into nutrients requires secretions from 5 different organs: Salivary glands, the stomach, the pancreas, the liver and the small intestine. These secretions, contain water & enzymes enter the GI tract at various points along the way.

Secretions of Digestion • Enzyme- is a protein that facilitates a chemical reaction. It is often identified by the organ they come from and the compounds they work on. Ex. Gastric lipase-stomach enzymes that work on lipids. Pancreatic lipase-comes from pancreas works on lipids. It is a catalyst as the enzyme it self remains unchanged. • The digestive enzymes facilitate a chemical reaction known as hydrolysis = the addition of water.

Secretions of Digestion • Salivary Glands-squirt just enough saliva to moisten each mouthful of food so it can pass easily down the esophagus. • Gastric Glands- secrete gastric juice which acts primarily in protein digestion. The strong acidity of the stomach prevents bacterial growth and kills most bacteria that enter the body with food. Mucus is secreted from the stomach cells to protect itself from gastric juices.

Secretions of Digestion • Pancreatic juice- contains enzymes that act on all three energy nutrients. This juice also contains sodium bicarbonate which is an alkaline, it neutralizes the acidic chyme arriving in the small intestine from the stomach. The cells of the intestinal wall also possess digestive enzymes on their surfaces.

Secretions of Digestion • Bile from the liver flows into the duodenum. Bile is an Emulsifier it brings fats into suspension in water so that enzymes can break then down into their components parts. • The Final Stage- At this point, the three energyyielding nutrients have been disassembled. • In the colon intestinal bacteria ferment some fibers producing water, gas, and small fragments of fat that provide energy for the cells of the colon.

Absorption • Most absorption takes place in the small intestine. A rush of circulating blood continuous washes the underside of the surface carrying the absorbed nutrients away to the liver and other parts of the body. • Anatomy of the Absorptive System • The small intestine surface under a microscope appears to be wrinkled into hundreds of folds or projections called villi each villi is covered with its own microscopic hairs called microvilli.

Anatomy of the Absorptive System • In the crevices between the villi lie the crypts which are glands that secrete the intestinal juices into the small intestine. • Any nutrient molecule small enough to be absorbed is trapped among the microvilli that coat the cells and then draw into the cells. • A closer look at the Intestinal Cells-Successive portions of the intestinal tract are specialized to absorb different nutrients.

A closer look at the Intestinal Cells • The myth of “Food Combining”. • Preparing nutrients for Transport- When a nutrient molecule has crossed the cell of a villus, it enters either the bloodstream of the lymphatic system. Water soluble nutrients and smaller products of fat digestion are released directly into the bloodstream.

A closer look at the Intestinal Cells • Larger fats and fat-soluble nutrients called chylomicrons are released into the lymphatic system enter the bloodstream at a point near the hear. • The circulatory Systems-The circulatory system deliver nutrients wherever they are needed. • Vascular System/ blood circulatory- is a closed system of vessels through which blood flows continuously, with the heart serving as the pump.

The circulatory Systems • As the blood circulates through this system, it picks up and delivers materials as needed. • Blood travels Heart to arteries to capillaries to veins to heart. • Lymphatic System-Provides a one-way route for fluid from the tissue spaces to enter the blood. No pump, lymph circulates between the cells of the body and collects into tiny vessels. The fluid moves from one portion of the body to another as muscles contract and create pressure and there.

The Health & Regulation of the GI Tract • GI tract health depends on adequate nutrition. • Gastrointestinal Bacteria-An estimated 10 trillion bacteria representing some 400 or more different species & subspecies live in a healthy GI tract. Most of these bacteria normally do the body no harm and may actually do some good. Diet is one of several factors that influence the body’s bacterial population. Yogurt contains probiotics. • Gastrointestinal Hormones &Nerve Pathways

Gastrointestinal Hormones &Nerve Pathways • Homeostasis-survival depends o body conditions staying about the same; if they deviate too far from the norm the body must do something to bring them back to normal. The body regulates its temperature, digestion, blood pressure etc. • The system at its best-To maintain health & promote the functions of the GI tract, you should make balance, moderation, variety, & adequacy features of every day’s menus.

Summary • References • Whitney, E. , Rolfes, S. , . Understanding Nutrition. Belmont, CA: Thomson. Wadsworth