© 2008 Thomson - Wadsworth Chapter 3 Digestion, Absorption, and Metabolism

What Happens to the Food We Eat? The food we eat undergoes three processes: 1.Digestion 2.Absorption 3.Elimination These processes occur in the (GI) gastrointestinal tract.

Carbohydrates, proteins, and fats are usually very large molecules. The goals of digestion: Break these large molecules down to smaller molecules Absorb the smaller molecules into the cells of the body Must overcome many challenges along the way……..

© 2008 Thomson - Wadsworth

Copyright 2010, John Wiley & Sons, Inc. Digestion and Absorption The mouth is the entry point for food into the digestive tract. Digestion in the mouth involves mastication (chewing), the stimulation of taste buds, and swallowing. Saliva contains salivary amylase, which helps to break down carbohydrates. Saliva also has an enzyme called lysozyme, which help to inhibit bacterial growth in the mouth and helps prevent tooth decay. Chewing mechanically breaks food down, increasing the surface area in contact with digestive enzymes. The epiglottis closes to prevent food from entering the pharynx. After swallowing the food is called a bolus.

Copyright 2010, John Wiley & Sons, Inc. Digestion and Absorption Esophagus - The esophagus is the tube that leads the bolus to the stomach. There is a sphincter at the upper and lower (also known as the cardiac sphincter) ends of the esophagus.

Digestion Anatomy of the Digestive Tract The stomach adds juices and grinds the bolus to a semiliquid mass called chyme. The pyloric sphincter regulates the flow of partially digested food into the small intestine. The small intestine receives digestive juices from the gallbladder and the pancreas. The three segments of the small intestine are the duodenum (1 foot), the jejunum (8 feet) and the ileum (11 feet). Majority of absorption occurs here!

Digestion\Absorption

Copyright 2010, John Wiley & Sons, Inc. Large Intestine The large intestine does not participate in digestion. The large intestine can absorb water and some vitamins and minerals. Undigested residues, including some fibers, continue through the digestive tract and form waste. Recycling of usable materials Water, nutrients and fecal material may spend up to 24 hours in the large intestine. Transit time: amount of time it takes food to pass the length of the GI tract Feces: body waste, including unabsorbed food residue, bacteria and dead cells

Transverse colon Ascending colon Descending colon Sigmoid colon Anus Rectum Appendix Opening from small intestine to large intestine End of small intestine Stepped Art

Digestion The Muscular Action of Digestion – under autonomic control Peristalsis pushes the digestive contents along. Stomach action involves circular, longitudinal, and diagonal muscles. Segmentation is contractions by circular muscles that contract and squeeze contents to promote mixing with digestive juices. Sphincter contractions open and close passageways. This prevents reflux and controls the passage of contents.

© 2008 Thomson - Wadsworth

Digestion The Secretions of Digestion Saliva from the salivary glands moistens foods Gastric juice from the gastric glands includes hydrochloric acid. The goblet cells of the stomach wall secrete mucus to protect the walls of the stomach from the high acidity levels that are measured by pH units. Pancreatic juice contains intestinal enzymes (carbohydrase, lipase, protease) and bicarbonate to help neutralize acidic chyme. Bile is produced by the liver, stored in the gall bladder, and acts as an emulsifier to suspend fat.

The brain is involved in regulating digestion and absorption through the nervous system and through hormones. Homeostasis is important in the functioning of the digestive and absorptive systems.

The Regulation of the GI Tract Gastrointestinal Hormones and Nerve Pathways Hormones act as messengers. Gastrin is secreted by the stomach. Gastrin stimulates secretion of HCl Secretin is secreted by the duodenum. This stimulates the pancreas to release bicarbonate secretion into intestine.

Regulation of the GI Tract Gastrointestinal Hormones and Nerve Pathways Pancreatic secretions change based on the content of the diet. Cholecystokinin (CCK) targets the gall bladder to release bile. Motility slows for foods that take longer to be digested. Nerves are triggered by the presence of food and cause the release of substances that speed up or delay motility.

Gastrointestinal Bacteria A healthy GI tract has many different non- disease-causing bacteria known as flora or microflora. (10 trillion bacteria, 400 species) Probiotics are bacteria found in the GI tract that can be beneficial to health. An example is the bacteria found in yogurt.

Absorption Nutrients can be absorbed through simple diffusion, facilitated diffusion, or active transport.

Carrier loads nutrient on outside of cell... Outside cell Cell membrane... and then releases it on inside of cell. Inside cell FACILITATED DIFFUSION Some nutrients (such as the water- soluble vitamins) are absorbed by facilitated diffusion. They need a specific carrier to transport them from one side of the cell membrane to the other. (Alternatively, facilitated diffusion may occur when the carrier changes the cell membrane in such a way that the nutrients can pass through.) ACTIVE TRANSPORT Some nutrients (such as glucose and amino acids) must be absorbed actively. These nutrients move against a concentration gradient, which requires energy. Stepped Art Some nutrients (such as water and small lipids) are absorbed by simple diffusion. They cross into intestinal cells freely. SIMPLE DIFFUSION

Absorption The enormous surface area of the small intestine facilitates nutrient absorption. Anatomy of the Absorptive System Villi are the fingerlike projections within the folds of the small intestine that move in a wave-like pattern to trap nutrients. Microvilli are the microscopic hairlike projections on each villi.

Absorption Water-soluble nutrients and small products of fat digestion are released to the bloodstream. Fat-soluble vitamins and larger fats form chylo- microns and are released to the lymphatic system.

The Health and Regulation of the GI Tract The System at Its Best: A healthy digestive tract is essential. Balance, moderation, and variety of meals help promote a healthy GI tract.

Heartburn and “Acid Indigestion” Gastroesophageal reflux is the backward flow of stomach contents into the esophagus. Antacids and acid controllers may help indigestion.

Ulcers Peptic ulcers can be gastric or duodenal and are regions of the GI tract that have been eroded by HCL and pepsin. The bacterium Helicobacter pylori contributes to the production of both gastric and duodenal ulcers.

Diarrhea Frequent, loose, watery stools Can be caused by food intolerances, toxins, infection of the GI tract, stress, or bowel disorders Can lead to severe dehydration Is more dangerous for children and the elderly Irritable bowel syndrome and/or colitis are possible GI disorders.GI disorders

Constipation Defecation habits are different among people. Many causes are possible. Hemorrhoids may be a problem. Diverticulosis is a condition in which the intestinal walls weaken and bulge. The bulging pockets are called diverticula. Diverticulitis is a worsened condition and requires intervention. Use of laxatives, enemas and mineral oil may not be necessary with lifestyle changes.

Chemical Reactions in the Body During digestion, the energy-yielding nutrients are broken down, then absorbed, and taken up by the body cells. In metabolism they are broken down further into energy, water and carbon dioxide.

Copyright 2010, John Wiley & Sons, Inc. Metabolism of Nutrients A series of reactions needed to transform food into a final product that can be used by the body. Energy is released from chemical bonds. Energy from food is converted into ATP. ATP is the energy currency of the cell.

Chemical Reactions in the Body Metabolism takes place inside of cells. The process is the basis of life, allowing cells to grow and reproduce, maintain their structures, and respond to their environments.

Breaking Down Nutrients for Energy occurs in 3 stages: The breakdown of glucose to energy starts with: 1)glycolysis to pyruvate. 2)TCA cycle or tricarboxylic acid cycle (or Kreb’s cycle) 3)electron transport chain