The Digestive System and Body Metabolism Digestion Breakdown of ingested food Absorption of nutrients into the blood Metabolism Production of cellular energy (ATP) Constructive and degradative cellular activities

Processes of the Digestive System Ingestion – getting food into the mouth Propulsion – moving foods from one region to another Mechanical digestion Chemical digestion Absorption Defecation One way disassembly line. Several steps done by dift organs. Some overlap in function.

Stomach Functions Acts as a storage tank for food Mechanical, chemical breakdown of protein begins Delivers chyme to the small intestine Chyme = processed food Convolutions of interior surface allow stomach to expand.

Food Breakdown in the Stomach Gastric juice is regulated by neural and hormonal factors Presence of food or falling pH causes the release of gastrin Gastrin causes stomach glands to produce protein-digesting enzymes Hydrochloric acid makes the stomach contents very acidic Protein digestion enzymes Pepsin – an active protein digesting enzyme Rennin – works on digesting milk protein The only absorption that occurs in the stomach is of alcohol and aspirin Necessity of an Extremely Acid Environment in the Stomach Activates pepsinogen to pepsin for protein digestion Provides a hostile environment for microorganisms

Chemical Digestion in the Small Intestine Pancreatic enzymes digest starch, proteins, nucleic acids, fats; reset pH. Gall bladder - Enzymes from the brush border Break double sugars into simple sugars Complete some protein digestion Pancreatic enzymes play the major digestive function Help complete digestion of starch (pancreatic amylase) Carry out about half of all protein digestion (trypsin, etc.) Responsible for fat digestion (lipase) Digest nucleic acids (nucleases) Alkaline content neutralizes acidic chyme Endocrine products of pancreas Insulin Glucagons Figure 14.6

Propulsion in the Small Intestine Peristalsis is the major means of moving food Segmental movements Mix chyme with digestive juices Aid in propelling food Esophagus moves food bolus by peristalsis Small intestine uses segmental movements to mix food, push it up agst lining for better absorption.

Absorption in the Small Intestine Site of nutrient absorption into the blood Suspended from the posterior abdominal wall by the mesentery Water is absorbed along the length of the small intestine End products of digestion Most absorbed by active transport Lipids are absorbed by diffusion Most absorbed by active transport through cell membranes Substances are transported to the liver by the hepatic portal vein or lymph

Absorption in the Small Intestine Site of nutrient absorption into the blood Villi = Fingerlike structures formed by the mucosa Microvilli on each cell Increases surface area Each villus is made of many cells. Each cell has tiny microvilli (1700) on its free surface. = brush border. Note vascular connections and lymph collecting vessels (green) Most absorbed by active transport through cell membranes Substances are transported to the liver by the hepatic portal vein or lymph Figure 14.7a

Absorption of Proteins and Carbohydrates Amino acids, sugars move by diffusion into blood vessels Next stop: liver, where glucose glycogen amino acids protein Excess molecules are converted to fats Figure 14.13

Absorption of Fats Lipids are absorbed into lymph system, which drains into heart Emulsified with bile salts Digested into fatty acids, glycerol With bile salts, diffuse into cells Reassembled into fats, exocytosis Figure 14.14

Food Breakdown and Absorption in the Large Intestine No digestive enzymes are produced Resident bacteria digest remaining nutrients Produce some vitamin K and B Release gases Water and vitamins K and B are absorbed Undigested fiber keeps materials moving and is eliminated via feces

Control of Digestive Activity Regulation dependent on volume and content of food Nervous system: stretch receptors in stomach Hormones: Gastrin: stimulates release of gastric juice Secretin: stimulates pancreas to secrete water and bicarbonate Cholecystokinin (CCK): signals pancreas to secrete digestive enzymes Chemical and mechanical receptors in organ walls trigger reflexes Table 14.1, p475. Stomach is stimulated by food entering it. Produces 3 hormones: Gastrin - stimulates release of gastric juice, stimulates small intestine Histamine - stimulates parietal cells to release HCl. pH drops Somatostatin - inhibits secretion of gastric juice and pancreatic juice. Inhibits emptying of stomach. 2-3 L of gastric juice per day, normally. Stimuli include: Stretch of the organ pH of the contents Presence of breakdown products Reflexes include: Activation or inhibition of glandular secretions Smooth muscle activity NOTE: this is not appetite control mechanism. Separate system. Appetite stimulated by: CCK (from pancreas) , low glucose levels in blood. Suppressed by leptin (from adipose cells). Phen-fen drug.

Nutrition Carbohydrates: major energy source, simple or complex Lipids: cell components and energy sources, saturated or unsaturated Proteins: 20 amino acids Vitamins: fat soluble and water soluble Minerals: recommended daily allowance Fiber Major biological polymers, made of repeated monomers. Same categories as listed on food nutrition labels. These molecules make up the vast bulk of our food, organic or otherwise. Carbs - energy, fiber Lipids, energy and membs Proteins - made up of amino acids.20 essential our bodies must ingest 9 of these, can make the rest. Vitamins - coenzymes. Essential to function of other enzymes, but not consumed in reaction. Minerals - specific functions. Fe in hemoglobin. Water - vital. Part of everything

Food Guide Pyramid Carbohydrates are The body’s preferred source to produce cellular energy (ATP) Glucose (blood sugar) is the major digestive product and serves as fuel to make ATP Brain uses 65% of blood glucose. Neurons take lots of energy, prefer to use glucose. Other cells will use fats, proteins if glucose is unavailable. Caffeine stimulates enzymes which break down fats AND those which make storage fats. No net gain. Figure 14.16

Body Energy Balance Energy intake = total energy output (heat + work + energy storage) Energy intake from food oxidation Energy output Heat is usually about 60% Storage energy is in the form of fat or glycogen Energy intake - combat obesity by blocking conversion of food. Olestra - fat which cannot be broken down (5 Carbon sugar with 5 fatty acids) gastric bypass surgery artificial sweeteners - same sweetness, fewer molecules = fewer Calories Enegy output - maintain body temp. Heat is usly lost to environment. So if two people weigh the same, which uses more energy for body temp stability: short, wide vs tall, thin? Why? Storage energy - carbo loading before a competition is putting glycogen into muscle cells and liver. Longer term storage uses fats in adipose tissue, usly underneath skin.

Regulation of Food Intake Mechanisms that may regulate food intake Levels of nutrients in the blood Hormones Body temperature Psychological factors Anorexia bullemia

Basal Metabolic Rate BMR– amount of heat produced by the body per unit of time at rest Factors that influence BMR Surface area Gender Age – children and adolescents have a higher BMR thyroxine from thyroid gland Give reasons why each one of these factors should influence BMR. Factors that influence BMR Surface area - loss of body heat. Surface area/volume Gender - males highe b/c of lower % BODY FAT. Adipose tissue w/ low metabolic rate. Age – children and adolescents have a higher BMR b/c of growth, not just repair of cells. thyroxine from thyroid gland.

Total Metabolic Rate (TMR) Total amount of kilocalories the body must consume to fuel ongoing activities TMR increases with an increase in body activity