(*) (*) Ingestion, digestion, absorption, and elimination. Uptake of nutrients by body cells (intestine)

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1 Human Digestive System Food is pushed along the digestive tract by peristalsis the rhythmic waves of contraction of smooth muscles in the wall of the canal Accessory glands. Main stages of food processing Ingestion, digestion, absorption, and elimination The act of eating, is the first stage of food processing (oral cavity) Uptake of nutrients by body cells (intestine) Occurs as undigested material passes out of the digestive compartment (rectum, anus) Breaking food down into molecules small enough to be absorbed Involves enzymatic hydrolysis of polymers into their monomers (stomach) 1

2 Waves of muscular contraction (peristalsis) move the bolus down the esophagus to the stomach The Oral Cavity where ingestion occurs Saliva has several functions 1. Protects mouth from abrasion and lubricates food 2. Buffers neutralize ph and prevent tooth decay 3. Antibacterial agents kill germs The Stomach where digestion (2-6 hours) occurs Secretes gastric juice, which converts a meal to acid chyme Gastric juice is made up of hydrochloric acid (HCl) and the enzyme pepsin The lining of the stomach Is coated with mucus, which prevents the gastric juice from destroying the cells The Small Intestine Absorption of nutrients to the blood takes place in the small intestine The circulatory system distribute the nutrients to all the cells in the body Gall bladder The gall bladder stores a digestive juice produced by the liver: bile Liver Pancreas Secretion on bicarbonate ions that neutralize ph coming from the stomach Produces proteases, protein-digesting enzymes When blood leaves the intestine it enters the liver. Functions of the liver: Filters any foreign organisms. Detoxifies many dangerous molecules that enter with blood To modify nutrient molecules, e.g., glucose to glycogen and regulate cholesterol levels Is also a storage organ for vitamins 2

3 How does the small intestine maximize absorption? Surface area limits the amount of material that can be taken up The more surface area available The more absorption per unit of time Solutions? (1). Small intestine is very long! (18 to 24 feet) (2). Fingerlike projections called villi (3). Membranes of cells in the villi also have folds (microvilli) (2) (3) (1) The Large Intestine The large intestine, or colon Is connected to the small intestine A major function of the colon Is to recover water that has entered the alimentary canal The wastes of the digestive tract, the feces Become more solid as they move through the colon Pass through the rectum and exit via the anus The colon houses various strains of the bacterium Escherichia coli Some of which produce various vitamins 3

4 Energy and Food: Amount of energy used at rest energy for breathing, heart rate, maintain the temperature constant, etc. Basal Metabolic Rate (BMR) What is the normal, basal rate? kcal Age Weight Height Percentage of weight that is fats Sex Climate Time of the year Altitude Most young adults = 1,800-2,200 kcal/day Higher in children than in elderly More weight higher the BMR ADDITIONAL kcal BY OCCUPATION Occupation kcal/day Sedentary (student) Light work (business person) Moderate work (laborer) More height higher the BMR Heavy work (professional athlete) Fat tissue has a lower metabolic rate than lean tissue Males in general have higher metabolic rate than women Individuals living in cold climates have higher BMR People have higher BMR during the cold season People living at higher altitudes have higher BMR How Obesity is defined Body Mass Index (BMI) Relation between a person s weight in kilograms (1 kg=2.2 pounds) and height in meters (1 m = 3.3 ft) BMI = weight in kg (height in m) 2 e.g. 73 kg = 73 kg = 24.1 kg/m 2 (1.74) m kg/m 2 Healthy BMI = 19 and 26 kg/m 2 Overweight BMIs = kg/m 2 Obese BMIs = kg/m 2 4

5 Chemical composition of the diet A person s diet must supply carbon skeletons and essential nutrients To build the complex molecules it needs to grow, maintain itself, and reproduce Humans, like all other animals, need to get the nutrients from the food Nutrients Carbohydrates: sugars Lipids: fats Proteins Vitamins Minerals Inorganic Water Stored in the body as reserve as glycogen Organic The only type of nutrient that does not have essential representatives Some nutrients that cannot be synthesized by the body Essential nutrients Fatty acids: LINOLEIC and LINOLENIC present in meat and vegetable oils Aminoacids: 10 of the 20 required are essentials Unlike carbohydrates and fats, proteins cannot be stored for later use. The total amount of protein necessary on a daily basis is about 50 grams (this is equivalent to a half of a chicken breast). Vitamins: Needed in minute amounts to maintain essential metabolic activities Minerals: Inorganic elements, none of them can be synthesized by the body Water Most plant proteins are incomplete in amino acid makeup Individuals who must eat only plant proteins need to eat a variety to ensure that they get all the essential amino acids 5

6 Vitamins Do not serve a s a source of energy Needed in a very small quantity Minerals Na+, K+, Ca++ transmission of nerve impulses Mg++ Energy release during reaction involving ATP Fe++ Hemoglobin, oxygen transport in red blood cells, anemia Ca++ osteoporosis (weak bones by loss of calcium) Eating disorders Obesity BMI > 26.5 kg/m 2 overweight BMI > 32 kg/m 2 obese 34% adults over 20 in US 12% children age % children age % young age Imbalance between metabolic needs (kilocalories needed) and amount of food ingested Induces diabetes, cardiovascular disease, and colon and breast cancer Bulimia Cycle of excessive eating followed by purging the body of the food by inducing vomiting or using laxatives Anorexia Severe, prolonged weight loss due to a voluntary restriction in food intake 6

7 Obesity in the United States: Percent of Obese (BMI > 30) in U.S. Adults Control of the Appetite Over the long term, homeostatic mechanisms regulate body weight Feedback circuits control the body s storage and metabolism of fat Several hormones regulate both long-term and short-term appetite by affecting a satiety center in the brain ADIPOSE TISSUE Produced by adipose (fat) tissue, leptin suppresses appetite as its level increases. When body fat decreases, leptin levels fall, and appetite increases. STOMACH Secreted by the stomach wall, ghrelin is one of the signals that triggers feelings of hunger as mealtimes approach. In dieters who lose weight, ghrelin levels increase, which may be one reason it s so hard to stay on a diet. INTESTINE The hormone PYY, secreted by the small intestine after meals, acts as an appetite suppressant that counters the appetite stimulant ghrelin. Leptin Ghrelin Insulin PYY PANCREAS A rise in blood sugar level after a meal stimulates the pancreas to secrete insulin In addition to its other functions, insulin suppresses appetite by acting on the brain. 7