The Human Body:The Digestive System

Textbook Chapter 35Review Book Topic 1

Function

• Ingest (eat) food

• Breaks food down to absorb nutrients

• Eliminates what cannot be digested as waste

Ingestion• Mechanical digestion – involves

physical breakdown of food into smaller pieces

• Teeth and tongue in the mouth

• Smooth muscles in the stomach

• Smooth muscles in the small intestines

• Chemical digestion – enzymes are used to break down large molecules into smaller molecules that can be absorbed by cells

• Ex. Amylase – enzyme found in saliva which causes the breakdown of starches into sugars

Esophagus

• As the tongue pushes chewed food to the back of the mouth, the swallowing reflex is stimulated

• Esophagus connects the mouth to the stomach

• Lined with smooth muscles that contract rhythmically to move food through the digestive system in a process called peristalsis

• Continues throughout the digestive tract

• Not even affected by gravity….you could still digest food upside down!

• When a person swallows, the epiglottis covers the trachea (windpipe)

• If food enters the windpipe, choking occurs

Stomach• A sphincter, a circular muscle, controls the

movement of food from the esophagus into the stomach

• Stomach walls are composed of three layers of smooth muscle

• Involved in mechanical digestion and chemical digestion

• Very acidic environment

• Gastric glands secrete an acidic solution at pH 2

• Any leakage of stomach acid through the sphincter muscles can cause acid to reach the esophagus, causing heartburn

• Pepsin – enzyme involved in the chemical digestion of proteins in the stomach

• Cells in the lining of the stomach secrete mucus to help prevent stomach acid from digesting the stomach’s cells

• Some substances like alcohol and aspirin are absorbed by stomach cells (but most absorption occurs in the small intestine)

• When empty, stomach capacity is about 50 mL

• When full, it can expand to 2 – 4 L

• Muscular stomach walls contract and push food farther along the digestive tract

• Empties through another sphincter into the small intestine

Small Intestine• 7 m in length

• Longest part of the digestive system

• Small intestine because the diameter is 2.5 cm compared to the diameter of the large intestine at 6.5 cm

• Smooth muscular walls continue the process of mechanical digestion and peristalsis

• Chemical digestion is completed by accessory organs

• Pancreas

• Produces enzymes that digest carbohydrates, proteins, and fats

• Produces hormones

• Secretes an basic solution to raise the pH in the small intestine to pH 7 (neutral) for the optimum environment for enzyme action in the small intestine

• Liver• Largest internal organ of the body

• Produces bile which breaks down fats

• About 1 L of bile is produced each day and any excess is stored in the gall bladder

• Gall bladder• Stores excess bile

• Releases bile into the small intestine when needed

• Nutrients from food are absorbed from the small intestine into the bloodstream through villi

• Fingerlike projections

• Increase the surface area of the small intestine

• Gives the small intestine as much absorption room as a tennis court!

• When digestion is complete, any remaining food, in the form of chyme moves into the large intestine

• Chyme is a semi-liquid solution composed of materials that cannot be digested or absorbed by the small intestine

Large Intestine• End portion of the digestive tract

• 1.5 m long

• Includes the colon, rectum and appendix

• Colon normally contains bacteria which produce vitamin K and some B vitamins

• Primary function is to absorb water from chyme

• Indigestible material becomes more solid as water is absorbed, forming feces

• Peristalsis continues to move feces to the rectum

• Walls of the rectum to stretch initiating a reflex

• Final sphincter muscle to relaxes, eliminating the feces from the body through the anus

Digestive Structure Primary Function Time Food in Structure

Mouth Mechanical and chemical digestion

5-30 seconds

Esophagus Transport (swallowing)

10 seconds

Stomach Mechanical and chemical digestion

2-24 hours

Small Intestine Mechanical and chemical digestion

3-4 hours

Large Intestine Water absorption 18 hours – 2 days

Digestive System BrainPop

Nutrition• Nutrition – process by which a person takes in

and uses food

• Food supplies building blocks and energy needed by the body to maintain body mass

• Daily input of energy from food should equal the amount of energy a person uses daily

• Calorie – unit used to measurethe energy content of food

• 1 calorie is the amount of heat needed to raise the temperature of 1 mL of water by 1°C

• Can be found by burning the food and converting its stored energy to heat

• To lose weight, more calories must be used than consumed

• Carbohydrate• Ex. cereal, pasta, potatoes,

strawberries, rice

• Broken down into simple sugars

• Stored in the liver in the form of glycogen

• Cellulose – a plant starch – cannot be digested but is required by humans to “flush” out and keep the digestive tract clean (also known as fiber)

• Fats• Essential for your diet in proper amounts

• The most concentrated energy source available to the body

• Protect internal organs

• Maintain homeostasis by providing energy, storing and transporting certain vitamins

• Not all fats are beneficial

• Saturated (solids)• Ex. Meats, cheeses, dairy• “Unhealthy”• Can result in high blood pressure or heart

problems

• Unsaturated (liquids)• Ex. Plants• “Healthy”

• Digested in small intestine

• Broken down into fatty acids and glycerol

• Proteins• Basic structural components of all cells

• Made up of amino acids• Absorb into the blood and are “rebuilt” when they

enter body cells

• 20 amino acids • 8 of these we cannot make in our body and

must obtain them from our food

                                            

      

• Form enzymes, hormones, neurotransmitters, membrane receptors

• Ex. Animal byproducts: meat, fish, poultry, eggs, dairy

• Ex: Plants (only cover some of the essential amino acids needed…ex. beans + rice)

• Food Pyramid

• Changed in 2005 by the Department of Agriculture

• Shows which foods and the amount of these foods is right for your diet

Old Food Pyramid

Modern Food Pyramid

• Vitamins and Minerals

• Vitamins – organic compounds needed in small amounts for metabolic activities

• Help enzymes function

• Obtained by your diet, cannot be made in large enough quantities by your body

• Some can be stored in your liver but many cannot

• Minerals – inorganic compounds used as building materials an involved in some metabolic functions

• Taking over the recommended daily amount of vitamins or minerals can be dangerous

• Nutrition Labels• Provided on commercially packaged foods

• Based on a diet of 2000 calories a day

• Required information:• Name of food• Net weight or volume• Manufacturer information• Ingredients• Nutrient content