The Digestive System & Body Metabolism Chapter 15 Biology Ch. 38-2

I. Overview of the Digestive System

A. Functions of the Digestive System 1. Ingestion – consuming food 2. Digestion – breakdown of ingested food into smaller molecules 3. Absorption – passage of nutrients into blood 4. Metabolism – converting nutrients into raw materials for production of cellular energy (ATP) 5. Elimination – removing indigestible wastes

B. Types of Digestion 1. Mechanical Digestion: –Chewing, mixing, churning, cutting, tearing, or mashing of food.

2. Chemical Digestion Enzymes break down food molecules into their building blocks –Carbohydrates are broken down to simple sugars –Proteins are broken down to amino acids –Fats are broken down to fatty acids and alcohols

II. Organs of the Digestive System

A. Two main groups of digestive organs: 1. Organs along the Alimentary canal (Gastrointestinal tract): continuous coiled hollow tube over 30 ft. long –Mouth, pharynx, esophagus, stomach, small and large intestine, rectum 2. Accessory digestive organs: organs that contribute secretions (fluids) & enzymes to aid in digestion –Salivary glands, liver, gall bladder, pancreas

B. Alimentary Canal Organs 1.ORAL CAVITY (Mouth, Teeth, Salivary Glands) a. Mouth: Food enters the mouth where digestion begins –Mechanical digestion: Mastication (chewing) of food by teeth –Chemical digestion: Saliva contains the enzyme amylase that breaks down carbohydrates & starches in the mouth Bolus – masticated food mixed with saliva

Frontal View of Mouth

b. Teeth Function: chew food Humans have 2 sets: 1. Deciduous (baby or milk) teeth 20 teeth are fully formed by age two 2. Permanent teeth Replace deciduous teeth between the ages of 6 to 12 A full set is 32 teeth, but some people do not have wisdom teeth

Classification of teeth Incisors – used for biting food Canines – used for tearing food Premolars (bicuspids) – used for grinding food Molars – Used for crushing food

Parts of a tooth Crown – exposed part of tooth made of enamel Neck – connects crown to root Root – Periodontal membrane attached to the bone, contains blood vessels and nerves

c. Salivary Glands c. Salivary Glands Produce saliva - mixture of mucus and serous fluids Produce saliva - mixture of mucus and serous fluids Helps to form a food bolus Helps to form a food bolus Contains salivary amylase to begin starch digestion Contains salivary amylase to begin starch digestion Dissolves chemicals so they can be tasted Dissolves chemicals so they can be tasted

2. PHARYNX The passageway for air and food Has 3 parts: (highlight parts in diagram in your notes)

Swallowing: –Reflex that causes the epiglottis to move blocking the trachea (windpipe) –The esophagus opens to receive the bolus

3. Esophagus “Food tube” links to the stomach through the diaphragm Muscle layers contract to squeeze food through to the stomach –Longitudinal inner layer –Circular outer layer

Peristalsis – alternating waves of muscle contraction

4. Stomach A. Structure & Path of Food –Thick-walled, J-shape, muscular organ on left side of abdominal cavity –Bolus (food) enters through the cardioesophageal sphincter Prevents backup of stomach acid into esophagus (acid reflux)

Bolus goes through the four regions of stomach: –Cardiac region – near heart –Fundic region –Body region – main part of stomach –Pyloric region – funnel shaped end Chyme (mix of bolus and digestive juices) exits through pyloric sphincter to small intestine

Mechanical Digestion: Muscles of stomach contract to churn and mix food with gastric juices Deep folds called rugae allow stomach to expand

B. Coverings of the Stomach –Peritoneum – membrane that covers the abdominal wall and organs Visceral peritoneum – covers organ Parietal peritoneum – covers cavity walls –Mesentery holds the stomach and intestines in place in the abdominopelvic cavity

C. Gastric Juices –Chemical Digestion: Lining of stomach secretes gastric juices that break down food. Pepsin breaks down proteins into amino acids Hydrochloric acid (HCl) is a very strong acid (pH 1 or 2) activates enzymes and kills bacteria Mucous protects the lining of the stomach from the acid environment –Replaced every few days

If the mucus layer is broken down by increased acid, an ulcer may form Peptic Ulcer: open sores on the lining of the digestive tract that may bleed and be very painful –Caused by bacteria (Heliobacter pylori), regular use of pain relievers or other medications

Gastric Bypass

Lap-band Surgery

5. Small Intestine A. Overview –Muscular tube extending from the pyloric sphincter to the ileocelcal valve –Up to 22 feet long!

Functions –Main digestive organ – most digestion occurs in the small intestine Carbohydrate and protein digestion is completed Fat digestion begins –Main site of nutrient absorption into the blood

b. Structure Small intestine has 3 parts: –Duodenoum First 25 cm, attached to the stomach Receives secretions from the liver and pancreas to aid in digestion –Jejunum Next 3 feet Contains folds and villi for absorption –Ileum Last 6-7 feet Extends from jejunum to large intestine

Walls of the small intestine Has 3 features that increase the surface area for maximum absorption of nutrients 1. Circular folds- permanent transverse folds in the mucosa and submucosa

2. Villi – fingerlike projections of the mucosa –Goblet cells in villi produce mucus to lubricate chyme –Villi contain absorptive cells & blood capillaries to absorb nutrients 3. Microvilli – microscopic extensions from the plasma membrane

c. Chemical Digestion in the Small Intestine Digestion is completed in the small intestine Accessory organs aid in digestion by secreting enzymes into the small intestine –Bile from the gall bladder Breaks down lipids (fat) –Pancreatic juices from the pancreas Sodium bicarbonate neutralizes the acid

d. Absorption in the Small Intestine When food has been broken down to its nutrients & waste products, the nutrients must be absorbed by the body Most nutrient absorption occurs in the small intestine –Most substances are absorbed by active transport through cell membranes –Lipids are absorbed by diffusion After absorption, nutrients are transported to the cells through the bloodstream

Once digestion & absorption of nutrients is complete, the chyme moves into the large intestine by peristalsis 6. Large Intestine

Also known as the Colon Large in diameter, but shorter than the small intestine

Absorption of water Does not participate in digestion of food Eliminates indigestible food from the body as Feces Goblet cells produce mucus to act as a lubricant a. Functions of the Large Intestine

Ascending Colon – includes the cecum (saclike structure) with a small projection called the appendix Transverse Colon Descending Colon Sigmoid Colon b. Structure

Bacteria breaks down indigestible materials –Source of odor –Produce some vitamin K & B –Release gases Water, vitamins K and vitamin B are absorbed Remaining materials are eliminated as feces c. Food Breakdown & Absorption in the Large Intestine

Feces moves from the sigmoid colon of the large intestine to the rectum by peristalsis Feces is stored in the RECTUM until the involuntary internal anal sphincter is triggered to open 7. Rectum

Defecation or expulsion of feces occurs when the voluntary external anal sphincter relaxes and the feces passes out of the body –Happens daily –Triggered by presence of feces in rectum

8. Accessory Digestive Organs 1. Pancreas –Produces a wide spectrum of digestive enzymes that break down all types of food –Enzymes are secreted into the duodenum of the small intestine Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

–a) Secretions of the Pancreas 1) Pancreatic Amylase: helps complete digestion of starch 2) Trypsin: carry out about half of all protein digestion 3) Lipase: responsible for fat digestion 4) Nucleases: digest nucleic acids 5) Sodium Bicarbonate: neutralize acidic chyme Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

–Enzymes are transported to the small intestine through the Common Bile Duct (shared with the liver) Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

Life: The Science of Biology, 4th Edition, by Sinauer Associates ( and WH Freeman (

2. Liver Largest organ in the body Has 2 main lobes –Larger right lobe –Smaller left lobe The liver also contributes to the digestion by producing bile

Bile is a greenish substance that emulsifies fat Stored in Gall Bladder Composed of bile salts & pigments (biliruben from hemoglobin breakdown), cholesterol, phospholipids, & electrolytes Helps to digest fatty foods a. Bile

Removes drugs and alcohol from blood Degrades hormones Produces cholesterol, blood proteins (albumin and clotting proteins) Plays a central role in metabolism Stores products of digestion as glycogen to be released as needed b. Role of Liver in Metabolism

Sac found in hollow fossa of liver Stores bile produced by the liver Bile is introduced into the duodenum in the presence of fatty food Gallstones can cause blockages 3. Gall Bladder

III. Control of Digestive Activity Mostly controlled by reflexes via the parasympathetic division of nervous system Chemical and mechanical receptors are located in organ walls that trigger reflexes

III. Control of Digestive Activity Stimuli include: –Stretch of the organ –pH of the contents –Presence of breakdown products Reflex include: –Activation or inhibition of glandular secretions –Smooth muscle activity

Digestion Animation

Processes of the Digestive System