Digestive System

Digestion: The chemical breakdown of large food molecules into smaller molecules that can be used by cells. The basic fuel molecules are amino acids, lipids, and sugars.

Steps to food processing Four steps to food processing: Ingestion Digestion Absorption Elimination

Two Types Mechanical Digestion- breaking food down into smaller pieces. Chemical Digestion- breaking food down into smaller molecules.

Mouth Chewing breaks food into smaller particles Food is moistened and lubricated in the mouth. Tongue mixes food with saliva. Saliva is secreted by salivary glands. Contains the enzyme salivary amylase to begin the breakdown of starch. There is continuous secretion to keep the mouth moist and to lubricate food. Secretion of saliva is stimulated by the presence of food.

Teeth Teeth are important to animal digestion for capturing, tearing, and chewing food. Carnivores possess pointed teeth for capture, cutting and shearing. Herbivores have large, flat teeth suited for grinding plant materials. Omnivores have both types, front like carnivores, back like herbivores.

Swallowing Mucus helps form bolus The tongue pushes the bolus to the back of the mouth The swallowing instinct is activated Epiglottis moves to protect trachea In the esophagus peristalsis moves the bolus Cardiac sphincter opens to allow bolus into the stomach

Stomach-preliminary Stomach lining Highly folded Expands to accommodate food Extra layer of muscle to churn food Bolus + gastric juices = chyme Secretes HCl (2 liters per day) 2 Kills most bacteria Lining protected by mucus (constantly replaced)

Secrete pepsinogen (inactive) Converted to pepsin (active) Pepsin breaks down proteins into smaller pieces (final breakdown in small intestine) Ulcers Excess gastric juice Bacteria Absorption Water Alcohol aspirin

Small Intestine Approx. 3 meters long Made up of three parts Duodenum Jejunum Ileum

Duodenum Approx. 25 cm long Pyloric sphincter allows small amounts of chyme to enter at a time Pancreatic enzymes and bile from the liver are mixed with the chyme The intestine must be protected from the acid contained in gastric juice. When chyme enters the small intestine, the cells of the duodenum release the hormone secretin. This hormone stimulates the pancreas to produce sodium bicarbonate, which neutralizes the acidic chyme and shuts off pepsin. It also stimulates the liver to secrete bile.

Another hormone (CCK, cholecystokinin), stimulates the gallbladder to release bile and the pancreas to produce pancreatic enzymes. Another hormone (GIP, gastrin inhibitory protein) inhibits gastric glands in the stomach and inhibits the mixing and churning movement of stomach muscles. This slows the rate of stomach emptying when the duodenum contains food.

Absorption Villi Microvilli Fats absorbed by villi into lymphatic system

Pancreas Located below the stomach Pancreatic duct connects to duodenum Pancreatic juice contains: Trypsin-proteins Pancreatic amylase-starch Lipase-fat Bicarbonate

Pancreas II Produce hormones Insulin Glucagon Produced by islets of Langerhans Controls sugar in blood stream

Liver Destroy red blood cells Hemoglobin makes bile Rest of the b.c eliminated with feces & give it its brown color Produces bile and stores it in the gallbladder Fatty foods in duodenum stimulate its release through common bile duct Bile salts are soluble in both lipids and water Breaks fat into smaller pieces More surface area for lipase

Liver II Cholesterol binds bile together to form crystals Can block common bile duct Can cause jaundice

Liver III Control of blood sugar levels Liver removes glucose in the blood, converting it into glycogen Insulin causes the liver to store sugar (glycogen) Glucagon stimulates the release of sugar (during fasting) Excess glycogen can be stored in muscle The liver stores enough glycogen for about 10 hours of fasting. If food is still unavailable after that, amino acids (from muscles) and fats (from fat stored in fat cells) are used as a source of energy.

Liver IV Liver removes toxins from the blood stream Converts toxinss to urea Removed by kidneys through urine Alcohol, Drugs, Pesticides, and Carcinogens are removed by the liver Chronic exposure of these toxins can lead to damage of the liver