THE DIGESTIVE SYSTEM

PLO’s C1 - analyze the functional interrelationships of the structures of the digestive system

Food – What’s in it for you?

Fig. 12.1

Digestion Digestion is the process of breaking down large molecules of food into a form that can be absorbed and utilized by cells of the body Consists of both physical and chemical processes (What’s the difference?)

Mouth Food is broken down physically by chewing Tongue tastes food and aids in swallowing

Salivary glands produce salivary amylase (digests starch)

Pharynx Passage for food and air Separates into two separate passages Swallowing occurs by reflex action Epiglottis prevents food from entering the trachea http://www.linkstudio.info/images/portfolio/medani/Swallow.swf

Fig. 12.3

Esophagus Food is pushed along by muscular contractions called peristalsis Cardiac (esophogeal) sphincter is a muscular ring located between stomach and esophagus

Stomach Stores food Muscular action mixes food with gastric juice Gastric glands produce mucus, pepsin, and HCl (pH 2) Begins digestion of proteins

Chemical digestion of protein begins

Fig. 12.5

Chyme (partly digested food + gastric juice) leaves the stomach through the pyloric sphincter

Duodenum First part of the small intestine, where food enters after it leaves the stomach

Sodium bicarbonate neutralizes the pH Bile enters from the gall bladder (emulsifies fats) Pancreatic enzymes digest starch, protein and fats

Small Intestine Most digestion and absorption occur here Length and large surface area allow for maximum absorption of food Material is moved along by peristalsis

Blood vessels surround the small intestine Cells in intestinal wall produces more digestive enzymes

Intestinal Villi Walls contain villi and microvilli to increase surface area for absorption (these contain blood and lymph vessels)

Sugars and amino acids enter the blood via capillaries leading to the hepatic portal vein Fatty acids and glycerol enter lacteals (lymph vessels)

Large Intestine (Colon) No digestion occurs here Consists of the ascending, transverse, descending and sigmoid colon Absorption of water, salts and some nutrients

Anaerobic bacteria break down indigestible materials and produce some vitamins (vitamin K and B)

Feces moves along by peristalsis, is stored in the rectum and exits via anal sphincter

REVIEW Animation and quiz List the structures that food passes through in your body, from beginning to end. What happens at each location?

Accessory Organs Several organs and glands contribute to the digestive process even though food does not pass through them

Pancreas Cells in the pancreas produce pancreatic juice, which contains sodium bicarbonate and several enzymes to digest carbohydrates, proteins and fats Pancreatic juice is secreted into the duodenum via the pancreatic duct

The pancreas also has an endocrine function (hormone secreting) insulin is produced in the pancreas and secreted into the blood when blood glucose is high (after eating) stimulates body cells to take up glucose (lowers and regulates blood sugar levels) Another hormone, glucagon, is secreted to increase blood sugar levels

Liver Is considered the “gatekeeper” to the blood as it regulates blood composition in several ways

Blood leaving the digestive tract first goes to the liver via the hepatic portal vein Blood leaves the liver via the hepatic vein

Liver functions: Contributes to digestion by producing bile (helps break down fats) Stores excess glucose as glycogen Converts glycogen to glucose when needed Stores iron and some vitamins Removes and breaks down toxins from the blood Regulates blood cholesterol levels

Synthesizes blood proteins Produces urea from amino acid breakdown (which is later excreted by the kidneys) Breaks down hemoglobin from old red blood cells; components of hemoglobin breakdown are excreted in bile, giving it its green colour

Gall Bladder Attached to the liver Stores bile and secretes it into the duodenum via the common bile duct when fats are present

Appendix Not really an accessory organ since it has no apparent digestive function Found at the end of the cecum (where the small and large intestine join) Thought to have functions associated with the immune system