Chapter 38

Functions of the Digestive System To ingest food Digest food Force food along digestive tract Absorbs nutrients from the digested food Eliminates undigested foods from your body

The Mouth In the mouth the tongue and teeth begin the first part of food digestion by chewing. Chewing is a part of mechanical digestion. Mechanical Digestion: the physical process of breaking food into smaller pieces Mechanical digestion increases the surface are of food particles which allows chemical digestion to be more effective. Chemical Digestion: process of structurally changing food molecules through action of enzymes

The Mouth Cont’d As you chew your salivary glands release saliva, which is a mixture of water and the enzyme amylase. Amylase breaks down the starch in food which are generally in polysaccharide form into disaccharides. Although swallowing happens quickly amylase still acts on starches for 30 minutes inside the stomach.

Swallowing Food Swallowing is a voluntary action in which your tongue helps push the food from the mouth to the esophagus. Esophagus: muscular tube that connects the mouth to the stomach Food gets moved through the esophagus very quickly (5-8 sec) by peristalsis. Peristalsis: involuntary muscle contractions along the walls of the digestive tract.

Swallowing Food Cont’d Have you ever had food “go down the wrong tube?” As you swallow food passes over the windpipe which leads to your lungs, the epiglottis temporarily closes when swallowing preventing food from going down the wrong direction. Breathing is temporarily stopped but if you talk or laugh while swallowing food can get into the wrong place. Epiglottis: flap of skin covering the windpipe during swallowing

The Stomach Food enters the stomach through a valve at the bottom of the esophagus. Stomach: is a muscular, pouch-like enlargement of the digestive tract. Made up of three layers of muscles lying across one another. When these muscles contract they churn or mix the food inside your stomach, another form of mechanical digestion.

Stomach Cont’d Chemical digestion also occurs in the stomach, the wall of the stomach is full of glands that secrete gastric juice. Gastric Juice: combination of Hydrochloric acid and Pepsin. Pepsin: digestive enzyme that begins the chemical digestion of proteins, only works in acidic environments. In such an acidic environment why doesn’t the stomach digest itself?

Stomach Cont’d To fight against the acidic environment the stomach secretes a mucus that helps line the stomach and create a protective layer. Ulcers form from holes in this protective layer. Food remains in the stomach for 2-4 hours until it is passed into the small intestine. When it finally passes food is in a liquid form with the consistency of tomato soup.

The Small Intestine Small intestine: muscular tube about 6m long and 2.5cm in diameter. Digestion is completed inside the small intestine. Mechanical and Chemical digestion take place on proteins and carbohydrates. Duodenum: is the first 25 cm of the small intestine The wall of the duodenum secrete enzymes that chemically breakdown food but most of them enter through a duct from the pancreas and liver. Both of these organs play major roles in digestion although food never passes through them.

Small Intestine Cont’d Pancreas: is a soft, flattened gland that secretes both digestive enzymes and hormones. Enzymes from the pancreas break down carbohydrates, fats, and proteins Also sodium hydrogen carbonate is secreted which makes the pancreatic juice alkaline. Meaning it neutralizes the acidic stomach acid stopping pepsin from continuing to work.

Small Intestine Cont’d Liver: is a complex gland that secretes bile. Bile: is a chemical that breaks fats into small droplets and also neutralizes stomach acids. Gallbladder: stores bile made from the liver. Liquid food will stay in the small intestine for 3-5 hours allowing for absorption. Food is slowly moved along by peristalsis. This pushes food over millions of villi. Villus: projection on the lining of the small intestine that helps aid in the absorption of nutrients.

The Large Intestine The undigested food will pass into the large intestine. Large Intestine: a muscular tube also referred to as the colon. 1.5 m long and 6.5 cm in diameter Water is absorbed through the walls of the large intestine from the undigested food, leaving behind a more solid material. Anaerobic bacteria residing in the large intestine will digest most of the material and synthesize some vitamin B and K needed by the body.

Large Intestine Cont’d After 18 to 24 hours in the large intestine, the remaining undigested food will have reached the rectum in the form of feces. Rectum: The last section of the digestive system. Feces is eliminated from the rectum through the anus. The entire process from beginning of digestion to end takes hours.

Vocabulary Words Mechanical Digestion Chemical Digestion Salivary Glands Saliva Amylase Esophagus Peristalsis Epiglottis Stomach Gastric Juice Pepsin Small Intestine Duodenum Liver Pancreas Bile Gall bladder Villus Large intestine Rectum