Biology 3A - Student Resources

Mouth

Teeth bite off and chew food into a soft pulp that is easy to swallow. Chewing mixes the food with watery saliva, from 6 salivary glands around the mouth and face, to make it moist and slippery.

Amylase is an enzyme that breaks down starch into sugars

Pharynx - a cavity that connects the mouth to the esophagus, it also connects the nose and mouth (via the larynx) with the trachea

Esophagus

The esophageus, or gullet, is a muscular tube. It takes food from the throat and pushes it down through the neck, and into the stomach. It moves food by waves of muscle contraction called peristalsis.

Stomach

The stomach has thick muscles in its wall. These contract to mash the food into a sloppy soup. Also the stomach lining produces strong digestive juices. These attack the food in a chemical way, breaking down and dissolving its nutrients.

Two valves control the entrances of food into and out of the stomach

Pyloric Sphincter - opening to the small intestine
Esophageal Spincter - opening between esophagus and stomach. It stops the acid in the stomach from flowing back up (reflux) into the esophagus. If there is damage or weakness to this valve, stomach contents, including hydrochloric acid, flow up into the esophagus and cause injury to the lining of the esophagus.Ê This causes pain, commonly called "heartburn" or "acid reflux".

A layer of mucus prevents the stomach from digesting itself.

Ulcers

When the mucus barrier of the stomach breaks down, the acids in the stomach erode a hole in the stomach, causing an ulcer. Causes of this problem vary, smoking and stress may be related to the development of ulcers. Recently it was discovered that a bacteria ( Heliobacter pylori ) was present in many people with ulcers. Treatment with antibiotics has eliminated the occurance of ulcers in many people.

Food in the stomach is converted into a thick acidic liquid called chyme, which then moves into the small intestine (via paristaltic contractions)

Some substances are absorbed directly into the blood stream from the stomach - alcohol and water.

Small Intestines This part of the tract is narrow, but very long - about 20 feet. Here, more enzymes continue the chemical attack on the food. Finally the nutrients are small enough to pass through the lining of the small intestine, and into the blood. They are carried away to the liver and other body parts to be processed, stored and distributed. The small intestine consists of minute fingerlike projects called villi, which increase the surface area of the small intestines. Nutrients are absorbed across the villi and into the blood stream (via capillaries) as chyme travels down the small intestine.
	Pancreas The pancreas, like the stomach, makes powerful digestive juices called enzymes which help to digest food further as it enters the small intestines. It is involved in blood sugar regulation, as it secretes the hormon insulin which is involved in the breakdown of sugars
Gall Bladder This small baglike part is tucked under the liver. It stores a fluid called bile, which is made in the liver. As food from a meal arrives in the small intestine, bile flows from the gall bladder along the bile duct into the intestine. It helps to digest fatty foods and also contains wastes for removal. Liver The liver secretes a substance called bile, which is essential for digestion. Bile is stored in the gall bladder and released into the small intestine via the common bile duct. Also, blood from the intestines flows to the liver, carrying nutrients, vitamins and minerals, and other products from digestion. The liver is like a food-processing factory with more than 200 different jobs. It stores some nutrients, changes them from one form to another, and releases them into the blood acccording to the activities and needs of the body. It also serves to detoxify blood of harmful substanced like alcohol and nicotine. Ê
	Large Intestine Any useful substances in the leftovers, such as spare water and body minerals, are absorbed through the walls of the large intestine, back into the blood. The remains are formed into brown, semi-solid feces, ready to be removed from the body. The large intestine has two parts: the colon and the rectum (the final last 15cm). The large intestine is home to bacteria that live on unabsorbed nutrients, these bacteria do not harm us, and in fact help us by synthesizing important vitamins. Waste is transported to the rectum, explansion of this chamber induces the urge to defecate.