The Human Digestive System

By Rene Fester Kratz from Biology Workbook For Dummies

Humans have a complete digestive tract: Food enters at one end and wastes exit from the opposite end. Digestion begins in the mouth and continues as food moves through your system:

Digestion in the mouth occurs by both chemical and mechanical means.
- Chewing, or mastication, mechanically breaks food into smaller pieces.
- Your taste buds stimulate the production of saliva to help moisten the food, physically preparing it for you to swallow.
- Saliva contains the enzyme salivary amylase, which chemically digests the complex carbohydrate starch into simple sugars (glucose).
Your tongue pushes the chewed food to the back of your throat toward your pharynx, the muscular chamber at the back of your throat. As you swallow, your palate raises until it’s pressed up against the wall of your pharynx, preventing food from entering your nasal cavity (unless someone makes you laugh while you’re swallowing!). Your epiglottis moves to cover the opening to your trachea, preventing food from “going down the wrong tube” into your respiratory tract.
Your muscles squeeze, in a process called peristalsis, the food mass, or bolus, into your esophagus, the tube that connects your mouth to your stomach.
In the stomach, peristalsis continues and gastric juices chemically digest the food to a thick liquid called chyme. Gastric juice is extremely acidic, with a pH range between 1 and 4, and contains the enzyme pepsin, which breaks proteins into smaller chains of amino acids.
Chyme passes through the pyloric valve, the gate between your stomach and small intestine, and into your small intestine. Your pyloric sphincter muscle occasionally opens the valve, allowing your stomach’s contents into your small intestine a little bit at a time.

Food arrives at your small intestine between one and four hours after you eat. After food molecules hit your small intestine, your liver and pancreas break them down into even smaller units:
- Your liver is the largest gland in your body. It’s a large, lobed structure that wraps around the gallbladder, a small, pear-shaped structure. The liver secretes diluted bile into the gallbladder, which stores and concentrates the bile, and then releases it into the small intestine.
  
  Bile helps to emulsify fats so they’re suspended in water and you can digest them more easily. (If you’ve ever shaken up a bottle of salad dressing and forced oil to break up into small droplets that mix with the water portion of the dressing, you have first-hand experience with emulsification.)
- Your pancreas has an irregular, almost triangular shape that begins with a larger end near the junction between the stomach and small intestine. Your pancreas releases pancreatic juice into your small intestines, contributing a mix of digestive enzymes to help chemically digest food molecules: Lipase breaks apart fat molecules, pancreatic amylase breaks apart long carbohydrates, and the enzymes trypsin and chymotrypsin break apart peptide fragments.

Don’t let the word small fool you. The small intestine is much longer than the large intestine (over 20 feet long versus about 5 feet long). The term small intestine refers to the fact that this part of the intestines is narrower in diameter than the large intestine; the large intestine is wider in diameter but shorter in length.

Your small intestine is the primary site of absorption of small food molecules into your cells. Your body absorbs the nutrients it can use into the cytoplasm of the cells lining your small intestine.
The rest of the material that you can’t further digest or use passes on to the large intestine, or colon. The large intestine absorbs water back into your body, concentrating the waste material into feces. Feces pass through your rectum and leave your body through the anus. A small, worm-like appendage called the appendix dangles off one part of your colon. For a long time, scientists thought the appendix had no function, but recent research suggests that it plays a role in immunity.