Main->Readings->4th Grade Readings->Table of Contents->Part 2: How Food Is Digested
Vocabulary |
There are some things that must happen to food so it can reach the cells.
Why must it be so small? This is because your digestive system is like a tube that goes from your mouth to your anus. It can be anywhere from 6 to 9 meters long, though it is coiled up. There is no opening between your digestive system and the rest of your body. The only way food can get to your body is through the walls of your digestive system. To do that, it has to be a very small, as small as a molecule. A molecule (MOLL-uh-kyool) is a tiny piece of matter made of atoms joined together.
Most people think that all your digestion happens in the stomach. That is not true. Digestion starts in the mouth. Actually, it starts even before then! When you smell or think of food, your mouth starts producing saliva (sal-EYE-vuh). Saliva is not just water. It contains enzymes (EN-zimes), substances made by the body which help changes happen. The enzyme in your saliva starts to break down the starch in your food. The food must be crushed and mixed with the saliva, and this is where your teeth come in. They are great at grinding, cutting, and tearing food. The food is chewed and moistened until it is a soft wet ball called a bolus (BOWL-us), and then it is ready to go on.
The tongue pushes the bolus to the back of the throat and, without your having to think about it, you swallow. Swallowing closes the windpipe so the food will not go into your lungs, and pushes the food down your esophagus. The esophagus is a muscular tube that goes down your throat to your stomach. Every part of your digestive system is lined with muscles to keep the food moving. Food does not fall through your system. It is pushed.
The way the esophagus moves the food is interesting. Again, without you having to think about it, the muscles squeeze and relax in waves, squeezing the bolus down to the stomach. This movement is called peristalsis (pair-uh-STALL-siss).
There is a tight place at the top of the stomach which must relax to let the food in. This place is called a sphincter, and it is a ring of muscles which can open and close. Your stomach holds only 50-100 mL normally, but when you eat a meal it can hold as much as a liter of food. It can hold even more, but you will feel uncomfortable.
Inside the stomach, the lining releases water, hydrochloric (hi-droe-KLOR-ick) acid, and another enzyme. Hydrochloric acid kills germs and makes it possible for the enzyme to work. The enzyme breaks down protein. Meanwhile, the muscles lining your stomach squeeze and mix the food. The acid does not hurt the lining of the stomach because it is protected with mucus.
Very little digestion happens in the stomach. The food is only there for 1-2 hours, though it takes longer if your meal was high in fat. Gradually, the stomach releases food into the small intestine.
The small intestine is where most digestion happens. Though it is called "small," the small intestine is actually very big. It is just not as fat as the large intestine. It is very long, 3-6 meters long, but it is all coiled and bunched inside the body. In the small intestine, the starches and proteins finish digesting, and fats are digested in the last third of the small intestine. Juices from organs called the pancreas and the liver come in to help digest the food. The bile from the liver is very important in breaking down fat. Also, there are many bacteria in your small intestine. Some of them make vitamins.
The small intestine is also where nutrients pass into the rest of the body. It is lined with tiny bumps called villi. Through the skin of the villi, the nutrients can pass into the blood and into a fluid system in the body known as the lymph.
All that is left now is fiber, water, bile, cells from the lining of the intestine, and bacteria. This material, which cannot be digested, passes into the large intestine. The large intestine is shorter than the small intestine, but it is thicker. Inside the large intestine, the water is taken out and returned to the body. What is left is passed out of the body as waste, known as feces (FEE-seez).
This whole process of digestion might take as long as 24 hours. In other words, when you are eating supper, you are still digesting your breakfast.
Questions: For your first assignment of the week, answer these questions in complete sentences on a sheet of loose-leaf paper, with a proper header:
Notes: For your second assignment of the week, in your journal on the next clean page, write the vocabulary words from this section and their definitions.
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Go on to Part 3: The Food Pyramid
This page last modified February 20, 2003
Copyright ©2000 Delia Marshall Turner. All rights reserved.
Questions? Send me a note at dturner@haverford.org