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How does your body make poop? Owen, 4, Wakefield, Massachusetts
Much of the food you eat is absorbed by your digestive system, which includes your stomach and your intestines.
But some of what you eat makes it all the way through those twists and turns and comes out the other end as poop. How does that happen?
Imagine you start your day by eating a bowl of crunchy cereal with milk. The process of digestion begins as you start to chew.
Your teeth grind up the cereal into smaller particles, making it easier to swallow and digest. Your saliva contains an enzyme, a kind of chemical, called amylase that starts breaking down the cereal on a molecular level.
I’m a doctor who regularly treats children and adults with digestive problems. Some of my patients have problems absorbing nutrients from their food and others poop too often or not often enough. When they describe their symptoms, I consider the process of how our bodies make poop and which steps can go wrong.
Table of Contents
Your stomach is full of enzymes and acid
Everything you eat contains three types of molecules that provide your body with the energy you need to live: carbohydrates, fats and proteins.
Amylase, an enzyme in saliva, begins breaking down the starches, a kind of carbohydrate, while the cereal is still in your mouth.
After you swallow, the milky cereal travels down your esophagus, a tube that carries swallowed food from your mouth to your stomach. That’s where digestion really gets going.
Your stomach contains hydrochloric acid, which breaks the food down into much smaller pieces. Over several hours, that acid and additional enzymes continue to pulverize the carbohydrates and protein from your bowl of cereal.
Veronika Zakharova/Science Photo Library via Getty Images
Your long and winding small intestine
Two or three hours later, your breakfast will leave your stomach and enter the small intestine, which is a long and coiled tube that is contained in your abdomen behind your belly button. By that point, the digestive process will have turned those big chunks of cereal into tiny particles that are small enough for your body to absorb.
By coursing through your bloodstream, these teeny particles will deliver energy and the building blocks for growth to the cells all over your body.
The small intestine is perfectly suited to perform the job of absorbing nutrients partly because it’s gigantic. Regardless of your height, it can be over 20 feet (6 meters) long, and its surface is covered with villi, tiny protrusions with a texture that resembles a shag carpet.
Those millions of villi create a huge amount of surface area, which is ideal for absorbing the nutrients in what you’ve eaten once it has been digested. The small intestine also contains many types of bacteria, which assist in breaking down the food particles.
The small intestine also produces more enzymes to help break down the carbohydrates in breads and pasta into simple sugars that are easily absorbed. As food enters into the small intestine, other organs also contribute their digestive juices to the mix.
The liver and gallbladder mix a greenish liquid called bile into the food.
Bile helps break down fats contained in food. Pancreatic enzymes help break down the carbohydrates, fats, proteins and the other nutrients in the food you eat.
Dmytro Lukyanets/iStock via Getty Images Plus
Your slow and short colon
The journey through your small intestine takes between two and six hours to complete. By this point, your bowl of cereal is unrecognizable. It has turned into chyme, a greenish liquid. Chyme gets its color from the bile made in the liver.
As the chyme reaches the end of the small intestine, it enters into your large intestine, also known as the colon. The large intestine gets its name due to being wider than the small intestine, even though it is much shorter.
The colon is about 5 feet (1.5 meters) long. Unlike the villi-lined small intestine, it doesn’t absorb any nutrients. Instead it does another important job: It absorbs water from the slimy green chyme your digestive system made from your breakfast. The small intestine also absorbs water into the bloodstream, where it is delivered to your kidneys to make urine.
So the intestines also play a small part in making your pee, as well as your poop.
This process is much slower than those earlier steps. It can take a whole day, and up to three days, to complete. By the time the chyme reaches the end of the colon, it has solidified and probably turned from green to brown.
The brown color of poop comes from the bile that is added by the liver to your bowl of cereal as it makes its way through the small intestine. The bile is changed by bacteria from green to brown. Without bile your poop would be a pale silver or clay color.
Lots of bacteria
What’s in your poop?
When it leaves your body, poop contains some leftover water, as well as undigested food such as plant fiber, as well as some dead intestinal cells. And, it may surprise you to learn, almost half of it, measured by weight, consists of bacteria.
Your intestines contain trillions of these bacteria, which help you digest what you eat. Unlike some other kinds of bacteria, they do not make you sick. The ones that come out as part of your poop give it that stinky smell.
Each part of your digestive system, from your mouth to your colon, plays an important role in extracting from what you eat the energy and water that your body needs. They all work together to help you absorb most of that energy and water, while eliminating what you do not need.
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