Smooth muscle tissue makes up much of the stomach. The muscles squeeze and relax repeatedly to mix the food and the gastric juices inside. These wavelike movements are called peristalsis (peri STAL sis).
Peristalsis also helps push the partially digested food out of the stomach and into the small intestine. The wavelike movements cause the food to be squirted out in small amounts—about three squirts a minute. At that rate, it can take anywhere from one to four hours for your meal to move into the next phase of digestion.
On to the small intestine
Okay, let’s talk about the small intestine—the site where your food is finally digested. Any remaining nutrients in the food are absorbed (or soaked up) through the lining of the small intestine and pass through the tiny blood vessels lining the intestinal walls.
When you think about your small intestine, imagine a 22-foot-long garden hose that’s about 1.5 inches wide. Now imagine neatly storing such a long hose in your yard. How would you do it? You’d probably coil it into many loops. And that’s pretty close to how the long hollow tube called the small intestine is stored inside your body.
No food particles can pass through your intestinal wall unless they are completely digested.
The liver, gall bladder, and pancreas get involved
Other digestive juices that help out in the small intestine are produced by organs not part of the digestive tract.
For example, your liver, the largest organ inside your body, rests on top of your intestines. Your liver makes chemicals that help digest food. One of the digestive juices your liver never stops producing is a thick, greenish-yellow fluid called bile. The bile moves from the liver into a small sac called the gall bladder, where it is stored, then emptied into the small intestine when needed.
Bile acts as a powerful detergent to break down fats in the small intestine. Then, a substance in the bile helps the small intestine absorb the fats.
The pancreas, a pinkish-yellow gland about 6-8 inches long, is connected to the top part of your small intestine. The pancreas produces and releases another digestive juice into the small intestine that helps break down starches, sugars, fats and proteins. Your pancreas also produces a substance that decreases the harshness of the digestive juices entering the small intestine from the stomach.
Your pancreas performs another very important task—it produces a hormone called insulin. While insulin has no direct role in digestion, it controls the level of sugar in your blood. When your pancreas does not produce enough insulin, the result is a disease known as diabetes.
Back to the digestive system
Even though the outside of your small intestine is very smooth, the inside is anything but. Bumpy or wavy in appearance, the lining of the small intestine consists of tissue that forms hills and valleys. The “hills” are tiny fingerlike bulges call villi (VILL eye; singular of VILL us). Hundreds of thousands of villi, each about 1 millimeter in height, line the small intestine. They create a greater surface area through which food can be absorbed.
But wait, there’s more. Each villus is covered by smaller bumps called microvilli. More than a thousand microvilli cover each villus, further increasing the surface area of the small intestine.
The process by which completely digested food is moved into the bloodstream is called absorption. Imagine you spilled some water on the floor and then placed a paper towel on top of it. The water would move from the floor to the towel by means of absorption. The towel would absorb, or soak up, the water.
You might think of the lining of the small intestine as acting somewhat like a paper towel, absorbing nutrients, minerals, water, digested carbohydrates, fats and proteins into the blood where they are carried to all parts of the body.
The large intestine
Once all the nutrients are absorbed from digested food in the small intestine, a very wet mixture of indigestible food material passes into the large intestine, also known as the colon. The large intestine is twice as wide as the small intestine—about three inches across—but is only about five to six feet long.
The food matter that reaches the large intestine consists mostly of water and dietary fiber, such as the tough plant fibers from vegetables and grains. The large intestine absorbs most of the water from this mixture. Then, during a six-hour period, it changes the indigestible food into solid waste which is eliminated from the body through the rectum and anus.
The entire process of digestion, from taking the first bite of food through absorption of its nutrients into the bloodstream, takes between four and five hours.
Putting it all together
To sum it all up, the digestive system consists of many organs working together to break down food into nutrients the body can use. It all starts at the mouth, and continues with the esophagus, the stomach, the small intestine and the large intestine, with help from the liver, gall bladder and pancreas along the way.
With such a finely tuned system, what could possibly go wrong?