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How Does Digestion Work?

Your digestive system has three main functions – digestion, absorption, and elimination.

Digestion: is the process by which the digestive system breaks down food into molecules that the body can use. Your digestive system has two ways to do this. With mechanical digestion, foods are physically broken apart into smaller pieces. Chewing is an example of mechanical digestion. Your body also uses chemical digestion to break large molecules into smaller ones that your body can use. Most of the chemicals involved in digestion are enzymes, substances that speed up chemical reactions.

Absorption: is the process by which nutrients pass through the lining of your digestive system into your blood. The blood then transports the nutrients throughout your body.

Elimination: is the process of removing materials that are not absorbed from the body as wastes through the excretory system.

Structure of the Digestive System

Digestion is an incredibly complex process that involves many organs and structures throughout your body, from your tongue to your liver. To get a better appreciation for this process, take a closer look at the specific functions of some of the main players involved.

Mouth

Mechanical digestion occurs in your mouth as your teeth tear your food and your tongue pushes it around. Chemical digestion also occurs, as enzymes in your saliva help to break down food.

Pharynx

The upper portion of the throat contains the junction between the digestive tract and the respiratory system, called the pharynx. As you swallow, a flap of tissue called the epiglottis seals off the trachea, preventing food and liquid from entering your lungs. Have you ever started coughing after taking a drink? Sometimes, the epiglottis doesn’t close soon enough and liquid drains into your trachea. Your body knows that air is to pass through the trachea, not food or liquid, so you cough to prevent liquid from entering the lungs.

Esophagus

The muscular tube that connects the pharynx to the stomach is the esophagus. Muscle contractions push the food through the esophagus and toward the stomach. Waves of muscle contractions called peristalsis continue to push food through the rest of the digestive system.

Stomach
The stomach is muscular pouch located in the abdomen. The stomach can hold about one gallon of food and liquid. Most mechanical and some chemical digestion take place in the stomach. A few hours after you eat, mechanical digestion in the stomach is complete. By that time most proteins have been chemically digested. Peristalsis moves the food into the small intestine, where it is now a thick liquid called chyme.

Small intestine
Most chemical digestion and absorption of nutrients takes place in the small intestine. The length of the small intestine is far from small—it measures around 20 feet long. However, it is only one-inch diameter. The lining of the small intestine is covered with millions of tiny fingerlike projections called villi. The villi absorb nutrient molecules and pass them onto cells for transport through the blood stream to the parts of the body that can use them.

Large intestine
The remaining undigested material from the small intestine is pushed into the large intestine. Here, waste material is compressed into a solid form and exits the body through a bowel movement.

Although food doesn’t pass through them directly, other organs play a role in digestion. The liver produces bile, a substance that breaks down fats for absorption. It also helps to break down waste products and to store nutrients. Bile is stored and released from the gallbladder into the small intestines. The pancreas secretes enzymes into the small intestine that complete the breakdown of carbohydrates, proteins, and fats.

The Process of Excretion

After your body has taken the nutrients it needs from the food you have eaten, it must get rid of the excess waste. Have you ever been on a road trip and felt the urge to “go?” If you had to wait to find a bathroom, you might have experienced lower abdominal cramps and a general feeling of discomfort. Your body had begun the process of excretion, collecting and removing wastes. Once these wastes are collected, your body wants to remove them. Holding them in for too long can cause wastes to be reabsorbed into the bloodstream through the colon. After all the work it did to collect them for removal, now you can see why the urge to “go” can be so strong!

Several organs in the body are involved in the collection and removal of waste.

The liver converts impurities and poisons in the body to less harmful substances. For example, the liver forms urea from a harmful waste product of protein breakdown. The lungs remove carbon dioxide and some water from the body. The skin uses sweat glands to excrete water and urea through perspiration.

You have two kidneys, both about the size of your fist, which filter urea and other wastes from the blood. The wastes are eliminated in urine, a watery fluid produced by the kidneys that contains urea and other wastes. Urine flows from the kidneys, through other organs of the urinary tract and out of the body.

How do your kidneys remove wastes from your body, but not substances that your body needs? Each kidney contains about a million nephrons, which are tiny filtering units that remove wastes and produce urine. The nephrons filter wastes in stages:

Stage 1—Both required materials and wastes are filtered from the blood. During this stage, blood enters the kidneys and flows through smaller and smaller blood vessels until it reaches a cluster of tiny blood vessels in a nephron called a glomerulus (gloh MUR yoo lus) for filtration.

Stage 2—Required materials are returned to the blood, and the wastes are eliminated from the body.

Kidneys that are damaged from injury, diabetes, uncontrolled high blood pressure, or other diseases may fail. When kidney failure occurs, the kidneys are unable to remove wastes and excess fluid. Kidney failure can be treated with dialysis, a process by which a machine filters wastes from the blood. The blood is carried through tubes to a machine and then is returned to the body. Less serious harm to the kidneys, such as kidney stones or a urinary tract infection that has spread to your kidneys, can also occur. Since the kidneys are located on the sides of the lower back, you might feel pain there. Contact your doctor immediately as prompt treatment can prevent infection from spreading to other parts of excretory system.