How the Digestive System Works

The goal of digestion is to break down food into its smallest chemical components, called molecules. Different food types have different building blocks. For example, carbohydrates are broken down into sugar molecules, proteins become amino acids and small peptides, and fats (or lipids) are broken down into fatty acids.

The breakdown of food particles into molecular nutrients that the body can use is a complex process. All food particles travel the same route through the gastrointestinal tract, but different locations and specific enzymes are designated to break down the various food types. Let’s review the overall process of digestion.


Chewing

Food is ground into small bits by the teeth in your mouth. Chewing increases the surface area of the food and reduces food particles into smaller parts, making them easier to swallow. The food is softened with saliva that is excreted from salivary glands in your mouth water and mucus to help you swallow. Saliva contains enzymes that start breaking down carbohydrates in your mouth. The lump of chewed, softened food is brought to the back of your tongue for the next step.

Swallowing

Your throat muscles contract to force the chewed, softened food out of your mouth and into your pharynx. Swallowing is initially an active, voluntary action with an additional reflex response to clear the esophagus of food. From your pharynx, the food then travels down your esophagus and into your stomach. Smooth muscles surrounding your esophagus push the food to your stomach by a process called peristalsis. Your body performs peristalsis automatically without any conscious effort on your part.

Digestion

Your stomach secretes gastric acid and digestive enzymes that begin the digestion of proteins. The food is churned with the acids and digestive enzymes until it is a semi-liquid form, called chyme. Your stomach then acts as a storage area for this chyme, releasing it in small amounts to your small intestine.

As food enters your small intestine, carbohydrate, protein, and fat-digesting enzymes from your pancreas, and bile from your liver and gallbladder are secreted into your small intestine through little tubes called ducts. 

Absorption

Once broken down into their smallest parts, simple sugars, amino acids, and small peptides (2-3 amino acids together) move across the walls of your small intestine in a process called absorption. Fat molecules cannot dissolve in water, but with the help of bile, they are hidden in little globules to be delivered to the wall of the small intestine and then move across the wall into the lymph system. The absorbed nutrients are then transferred to tiny blood vessels, called capillaries, lining your small intestine, and are carried through your bloodstream to your body’s cells.

Ridding the Waste

Whatever is not absorbed by your small intestine, like dietary fiber, is carried to your large intestine. This organ compresses the waste products it receives from your stomach and small intestine and removes a lot of the water from this material.

When fecal matter collects in the rectum, the anal muscles contract and you feel the sensation that you need to use the bathroom and defecate. This process is extremely important for the health of your body. Fecal matter that remains in your body too long is toxic and can be deadly if it gets into the bloodstream.