In this chapter... The digestive system prepares the food we eat to be used by cells. Food contains energy that powers the cell, but food molecules must be broken up in order to get to cells. The mouth and stomach physically chop and churn food, then enzymes break it down on the molecular level. Plant-based foods contain a lot of carbohydrates, while animal-based foods contain many of the proteins we need. Bacteria and archaea in our large intestine also help with digestion. Getting food across the cells lining the intestines and into the bloodstream requires several types of membrane transport.

The digestive system has one main job – digest food. That means that it breaks the food we consume into pieces that can dissolve in the blood to be carried to cells. This process happens over many steps through a long series of tubes – about 10 meters altogether – which work like an assembly line in reverse. It can take up to a few days for the food to complete its journey.

Why this is important…Digestive system “issues” are a common medical complaint and can range from mild to life-threatening. All energy that the body uses comes through food, so if the digestive system is having trouble, it can easily affect every system in the body.

Foundations

The digestive system is composed of a long system of muscle-lined tubes that run through the body.

Diagram of human digestive system. Major organs are labeled.

FIGURE 5-1. Human digestive system. Overview of major organs of the digestive system. ("Digestive system" by BruceBlaus is licensed under CC BY 3.0)

Each digestive organ makes its own contribution to the digestive process.

Mouth – physically break food into small pieces and start chemically breaking down starches
Stomach – liquefies food by churning and strong acids, and chemical digestion of proteins
Small intestine – finishes chemical digestion of all major nutrients (biomolecules) and absorbs most of the food molecules into the bloodstream
Large intestine (colon) – bacteria help break down more chemicals like fiber; it also absorbs lots of water and several nutrients

Glands help by secreting digestive enzymes to help break down food molecules efficiently and in specific ways.

Salivary glands – secrete amylase to digest starches in the mouth
Pancreas – secretes a variety of enzymes to digest nutrients in the small intestine
Liver – produces bile acids that break up fats in the small intestine
Gall bladder – stores bile acids from the liver

Details

As we take a bite of food, we taste it to evaluate its qualities. At least 80% of what we “taste” is actually smell. Taste, as sensed by the tongue, is limited to just 5 chemical signatures.

Sweet signals that the food is a good source of easily-digestible calories in sugar.
Umami (savory) signals that it is a good source of proteins.
Salty tells us sodium is present, which is a critical mineral for normal body function.
Sour and bitter both give us potential warning signals about potential contamination or toxins.

If our sense of taste tells us the food is worth eating, we chew and swallow it. This is the beginning of the digestive process. Digestion involves grinding, churning and chemically breaking down food into molecules that can dissolve in the bloodstream.

Where does all the food go once it is digested? A common misconception is that most of it becomes feces – not true. We can’t extract energy from food and leave the empty shells of it to exit our bodies. For energy to get to cells, food molecules must go into the bloodstream to be distributed to cells of the body. Food molecules are broken down to extract energy, and in the process, forms carbon dioxide. So, most of our food leaves the body in our breath. The material that leaves the digestive system as feces is a mix of bacteria, undigestible parts of food, and waste material that the body has dumped into the digestive tract.

Learning objectives

Name or identify the major organs of the digestive system
Describe the main function of the digestive system