Digestive system

Introduction

Humans obtain their energy from the food and drink they consume. To be able to use these consumed nutrients, our body has to break them down into smaller molecules. These smaller molecules are absorbed into the blood and carried to different cells throughout our body. Food nutrients are changed into smaller particles in a process called digestion. This chapter looks at the human digestive system.

Food movement through the system

The organs of our digestive system move nutrients through with their wall muscles. The typical movement of the oesophagus, stomach and intestine is called peristalsis. Peristalsis is the rippling motion of muscles in the digestive tract. In our stomach this motion mixes food with gastric juices, turning it into a liquid that is more easily absorbed. The first major muscle movement occurs when food or liquid is swallowed. See image 1.

Main components of the digestive system

The mouth

Food enters the digestive system through our mouth. The mouth is an oral cavity lined with a mucous membrane. First our food is physically processed (bitten off and chewed) by teeth and then chemically processed by saliva.

Swallowing

Swallowing involves the coordinated activity of many organs like the tongue, palate, throat and oesophagus. The first stage of swallowing can be controlled, but once food is inside our throat its further movement is controlled by our peripheral nervous system.

The oesophagus

The oesophagus is the organ into which the swallowed food is pushed. It connects the throat above with the stomach below.

The stomach

The food then enters the stomach, which has three mechanical tasks to perform. First, the stomach must store the swallowed food and liquid. This requires the muscle of the upper part of the stomach to relax and accept large volumes of swallowed material. The second job is to mix the food, liquid, and digestive juice produced by the stomach. The lower part of the stomach mixes these materials by its muscle action. The third task of the stomach is to empty its contents slowly into the small intestine.

The intestines

All of the digested nutrients are absorbed through the intestinal walls. The waste products of this process include undigested parts of the food. These waste products move into the colon, where they remain for a couple of days until they are expelled from our bodies, in the form of faeces by our bowel movement.

In the process of evolution, our intestines became very long in order to allow time to absorb as many food nutrients as possible.

Production of digestive juices

In the process of digestion, different food and drink compounds are changed into smaller and simpler molecules by our digestive juices. These juices are made of enzymes that act as chemical catalysts of biochemical reactions of our bodies. These chemically-reactive solutions are sometimes referred to as 'juices' because they are liquid. Even though it is commonly used, the term 'digestive juice' is not a scientific term.

Our bodies have several glands that produce digestive juices. The glands that act first are in our mouth. They are called the salivary glands. Saliva produced by these glands contains an enzyme that begins to break down a nutrient's polymers into smaller molecules.

The next set of digestive glands is in the stomach lining. They produce stomach acid and an enzyme that digests protein. As our food moves into the intestines, the juices of two other digestive organs mix with the food to continue the process of digestion. One of these organs is the pancreas. It produces a juice that contains a wide range of enzymes for breaking down carbohydrates, fats and proteins in food. Other enzymes that are active in the process come from glands in the wall of our intestines.

Our liver produces digestive juice called bile. Bile is a watery, greenish fluid which is secreted via the hepatic duct and cystic duct to the gall bladder. Between meals, bile is stored in the gallbladder. It contains bile salts, bile pigments and cholesterol. It is believed that high levels of fat in our food lead to collection of cholesterol in the arteries, possibly leading to serious health risks. When we eat, bile is squeezed out of the gallbladder into the bile ducts. Later it reaches the intestine where it mixes with the fat in our food. The bile acids change food fat into the watery contents of the intestine. After the fat is dissolved, it is digested by enzymes from the pancreas and the lining of the intestine.

Transport of nutrients

Most absorbed materials cross the mucosa (the mucous membrane that lines our digestive organs) and are carried off by our bloodstream to other parts of the body. These digested nutrients might be stored or further chemically changed.