Digestive system

How does the body convert food into energy?

The human body needs energy, to be able to perform all the vital functions to survive. The digestive or gastrointestinal system converts the food that we eat into a form that can be processed and used as energy for all the activities carried out by the body. The type of foods that we ingest has an effect on the way that it is processed in the body and the amount of energy that is produced.

As food passes through the digestive tract it is broken down, both physically and chemically into a form that can be absorbed into the bloodstream and used by the body for energy. Those particles that are unable to be digested are excreted in the form of faeces.

Structure

There are two main groups of organs and these are:

Gastrointestinal tract (GIT) (also called the alimentary canal ) extends from the mouth to the anus.
Accessory structures which assist in the mechanical and chemical breakdown of food - these include teeth, tongue and glands lining the GIT.

Activity - Digestive system

Explore the female and male digestive systems.

Function

The Digestive System performs the following vital activities:

Ingestion which involves the taking in of food.
Digestion is controlled by the autonomic nervous system and may take two forms:
- mechanical breakdown of food by chewing and the action of muscles within the digestive tract. Food moves along the GIT by a wave like motion called peristalsis that breaks down the food mechanically.
- chemical breakdown of food by enzymes produced at various stages of the digestive tract as well as bacteria. In the duodenum, food is acted upon by bile that is secreted by the gall bladder, juices from the pancreas and secretions from the wall of the duodenum. Fats are changed to fatty acids and glycerol, carbohydrates to simple sugars and proteins to amino acids.
Absorption is where substances pass through the walls of parts of the digestive tract into the bloodstream. 90 % of the absorption of all nutrients takes place in the ileum of the small intestine through small projections called villi. The other 10 % take place in the stomach and large intestines.

Elimination is the process by which undigested foods leave the body. Any undigested food and water moves into the colon. Movement is slow in this section and it is anywhere from 16-24 hours before waste is evacuated. This evacuation is a reflex action and is called defecation. It occurs as the rectal sphincter responds to a filling of the colon and can be voluntarily inhibited by keeping the external sphincter contracted.

Activity - Structure and function

Process of digestion

The digestive system is designed to turn food that is eaten into nutrients, which the body uses for energy, growth and cell repair.

The processes of digestion include six activities:

ingestion
propulsion
mechanical or physical digestion
chemical digestion
absorption, and
defecation.

Mouth

The process of digestion begins in the mouth where food is chewed until it reaches a consistency whereby it can be swallowed. The following accessory structures aid this early stage of digestion:

Tongue — a muscle that is covered by tastebuds. It assists the process of chewing and manoeuvres food to a position where it can be swallowed easily.
Salivary glands — begin the process of chemical digestion through the secretion of the enzyme, salivary amylase. This enzyme begins the process of breaking down carbohydrates. Saliva also moistens food which helps it to be swallowed more easily.
Teeth — break food down mechanically into smaller particles that may be ingested more easily.
Pharynx — allows the passage of both food and air.
Oesophagus — tube that leads to the stomach.

Oesophagus

The oesophagus is the tube or gullet connecting the mouth to the stomach. It lies in front of the vertebral column and behind the trachea (breathing tube) and heart.

Stomach

Food remains in the stomach for 3 to 4 hours. During this time it is further broken down by the muscular churning action of the stomach. Powerful gastric juices that contain enzymes are secreted by the cells of the stomach, contributing to chemical digestion. The food ends up in a semi-liquid form that is called chyme.

Intestines

Digested material is moved through the intestine via a process of wavelike muscular contractions called peristalsis. The process of digestion is completed in the small intestine. At this stage the nutrients that the body needs are absorbed through the walls of the small intestine (ileum).

The waste material then moves into the large intestine where water is reabsorbed; thereby changing it into a more solid form, ready to be excreted through the rectum.

Liver

The liver is a large gland that is divided into four lobes. It carries out many vital metabolic functions such as:

manufacturing bile which breaks down fats.
helping to maintain normal blood glucose levels.
producing the blood proteins, prothrombin and fibrinogen which have a role in blood clotting.
storing iron derived from food or the by-product of the breakdown of worn out red blood cells.
storing vitamins A, D, E & K, that have been extracted from food ingested.
heat production due to the high amount of metabolic activity; the liver is the main heat procuring organ of the body.

Gallbladder

The gallbladder concentrates and stores bile that is produced in the liver. It then releases the bile when it is needed for digestion after a fatty meal.

Pancreas

The pancreas is what is called both an endocrine and exocrine gland. It produces pancreatic juices, containing enzymes, which play an important role in the chemical digestion of food. The pancreas also produces the hormones insulin and glucagon directly into the bloodstream, which is termed an endocrine function.

Activity - Digestive system

Drag the labels to the correct place to test your understanding of the digestive system.

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