Digestive System

 

The digestive system is a varied collection of organs and tissues, which participate in some way to digestion and absorption of food.

 

Organs that make up the digestive system include:

 

  

The organs of the alimentary canal

• Mouth

• Oesophagus

•  Stomach

• Duodenum (small intestine)

• Iliem (small intestine)

• Colon

• Rectum and Anus

 

Accessory organs

• Salivary glands

• Pancreas

• Liver

• Gallbladder

 

The function of the digestive system

 

Food is vital for life because it is the source of energy that drives the chemical reactions in cells, and provides matter that is used to form new tissue or to repair damaged tissue.

 

Food, as it is consumed, is not in a state suitable for use as an energy source by any cell. It must be broken down into molecules small enough to be transported through the cell (plasma) membranes. The process that allows this to happen is called digestion.

 

Digestion

 

Chemical digestion is a series of catabolic reactions that break down large carbohydrate, protein and lipid (fats) molecules into smaller molecules that are absorbable and usable by the cells. These molecules need to small enough to pass through the epithelial cells of the wall of the small intestine (iliem), into the blood capillaries and eventually into the cells.

 

Mechanical digestion is the process of preparing the food with our teeth and tongue to form a bolus, which can be swallowed and transferred down the oesophagus by a process called peristalsis. This involves combined muscle contractions and relaxation, which causes the bolus of food to move in the right direction to the stomach. 

 

Mechanical digestion also involves the churning of the food, mixed with juices that contain enzymes, that digest the food in the stomach and small intestine (duodenum).

 

Absorption

This process involves the passage of digested food from the alimentary canal, into the cardiovascular and lymphatic systems for distribution to the cells.

 

The Mouth 

Ingestion of food occurs at the mouth, where food is mixed with the enzyme salivary amylase, which begins the process of digestion by starting the breakdown of carbohydrates (sugars and starches). 

 

Food is chewed by the teeth and tongue and rolled into a small ball called a bolus, ready to be swallowed.

 

The Oesophagus

Structure

The oesophagus is a muscular tube that lies behind the trachea and is about 23 to 25 cm long.

 

It begins at the end of the laryngopharynx, passes down in front of the spinal column, through the diaphragm to the stomach

 

Function

Food is swallowed and passes through the pharynx and into the oesophagus, where it passes to the stomach by a process called peristalsis. There are no enzymes in the oesophagus, but the enzymes present in the saliva, which has been mixed with the food, continue to slowly digest the carbohydrates.

 

The Stomach

Structure

The stomach is the widest part of the alimentary canal, and food can be stored in here for 1 - 4 hours. It is J shaped and lies directly under the diaphragm, on the left side of the body.

 

The pH of the stomach is acidic (2), because of the hydrochloric acid that is secreted by the glands in the stomach wall.

 

The stomach has sphincters (valves) at either end, to make sure the food moves in the right direction. The sphincter at the top of the stomach, which separates it from the oesophagus, is however quite weak and the chyme can be regurgitated back into the oesophagus.

 

Function

Food in the stomach is mixed with gastric juices, which contain enzymes that start to break down proteins, and hydrochloric acid that kills most of the bacteria in the food. 

 

Very little digestion occurs in the stomach; its main function being to churn food into chyme and destroy bacteria. 

 

The Duodenum

Structure

This C shaped structure makes up the first part of the small intestine and is about 25 cm long. It is connected to the stomach and continues as the rest of the small intestine.

 

Function

It is in here that most of the digestion occurs. Intestinal glands secrete intestinal juice, which contains enzymes, which continue to break down protein and carbohydrates. 

 

In adults, almost all digestion of lipids occurs in the small intestine. Before lipids can be broken it is necessary to increase their surface area, so the enzyme can get to it more. This process is called emulsification and requires bile. 

 

Bile is produced in the liver and stored in the gallbladder. When required it is released into the duodenum via the bile duct.

 

Digestion in the duodenum is also supported by the pancreas. Pancreatic juice, that contains enzymes, is secreted into the duodenum to help break down the proteins, lipids and carbohydrates.

 

The Ilium 

The ilium is the part of the small intestine where absorption occurs. Its structure has been adapted to allow this process to occur more efficiently. 

• Long length 

• Folded interior

• Modified epithelial cells with microvilli, to increase the surface area

The extensive capillary network which drains the products of proteins and carbohydrates into the hepatic portal vein, where they are transported to the liver in the blood.

 

The Colon

This is the large intestine, and it is here that any water left that hasn't already been absorbed is absorbed back into the body.