The human digestive system

The human digestive system

Food must be digested because the molecules are insoluble and too big to cross membranes and be asborbed into the blood. They're also polymers and must be converted to their monomers so they can be rebuilt into molecules needed by body cells.

Digestion and absorption occur in the gut, a long, hollow, muscular tube. It allows movement of its contents in one direction only. Each selection is specialised and performs certain steps in the processes of mechanical and chemical digestion and absorption. The food is propelled along the gut by peristalis (a wave of muscular contraction).

Functions of the gut

The human gut performs four main functions. These are:

• Ingestion: taking food into the body through the mouth
• Digestion: the breakdown of large insoluble molecules into soluble molecules that are small enough to be absorbed into the blood.  Mechanical digestion is the cutting and crushing by teeth and muscle contractions of the gut wall, increases the surface area over which enzymes can act. Chemical digestion is by the secretion of digestive enzymes. Bile and stomach acid contribute to chemical digestion.
• Absorption: the passage of molecules and ions through the gut wall into the blood
• Egestion: the elimination of waste not made by the body, including food that can't be digested, e.g. cellulose

Functions of parts of the digestive system

• Mouth - ingestion; digestion of starch
• Oesophagus - carriage of food to the stomach
• Stomach - digestion of protein
• Duodenum - digestion of carbohydrates, fats and proteins
• Ileum - digestion of carbohydrates, fats and proteins; Absorption of digested food
• Colon - absorption of water
• Rectum - storage of faeces
• Anus - egestion

The structure of the gut wall

The gut wall consists of four tissue layers surrounding a cavity, the lumen of the gut. The proportions of the different layers in the gut vary depending on the function.

The outermost layer, the serosa, is tough connective tissue protecting the gut wall. The gut moves while processing food and the serosa reduces friction with other abdominal organs.

The muscle compromises two layers in different directions, the inner circular muscles and the outer longitudinal muscles. They make coordinated waves of contractions, peristalsis. Behind the ball of food, circular muscles contract and longitudinal muscles relax, pushing the food along.

The submucosa is connective tissue containing blood and lymph vessels, which remove the absorbed products of digestion, and nerves that co-ordinate perstalsis.

The mucosa is the innermost layer and lines the gut wall. Its epithelium secretes mucus, lubricating and protecting the mucosa. In some regions of the gut it secretes digestive juices and in others, absorbs digested food.

Digestion

Absorption of nutrients by gut epithelial cells is only possible if the macromolecules are first digested into smaller molecules. Different enzymes digest different food molecules, more than one type is needed for the complete digestion of a particular food.

• Carbohydrates -polysacharrides are digested into disaccharides and then monosaccharides. Amylase hydrolyses starch…