Digestion

Digestion plays a major role in the transformation and assimilation of food, and includes all chemical and mechanical processes that occur inside the body: the synthesis of food through chemical reactions (anabolism) and the breakdown of food by a chemical process (catabolism, basal metabolism, additional metabolism and other more or less complex mechanisms).

The role of the digestive system is to provide cells with the substances they need, and to provide energy, water and essential vitamins to the body in a form that can be easily assimilated.

It is made up of hollow organs that follow one another, and is commanded both by the autonomic system and by a conscious command: the act of eating, for instance, is made necessary by hunger and habit, and is also a source of pleasure for the senses of taste and smell.

Before dealing with the main functions of the digestive system, it is important to give a proper definition of the term NUTRIENT, a basic element referred to throughout this blog.

Nutrients are substances that can be incorporated as such into the nourishment of living cells, (carbon, trace elements, etc.), or assimilated without prior digestion (glucose, amino acids, etc.).

Their function is to provide cells with basic matter (amino acids, simple lipids, glucides, etc.) to be used in the synthesis of their own substance.

Part of these nutrients will also be oxidised in the catabolism, notably in the mitochondria (part of the cell), to provide the chemical energy that is required for synthesis phenomena.

The digestive system

The digestive system includes the digestive tract and a certain number of annexed glands. The digestive tract, which measures from 5 to 8 metres, is located between the mouth and anus. It is continuous and segmented into areas that are morphologically and functionally different: the mouth, oesophagus, stomach, small intestine, large intestine (colon), rectum and anus. The void in this tube is called lumen. The substances that are located in the lumen are considered to be outside the body.

In order for these substances to find their way into the body, the substances must cross the intestinal epithelium and enter the blood flow. Digestion and absorption occur in the digestive tract, which is made up of muscle fibres. The myogenic activity that takes place is controlled by the nervous system, and its role is to modulate contractions according to stimuli from the nerves and hormones.

The different actions of the digestive system can be summed up as follows:

  • Digestion of food: decomposition carried out by specific enzymes.
  • Absorption: molecules travel through the intestinal epithelium.
  • Secretion: of hormones and mucus.
  • Motility: for mixing and moving food along the different parts.Division

    Activities which occur in different segments of the digestive tract:

     

The digestive tract starts in the oral cavity and continues to the pharynx (oral and laryngeal parts), the oesophagus, stomach, small intestine, caecum, colon, rectum and anal canal. Annexed glands are the salivary glands, the liver and the pancreas.

Functions

 The internal lining of the digestive tract is covered with epithelium which forms the digestive mucous with underlying tissue layers. Annexed to this tube are glands whose secretions enter the lumen.

The digestive system has two main functions:

  • Digestion: The majority of food groups are made up of molecules that are not used by the cells. These molecules are simply broken down by digestive enzymes, which reduce them to nutrients (simple components of living matter).
  • Absorption: The nutrients are absorbed by the digestive wall and into the interstitial liquid through blood and lymphatic circulation.

Digestive mechanisms

First Stage

Evaluation of the food, by using taste and smell.

Second Stage

  • Buccal Stage
    • Crushing and dispersion of food by mastication.
    • Saturation and partial dissolution of food in water.
  • Pharyngeal and Oesophageal Stages: Transport of the alimentary bolus.

Third Stage

Digestion and Transformation Stages

Food is transformed into nutrients by digestive enzymes:

  • Absorption of nutrients into the circulation through the digestive mucous membrane.
  • Elimination of indigestible residue.

Particularities of the Small Intestine

The small intestine is the longest and most folded section of the digestive tract, and is subdivided into three different areas:

  • Duodenum: the first 25 cm after the stomach
  • Jejunum: approximately 2.5 m
  • Ileum: approximately 3.5 m

The wall of the small intestine is made up of different levels of folds that allow its surface to be increased and absorption maximized.

  1. Plicae circulares: permanent circular folds that cover the entire surface of the wall and multiply its surface by 3.
  2. Villi: finger like projections which multiply the surface by 10.
  3. Microvilli: membrane projections of the epithelial cells which multiply the surface by 20.

Due to these different fold structures, the absorption surface is multiplied 600 times.

Anatomy and functional organisation

Structural modifications of the small intestine.

a) Several circular folds (plicae circulares), seen on the inner surface of the small intestine.

b) Enlargement of one villus extension of the circular fold.

c) Enlargement of an absorptive cell to show microvilli.

Particularities of the colon

The large intestine is subdivided into four sections before it reaches the rectum: the ascending colon starts from the ileum (last part of the small intestine) and climbs up towards the lower part of the pancreas and stomach. The transverse colon goes from the right side of the abdomen to the left. The descending colon reaches down to the lower part of the abdomen where it forms a small S-shaped part of the intestine called the sigmoid colon. It is followed by the rectum, whose main role is to store feces, and the anus, which is composed of an internal and an external sphincter. Seen from the outside, the colon shows permanent bumps.

The large intestine. A section of the cecum is removed to show the ileocecal valve.

Particularities of nervous control along the digestive tract

The digestive tract is controlled by nervous signals from the brain (autonomic nervous system).

However, the digestive system has its own intrinsic nervous control system.

Indeed, two “nervous centers”, called plexuses, are located in the intestinal wall. These nervous centers can receive messages and transmit stimuli to other areas along the tract. This characteristic enables the digestive tract to control its movements more precisely, and synchronize these movements according to the different stages of digestion.

This leads us to the basic principle behind all the activities of the digestive system: any individual segment must not receive substances to digest if it has not finished digesting the substances that it already contains, and if it is not ready to receive new substances.

In other words, the movement of food must be synchronized with the system’s capacity to digest and absorb.

Digestive Absorption

Chemical Principle

Large molecules are broken down into small molecules which are absorbed more easily. This breaking down is done by rupturing chemical bonds and requires the presence of water.

 

Digestive Enzymes

Digestive enzymes come from the salivary glands, stomach, pancreas and small intestine.

  • Saccharidases: break down saccharides (sugars).
  • Peptidases: break down proteins.
  • Lipases: break down lipids and fats.

Absorption

Absorption per se concerns the passing of the components of broken down molecules through the intestinal epithelium and into the blood (carbohydrates and amino acids) and lymph (lipids). Different mechanisms allow these substances to pass through; some require energy, others not.