The Digestive System
The Digestive System is the process of simplifying the food we eat into a form that is suitable for absorption into the body. Made possible by the physical and chemical breakdown of foods into substances that can pass into the bloodstream to be distributed round the body's cells.
The nutrient material produced from the digestive process is used to provide the raw materials for the manufacture of new cells (which is going on all the time), hormones and enzymes. Certain minerals such as salts can be absorbed directly into the circulation.
Foods - fats, carbohydrates and proteins and fluids are processed by the digestive organs and broken down so that nutrients can be absorbed and circulated around the body. Any food residue not digested is solidified and eliminated from the body in the form of faeces.
The digestive system is made up of the alimentary canal, some related organs and various digestive processes which take place in the canal to prepare the food we eat for absorption into the body.
Don't forget - Your Body Needs Food! and to get the food (nutrients) it needs the food has to be broken down and processed. Hence the importance of a healthy digestive system.
The alimentary canal or tract begins in the mouth and passes through the pharynx, oesophagus, stomach, small intestine, large intestine and finally the rectum and anal canal. It is basically a food processor about 9 metres (30 feet) long. The related organs mentioned include salivary glands, pancreas and liver and the biliary tract.
The digestive system has four main functions:
- Ingestion (taking in food)
- Digestion (the mechanical breakdown of food by chewing and
chemical breakdown by enzymes produced by the glands in the digestive system)
- Absorption (the substances produced from digested food passing into the blood and lymph capilliaries for circulation round the body.)
- Elimination ( food substances not digested and absorbed but excreted as faeces)
As we have already said, everything is interrelated so there is also the need for a good circulation system alongside a good digestive system.
Digestion has two stages - mastication (chewing) and chemical (by enzymes present in the glands of the digestive system.)
Mastication is important and food should be chewed properly and thoroughly to make things easier for the chemical process to work. Generally we don't chew our food enough. A large lump of steak swallowed whole is obviously going to take more processing.
The chemical process of digestion includes:
- secretion from the salivary glands
- gastric juices from the stomach
- Intestinal juice from the pancreas
- bile from the liver.
Quite a cocktail and again we see the importance of overall health with the pancreas and liver involved in the digestive process.
The food enters the mouth and is moistened and lubricated by saliva before swallowing which takes place in three stages to prepare the food for digestion by stomach and gastric juices.
Stomach and Gastric Juices: Organs associated with the stomach are the liver, pancreas, spleen, kidneys, adrenal glands, duodenum, colon and small intestine. All have their own nutritional requirements and their own part to play to function properly. Therefore all need to be in good working order.
The size of the stomach varies with the amount of food it contains. Muscle contraction churns and breaks down the chewed food and mixes it with gastric juices. Mixing with the gastric juices takes place gradually.
Gastric juices are secreted by the gastric glands and consist of water, salts, mucus secreted by glands on the stomach surface plus hydrochloric acid, intrinsic factor and enzymes.
The water liquifies food, the hydrochloric acid kills harmful microbes, acidifies the food and provides the acid environment needed for digestion by enzymes. Intrinsic factor is a protein compound which is needed for absorbing Vitamin B12 while mucus prevents injury to the stomach wall.
There is always a small amount of gastric juice in the stomach even when there is no food. Maximum levels of gastric juices are reached about 1 hour after a meal then drop back to the minimum level after 4 hours. Hence the advice to wait at least 4 hours between meals.
The juices start to flow before food reaches the stomach because of reflex action by the sight, smell and taste of food. As the partially digested food reaches the small intestine the release of gastric juices slows down and also slows down the emptying of the stomach.
Stomach Function:
The stomach acts as a temporary reservoir for food, allowing digestion to take place. It produces gastric juices which start the digestive process. Muscular action mixes the food with the juices. The rate at which the digestion takes place and the stomach empties depends to a large extent on the type of food eaten. A carbohydrate meal leaves the stomach in 2 or 3 hours, a protein meal takes longer and a meal containing high fat levels takes longer still. So you can see how too much food to soon, or the wrong sort of food, strains the system.
The Small Intestine:
Leads into the Large Intestine. It is 5 metres long approximately and in the small intestine the chemical digestion of food is completed and most of the absorption of nutrients takes place in the small intestine. Secretion of intestinal juices continues, digestion of carbohydrates, proteins and fats occurs as well as the absorption of nutrients.
The Large Intestine:
or Colon is about 1.5 metres long. The rectum is a slightly dilated part of the colon leading into the anal canal. Though some water is absorbed in the small intestine, in the large intestine water continues to be absorbed to help with elimination. Mineral salts and vitamins are also further absorbed into the blood stream.
I could go into more technical details of the chemistry of digestion, the molecular breakdown of enzymes, the structure of everything from teeth to intestines etc, but this book is about better nutrition as diet not anatomy and physiology. There are plenty of more technical books and I just want to give you an idea of what is involved in achieving good nutrition without confusing you with science.
There are other organs important to the digestive process so a mention is worthwhile. The Pancreas, Liver and Gallbladder each have an important role to play.
The Liver is a large organ that processes absorbed nutrients, detoxifies harmful substances (there are plenty of those around now) and produces bile, a digestive fluid produced from the breakdown products of dietary fat and old red blood cells. The liver also produces proteins and stores some nutrients and vitamins.
The Pancreas secretes digestive enzymes that breakdown fats, proteins and carbohydrates. Sodium bicarbonate in the juices also helps neutralise stomach acid. Therefore better to have a healthy pancreas doing the work instead of taking bicarbonate of soda in anti-acid medication.
The Gallbladder is a small fig shaped bag lying under the liver, in which bile from the liver is stored. Bile is partly waste from the chemical process in the liver. It plays an important part in digestion of fats.
