Nutrition and Weight Management
Information Sheet
Functions of the digestive system
Motility
Motility is the mechanism that moves food along the digestive tract from one end of
the digestive system to the other. The walls of the digestive system are made up of
smooth muscle that maintains tone, keeping pressure on the digestive system and
preventing permanent stretching of the digestive tract after distension from the
ingestion of food. It is controlled by contractions of the smooth muscle in the walls of
the digestive tract.
There are two types of motility. The first is propulsion. Propulsive movements move
the food through the digestive tract. Depending on where the food is in the digestive
tract, different rates of propulsive movement occur, allowing the various areas to
complete their particular tasks.
Looking at the diagram of the digestive tract, the food moves through the
oesophagus fairly quickly, as it is just a pipe to get food to the stomach and serves
no digestive function. However, by the time the food enters the small intestine the
speed of propulsion is much slower, as this is where the main processes of digestion
and absorption take place. The slower movement allows the breakdown of food to be
properly completed.
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©Future Fit Training, 2013
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Nutrition and Weight Management
The second type of motility is the mixing movement, which traps digestive enzymes
(more of which later) and allows them to do their job. It also exposes more of the
food to the absorbing surfaces of the digestive tract.
Secretion
Secretion refers to the digestive secretions that are released into the digestive tract
by various glands. These all play various specific roles in the breakdown of food as it
travels along the digestive tract.
The digestive secretions are controlled by neural or hormonal stimulation in
response to the presence of various food groups. The production of these secretions
requires energy and the amount of energy required to absorb a food’s energy is its
calorific value. This value is known as the food’s “thermogenesis”, and varies from
food to food.
Neural – controlled by the nervous system.
Hormonal – controlled by the hormones in the body e.g. insulin.
The digestive secretions can consist of enzymes, mucus and bile salts, as well as
water and electrolytes. These are all reabsorbed back into the blood as part of the
digestive process.
Digestion
Digestion is the process in which digestive secretions break down food into its
constituent parts. As mentioned previously the structurally complex food molecules,
or macronutrients, we eat are too big to be absorbed as they are, so we break them
down into smaller building blocks. This is achieved by physical means such as
mastication (or chewing) and by chemical breakdown, using the digestive secretions
described in the previous step.
Later in the module we will take a more detailed look at each of the macronutrients.
We will look at the large molecule that makes up each macronutrient as well as its
component smaller molecules.
The smaller molecules making up large macronutrient molecules are bound together
by the removal of a molecule of water. During digestion, this process is reversed,
and the water molecules are replaced, freeing up the smaller molecules. This
process of breaking down using water is called ‘enzymatic hydrolysis’. This is
another important function of water in digestion.
As the nutrients move through the digestive system, the breaking down process
continues until they become small enough to be absorbed.
May 2013
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Nutrition and Weight Management
Absorption
When the process of digestion is completed, the products are small enough to be
absorbed. The main site of absorption is the small intestine. Here, the digestive
products leave the digestive system into either of the two transport systems of the
body, where they will be either used or stored. Although the products of
carbohydrate and protein digestion, along with some vitamins and minerals, pass
into the blood stream, the products of fat digestion cannot pass into it. This is
because fat cannot mix with water and the blood contains water. Consequently, the
digestive products of fat (fatty acids and glycerol) and any fat-soluble vitamins (A, D,
E and K) pass into the lymph system.
May 2013
©Future Fit Training, 2013
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