LO9 of Unit 1 – The structure and function of the digestive system (2021)

The structure and function of the digestive system

Learning outcomes
By the end of this section, you will be able to:

9.1 Describe the function of each section of the digestive system

9.2 Explain how fats, proteins and carbohydrates are digested and absorbed

9.3 Describe the role of dietary fibre in the maintenance of gut function

9.4 Describe the role of the liver and pancreas in digestion

9.5 Explain the importance of maintaining fluid levels for health and exercise performance

The structure and function of the digestive system


Image of the digestive system to be labelled


Digestion starts here with mastication (chewing). Saliva (1-1.5 litres per day), which contains digestive enzymes such as salivary amylase, and aids in the chemical breakdown of CHO) is released from the salivary glands and mixes with chewed food.

Oesophagus (hollow muscular tube)

The oesophagus delivers food to the stomach through muscular contractions (peristalsis).


The stomach is a holding container allowing food to be mixed with a strong acid (which breaks down proteins) and enzymes (pepsin) to continue digestion. Food is turned into a soupy mixture of partially digested food called chime.


The liver is a vital organ having multiple functions such as processing nutrients (CHO, protein and fat) absorbed from the small intestine, bile secretion, and detoxification of potentially harmful chemicals.


The pancreas secretes enzymes into the duodenum to break down carbohydrates, proteins and fats.


The gallbladder stores bile acids and secretes them into the duodenum to break down fats.


The duodenum is the first part of the small intestine; a muscular tube where enzymes (amylase, lipases and proteases) from the pancreas and bile from the liver


The jejunum is the second section of the small intestine. Sugars and amino acids are absorbed through its walls and into the body.


The ileum is the final section of the small intestine. Vitamin B12 is absorbed through its walls into the body.

Large intestine (Colon)

The large intestine consists of ascending, transverse and descending portions. It connects the small intestine to the rectum. Waste is processed here, water is removed, and its contents become solid ready for elimination.


The rectum holds stools ready for elimination.


A 2-inch canal containing two anal sphincters.

How fats, proteins and carbohydrates are digested and absorbed

Carbohydrate digestion (CHO)

CHO digestion starts in the mouth where mastication (chewing) breaks down the food into smaller sugar pieces.

The salivary glands in the oral cavity secrete saliva that coats and act on the food particles. Saliva contains the enzyme, salivary amylase, which breaks the sugar units’ bonds.

Food then enters the stomach via the oesophagus and then makes its way to the small intestine where digestion of CHO continues, resulting in smaller subunits called glucose (monosaccharide; one-unit sugar). The glucose molecules are then transported through the blood to all body tissues.

Glucose is taken up by the muscles and stored in the muscle cytoplasm (400g) and liver (100g) where it’s placed into its storage form called glycogen. Here it remains until it is needed to form ATP. Liver glycogen stores can be viewed as the body’s backup when muscle glycogen stores become depleted. It must be first converted to glucose and is then transported to muscle tissues via the bloodstream. On average, the body stores 500g of CHO at any one time, so any excess of blood glucose is converted and stored as fat in adipose tissue.


Fat digestion

Fat digestion starts in the mouth through chewing and then in the stomach where the stomach’s muscles contract in a rhythmical manner to churn the food over and over, stimulating further breakdown. However, the vast majority of fat digestion occurs in the small intestine with support from the accessory organs; liver, gallbladder and pancreas, where bile salts and specialist enzymes called lipases continue to digest fat, resulting in sub-units called fatty acids. Fatty acids are long carbon chains with hydrogen atoms attached.

Due to fats being hydrophobic, they are transported through the blood. However, fat digestion is a complex process, and unlike CHO (glucose), cannot be transported in the blood unless the fatty acids are packaged inside structures called chylomicrons. Chylomicrons’ role is to transport dietary fats to other locations in the body where the majority is stored as adipose tissue. A small amount is stored in the muscles as triglycerides.


Fat digestion





Protein digestion

The majority of protein digestion occurs in the stomach where hydrochloric acid and enzymes called proteases, break the bonds holding proteins together. Proteins consist of chains of amino acids, and through digestion, large proteins are broken down into smaller proteins called polypeptides. Protein digestion finishes in the small intestine where pancreatic enzymes break the polypeptides into subunits called amino acids. Amino acids are tiny and can easily pass through the small intestine lining and bloodstream. They can then be used by the body cells for growth and repair or energy. The body does not store any protein surplus, so any excess is converted into fat and stored in adipose tissue.

The role of dietary fibre in the maintenance of gut function

Fibre (Non-starch polysaccharides)

Fibre (or roughage) is the indigestible part of plants that maintains the plant’s structure. It is this indigestibility which makes fibres important in both the normal functioning. Fibre is found in all plant-based foods such as fruits, vegetables, grains and beans. Unlike other food components, such as fats, proteins, or carbohydrates, your body breaks down and absorbs — your body doesn’t digest fibre. Instead, it passes relatively intact through your stomach, small intestine and colon and out of your body.

Fibre doesn’t provide any energy but aids in gut health by creating bulk. The bulk makes it easier for food waste to move through the digestive tract, thus decreasing intestinal transit time to eliminate faeces.

Many healthcare professionals highlight fibre’s importance as part of a healthy balanced diet.

It can help prevent:

  • Heart disease
  • Diabetes
  • Weight gain
  • Some cancers (particularly colorectal cancers)
  • Improve digestive health

However, many people don’t get enough fibre. According to the NHS website, most people in the UK get about 18g of fibre per day, which is just over half the daily recommended amount needed to maintain gut health. The daily recommended amount of fibre for adults is at least 30g per day. There are two types of fibre:

Soluble fibre

Dissolves in the water in your digestive system. It may help to reduce the amount of cholesterol in your blood.

Foods that contain soluble fibre include:

  • Oats, barley and rye
  • Fruit, such as bananas and apples
  • Root vegetables, such as carrots and potatoes
  • Golden linseeds

Insoluble fibre

Doesn’t dissolve in water and passes through your gut without being broken down. Insoluble fibre keeps your bowels healthy and helps prevent digestive problems.

Good sources of insoluble fibre include:

  • Wholemeal bread
  • Bran
  • Cereals
  • Nuts and seeds (except golden linseeds)

Describe the role of the liver and pancreas in digestion

The liver

The liver is the largest organ in your body involved in the digestion and processing of nutrients absorbed by the small intestine. It plays an essential role in fat digestion by creating bile stored in the gall bladder. Bile is the chemical that breaks down fats and allows the nutrients within fat (such as fat-soluble vitamins like A, D, E, and K). Bile is released into the duodenum (first section of the small intestine) where it acts and digests fats.

The liver also takes harmful chemicals consumed, such as alcohol and drugs, and detoxifies them, thus protecting the body.

The pancreas secretes digestive enzymes into the duodenum that break down protein, fats and carbohydrates.


Roles of liver and pancreas in digestion

The pancreas

The pancreas is located just behind the stomach. It plays a significant role in digestion by functioning to secrete digestive enzymes into the duodenum that breaks down protein, fats and carbohydrates. The enzymes include:

  1. Lipase – help your body digest fats.
  2. Protease – protein digestion starts in the stomach with the release of pepsin. However, the majority of protein digestion is due to the pancreatic proteases. The two major pancreatic proteases are trypsin and chymotrypsin which digest proteins into peptides and peptides into smaller peptides.
  3. Amylase – acts of sugars and starches to break them down into smaller carbohydrate molecules such as maltose (composed of two glucose molecules), which is the first step. Ultimately, starches are broken down by other enzymes into glucose molecules, which are rapidly absorbed through the intestinal wall.

The pancreas also makes insulin, passing it directly into the bloodstream. Insulin is the primary hormone in your body for dealing with glucose metabolism.

Fluid levels for health and exercise performance

Water is an essential natural resource and necessary for life. Without it, humans will survive for only a few days. Water comprises around 75% of an infant’s body weight and 55% of an older person’s body weight. A water loss of 9% to 12% of an individual’s body weight can lead to death. Fortunately, water balance is regulated in the body, and if levels start to drop physiological thirst is triggered.


The body loses water through the following mechanisms:

  1. Excreted as urine – this amounts to around 60% of the total daily fluid loss from the body
  2. Breathing – around 28% of the total fluid loss is through breathing
  1. (evaporation), and the skin
  2. Sweating – around 6% of the total daily fluid loss from the body
  3. Faeces – around 6% of the total daily fluid loss from the body

As you can see, a lot of water is lost through the above mechanisms. This is why it’s important to rehydrate by drinking fluids and eating foods that contain water.

The amount of fluid you need depends on many things, including:

  1. The environment, temperature and weather – you need to drink more if it’s hot and you’re sweating more water out of your body
  2. The amount of daily physical activity or exercise you do – drink more if you exercise more
  3. Age – how well your body can balance water is affected by age. As we get older, we store less water
  4. Diet – if you’re following a special diet or very low-calorie diet, you need to drink more


As a basic guide, most people need about 1.5 to 2 litres of fluid a day, which is about eight to 10 glasses (http://www.bupa.co.uk/ health-information/directory/h/hydration). You can get this fluid from water and other drinks, such as milk and fruit juice. Water in food also counts; fruit and vegetables contain lots of water. Cucumber and lettuce have the highest water content of any food (a massive 96% water). Tomatoes are also packed with water at 94%. Keep your hydration levels topped up by adding them to a salad or a sandwich.

The effects of dehydration

Any level of dehydration impairs physiological function and performance. Dehydration leads to decreased plasma volume, a drop in peripheral blood flow and sweating rate. A reduced blood volume lowers the ventricular filling rate resulting in elevated heart rates. Another effect is an increased core temperature due to a reduced sweating rate.

A fluid loss equivalent to only 1 % of body mass can affect temperature regulation. A water loss of 4 to 5 % impairs physical performance. It’s essential to stay hydrated to perform at your best during exercise or physical activity.


The signs and symptoms of dehydration

Signs of dehydration are visible in your urine. Dark and strong-smelling urine is a clear sign that you need to drink more fluids.

Other symptoms of dehydration include:

  • dry mouth
  • thirst
  • dizziness
  • headaches
  • tiredness
  • lack of concentration

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