The function of the digestive system

The human alimentary canal, also known as the digestive tract/system, is the pathway in which food enters and leaves the body. Although our food contains carbohydrates, lipids and proteins, it is not the same as the carbohydrates, lipids and proteins contained in human tissues. For these components to be utilised by the body they must first be broken down by the process of digestion.

During digestion, food is broken down into its basic and simplest forms; this is done through processes known as mechanical digestion and chemical digestion. Some of the most basic forms that food will be broken down into include the digestive molecules of amino acids, vitamins, minerals, water, sugars, fatty acids and glycerol. The walls of the digestive system are unable to absorb whole foods; however, they are able to absorb these digestive molecules. Once absorbed, these molecules will be carried through the blood to certain tissues in the body that require them. The molecules will then be reassembled into the molecules that make up the cells of the human body.

The following diagram is a simplified version of the human digestive system. It illustrates the positions of the mouth, oesophagus, stomach, liver, small intestine (duodenum and ileum) and the large intestine (colon and rectum) on the human body. However, the actual gut is larger than what is depicted in the diagram. It fills the whole space of the abdomen, is much longer and much more coiled up.  The estimated length in the average human is 8 metres which allows plenty of time for food to be broken down by chemical and mechanical digestion as it passes through the digestive system and is absorbed.

We will now look at these areas of the body individually and in more detail to discuss their involvement in the breakdown of food.

The mouth

The mouth is the first section of the alimentary canal; it is where the food enters the body and the digestion process begins. This is called ingestion. Inside the mouth, we have teeth to mechanically grind down the food by chewing; this is known as a form of mechanical digestion. The grounded down pieces of food are then mixed together with saliva which forms a bolus (a ball of food).

As well as breaking down food from chewing, the saliva that is secreted into the mouth from salivary glands contains enzymes which also break down food; this is known as a form of chemical digestion. The mechanical and chemical digestion processes in the mouth work together, as chewing allows for the enzymes in the salvia to break down food particles much quicker.

The teeth in an adult human’s mouth have different structures and functions. The diagram below represents one side of a human mouth:

Incisors are located at the front of the mouth; their narrow-edged shape is specially adapted for cutting through food. Human beings have four incisors in each jaw; making eight altogether in the whole mouth.

Canines are located behind incisors and in front of the premolars in the human mouth. Canines often have a long, pointed shape which is specifically designed for firmly holding food in the mouth to tear it apart. There are two canines on each jaw; making four altogether in the whole mouth.

Premolars are located behind the canine teeth and in front of the molars. Premolars can be seen to represent features of both canine and molar shapes. These teeth are specially designed like this for moving food around the mouth; specifically to the back of the mouth where the molars are used for chewing. Because these teeth have slightly pointed edges, they are also able to cut through food. Humans have four premolars in each jaw; making eight altogether in the whole mouth.

Molars are located at the back of the mouth and are used for chewing and grinding down the food. They are located behind the premolars. Humans have six molars on each jaw; making 12 molars altogether in the mouth.

Some people have teeth taken out for various reasons, meaning they will not contain all the teeth illustrated in the previous diagram. Tooth decay is a common reason for the extraction of a tooth or a filling. Tooth decay occurs when certain bacteria in the mouth convert sugars into acid which breaks down the enamel (a protective layer covering each tooth). Bacteria is then free to enter the softer areas of the tooth inside (the dentine). This is one reason why consuming too many foods and drinks that have a high sugar content is bad for you. Tooth decay can be prevented by avoiding foods and drinks that are high in sugar; brushing, flossing and using mouthwash on your teeth twice a day; and drinking water containing fluoride (fluoride is found in toothpaste, and most tap water now contains a small amount of fluoride).

Tooth decay due to acid occurs because of the damage it makes to calcium and phosphate ions located in the enamel. Fluoride reduces tooth decay by helping to replace these calcium and phosphate ions that are lost through acid attacks. Fluoride also helps to reduce bacteria in the mouth from producing acid on plaque.

Although fluoride has many positive effects on the teeth it has been proven that too much fluoride can cause brown and grey spots to appear on the teeth. Some also argue that it should not be added to tap water as people should not be forced to consume fluoride if they do not want to.

The salivary glands

Salivary glands in mammals are located in and around the mouth. Salivary glands are exocrine glands that have ducts which produce saliva. Saliva is formed of several components and this includes an enzyme known as amylase. Amylase is an important digestive enzyme that is responsible for the breakdown of carbohydrates, for example starch molecules into glucose. When amylase breaks down these particles it is known as chemical digestion.

The salivary glands of the mouth

The major salivary glands of the mouth are the parotid, sublingual and submandibular. The diagram provided illustrates the positions of these glands and their ducts in the mouth. These glands secrete and drain saliva in the mouth through their ducts.

The parotid glands are the largest of the salivary glands and are located at the back of the mouth just below the ears and their ducts are near the upper back teeth.

The sublingual glands are located just below the mouth with the ducts on the floor of the mouth.

The submandibular glands are located under the tongue and their ducts are just above them.

The oesophagus

The oesophagus, sometimes called the gullet, is a long muscular tube connecting the mouth to the stomach. When food is broken down in the mouth and swallowed, the smooth muscle walls of the oesophagus contract behind the food to push it down further and to prevent it from leaving the body via the mouth; this is known as a process called peristalsis, which we will go into in much more detail in a few chapters.

The lining of the oesophagus is made up of moist tissue known as mucosa. This moist tissue allows for a bolus of food to pass down smoothly without causing any damage to the lining of the oesophagus.

The epiglottis

Although small, the epiglottis is an essential part of the upper alimentary canal. The epiglottis is made of cartilage tissue and acts as a flap attached to the opening of the larynx (the tube that allows air to pass through to the lungs). When we inhale air the epiglottis remains open but automatically closes when we swallow; this helps keep the airways clear and prevents any food particles or saliva from entering the lungs.

The below diagram illustrates the swallowing process.

The stomach

The stomach is located on the left side of the upper abdomen. After food has travelled along the oesophagus, it reaches the stomach and enters it via the lower oesophageal sphincter (a muscular valve). In the stomach both mechanical and chemical digestion takes places. The stomach secretes acids and enzymes to digest food whilst also contracting regularly to churn the food which helps in its digestion; this contraction is known as peristalsis.

The initial digestion of protein takes place in the stomach and thus food is held here for several hours before moving further down the digestive system. A protease enzyme that is produced in the stomach is known as pepsin. Pepsin breaks down proteins that enter the stomach into smaller peptides. Pepsin is unlike other enzymes in the body as its optimum pH value is around 2. This is essential as the stomach not only secretes digestive enzymes but also hydrochloric acid.

Hydrochloric acid is secreted by the stomach to help in the breakdown of food; this means that the stomach has highly acidic pH levels. The stomach being highly acidic has several benefits; one of them being that it helps protect against food poisoning as the acid kills any bacteria that might have entered the stomach along with the food. However, to make sure the hydrochloric acid in the stomach does not damage the opening to the oesophagus or the beginning of the small intestine (the duodenum), the lower oesophageal sphincter and a pyloric sphincter keeps the acid contents of the stomach inside. These sphincters are depicted in the following diagram:

Gastric rugae provides the stomach with an increase in surface area. The rugae gradually smooths out and expands when food enters the stomach so that it can be absorbed more sufficiently.

Semi-digested food enters the duodenum via the pyloric sphincter when it relaxes.

The liver

The liver is located on the upper right side of the abdomen. It is the largest solid organ and gland in the body.

On average, the liver holds around 13% of a human’s total blood supply and has numerous functions within the body. Some of these include:

• Controlling the levels of amino acids, fats and glucose in the blood
• Detoxifying and removing harmful substances from the blood
• Producing and regulating certain hormones
• Producing important enzymes and proteins
• Regulating glycogen storage
• Storing essential vitamins and minerals, for example iron
• The synthesis of plasma proteins

The gallbladder

The gallbladder is located on the right side of the abdomen under the liver. The liver produces a substance known as bile. The gallbladder has a number of functions but its primary function is to store and concentrate the bile that is produced by the liver.

Bile is a think green substance that is secreted by the gallbladder down the bile duct when food enters the duodenum from the stomach. Because the stomach is highly acidic, secretions left on the food need to be neutralised. Bile is a highly alkaline substance and so neutralises the food. Bile also helps break up lipids during digestion:

Bile does not contain enzymes but transforms any large lipid globules into an emulsion of small droplets which increases the surface area of the lipid and allows lipase enzymes to break them down more quickly.

The gallbladder also drains waste products that have built up in the liver into the duodenum so that they can be released from the body.

The pancreas

The pancreas is located in the upper left abdomen behind the stomach. The pancreas has exocrine glands that produce different enzymes. These digestive enzymes are added to the food when it reaches the duodenum. Their main function is to help the body digest carbohydrates, lipids and proteins but they also act to neutralise the acidic secretions left on the food when it enters the duodenum. We will discuss digestive enzymes in detail later in this topic.

The pancreas also has another important function – regulating blood sugar. The pancreas regulates blood sugar using two main hormones: insulin (lowers blood sugar) and glycogen (increases blood sugar). The pancreas contains islet cells which produce and release these important hormones directly into the bloodstream.

It is essential that healthy blood sugar levels are maintained as otherwise it can affect the functioning of the brain, liver, the kidneys and other key organs in the body.

The small intestine

The small intestine is connected to the stomach and is responsible for the majority of the digestion process.

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The first part of the small intestine is known as the duodenum. The duodenum receives food from the stomach. Once in the duodenum, bile is released from the gallbladder and digestive enzymes from the pancreas to neutralise the food’s toxicity and to help the body digest carbohydrates, lipids and proteins. As food travels through the duodenum, more enzymes are added until the food is broken up into soluble end products that can be absorbed in the final section of the small intestine known as the ileum.

A process known as peristalsis occurs in the small intestine. Peristalsis is the movement of food through the digestive system. We will discuss this process in detail in the section titled ‘How food is moved through the gut by peristalsis’.

The final section of the digestive system is known as the ileum. The lining of the ileum is adapted so that it has maximum contact with the digested food, increasing its absorption. It has a very large surface area, both due to the length and also the specific lining of the ileum. The lining of the ileum contains villi. There are millions of villi along the lining of the ileum, each of them measuring around 1-2mm in length. The entire area of the lining is estimated to be around 300m². The following diagram illustrates the villi in the ileum:

Digested food enters the network of capillaries contained in each villus. However, the products of lipid digestion enter the lacteal (a tube in the middle of the villus). Lacteals form part of the body’s lymphatic system. The lymphatic system’s primary function is to transport a fluid containing infection-fighting white bloods cells around the body; this fluid is known as lymph.

In some people, the lining of the small intestine can become damaged because they have an adverse reaction to gluten. Gluten is a type of protein found in three types of cereal:

• Wheat
• Barley
• Rye

Many foods contain the above cereals, for example:

• Most types of bread
• Cakes
• Pasta
• Some ready meals
• Breakfast cereals

When the lining of the small intestine is damaged by gluten it can cause coeliac disease. Coeliac disease is a common condition where the small intestine is damaged so much that it becomes difficult for it to absorb nutrients. The following diagram illustrates healthy villi compared to villi that have become damaged because of coeliac disease:

As you can see from the above diagram, the damaged villi are blunted. This greatly affects the small intestine’s ability to absorb nutrients as its surface area has drastically decreased as a result of coeliac disease.

The large intestine

The large intestine consists of the colon and the rectum. The colon is where stool is formed. The colon absorbs water, salt and other nutrients from the food whilst the remaining waste material is turned into stool. This stool then travels through the rest of the colon and the anal canal before leaving the body. The process of waste leaving the body is known as defecation.

Peristalsis occurs in this part of the digestive system as the colon has muscles lining its walls, pushing and squeezing the content along.

The colon is split into different sections:

• The cecum
• The ascending colon
• The transverse colon
• The descending colon
• The sigmoid colon

How food is moved through the gut by peristalsis

Peristalsis is the process by which food is moved along the digestive system. Two sets of muscles known as circular muscles and longitudinal muscles work together to produce wave-like contractions that push the bolus of food down through the digestive system. When circular muscles contract it reduces the diameter of the gut; when longitudinal muscles contract it reduces the length of the gut.

The process of peristalsis can be compared to squeezing toothpaste out of a tube. The following diagram illustrates the process of peristalsis:

Peristalsis begins in the oesophagus and carries on through the stomach, small intestine and large intestine before the leftover waste material is defecated.

As mentioned previously, fibre is an essential dietary requirement. Dietary fibre is found exclusively in plant based foods, such as fruit and vegetables. Dietary fibre is a type of carbohydrate that is not digested in the small intestine of humans. Instead, it passes through the small intestine and into the colon where it is then fermented by certain bacteria. Dietary fibre still provides the body with a small amount of energy but not as much as other types of carbohydrates that are digested. Instead of providing the body with energy, dietary fibre’s most important function is to help protect the digestive system and keep it healthy. Fibre allows food to pass through the digestive system smoothly, avoiding any possible damage caused to the tissues and muscles, which helps the process of peristalsis.

The role of digestive enzymes

The digestion of food involves both mechanical and chemical processes. When food first enters the mouth, the molecules are large and insoluble; however, along the digestive system the food is broken down into small, water-soluble molecules using mechanical and chemical digestion processes.

Mechanical digestion refers to chewing in the mouth and churning in the stomach.

Chemical digestion refers to digestive enzymes. Enzymes are proteins that function as biological catalysts. Although most enzymes remain inside cells, digestive enzymes are produced in the tissues and glands of the gut. They are essential in the breakdown of food into smaller, soluble molecules. There are a number of different digestive enzymes within the human body: