Human Biology · Intermediate

The Systems Series: Digestion


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The digestive system consists of the gastrointestinal tract and accessory organs that allow us to digest food that we consume. The GI tract is the path from your mouth, down your oesophagus, into the stomach, through the small intestine until you get to the large intestine. Accessory organs are the glands and organs that release digestive enzymes into the GI tract, such as the salivary glands, pancreas and liver. See figure 1 for an image of the GI tract. Digestion is the breakdown of large, insoluble molecules of food into smaller, soluble food molecules that can be absorbed into the blood. This allows the body to absorb nutrients we take in from food. Digestion can be divided into two processes – mechanical and chemical – that take place throughout the GI tract.

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Figure 1. The gastrointestinal tract is made up of the eosophagus, stomach, small and large intestines. 

Mechanical digestion is the physical breakdown of food into smaller pieces. The first stage of digestion is chewing your food, which is a form of mechanical digestion. The reason for this first stage is because chewing food into smaller pieces increases the surface area of food molecules, allowing enzymes to access the food particles more easily. When we chew up our food, saliva is released from the salivary glands to make the food softer. Saliva also contains salivary amylase, the enzyme that begins to break down starch into the simpler component maltose. The processes that involve digestive enzymes make up chemical digestion. If you’re not sure what enzymes are, make sure to check this link. The “biological” in biological detergent is because it contains enzymes that are found in our digestive system. These enzymes digest food marks in fabric, making it easier to remove stains even at low washing temperatures.
The breakdown of starch into maltose allows for enzymes further on in digestion to break maltose down into glucose, one of our key energy sources. Chewed up food then enters the oesophagus, the pipe in your throat, by a process called peristalsis, which is the fancy term for wave-like muscle contraction that moves food from A to B. The food is swallowed into the stomach to enter the next stage of digestion.

The stomach is responsible for churning up the food, another process of mechanical digestion. It also contains pepsin and hydrochloric acid; two very acidic chemicals that start digesting proteins from the swallowed food. These two acids can be harmful to the stomach as they are corrosive. To prevent this, the stomach secretes mucous which forms a layer to protect the lining of the stomach. This mechanical and chemical digestion continues in the stomach until the food becomes a thick liquid, which is called chyme. When food becomes chyme, it can pass on to the next stage.

A valve between the stomach and the duodenum, the top part of the small intestine, opens and allows chyme to enter. Fun fact: the small intestine is 7 meters long, making its surface area huge. This optimises absorption of nutrients. Bile juice from the liver and digestive enzymes from the pancreas are released and mix with the chyme. When chyme is fully digested from food into nutrients, it is absorbed into the blood. 95% of absorption takes place in the small intestine. Leftover chyme moves through the large intestine, where water and minerals are absorbed into the blood. Any waste materials are excreted from the body when you go to the loo.

Digestion is a very fine-tuned process that ensures that you absorb the optimum amount of nutrients from the food that you eat. However, there are several diseases and syndromes that can affect both the upper (oesophagus and stomach) and lower (small and large intestine) GI tract.

Dyspepsia, commonly known as indigestion, occurs when digestion is impaired. Symptoms typically include heartburn, a feeling of fullness, nausea, belching and pain. Causes are likely unknown, and there may be multiple causes that are difficult to identify. Treatment includes watching the diet and identifying foods that may trigger indigestion and using acid-lowering medication (antacids such as Rennie and Gaviscon). Because of a lack of research around dyspepsia, little is known about causes and treatments.

Irritable bowel syndrome (IBS) is characterised by a range of symptoms including abdominal pain, diarrhoea, constipation, and irregular bowel movements. The causes of IBS are not clear, although there is evidence that communication between the GI tract and the brain is skewed in IBS sufferers, and food sensitivities trigger IBS symptoms. Food sensitivities are different in every patient, although IBS sufferers are encouraged to follow a low FODMAP diet, as FODMAPs are thought to trigger IBS. Over the counter drugs are available to relieve the symptoms of IBS.

Problems and diseases of the GI tract are often difficult to diagnose and treat, due to a lack of research into the digestive system. This is because there are several factors that impact digestion, making it difficult to single out causes. Most GI tract problems can be treated with medication, and sometimes even hypnotherapy, although most problems have no solid cure.

If you have any requests for what system to cover next, or any other blog posts, please leave them in the comments section or DM me on social media!


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