The Digestive System

The digestive system allows for the breaking down of chemicals in the body that can be absorbed and contains a number of hollow organs which runs from the mouth to the anus. There are a number of stages to digestion as it follows its route through the digestive tract, which takes from 20 – 30 hours, and we are going to look at them in turn as we follow that journey. The digestive systems main aim is to allow for mastication, digestion, absorption and then elimination of food.

The Mouth

When we think of food or before it even enters the mouth, saliva is released from the salivary glands which are passed around the mouth by the tongue. The saliva, which is secreted at around 1 – ½ litres a day, contains an enzyme called amylase that assists with chemically breaking down some carbohydrates. The saliva also moistens the food, making it easier to swallow. Our teeth break down food into smaller manageable pieces by tearing and shredding.

Movements by the tongue and the jaw push the food to the back of the pharynx (throat), where a tiny flap of skin called the epiglottis closes over the entrance of the trachea to prevent choking. This swallowed food, now called a bolus, is pushed down into the esophagus, where wave like contractions, called peristalsis push the food further down to the stomach. The food passes through a muscular ring, called the cardiac sphincter into the stomach, which then quickly shuts to prevent food travelling back up the esophagus.

The Stomach

Once in the muscular J shaped sac, the food and liquids are stored and mixed with strong digestive juices that are secreted by the lining of the stomach. The bolus is churned and squeezed by the powerful muscular contractions of the stomach wall. Hydrochloric acid breaks down the bolus into chyme, which is a liquid. The acid does not damage the stomach walls due to a thick layer of protective mucus, but if this mucus becomes limited, then an ulcer may form. With the exception of water, alcohol and certain drugs, very little of the chyme is absorbed into the blood from the stomach.

The stomach walls contain three layers of smooth muscle arranged in longitudinal, circular, and diagonal rows, which allows the stomach to squeeze and churn the food during mechanical digestion. Whilst this digestive process in the stomach is occurring, which can take several hours, a stomach enzyme called pepsin is breaking down proteins. The chyme is then transported a little at a time through the pylorus into the small intestine, via the pyloric sphincter.

The Small Intestine

Sometimes called the small bowel, the small intestine is the longest portion of the digestive tract and is approximately 20 feet long is made up of the duodenum, the jejunum and the ileum and is a narrow tubed structure that fills most of the lower abdomen. Once the chyme is in the duodenum, bile from the gallbladder and enzymes from the pancreas all combine to complete the final stages of digestion. The acid from the stomach is neutralised in the duodenum’s alkaline environment.

Gland cells in the small intestine secrete digestive enzymes that chemically break down complex food molecules into simpler ones.

The chyme leaves the duodenum and it enters the jejunum and ileum. Here the nutrients are absorbed through the lining of the small intestine and transferred to the bloodstream and liver by tiny villi which cover the walls of the small intestine.

These finger-like projections allow for a greater surface area, allowing the chyme to be absorbed. Such products as fibre and water, that have not been digested in the small intestine travel to the large intestine. The ileum is the final portion of the small intestine, which leads into the large intestine.

The large Intestine

Sometimes called the large bowel, the large intestine collects and stores all waste products before processing them into faeces to be removed from the body. This part of the intestines is around 5 feet long and is made up of the caecum, appendix, colon and rectum.

The caecum is shaped like a pouch and is found in the right lower abdomen and stores all the material; fibre, water salts and some vitamins from the small intestines before moving it along to the colon. The material enters the expanded caecum through a valve that separates the small intestines from the large intestine. A small projection, the appendix, emerges from the caecum, and although it has no known function it can become troublesome if it becomes infected.

Starting at the caecum, the ascending colon travels up the abdomen towards the liver. The colon then becomes transverse as it travels across the abdomen, and then descends down the left side of the abdomen to the sigmoid colon. This S shaped organ is the largest part of the intestine and joins onto the rectum. All the time that the processed mixture is in the colon, mucus and bacteria from within the large intestine mix and starts to form faeces. This water and some vitamins and minerals from the faeces are then absorbed into the colon.

The faeces are pushed along to the sigmoid colon and finally the rectum by muscular action, where they are stored until being passed as a bowel motion.

The Pancreas

This 12-15cm organ is located just below the stomach, and slightly behind it. Around 99% of the pancreas is made up of small clusters of glandular epithelial cells called acini, which is responsible for producing the clear pancreatic juice which has many functions. These enzymes enter the duodenum via two pancreatic ducts, which classifies it as an exocrine gland. Pancreatic amylase is secreted for digesting carbohydrates, trypsin to digest proteins and lipase to digest fats. The remaining 1% of the pancreas contains cells which are arranged into clusters called Islets of Langerhans. These cells directly secrete the hormones Insulin and glucagon, needed to control the blood sugar level into the bloody which also means the pancreas is an endocrine gland.

The Liver

Found in the upper abdominal cavity towards the right and above the diaphragm, the liver is the heaviest gland in the body. The livers cells or hepatocytes process chemical changes and its main function is to regulate the composition of blood, so is therefore highly vascular, receiving oxygenated blood via the hepatic artery and deoxygenated blood from the stomach and intestines via the hepatic portal vein. Here, excess glucose from the blood is removed and stored in the form of glycogen, until all the glucose in the body has been used up and blood sugar levels fall. The liver then re-converts the glycogen back into glucose. Filtering the blood of any harmful substances is crucial and this is done by the liver extracting it from the blood. Blood is transported back to the heart via the inferior vena cava.

As well as the mentioned functions, the liver also secretes bile, which consists of cholesterol, pigments, salts and traces of other substances. Bile is removed from the liver to the gall bladder for storage via the canaliculi.

The Gall Bladder

Acting as a storage vessel for bile produced in the liver, this small 4 inch sac is located behind the liver and has an important function of adding mucus to the bile which increases its concentration. As the body requires bile to emulsifier fats, the gall bladder contracts and releases bile into the bile duct. Along with the pancreatic duct, bile enters the duodenum.

Enzymes

Enzymes are biological catalysts, made up of proteins, which speed up chemical reactions in all living things. They are needed to digest food and only work for one specific reaction. Human saliva contains an enzyme called amylase which breaks down starch into a sugar called maltose. The pancreas is the main digestive gland in the body

In the stomach the gastric enzymes pepsin, gelatinase, gastric amylase and gastric lipase are secreted. All of these enzymes have a different role, from breaking down proteins to degrading starch

Digestive Hormones

As well as releasing enzymes, at least four digestive hormones are released which help aid and regulate the digestive system. These consist of gastrin found in the stomach, secretin, cholecystokinin and gastric inhibitory peptide also found in the duodenum.

Absorption and Transportation of Nutrients

The human body is made up of two-thirds water and it is an essential nutrient that is involved in every function of the body. Water helps transport nutrients and waste products in and out of cells and is necessary for all digestive, absorption, circulatory, and excretory functions. Maintenance of the proper body temperature is determined by water and it is recommended that you drink at least eight 8-ounce glasses of water each day.

Carbohydrates, such as starch and sugars need to be broken down into simpler molecules by enzymes in the saliva and pancreatic enzymes. Starches are digested by the enzyme in saliva and pancreatic juices and sugars are digested by an enzyme found in the lining of the small intestine.

Fibre is indigestible and does not get broken down at all by enzymes. Soluble fibre can be dissolved in water, whereas insoluble fibre passes through the intestines unchanged.

Fats need to be broken down by being dissolved in the intestine and then by the bile acids produced by the liver, so they form tiny droplets. These droplets are then carried into the cells of the mucosa before changing back into large molecules. They then pass into the lymphatic’s to be carried to the veins of the chest, and by blood vessels to the fat deposits in the body where they are laid down as storage.

Proteins must start to be digested by an enzyme in the stomach before they can be used to help build and repair the bodies cells and tissues. Once in the small intestine, pancreatic enzymes complete the breakdown of larger protein molecules into smaller ones called amino acids. Once absorbed, they can be carried to all parts of the body for growth and repair

Vitamins are classified by being either water-soluble vitamins (the B vitamins and vitamin C) or fat-soluble vitamins (vitamins A, D, E, and K). It is difficult to store water-soluble vitamins, and any excess are flushed out in the urine. Fat-soluble vitamins can be stored in the liver and the fatty tissue of the body.

Minerals are needed in small amounts. Minerals are classified into essential minerals and trace minerals, with the body only needing very small amounts of trace minerals. Those classified as essential are magnesium, sulphur, sodium, potassium, phosphorus, iron and calcium. The main function of minerals is to control body fluids, build strong bones and teeth and to assist with converting food into energy that can be used.

Water is an essential nutrient which makes up around 50-75% of our body weight. It removes toxins in the body, regulates temperature and is essential for growth of the body.

Recommended Daily Allowance (RDA)

The RDA was originally set in 1979 by the Government of Health to state what the minimum amount of nutrients needed by the body but the term Dietary Reference Value (DRV) can also be used. The rates are different for adults, children, male and females. The law now ensures that food is labelled to state what percentage it is providing against the RDA, and also many foods now contain the “traffic light” symbols.

Pathologies of the Digestive System

DiseaseMeaning
Crohn’s DiseaseInflammation in the gut, a long-term disease, affecting any part of the digestive system.
Coeliac’s DiseaseAn intolerance to gluten which causes indigestion, bloating, weight loss
Gall stonesPebble like deposits, usually made of cholesterol, that form inside the gall bladder.
Irritable bowel syndromeBouts of stomach cramps, bloating and a change in bowel motion.
HepatitisInflammation of the liver usually caused by a virus.
HerniaThe lining of the abdominal cavity weakens and protrudes through a weak area of the abdominal wall.