What is digestion?
Digestion is the process of breaking down food so that it's small enough to be absorbed and used by the body for energy or in other bodily functions.
Digestion involves a number of different stages. The first phase is known as the cephalic (head) phase. It starts before food has even entered your mouth. The sight, smell, taste or even the thought of food will activate saliva in the mouth as well as digestive juices, which contain enzymes to break down food.
In the mouth
Once food is in the mouth, the tastebuds begin determining the chemicals within the food via their nerve endings, in order to give you the taste sensations of salt, sweet, sour or bitter. As your teeth chew and grind the food, breaking it down, it's mixed with saliva. This comprises many enzymes including salivary amylase, which begins to break down the long chains of starch found in foods such as bread, cereals, potatoes and pasta. Saliva also contains mucin, which moistens the food so it can pass easily through the digestive (gastrointestinal) tract.
The oesophagus
After the food has been swallowed, it's carried down the oesophagus (a muscular tube) towards the stomach. The oesophagus can contract and relax in order to propel the food onwards, and each mouthful of food takes about six seconds to reach the stomach once swallowed.
The stomach
The stomach is a sack made of muscle and, when it's empty, it has a volume of only 50ml but this can expand to hold up to 1.5 litres or more after a meal. The walls of the stomach are made of three different layers of muscle that allow it to churn food around and make sure it's mixed with the stomach's acidic digestive juices. The presence of hydrochloric acid in the stomach prevents the action of salivary amylase and helps to kill bacteria that might be present. The stomach also produces the enzyme pepsin, which breaks down proteins (mostly found inmeat, fish, eggs and dairy products).
The hormone ghrelin is produced by cells lining the stomach. Ghrelin stimulates hunger and tends to increase before a meal and decrease after eating. This hormone forms part of the communication system between the gut and the part of the brain that controls hunger and satiety (how full you feel).
Food can stay in the stomach for a few minutes or several hours in the gastric phase where numerous acids and enzymes are released, including the hormone gastrin. When the food has been churned into a creamy mixture known as chyme, the pyloric sphincter (an opening controlled by muscle) opens and chyme passes gradually into the small intestine.
The small intestine
Prebiotics are mainly indigestible carbohydrates called oligosaccharides. On reaching the large intestine, they selectively stimulate the growth and/or activity of beneficial microorganisms already in the colon, such as bifidobacteria and lactobacilli.
About 3ml of chyme is squirted into the small intestine at short intervals as the pyloric sphincter opens. This is known as the intestinal phase and causes the secretion of many hormones, which all aid the digestive process. The sphincter is designed to open partially so that large particles are kept in the stomach for further mixing and breaking down.
Digestion and absorption of fats, protein and carbohydrates occurs in the small intestine. Three important organs are involved:
- The gall bladder provides bile salts that help to make fats easier to absorb.
- The pancreas provides bicarbonate to neutralise the acidic chyme from the stomach, and also produces further digestive enzymes.
- The intestinal wall contains cells that make up the wall of the small intestine. These cells help to neutralise the acid and also produce enzymes to digest food.
The inner surface of the small intestine is folded into finger-like structures called villi, which greatly increase the surface area available for absorption - in fact the surface area of the villi is equivalent to that of a tennis court! Blood vessels receive the digested food from the villi where it's then transported through the blood stream to the liver via the hepatic portal vein.
Probiotics are live bacteria similar to the bacterial micro-organisms that live in the large intestine. They are often referred to as 'friendly bacteria', and come from food sources or dietary supplements. The mix of these 'friendly' bacteria and other gut microorganisms is important for good health, and many factors can alter this delicate balance, such as infection or use of antibiotics. Friendly bacteria are vital for proper development of the immune system, to protect against micro-organisms that could cause disease, and to aid the digestion and absorption of food and nutrients.
Fat can take much longer to be broken down, with the process of fat digestion and absorption taking between three and five hours.
The unabsorbed residue of this process finally reaches the end of the small intestine and enters the large intestine.
The large intestine
This is one of the most metabolically active organs in the body. It measures about 1.5 metres and contains over 400 different species of bacteria that break down and utilise the undigested residues of our food, mostly dietary fibres. As the watery contents move along the large intestine, water is absorbed and the final product - faeces - is formed, which is stored in the rectum before excretion from the body.
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