The Digestive System
How The Digestion System Works

digestion system

How Food Is Broken Down In The Body


What Is The Digestive System?
What Are The Stages Of Digestion?
Stage 1: Mouth and Esophagus
Stage 2: The Stomach
Stage 3: The Small Intestine
Stage 4: Large intestines And Waste
Other Organs Involved In The Digestion Process
Diseases Of The Digestive System
Other Systems It Links To

Related Articles

Female Body
Human Body
Bowel Disorders
Stomach Problems

What Is The Digestive System?

It is a group of organs in the body which work together to digest food. Digestion is the process by which our body breaks down food into substances we can take in (absorb) and use. Food travels through the alimentary canal which starts at mouth and ends at the anus. The alimentary canal is also known as the digestive tract or gut. As well as the mouth and anus, it includes the esophagus, stomach and intestines. If you could lay your gut out straight it would be 6 times your height.


What Are The Stages Of Digestion?

There are four stages of digestion:

Mouth: Food is ingested (the taking in of food or liquid into the body), chewed and swallowed. It is softened by chemicals in your salvia (spit). When you swallow food travels down the esophagus into the stomach. The food that leaves the mouth is called bolus.

Stomach: The stomach is a bag with a muscular wall. It mashes food into a pulp, helped by chemicals called gastric juices. When empty it is about 0.5L in size, but when it is full after a meal it can stretch to 4L in size. Food that leaves the stomach is called chyme. It passes into the small intestines after about 4 hours of eating.

Small intestine: A 6 meter long tube where chyme is further broken down. Carbohydrates and fat are broken down and absorbed through the intestine walls into the blood.

Large intestine: Food that can't be digested passes into the large intestines. It is then pushed towards the anus where it excreted as feces.

What Are The Bowels? Bowels is another name for the small and large intestines combined.

Studies estimate that it takes food 50 to 53 hours to travel from the mouth to out the other end as feces.

Stage 1: The Mouth and Esophagus

In the mouth the action of teeth and saliva combine in the first stage of breakdown, chewing and partially digesting the food so that it will pass more easily along the esophagus. The ball of food that leaves the mouth is known as a bolus.

What Is Saliva?

Saliva (spit) is a liquid secreted by 3 pairs of salivary glands: the parotid gland (situated below the ear), the submandibular gland and the sublingual gland (both situated below the tongue). It contains water, mucus and the enzyme salivary amylase.

Saliva has 3 functions. It:
1. Lubricates food with mucus making it easier to swallow.
2. Starts digestion: the enzyme salivary amylase acts on cooked starch turning it into shorter polysaccharides.
3. Keeps the mouth and teeth clean.

What Is An Enzyme?

If we think about the food we eat, and the difference in size between it and the microscopic cells and tissues that it will feed in our body, it is easy to understand why a digestive system that breaks food down into different units is needed. Enzymes are an important part of the process. If the digestive system is a conveyor belt, enzymes are the machines and workers which slowly change whatever is on the belt to make it smaller and smaller so that, eventually, it can be carried around the body in blood. They are made of protein and act as catalysts - that is, they make chemical changes happen in other substances, while themselves remaining unchanged. They act on food, changing it into smaller particles.

What Is The Tongue?

The tongue is a muscular organ, covered with a membrane. It is held in place by attachments to the lower jaw (mandible) and the hyoid bone. Tiny projections known as papillae cover the top, increasing its surface area and producing a rough texture. Sensory nerve endings in the papillae form what we commonly know as taste buds.

The tongue has 3 digestive functions — taste, chewing and swallowing:
- Taste: The tongue is covered with thousands of taste buds which are sensitive to salt, sweet, sour and bitter chemicals in food and drink. They help us enjoy what we eat and drink and act as the first line of defense, warning us when food, drink or foreign matter are off or inedible,
- Chewing: The tongue aids chewing by moving food around the mouth, pushing it between the teeth and covering it with saliva, which contains enzymes that start the digestive process. The food is turned into a partially digested mass known as a bolus.
- Swallowing: When the food is ready to travel to the stomach, the tongue pushes it to the back of the mouth.

How Does Food Get From The Mouth To The Stomach?

Through the action of swallowing and through the portion of the gastrointestinal tract known as the esophagus. The tongue pushes the bolus to the back of the mouth, towards the pharynx, a muscular tube behind the mouth. The food passes into the pharynx and down to the esophagus. The epiglottis, a small flap of cartilage which forms part of the larynx (the windpipe) moves upwards and forwards, blocking the entrance to the larynx. This stops the food from 'going down the wrong way' and prevents choking.

What Is The Esophagus?

The esophagus is a muscular tube which leads from the pharynx, at the back of the mouth, to the stomach, the first main organ of digestion.

To carry chewed food from the pharynx to the stomach. Food moves along it by a muscular contraction known as peristalsis. The muscle fibers contract and relax pushing the bolus forward. The lining of the esophagus secretes mucus to ease and lubricate the passage of food.

Stage 2: What Is The Stomach?



The stomach is a J-shaped, elastic organ which expands and contracts depending on what is in it. Food enters it from the esophagus via the esophageal sphincter, a valve that stops back flow of the stomach's contents. It leaves the stomach through the pyloric sphincter into the duodenum, the first part of the small intestine. The wall of the stomach is a combination of layers of muscle fiber with an inner mucous membrane. The inner mucous membrane has lots of folds, called rugae. When the stomach is full they stretch out, enabling expansion, then they contract when it empties.
What Does It Do?

- Digests proteins through the action of enzymes.
- Churns food with gastric juices.
- Helps to lubricate the food by producing mucus (from the mucous membrane).
- Absorbs alcohol.
- Kills bacteria in the food by producing hydrochloric acid.
- Stores food prior to it passing to the small intestine.

Gastric juices contain:
- Hydrochloric acid: neutralizes bacteria and activates pepsin.
- Rennin: enzyme that curdles milk protein (only in infants).
- Pepsin: enzyme that acts on proteins turning them into polypeptides.

At this stage proteins have been partially digested and, along with the carbohydrates such as starch which were partially digested in the mouth, they have to wait until the small intestine to complete digestion.

The Chemistry Of Digestion

The whole digestive process is a combination of different chemical reactions that act on the food we eat, reducing it to the building blocks of nutrients for absorption and use by the body. Every piece of food we eat is composed of fats, carbohydrates and proteins. These must be broken down into their relative chemical compounds in order for the body to use them i.e. by the time the bread you eat reaches your muscles as energy it has been chewed, churned, liquefied and the starch changed to useable glucose. The following shows the main chemical reactions and breakdowns at different stages of digestion.

What Are Proteins?

Proteins foods include dairy products, meat, fish and beans. Proteins are large molecules that are made up long chains of polypeptides. They are the building material for the body. In order to be used by the body they must be broken down into their smaller components - amino acids. There are approximately 20 amino acids classified by whether they are essential (those the body cannot make, that must therefore be supplied in the diet) and non-essential (those the body can make).
Proteins are broken down in the body by the following processes:
• In the stomach, the enzyme pepsin begins the digestion of proteins by breaking them down into large polypeptides.
• In the small intestine, enzymes from the pancreas break the large polypeptides into smaller chains.
• Finally, still in the small intestine, enzymes from the intestine, including aminopeptidase, break up the small polypeptides into individual amino acids ready for absorption.

What Are Fats?

Fats are classified as saturated, monounsaturated or polyunsaturated. Saturated fats can be found in dairy products and meat. Monounsaturated fats can be found in olive oil and avocados. Polyunsaturated fats can be found in sunflower oil and oily fish. Some polyunsaturated fats cannot be made by the body and are therefore also classified as essential fats and must be consumed in the diet. In order to be used by the body, fats must be broken down to fatty acids and glycerol.
Fats are broken down in the body by the following processes:
• In the small intestine, fat are emulsified by bile salts from the liver.
• In the small intestine, lipase from the pancreas breaks down emulsified fats into fatty acids and glycerol ready for absorption.

What Are Carbohydrates?

Carbohydrates are classified as monosaccharides, disaccharides or polysaccharides. Monosaccharides include fructose in fruit. Disaccharides include lactose in milk. Polysaccharides include starch and fiber in cereals, potatoes and other plant sources, and glycogen in meat. All carbohydrates are broken down to monosaccharides for absorption and all eventually become glucose to supply the body with energy.

Carbohydrates are broken down in the body by the following processes:
• In the mouth, salivary amylase begins the breakdown of polysaccharides.
• In the small intestine, intestinal amylase breaks down polysaccharides to disaccharides.
• In the small intestine, maltase, lactase and sucrase convert disaccharides to monosaccharides ready for absorption.

Stage 3: The Small Intestine

The small intestine, ironically, is not that small. It is seven 7 long and divided into 3 different parts: the duodenum, the jejunum and the ileum. The walls have several layers, including a muscular layer, a layer containing blood vessels, lymph vessels and nerves and an inner mucous membrane. The inner wall is covered with villi, tiny finger-like projections which increase the surface area for absorption and contain a network of blood and lymph vessels.

What Does The Small Intestine Do?

It completes the chemical digestion of food and the subsequent absorption of nutrients takes place in the small intestine. Nutrients are absorbed through the villi into the blood and lymph vessels. Hardly any food is absorbed elsewhere in the digestive system.

How Does Digestion And Absorption Happen In The Small Intestine?

1. Waves of muscular contractions called peristaltic movements mix food with intestinal and pancreatic juices as well as bile. The movements push the food against the villi. Intestinal juices are composed of enzymes:
- Maltase, sucrase and lactase which split disaccharides into monosaccharides
- Enterokinase which activates trypsin in pancreatic juice
- Peptidases which split polypeptides into amino acids.

2. Several hormones in the small intestine help digestion by stimulating the production of pancreatic or intestinal juices and regulating acidity levels.

3. Absorption: Digested food is absorbed by either active transport or diffusion.
- Most nutrients, including amino acids and sugars, are absorbed by active transport through the walls of the villi where they enter the bloodstream and are carried to the liver in the hepatic portal vein.
- Fats, fatty acids and glycerol diffuse into the lacteals (lymphatic capillaries). They are called lacteals because the fat passes into them in suspension, causing the lymph to look milky.

Other Functions Of The Small Intestine

To protect the digestive system from infection. It is the only section of the digestive system with a direct link to the protective lymphatic system.

Stage 4: Large intestines And Waste

What Is The Large Intestine?

The large intestine deals with waste. It is about 1.5 meters long and sits draped around the small intestine. It consists of the caecum, appendix, colon, rectum, anal canal and anus.

To reabsorb water and nutrients from digestive waste and to get rid of waste. Whatever remains of the food, once it has been through the processes of mixing, conversion and absorption carried out in the stomach and small intestine, is passed into the large intestine. Any remaining nutrients are removed and the result is feces.


Feces is the unwanted leftovers from food, combined with cellulose (roughage which is indigestible, found in foods like vegetables and bran), dead blood cells, bacteria (both living and dead), fatty acids and mucus used to help move the feces through the large intestine. The color comes from the dead blood cells and bilirubin, a bile pigment.

Summary: Functions Of The Large Intestines

• Absorbs nutrients, vitamins, salt or water left in digestive waste.
• Secretes mucus to help passage of feces.
• Stores feces in rectum (short-term because the arrival of feces in rectum tells the brain we need to defecate).
• Micro-organism/bacteria activity: Lots of bacteria live in the large intestine. Though they can cause disease they are usually harmless in the colon and may even be useful.
• Defecation: a 'mass movement' pushes waste along the colon, often stimulated by food arriving in the stomach. It is a reflex but humans have control of it. If the reflex is ignored, more water will be absorbed from the feces which may cause constipation.

Other Organs Involved In The Digestion Process

There are several other organs involved in the digestive process: the tongue, teeth and salivary glands, liver, pancreas and gall bladder. They are known as accessory organs because, although they do not form part of the gastrointestinal tract, they help the digestive process by breaking down foodstuffs and the toxins/ waste produced during digestion. The role of the tongue, teeth and salivary glands has been mentioned earlier in the chapter.

What Is The Liver?

The liver is the largest gland in the body. It is located on the right side of the tummy, just below the diaphragm. Its role is to regulate, convert, store and process countless substances that we eat, breathe in and absorb through the skin. It is vital because it performs many essential functions.


- Toxic substances like drugs and alcohol.
- Nitrogen from amino acids.

- Vitamins K, E, A and B12
- Glycogen, a substance that stores energy
- Iron, from food and from the breakdown of red blood cells
- Fats.

- Heat. The liver acts as a radiator, producing more heat than any other organ as a result doing its various jobs
- Vitamin A, found in green-leafed vegetables and carrots
- Vitamin D
- Plasma proteins
- Heparin
- Bile
- Uric acid and urea

- Stored saturated fat into cholesterol
- Glycogen into glucose when energy is needed
- Glucose back to into glycogen when prompted to do so by the presence of insulin
- Metabolizes protein.

What Is The Gall Bladder?

A pear-shaped sac attached by bile ducts to the back of the liver. Whenever there is excess bile secreted by the liver which can't be used immediately for digestion, it passes along the bile ducts to the gall bladder where it will be stored until needed.

- Storage facility for bile (from liver)
- Secretes mucus to add to bile
- Absorbs water from bile, making it more concentrated
- Contracts to empty bile into duodenum.

What Is The Pancreas?

The pancreas is a gland situated behind the stomach, between the duodenum and the spleen. It delivers pancreatic juices to the duodenum through the pancreatic duct. The cells of the pancreas are divided into the islets of Langerhans (which produce insulin and glucagon) and a network of alveoli (small sac-like cavities). The alveoli are lined with cells that produce enzymes.

The pancreas works with both the digestive and the endocrine system. It produces enzymes to break down food, the hormone insulin which regulates the blood sugar level after eating by causing the conversion of glucose to glycogen for storage in the liver and muscles, and the hormone glucagon which converts glycogen back to glucose.

What Is Insulin?

Insulin is a hormone secreted by specialized cells in the pancreas known as the islets of Langerhans. It regulates blood sugar level. When we eat, the blood sugar level rises. The sugar in the blood is in the form of glucose. Insulin helps cells absorb glucose and turns any excess glucose into glycogen, an insoluble sugar which is stored in the liver until the body needs it. Thus the blood sugar level drops. A lack of insulin causes diabetes. Type 1 diabetes is caused by auto-immune damage to the pancreas resulting in low or no insulin production. Type 2 diabetes is strongly linked to obesity and although insulin may still be produced it is unable to work properly in the body (insulin resistance). In either type, glucose cannot be properly absorbed into the body resulting in the following symptoms — a dangerously high level of blood sugar (hyperglycemia), the loss of glucose through excretion, thirstiness and excessive urine production. Low levels of blood sugar is known as hypoglycemia.

Diseases Of The Digestive System

Anorexia is a loss of appetite. Anorexia nervosa is a psychological condition which often affects teenage girls and young women. The sufferers have a fear of gaining weight or being fat and refuse to eat very much or stop eating altogether. It can be severely debilitating and sometimes fatal.

Acid Reflux Disease
Also known as GERD, it is a condition where food in liquid form leaks backwards from the stomach into the esophagus.

Acute inflammation of the appendix, usually treated by removal of the organ.

A bloated stomach causes a feeling of fullness, even if you have not eaten. It may also cause a tummy ache and cramping.

Bulimia is where someone regularly eats large amounts of food in a short period of time and then follows it up with self-induced vomiting and excessive use of laxatives. Bulimia is a psychological condition which often affects teenage girls and young women, and increasingly young men.

Chronic damage to an organ causing hardening. Several types of cirrhosis exist but the most common is cirrhosis of the liver, which is frequently caused by excessive alcohol consumption.

Constipation is the infrequent or uncomfortable bowel movements, causing hard feces to block the rectum. Caused by lack of fiber in the diet, lack of fluids and lack of exercise. Sometimes caused by stress.

Candida is a yeast infection caused by the fungus Candida albicans. It can cause an infection in the vagina or thrush in the mouth, throat and intestines.

An inflammation of the large intestine (the colon).

Colitis/Ulcerative Colitis
Ulcerative colitis (image) is a serious form of inflammatory bowel disease (IBD) and occurs just as often in men as women. A common characteristic is blood filled or pus covered diarrhea. It occurs when the colon becomes red and swollen.

Crohn's Disease
A disease of the small intestine that often spreads to the colon. Crohn's disease is characterized by diarrhea, cramping and loss of appetite and weight, with local abscesses and scarring.

Celiac Disease
Is a common bowel condition that is caused by intolerance to a protein in foods called gluten. Gluten is found in wheat, barley, rye and possibly oat products.

Diarrhea is where a person suffers episodes of frequent and watery bowel movements. See, how to treat diarrhea.

It is a disease which affects the lining of the large intestine. It is caused by the infection or rupturing of the wall of the digestive tract, usually the colon. It leads to herniation or outpouching of the intestinal lining.

An inflammation of the intestine (especially the small intestine) usually characterized by diarrhea.

An inflammation of the lining of the stomach which is characterized by nausea, loss of appetite and discomfort after eating.

Inflamed Gall Bladder
An inflammation of the gall bladder.

Gastroesophageal reflux disease (GERD) is a condition where food in liquid form leaks backwards from the stomach into the esophagus. It is also known as acid relux disease.

Stones formed from bile, cholesterol and calcium salts, found in the gall bladder.

Burning sensation in esophagus or throat, caused by back flow and regurgitation of acidic stomach contents.

A rupture, in which an organ pushes through the surface of the structures which normally hold it in. A Hiatal hernia is where part of the stomach bursts into the chest.

Excessive levels of bile pigments in the blood cause skin to turn yellow. Caused by a faulty gall bladder or blockage in the flow of bile.

Irritable Bowel Syndrome
No exact cause is yet known for irritable bowel syndrome (sometimes referred to as IBS), though stress and low-fiber, high fat diets are said to contribute. Symptoms include stomach and bowel pain and alternate bouts of diarrhea and constipation.

The following artical explains a list of digestive problems in pregnancy.

The most common effect of stress on the digestive system is ulcers. Anxiety and lack of relaxation cause overproduction of gastric juices and if they have nothing to work on they will start to attack the lining of the stomach or other structures. In short, the stomach starts digesting itself! Read more about the dangers of stress or take our online stress quiz to see if you suffer from it.

Painful sores that occur on the lining of the stomach walls. They are often caused by too much acid or infection from bacteria called H. pylori.

Flatulence (Farting)
Releasing excessive digestive gases through the anus. The average person farts about 15 times a day, they may not even be aware of it.

Inflammation of the gums.

Hiccups are caused by the sudden contraction of the diaphragm. Hiccups that last 48 hours or more or rare, but should be investigated. Short bouts are harmless.

Indigestion (Dyspepsia)
A disorder that causes discomfort in the upper chest, heartburn, burping and nausea. Most cases are caused by eating and drinking something, occasionally it can be the side effect of a medication or infection. See, how to treat indigestion.

Nausea is the sensation of discomfort and sickness in the stomach that makes us feel like we want to vomit.

Obesity is a medical condition in which excess body fat has accumulated to the extent that it may have an adverse effect on health, leading to reduced life expectancy.
Are you overweight? Calculator

Pernicious Anemia
A chronic progressive anemia of older adults, thought to be caused by impaired absorption of vitamin B-12 due to the absence of intrinsic factor.

An ulcer is a sore that appears on the tissue of the digestive tract. Ulcers can appear in the stomach (stomach ulcers), on the esophagus (esophageal ulcer), and the Duodenum - first section of the small intestines (duodenal ulcer).

How to stop someone choking: First aid treatment.

Other Systems It Links To

The digestive system links to:
All systems in the body because it provides nutrition to the whole body.

Circulatory System: This system transports nutrients from the digestive system to every system of the body.
Endocrine System: The endocrine system secretes certain hormones, which help metabolism.
Lymphatic System: Lymphatic vessels are found in the villi in the small intestine and help with the absorption of fats.
Nervous System: all the organs of the digestive system are stimulated by nerve impulses.
Urinary System: Excretes excess liquids taken from food from the body.
Muscular System: Sphincter muscles contract along the alimentary canal to push food along - known as peristalsis.


The digestive system:

• Transforms food and drink into nutrients and waste.
• Consists of every process from eating (ingestion) to excretion.
• Relies on chemicals (enzymes) to carry out the breaking down of food.

Other Systems Of The Body

Respiratory System: How we breathe.
Female Reproductive System: How babies are made.
Skin Structure and Function: What the skin does.

Other Useful Guides

Recommended health screenings for women: Tests for every woman, every age.
Main causes of death in women: What diseases kill women?
How menopause affects the body: Slows the digestion and a host of other problems.
Hospital departments explained: A to Z of departments.
Womens health books: List of useful resources and books.

Back To Homepage: Womens Health Advice

Please Note: Information provided on this site is no substitute for professional medical help. See Disclaimer.
Copyright. All rights reserved.