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Leaving Cert Biology Revision: Digestive System

Digestive System


The process through which organisms get and use food for development, energy, and the upkeep of body functions is called nutrition.

Types of nutrition

  • Autotrophic organisms, such as certain bacteria and plants, that produce their nourishment.

  • Heterotrophic organisms ingest food from other sources, such as bacteria, fungi, and mammals.

One can be a vegetarian, carnivore, or omnivore who is heterotrophic.

Ingestion: consuming food. Food enters your mouth and travels through your digestive tract.

During digestion, your body breaks down your food into smaller bits. 

There are two ways that it occurs:

Mechanical digestion is comparable to using your teeth to eat food. It aids in the food's reduction in size so your body can process it more easily.

Chemical digestion: During this process, several chemicals in your body, such as acids and enzymes, aid in further breaking down the food.

Absorption: Your body absorbs the beneficial components of food once it has been broken down into tiny bits. 

After entering your circulation, your cells utilize these nutrients to support growth and provide you with energy.


Egestion: Your body gets rid of things it can't use through the process of egestion. The portions of food that your body cannot utilize or absorb are expelled as waste through your anus.

The Digestive System's Components

Oral Mechanized Digestive Process

Food goes through mechanical digestion in the mouth. 

Here, the food is physically broken down into tiny bits by the teeth. Enzymes have more surface area to act on as a result of this process, which also makes the food easier to swallow.

There are four different kinds of teeth in the front of the mouth, starting with:

  1. Incisors: These teeth have a chisel form and are employed in biting, slicing, and cutting operations.

  2. Canines: These teeth resemble fangs and are long and pointed. They seize, pierce, and rip food.

  3. Premolars: These teeth have cusps, which are surface projections. They chew and break food.

  4. Molars: The back of the mouth contains these massive teeth. They chew and smash food as well.

Milk Teeth:

We have milk teeth, which are our first set of teeth when we are young. Eventually, these erupt and are replaced by our permanent teeth. 

Twenty milk teeth and thirty permanent teeth are present. The set of milk teeth is devoid of molars.

The two lower central incisors, or cutting teeth, often erupt first at six months. The two upper central incisors come after these. Teeth erupt at the rate of about one each month after they begin to erupt. 

Your baby might thus have six teeth by the time he turns one.

Chemical Digestion in the Mouth

  1. In the mouth, there is an enzyme called amylase, also known as ptyalin, which is present in saliva. Saliva is produced by three pairs of salivary glands situated beneath the tongue, behind the jaws, and in the cheeks.

  2. Saliva contains lysozyme, which aids in the destruction of microorganisms. Its role is to moisten the food, making it easier to swallow. Amylase works to break down starch into maltose.

  3. Food is pushed in the direction of the pharynx after it has been chewed into a ball or bolus. Food leaves the mouth at this moment and enters the esophagus. The epiglottis, a little flap that closes over the trachea, keeps food from getting inside the windpipe.

  4. The brain is in charge of voluntary swallowing and feeding during the early stages. 

  5. Food is easier to swallow and has more surface area for saliva to operate on once it is broken down into tiny bits by the teeth. Food is moved around the mouth by the tongue, lips, and cheeks to help the teeth.

  6.  After this mechanical process, the bolus becomes softer and is ready to be swallowed.

  7. The tongue then forces the bolus into the upper portion of the throat. It naturally pushes the epiglottis lower as it passes into the esophagus, further closing up the airway. The bolus finally passes into the esophagus.

The Oesophagus

The stomach and mouth are joined by the esophagus. Peristalsis, an involuntary wave-like muscle movement, is how food passes down the esophagus.

The esophagus is not the only organ where peristalsis occurs. It passes through the big, small, and stomach intestines. Lastly, food is forced into the rectum by peristalsis. It is now prepared to be released from the anus.

The Stomach

The stomach is like a bag made of muscles where food is stored and broken down. At the end of the tube that connects your mouth to your stomach, there's a muscle that opens and closes to let food into the stomach and prevent it from going back up.

Inside the stomach, there's a lining called the mucosa, which has many folds with tiny glands. These glands release a liquid called gastric juice, which has three main parts:

1. Mucus: This is like a protective layer that stops the stomach from harming itself.

2. Pepsinogen: It's a substance that turns into pepsin, which helps break down proteins in food.

3. Hydrochloric Acid (HCl): This acid makes the stomach very acidic, which helps with digestion and kills bacteria.

When you eat, the stomach muscles squeeze and mix the food, turning it into a thick, creamy mixture called chyme. Then, the chyme moves into the small intestine through another muscle called the pyloric sphincter.

The Small Intestine

The body's cells are nourished by nutrients that are taken into the blood in the small intestine. The duodenum, jejunum, and ileum are its three portions, and it is roughly 6 meters long.

1. The Duodenum: This part is essential to finishing the process of digestion. It gets nutrients from the pancreas and liver to help with digestion, and its lining cells also create digestive enzymes.

The inner surface of the small intestine is covered in many folds called villi; each villus has about 600 microvilli. These structures greatly increase the colon's surface area, improving absorption and digestion.

Intestinal glands located between the villi produce a variety of enzymes referred to as intestinal juices.

The arrow points to the intestinal gland at a villus's base.

Pancreatic juice, which the pancreas secretes to aid in digestion, contains the following nutrients:

1. Sodium hydrogen carbonate, a salt that balances the stomach's acidic chyme.

2. Digestive enzymes, such as amylase (which converts starch to maltose).

 3. Lipase, which converts fats into glycerol and fatty acids.

These digestive enzymes move through the pancreatic duct and function in the duodenum.

The Liver

There are many functions of the liver. Some of the most important are:

1. The production of bile.

2. Detoxifying the body, i.e. breaking down alcohol and drugs.

3. Breaking down excess amino acids to form urea.

4. Converting glucose to glycogen for storage.

5. Converting excess carbohydrates to fats.

6. Storing vitamins.

7. Storing minerals such as Iron, Copper, and Zinc.

8. Making plasma proteins such as fibrinogen which is used in blood clotting.

9. Making cholesterol which is used to form many hormones.

10. Producing heat for the blood and body.

The Bile

The yellow-green substance known as bile, which is produced in the liver and kept in the gall bladder, has various uses.

1. This process increases the surface area of big fat and oil molecules for improved enzyme action by breaking them down into little droplets.

2. Sodium hydrogen carbonate, found in bile, aids in the neutralization of stomach chyme.

3. The bile removes these pigments, which are made from red blood cells that have died.


Food is completely broken down and then absorbed in the jejunum and ileum, the last sections of the small intestine. Villi, which have thin walls and are found in large numbers in certain areas, allow for effective absorption. 

Nutrients such as glucose, amino acids, vitamins, and minerals are absorbed by the microscopic blood arteries (capillaries) found in each villus and subsequently released into the bloodstream.

Through the hepatic portal vein, these nutrients are then transported to the liver where they can be stored or used by the body right away. The liver converts surplus amino acids through a process known as deamination to urea. 

Via the hepatic vein, waste products from the liver are excreted by the kidneys.

Fatty acids and glycerols are two types of fats that are absorbed in different ways. They enter the lymph fluid-filled lacteals, which are specialized veins found within the villi. 

Fats leave the lacteals and pass through the lymphatic system before entering the bloodstream through the veins in the neck area known as the subclavian veins.

The Large Intestine

The big intestine is just 1.5 meters long but has a breadth of roughly 6 centimeters, making it broader than the small intestine. Food can stay here for anywhere from ten hours to many days. 

The appendix and caecum are located in the large intestine. The appendix is regarded as a vestigial organ since it may have once assisted in the digestion of cellulose. But at the moment, it mostly produces white blood cells and harbors bacteria.


Undigested food is extracted from its water content by the colon, which is a part of the large intestine.

The stool is held in the rectum until the anus is used to evacuate it.

Excess water in the stool causes diarrhea, which is frequently brought on by food passing through the colon quickly.

Constipation is caused by the removal of excessive amounts of water from the stool, which usually happens when food passes through the colon slowly.

Food Groups

Food is divided into four major types, each of which provides comparable nutrients. Please find these categories and the suggested daily servings in the table below.

Disorders of Eating

Anorexia nervosa and bulimia nervosa are two common eating disorders. People have a strong desire to regulate their weight and body shape and to be slender in both scenarios.

Symptoms of anorexia nervosa include extreme obsession with food and body image, dread of gaining weight, and abrupt weight reduction. Episodes of binge eating are followed by periods of self-induced vomiting and dietary restriction, which is known as purging in bulimia nervosa. 

Psychological counseling and education for the afflicted individuals and their families are common treatments for these diseases.

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