Human Digestive System- Parts and Functions | Process of Digestion

You have learned that plants prepare their own food by photosynthesis. Animals get their food from plants either directly by eating plants or indirectly by eating animals that eat plants. Animals take food and digest them with the help of their digestive system. Are you familiar with different parts of your digestive system? Let’s see the journey of food inside the human digestive system.

We take in food through the mouth. The food passes through a continuous canal which begins at the buccal cavity (oral cavity) and ends at the anus forming an alimentary canal or digestive tract. The canal can be divided into various compartments:

1) The buccal cavity
2) Food pipe or esophagus
3) Stomach
4) Small intestine
5) Large intestine ending in the rectum
6) Anus

The food components gradually get digested as food travels through the various compartments
Salivary glands, liver, inner walls of the pancreas, and the small intestine secrete digestive juices. These digestive juices convert complex substances into simpler substances. The digestive tract and the associated glands together constitute the digestive system.

The Salivary Glands and the Tongue

Let’s learn the importance of salivary glands and the tongue in the human digestive system. The food enters the digestive tract through the mouth. Let us learn about the salivary glands and the tongue- the two important parts that are present inside the buccal cavity. The human mouth possesses salivary glands which are present inside the buccal cavity.

These salivary glands secrete saliva which plays an important role in breaking down complex food components such as starch into simplified forms such as sugars. The saliva also binds the masticated food into a slippery bolus that usually slides down easily through the esophagus.

Let us see some more important functions of saliva.

• The saliva solubilizes dry food in order to be tasted.
• It keeps the mouth moist.
• Another important function of saliva is that it provides an alkaline medium for food digestion.

The muscular organ attached at the back to the floor of the buccal cavity is called the tongue.

  • It is free at the front and can move in all directions.
  • It helps us in talking. It is also used in moving the food in different directions and it also mixes saliva with the food during chewing, thus helps in swallowing food.
  • The tongue possesses taste buds that detect the different tastes of food. Taste buds, therefore, play the most important role in helping us enjoy the many flavors of food.

Our taste buds can basically recognize four different kinds of tastes namely: Sweet, Salty, Sour, and Bitter.

The salty and sweet buds are located near the front of our tongue, while the sour taste buds line the sides of the tongue, and the bitter taste buds are found at the back of our tongue. The buccal cavity ends into a region common for both food and air called the Pharynx.

The Buccal Cavity

The oral cavity or the buccal cavity is the entrance to our alimentary canal. The buccal cavity is the space between our teeth and cheeks. It is bounded by the upper and lower lips in the front, by the roof of the mouth on the top, by the tongue in the bottom, and by the cheeks on the sides. Learn more about the role of the buccal cavity in the human digestive system.

The food is taken in through the mouth and this process is known as Ingestion. This food is then crushed and chewed in the buccal cavity with the help of the teeth and the tongue. You must be aware that adults have 32 teeth that break down the ingested food into smaller pieces.

Each tooth is rooted in a separate socket in the gums. The human teeth vary in appearance and perform different functions. Accordingly, there are four different types of teeth.

Let us see the names of different types of teeth and its functions.

  1. Incisors- They are mainly used for cutting and biting food.
  2. Canines- These are sharp and pointed and are used to pierce or tear food.
  3. Premolars- These are used for chewing of the food, and lastly…
  4. Molars- These are used for the final chewing and grinding of the food before swallowing it into the food pipe.

The last molars present at every side of each jaw are referred to as the Wisdom teeth. You must have noticed and wondered why your mother or teacher always asks you to brush your teeth twice daily. Here is the reason: Normally, bacteria are present in our mouth which are usually harmless.

However, if our mouth and teeth remain unclean, many harmful bacteria start growing therein. These bacteria break down the sugars present from the leftover food and release acids. These acids gradually damage the teeth. This is called tooth decay which if not treated on time may cause severe toothache, and sometimes even results in tooth loss.

As a precaution to prevent and avoid tooth decay and damage, we should brush our teeth twice daily and should also rinse our mouth after every meal. We should also avoid chocolates, sweets, soft drinks, and other sugar products which cause and hasten tooth decay so as to maintain oral hygiene.

Process of Digestion – The Oesophagus

You already know that the digestion of food starts in the mouth and you have also seen how it is partially¬ digested in the buccal cavity with the help of teeth, tongue, and salivary glands.

Now, the swallowed food passes into the food pipe, also known as the esophagus. The esophagus runs along the neck and the chest and is 25 cm long. The esophagus conducts the food from the throat to the stomach. Movement of the food inside the esophagus is followed by peristaltic movements throughout the alimentary canal which are actually responsible for pushing the food downwards.

You must have wondered why your parents always tell you not to talk while eating. This is because both the oesophagus and the windpipe which carry air start from the pharynx. When the food moves into the oesophagus, a flap-like valve called the Epiglottis closes the windpipe.

This prevents the entry of food particles into the trachea. However, if you continue to talk while eating, there are chances of the valve not closing fully. This may lead to the entry of some food particles into the windpipe and may lead to coughing or hiccups.

The process of digestion – Stomach

We have already learned the structure and role of the buccal cavity and esophagus in the digestion of food and the human digestive system. Let us now see how to further digestion of food occurs in the stomach. Through the esophagus, the food enters the stomach. It is the widest part of the alimentary canal and is located between the esophagus and the intestine. It has a sac-like shape like a flattened ‘U’.

The human stomach is muscular and elastic, and lies crosswise in the abdominal cavity. It is about 12 inches long and 6 inches wide at its widest point, and can hold approximately 2-3 liters of food at a time in adults.

A ring of muscular valve closes off the stomach at each end. The valve between the stomach and the oesophagus prevents the food from passing back to the oesophagus while the lower valve separates it from the duodenum.

Let us now have a look at how the food is digested inside the stomach. The inner lining of the stomach secretes mucosa, hydrochloric acid, and digestive juices. Each of these has a specific function. The mucous protects the lining of the stomach.

Whereas the role of acid is to kill the harmful germs and bacteria which enter along with the food and makes the medium in the stomach acidic, thereby enabling the digestive enzymes to act. On the other hand, the function of digestive juices is to break down the proteins into simpler substances. The stomach then opens into the small intestine.

Process of Digestion – The Small Intestine

We have seen how the food is digested by the secretions of the inner lining of the stomach. From the stomach, the partially digested food, also called the chyme enters the small intestine. The small intestine is a highly coiled tunnel-like tube which is the longest section of the digestive tract.

It is approximately 7.5 meters long and is divided into 3 segments namely duodenum, jejunum, and ileum, each with a different digestive function. The small intestine receives secretions from the liver and the pancreas, besides secreting digestive juices itself.

The liver is the largest gland in the body and part of the human digestive system. It is reddish-brown and is situated in the upper part of the abdomen on the right side. It secretes bile juice which is temporarily saved in a sac called the gall bladder. Bile juice plays an important role in the digestion of fats.

The pancreas is a large cream-colored gland positioned just below the stomach. It has a narrow tube that joins the bile duct. This tube is called the pancreatic duct. The pancreas secretes the pancreatic juice that helps to digest carbohydrates and proteins.

The food then reaches the lower part of the small intestine where the intestinal juice completes the digestion and converts the carbohydrates into simple sugars and glucose, proteins into amino acids, and fats to fatty acids and glycerol.

Now, the digested food passes into the blood vessels in the wall of small intestine. This process is known as absorption. The inner walls of the small intestine possess millions of small finger-like projections called villi which increase the surface area for absorption of digested food by almost 8 times.

Each villus has a network of thin and small blood vessels close to its surface. The absorbed digested food is then transported via the blood vessels to different organs of the body where they are used to build complex substances required by the body such as proteins.

This is called assimilation. The energy needed for various activities is obtained by the breakdown of glucose. In the cells, glucose breaks down with the help of oxygen into carbon dioxide and water, and energy are released for various metabolic activities of the body. The remaining undigested and unabsorbed food then enters the large intestine.

Process of Digestion- The large intestine

We have learned how the digestion of food takes place in the digestive tract till the small intestine. Let’s learn about the importance of the large intestine in the human digestive system. We also know that the undigested food from the small intestine enters the large intestine. Let us now understand the process of digestion of food in the large intestine.

The large intestine is wider and shorter than the small intestine. It is about 1.5 meters in length,
The undigested food that enters the large intestine has a lot of water and salts in it. The large intestine absorbs the water and some salts from the undigested food material.

This occurs in the following manner. The undigested food travels upwards in the ascending colon of the large intestine. It then passes across the abdomen in the transverse colon and finally moves downwards in the descending colon.

The remaining undigested semi-solid waste passes into the rectum and is called feces. The fecal matter is then removed through the anus from time to time through a process called ‘Egestion’.

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