Digestive System

Digestive System plays a very important role in our body. We eat food to stay healthy, energetic and alive. Well, so how does the food that we eat convert into energy? What turns food into energy? What exactly happens inside our bodies when we consume food? In this article, let’s get to know the journey of food inside our bodies.

The digestive system helps obtain nutrition that is needed for our body’s growth, maintenance and energy from the food that we eat. We eat food to get energy. The journey from eating food and getting energy is a bit long! However, this article helps you understand the whole process of the digestive system: what is digestion, parts of the digestive system and associated glands and their function and more.

Digestive System: Definition

The digestive system is a well-defined system that helps digest the food, which will, in turn, be absorbed and used by cells to liberate energy. This system carries out complete digestion of food, i.e., complex food is broken into its simpler, absorbable form. The digestive system consists of the alimentary canal and its associated digestive glands.

What is Digestion? – Digestion is the catabolic process in which large food molecules are broken down into simpler absorbable molecules in the digestive system with the help of digestive enzymes.

For example, carbohydrates are broken into simple sugars, i.e., glucose. These simpler molecules are then absorbed in the blood or lymph and assimilated by the cells. These are then used up by the cell to produce energy in the form of ATP.

Nutrition and Digestive System

Our digestive system and the process of nutrition are closely associated. Food contains vital nutrients like carbohydrates, proteins and fats, which our body needs for growth and maintenance. These complex nutrients cannot be used up by the body directly, so they are broken down into their simpler form by the digestive system.

Like carbohydrates are broken down into simple sugars, fats and lipids are broken down into fatty acids and glycerol; proteins are broken down into amino acids in a sequential manner in the digestive system or the alimentary canal. These simpler forms of nutrients now can be absorbed by the body and can be assimilated to produce energy.

Digestive System Diagram

The alimentary canal is about 9 m long in humans, which facilitates the gradual and sequential breakdown of food and absorption of simpler molecules as well as water. The digestive system comprises the alimentary canal working in coordination with the digestive glands to facilitate the process of nutrition in humans.

Parts of Human Digestive System

The digestive tract of humans and its associated glands that allow complete digestion of food consists of the following parts:

The alimentary canal is a 9 m long muscular tube, which starts from the mouth and ends at the anus. It has the following parts:

Mouth

The alimentary canal of the digestive system starts with the mouth. It is surrounded by lips, which guard its opening. Lips are sensitive to heat and cold. It helps in sipping liquids and in speech. The mouth leads to the buccal cavity. Inside the buccal cavity, there are teeth, a tongue, and three pairs of salivary glands.

Functions of Mouth: It is the cavity where food is chewed by teeth and is mixed with saliva with the help of the tongue. The digestion of carbohydrates starts in the buccal cavity with the help of salivary amylase.

Tongue

It is a muscular, movable, protrusible sense organ that is attached to the lower jaw at the posterior end that helps us to sense the taste. The tongue has taste buds that help to sense taste. In the given figure, different taste sites have been shown.

Functions of Tongue: The tongue helps in:

1. Mixing the food with saliva while chewing
2. Tasting the food
3. Pushing the food down the buccal cavity
4. Cleaning the teeth after eating
5. Speaking

Teeth

In humans, a set of temporary milk or deciduous teeth is replaced by a set of permanent or adult teeth. Organisms with such a type of dentition are called diphyodont.

Each tooth is embedded in the socket of the jawbone. This type of morphological arrangement of teeth is called the thecodont dentition.

An adult human has 32 permanent teeth which are of four different types. Such a type of dentition is called heterodont dentition.

1. Incisors (I): These are sharp, front teeth with a flat edge. These help in cutting or biting substances.
2. Canine (C): These are sharp and pointed teeth, located next to the incisors, and help in tearing food. This type of teeth is poorly developed in humans.
3. Premolars (P): These are pointed and broad teeth and are also known as bicuspids. These help in chewing and crushing food.
4. Molars (M): These are flat and broad teeth located at the back of the jaw. These help in chewing and crushing food efficiency because of the large surface area. The third pairs of molars are called the wisdom teeth which develop between 17-25 years.
5. The dental formula for a permanent set of human beings is ICPMICPM=21232123. The dental formula for the temporary set of human beings is ICPMICPM=21022102

Pharynx

The oral cavity leads into the next part of the digestive system called the pharynx. It is a common passage for food and air (breathing). The region of the pharynx is divided into three regions:

1. Nasopharynx: The region of pharynx present at the junction of the nasal chamber.
2. Oropharynx/Glossopharynx: The region of pharynx present at junction of epiglottis.
3. Laryngopharynx/ Hypopharynx: The region of pharynx present at the junction of the larynx.

Oesophagus or Food Pipe – (25 cm long)

The oesophagus (also known as the food pipe) is a 25 cm long, tubular, muscular structure of the digestive system that leads to the stomach. It carries the food bolus from mouth to stomach. It is not involved in the digestion of food. The food can travel all along the food pipe due to the contraction and expansion of the oesophagus which is called peristalsis.

Stomach – (30 cm long)

It is a ‘J-shaped’ muscular structure located below the diaphragm, present between the oesophagus and small intestine.

Structure of Stomach

Parts of the Stomach

1. Cardia
2. Fundus
3. Body
4. Pylorus

Functions of Stomach

1. It is mainly involved in the digestion of proteins and in a very small amount digestion of lipids.
2. Muscles of the stomach contract due to which food inside it gets churned.
3. Complex food molecules get broken down due to the secretion of few enzymes like pepsin, rennin, lipase, etc.
4. The stomach contains an oesophageal sphincter and pyloric sphincter. The former prevents the backflow of a food bolus from the stomach to the oesophagus. While later prevents, the semi-digested food from passing from the stomach to the intestine until it is properly churned up.
5. In the stomach, partial digestion of food takes place. This partially digested food is called chyme which enters the small intestine, where complete digestion occurs.

Small Intestine – (6.25m long)

Parts of Small Intestine: Next in the digestive system comes the small intestine. It is a tubular structure that is 7 meters long and 2.5 cm wide. The intestine is coiled and folded in the abdomen and has the following three parts, namely:

1. Duodenum
2. Jejunum
3.  Ileum

Duodenum – (25 cm long)

Structure: It is the short upper part of the small intestine. It starts after the pyloric end of the stomach. It is a ‘U’ shaped structure and receives the semi-digested food called here ‘chyme’ from the stomach.

Jejunum – (2.5 m long)

Structure: It is a 2 m long-coiled tube that is the middle part of the small intestine. Primarily absorption occurs here.

Ileum – (3.5 m long)

Structure: It is a 3.5 m long coiled tube. It is the last part of the small intestine where both digestion and absorption occurs. Here complete digestion of food takes place.

Functions of Small Intestine

1. In the small intestine, both digestion and absorption occur.
2. Simpler forms of food like glucose, amino acids, etc., are absorbed here as it has numerous finger-like projections called villi.
3. These projections increase the surface area of the intestine so that more nutrients can be absorbed.
4. The villi are provided with blood vessels and lymphatic vessels. These lymphatic vessels are known as lacteals.
5. The blood vessels absorb the digested food, which will be transported to the cell and used by the same for liberating energy.
6. Lacteals are responsible for absorbing the digested fats.

Large Intestine – (1.5 m long)

Parts of Large Intestine: After the small intestine, the large intestine is the next part of the digestive system. This is a tubular structure that is larger in diameter than the small intestine. So it is named as a “large” intestine, although it is small lengthwise as compared to the small intestine. It can be divided into three parts, namely, caecum, colon and rectum.

Caecum Structure: It is present at the beginning of the large intestine, which is the connection between the large and small intestine. It is a pouch-like structure and from its blind end, the vermiform appendix is projected. Researchers believe that it is a vestigial organ, which means that this organ was useful to humans in the past.

Functions of Large Intestine

1. Solid waste products and liquids are passed on to the large intestine from the small intestine. So, it acts as the reservoir for liquids.
2. Salts and electrolytes are separated from the liquids and are absorbed here in the caecum.
3. Cecum also mixes the solid waste with the mucus and lubricates it.

Colon Structure: It is divided into ascending colon, transverse colon and descending colon. It is the longest part of the large intestine. Here, absorption of water takes place.

Functions of Colon

1. Absorption of water takes place here.
2. After absorption of water, the solid waste is moved to the rectum by muscular contractions.

Rectum Structure: It is the last part of the large intestine and is 15 cm long which opens at the anus. It has anal sphincters which keep it close and open when needed to pass the bowel to the anus.

Functions of Rectum

1. It receives the solid wastes from the colon and stores it for some time.
2. It plays an important role in the process of defecation.

Anus

It is the last part of our digestive system which is 4-5 cm. It helps to eject out the faecal matter.

Glands of Human Digestive System

Salivary Glands

Salivary glands are exocrine glands and secrete saliva, which initiates the digestion of food in the digestive system. There are three pairs of salivary glands present in our digestive system. These are

1. Parotid Gland: These are the largest salivary glands present beneath and in front of each ear.
2. Submandibular Glands: Found at lower jaw on each side.
3. Sublingual Glands: Found below the side of the tongue, these are the smallest salivary glands.

Composition of Saliva: Saliva is slightly acidic (pH about 6.8) in nature and consists of water, salts, mucus, an enzyme called salivary amylase (ptyalin). Saliva is 99% water. It comprises:

1. Salivary amylase (Ptyalin): It breaks α (1-4) glycosidic bonds of amylopectin and hence helps in the digestion of carbohydrates.
2. Lysosomes: It has bactericidal action.
3. Ions: Na⁺, K⁺, Cl^–, HCO₃^–

Functions of Saliva

1. Moistens food so that it can get easily passed through the buccal cavity of the digestive system.
2. Digests carbohydrates as it contains a digestive enzyme called salivary amylase or ptyalin.
3. Helps in sticking the food particles to one another to form a bolus like structure so that it can be easily passed down the food pipe and into the digestive system.
4. Helps in killing germs of the mouth and prevents tooth decay.

Gastric Glands

The stomach is a muscular structure of the digestive system that consists of gastric glands, which secrete gastric juices to digest food. The gastric juice is formed by the secretion of three types of gastric glands. The secretions and cells are:

Lipase: Digests lipids or fats.

1. Oxyntic Cells or Parietal Cells: It produces HCl and castle intrinsic factor.
2. HCl or Hydrochloric Acid:  Lowers the pH (1-3) of the stomach to kill the germs and activate the digestive enzymes.
3. Castle Intrinsic Factor: Helps in absorption of vitamin B₁₂
4. Neck Cells: It produces mucus that protects the lining of the stomach from an acidic environment created by HCl.
5. Chief Cells: It mainly produces proteolytic enzymes that help in digestion of protein in the stomach part of the digestive system. It mainly produces pepsinogen (in adults) and prorennin (in children).
6. Pepsin: Digests proteins.
7. Rennin: Digest casein protein of milk.
8. Lipase: Digests lipids or fats.

Functions of HCl in Stomach

1. It creates an acidic medium in the stomach which can kill the microorganisms coming along with food.
2. This acidic medium is required for the activation of pepsin from pepsinogen for the digestion of proteins into peptides.

Liver

It is the largest gland of the digestive system as well as the largest exocrine gland of the human body. It secretes bile which is stored in the gallbladder till the chyme arrives at the duodenum. Bile helps in the breakdown of large globules of fats into smaller globules so that fat-digesting enzymes can easily act upon them.

It contains bile salts and bile pigments (biliverdin and bilirubin). Bile also changes the acidic medium of the chyme to the alkaline medium, so that enzymes can act upon it.

Pancreas

It is present behind the stomach. It is a mixed gland, i.e. can function both as an endocrine and exocrine gland. Its endocrine part, the cells of islets of Langerhans, releases blood-glucose regulating hormones, i.e. glucagon and insulin. On the other hand, the exocrine part secretes pancreatic juices which help in the digestion of food.

Pancreatic juices contain trypsin, amylase and lipase. Trypsin is secreted as trypsinogen, which is an inactive form and gets activated in the alkaline medium in the duodenum. It, along with another proteolytic enzyme named chymotrypsin, can digest the leftover proteins into smaller peptides and amino acids. Lipase named steapsin acts on the emulsified fats and the pancreatic amylase called amylopsin here acts on the carbohydrates.

Digestion Process

In the human body, the process of digestion takes place as follows:

1. Ingestion: It refers to the process of intake of food.
2. Formation of Bolus and its movement: A mass of food, bolus, is formed by mixing food with saliva. This is then moved down through the pharynx into the oesophagus. The bolus further passes down from the oesophagus through successive waves of muscular contractions called peristalsis.
3. Digestion: It involves the breakdown of complex food into its simpler form in the digestive system.
4. Absorption: It involves the absorption of simpler substances into the blood and the lymph (mostly the fats). It occurs mainly in the ileum part of the digestive system.
5. Assimilation: It is the process by which the absorbed food is used by the cells to generate energy.
6. Egestion: It refers to the process by which undigested food is egested out of the body.

Importance of Digestion

1. Our digestive system helps in breaking down the complex food materials into their simpler forms which are easily absorbed by the villi present in the small intestine. From here, these simpler forms of food are transported to the cells through blood where these are used to release energy.
2. This energy is used by us to do several tasks in our day to day life. So, when we do not eat food we feel lethargic.

Diseases or Disorders Related to Digestive System

Gastroesophageal Reflux Disease (GERD): If a person gets acid reflux for more than a week, then he or she may be suffering from GERD. The oesophagus opens in the stomach through a lower oesophageal sphincter or LES. In this disease, this LES gets weakened and often opens whenever the stomach is loaded with food. This leaks the acid from the stomach to the oesophagus, which causes heartburn.

Jaundice: It is the diseased condition of the body when the skin turns yellow. This is due to the increased level of bilirubin present in the bloodstream resulting from liver disease. When RBCs die in 120 days, bilirubin is produced as a by-product. This yellow pigment, bilirubin is filtered from the blood by the liver. But if there is too much bilirubin, it leaks to the surrounding tissues resulting in yellowing of skin and eyes.

Peptic Ulcer: It is a sore that develops in the lining of the oesophagus or various parts of the stomach. When there is a breakdown of the protective mucus layer of the stomach lining, then acid attacks the lining of the stomach which causes a sore in it resulting in an ulcer.

Diarrhoea: The abnormal frequency of bowel movement and increased liquidity of the faecal discharge is known as Diarrhoea.

Constipation: In this disorder, the faeces are retained within the rectum as bowel movements occur irregularly.

Summary

In a nutshell, the food journey starts at the mouth and travels through the digestive system and ends at the anus. With the help of the digestive organs, the digestive system performs the process of absorption and digestion. This process extracts the nutrients required for our body from the food we consume, thus helping our body stay healthy and fit.

Related Topics to Study

FAQs

Q.1. What is the digestive system and its function?
Ans: The digestive system consists of the alimentary canal or gastrointestinal tract and its associated glands. Its function is to convert complex food into a simple form so it can be absorbed and used by cells to liberate energy.

Q.5. What are the 12 parts of the digestive system?
Ans: The digestive system includes the alimentary canal and its associated glands. The alimentary canal or gastrointestinal tract includes the mouth, pharynx, oesophagus, stomach, intestines, rectum and anus. Its associated glands are salivary glands, liver and pancreas.

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