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Key Highlights
- The small intestine is an organ found in the gastrointestinal tract which involves the absorption of nutrients.
- It is located in the lower abdominal cavity beneath the stomach.
- The small intestine is categorized into three main parts: the duodenum, jejunum, and ileum.
- It is the longest section of the gastrointestinal tract where processing of food takes place.
- Some important functions of the small intestine include the breaking of food particles and the removal of unnecessary components from the body.
The small intestine is part of the digestive system and is considered the primary organ in the gastrointestinal tract. It lies between the stomach and the large intestine.
- The small intestine consists of three main parts, namely Duodenum, Jejunum and Ileum.
- It is about 5.5 metres (18 feet) long with a vast surface area that is squeezed into folds and peaks.
Key Terms: Small Intestine, Human Body, Digestive System, Duodenum, Jejunum, Ileum, Stomach, Large Intestine, Liver, Pancreas, Pancreatic juice
Small Intestine
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The small intestine is an organ situated in the gastrointestinal tract where the food nutrients are absorbed completely. It involves the complete digestion of fats, proteins, and carbohydrates.
- It is also known as small bowel.
- The small intestine receives two main intestinal juices from the liver and the pancreas.
- The liver produces bile juice, and the pancreas secretes pancreatic juice.
- The length of the small intestine found in males is slightly greater than in females.
- It controls 90% of the digestion and absorption process, whereas the remaining 10% occurs in the large intestines.
- The intestine is composed of layers of tissues and muscles, which contain nerves, blood vessels and lymphatic glands.
- The small intestine is inflamed due to enteritis caused by viral and bacterial infections.
Small Intestine Diagram
The structural diagram of the small intestine is as follows:
Small Intestine Diagram
Structure of Small Intestine
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The small intestine forms a tubular or coiled-like structure that is present between the stomach and large intestine. It is considered a small bowel. The food will enter into the intestine after passing through the stomach.
- It is located at the inferior portion of the abdominal cavity following the stomach and surrounded by the large intestine.
- The small intestine’s length varies from as short as 10 feet to as long as 34+1⁄2 feet.
- It has a large surface area, which is caused by folds, villi and microvilli.
- The intestine has an appearance of long, pink or red tubes the width of your pointer finger.
Parts of Small Intestine
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The small intestine of the human body is made up of three sections which are given below.
Duodenum
The duodenum is the proximal or first part of the small intestine, which allows food to pass from the stomach into the small digestive tract. It circles around the pancreas before connecting to the rest of the intestines.
- The duodenum is divided into four parts: superior, descending, horizontal and ascending part.
- It is shaped like a "C" whose length ranges from 20–25 cm (8–10 in) in length.
Function of Duodenum
The function of duodenum are as follows:
- When the process of digestion is initiated, the duodenum prepares the chyme so that food can be broken down into simpler substances.
- It receives digestive juices from other organs, including your liver, gallbladder and pancreas, to break food particles.
- This part contains Brunner's glands, which help in the production of mucus-rich alkaline-containing bicarbonate.
Jejunum
The jejunum is the second or middle part of the small intestine, located between the duodenum and ileum. It consists of circular folds and intestinal villi that increase its surface area.
- There is a cell lining which is responsible for the absorption of nutrients.
- The villi are finger-like projections.
- It is characterized by blood vessels, which give this part of the small intestine a deep red colour.
Function of Jejunum
The functions of jejunum are as follows:
- Its function is to absorb some of the dietary nutrients such as amino acids, fatty acids, and sugars.
- Nerves will trigger the muscles to churn food back and forth with digestive juices.
IIeum
Ileum is the third or the last part of the small intestine in the digestive system. It further opens into the large intestine. This section of intestine is 3 m long, and also contains villi similar to the jejunum.
- The walls of the ileum become thin and narrow, and the supply of blood is reduced.
- It absorbs vitamin B12 and bile acids.
- Since food stays mostly in the ileum, most water and nutrients are absorbed in this part of the intestine.
Functions of Small Intestine
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The final process of digestion takes place in the small intestine. Some important functions are as follows:
Digestion of Food
The duodenum absorbs the partially digested food and receives digestive juices such as bile juice and pancreatic enzymes from the liver and pancreas.
- Bile juice from the liver converts fat into small particles, making digestion easier.
- The pancreas secretes pancreatic juice that converts fats into fatty acids and glycerol.
- The linings of the small intestine produce intestinal juice, which helps break down carbohydrates and starch into simple sugars.
- The sugars, also known as glucose, convert proteins into amino acids.
- The simpler form of food nutrients is called digested food.
Absorption of Digested Food
Digested food is absorbed in the walls of the small intestine by tiny finger-like projections known as villi. The small intestine transfers the broken-down food materials to the blood, which carries them to different parts of the body.
- The glucose breaks down in the small intestine to produce oxygen and carbon dioxide.
- It will release the energy required for various life processes.
Absorption of Proteins
The small intestine acts on proteins, peptides, and amino acids, which are followed up by catalysts like trypsin and chymotrypsin emitted by the pancreas. This results in breaking food particles into compounds in the stomach and goes on until the digestive organ.
Sample Questions
Ques. What do you mean by digestive system? (3 marks)
Ans. The human digestive system comprises gastrointestinal passages with some accessory organs like the tongue, mouth, salivary glands, liver, pancreas, etc. The whole system plays an important role in the absorption and digestion of food nutrients. The journey of food starts from the mouth and ends at the anus. It produces energy after the conversion of food into nutrients. The energy is needed by our body for metabolic activities.
Ques. What are the parts of the digestive system? (2 marks)
Ans. The human digestive system consists of an alimentary canal and its associated digestive glands. The accessory digestive glands consist of the salivary glands, liver, and pancreas. They all are responsible for the digestion of food particles. The process starts from the mouth and ends with the posterior part i.e anus. The alimentary canal is classified into different parts. They are as follows.
- Mouth
- Buccal cavity
- Pharynx
- Esophagus
- Stomach
- Small intestine
- Large intestine
- Rectum
- Anus
Ques. What are the functions of the small intestine? (2 marks)
Ans. The small intestine is the site where different types of food nutrients and water are absorbed. This produces digestive juices for the proper digestion and absorption of food nutrients. It works with the associated digestive glands. They all play an important role in incomplete digestion of food particles.
Ques. What are the functions of the duodenum? (2 marks)
Ans. Duodenum is the primary portion of the small intestine. Its size is about 25.4 cm. The duodenum makes the chyme for the process of conversion of complex food into simpler substances. That's why the nutrients can be absorbed quickly.
Ques. What are the functions of jejunum? (2 marks)
Ans. The jejunum is the middle part of the small intestine. This is having a length of about 0.9 m. There is a cell lining in the jejunum which is also helpful in the absorption of the nutrients. The amino acids, fatty acids, and sugars are absorbed by the jejunum.
Ques. What are the functions of Liver? (3 marks)
Ans.Liver is the largest gland of the body and consists of hepatic cells. Besides being a digestive gland, the liver performs a number of functions for the welfare of body. Its varied functions are as follows.
- Liver secretes bile juice.
- It stores fat, glycogen, vitamins like A, D, E, K and B12, blood, water, etc.
- The gland helps in the synthesis of urea.
- It helps in the deamination of amino acids.
- Liver plays a part in the elimination of excretory substances.
- The gland etoxifies harmful substances.
- It assists in the formation and breakdown of blood corpuscles.
- Liver secretes blood proteins, i.e., prothrombin and fibrinogen.
- It secretes enzymes.
Ques. What are the functions of the ileum? (2 marks)
Ans. This is the third and last part of the small intestine. Its length is about 1.8 meters. This is responsible for the absorption of the nutrients such as fats, carbohydrates, vitamins and minerals, and water from the food to provide energy to the body. Those products that were not absorbed by the jejunum are finally absorbed by the ileum.
Ques. What do you mean by digestion and absorption of food particles? (3 marks)
Ans. Digestion: This is the process in which the complex food particles are broken down into simple forms. The process involves two steps i. e. mechanically and chemically.
Absorption: This is the process in which the digested food nutrients are absorbed by the blood. The blood is then able to supply to all parts of the body. Finally, the nutrients are digested and absorbed in the small intestine.
Ques. What is the role of pancreatic juice in the digestion of proteins? (2 marks)
Ans. The pancreatic juice contains inactive enzymes trypsinogen, chymotrypsinogen, procarboxypeptidase. Trypsinogen is activated by an enzyme enterokinase, (secreted by the intestinal mucosa) into active trypsin, which in turn activates the other enzymes of the pancreatic juice. Proteins, proteoses and peptones (partially hydrolysed proteins) in the chyme reaching the intestine are acted upon by these proteolytic enzymes of pancreatic juice.
Ques. Describe the process of digestion of protein in the stomach? (3 marks)
Ans. The gastric glands of the stomach secrete gastric juice that contains HCl and proenzymes – pepsinogen and prorenin. The proenzyme pepsinogen, on exposure to HCl gets converted into the active enzyme pepsin, the proteolytic enzyme of the stomach. The pepsin converts proteins into proteases and peptones (peptides). Prorenin is found in the gastric juice of infants and is activated by pepsin into active rennin. It helps in the digestion of milk protein casein.
Ques. Why bile juice is important for digestion? (2 marks)
Ans. Bile has no enzymes but contains bile salts, namely, sodium bicarbonate, sodium glycocholate and sodium taurocholate that reduce the surface tension of large fat droplets and break them into many small droplets by a process known as emulsification. These small fat droplets present a large surface area for lipase (fat-digesting enzyme) to act upon them. Moreover, bile also activates lipases.
Ques. How does butter in the food get digested in the human body? (3 marks)
Ans. Butter is saturated fat. Fats and oils of the ingested food are triglycerides. They are digested by lipases. The small intestine is the principal organ for fat digestion. In the small intestine, food meets three secretions, bile, pancreatic juice and intestinal juice, all alkaline in nature. Bile contains no enzyme but it contains bile salts which reduces the surface tension of large fat droplets and breaks them into smaller ones (emulsification).
Emulsified triglycerides Pancreatic juice contains pancreatic lipase, which is the principal fat-digesting enzyme. It is activated by bile.
Emulsified Triglyceride → Fatty acid + Diglyceride
Diglyceride → Fatty acid + Monoglyceride
Monoglyceride → Fatty acid + Glycerol
Ques. What is intestinal lipase? (5 marks)
Ans. Intestinal lipase found in intestinal juice hydrolyses some triglycerides, diglycerides and monoglycerides to fatty acids and glycerol like pancreatic lipase. Fatty acids, glycerol and monoglycerides are the end products of fat digestion and is insoluble in water. It cannot be directly absorbed from the intestinal contents.
- So they combine with the bile salts and phospholipids to form micelles (water soluble).
- From the micelles, fatty acids, glycerides, sterols and fat-soluble vitamins are absorbed into the intestinal cells by diffusion where they are resynthesised in the ER and are converted into very small protein coated fat molecules (droplets) called chylomicrons.
- The latter is released from the intestinal cells into the lymph present in the lymphatic capillaries, the lacteals.
- These lacteals ultimately release the absorbed substances into the bloodstream.
Ques. Describe the digestive role of chymotrypsin. Which two other digestive enzymes of the same category are secreted by its source gland? (2 marks)
Ans. Chymotrypsin is a proteolytic enzyme of pancreatic juice secreted by the exocrine part of the pancreas. It helps in the digestion of proteins. It converts proteins, peptones and proteoses into oligopeptides and dipeptides. Two other proteolytic enzymes present in pancreatic juice are trypsinogen and procarboxypeptidase.
Ques: What are the two main functions of the small intestines? (2 marks)
Ans: The small intestine has 2 main functions:
- Break down food and absorb the necessary nutrients from it. The duodenum, jejunum and ileum are used to absorb nutrients.
- Getting rid of unecessary components of the food.
Ques: What is the difference between small and large intestine? (2 marks)
Ans: The main differences are tabulated below.
| Parameters | Small Intestine | Large Intestine |
|---|---|---|
| Length | Measure around 4.5 to 7.0 m in length | Measure around 1.5 m in length |
| Width | It is narrow in width of around 3.5 to 4.5 cm | It has a width of 4 to 6 cm in diameter |
| Parts | It is divided into three sections: duodenum, jejunum, and ileum. | It is divided into 4 sections that are colon, rectum, caecum, and anal canal. |
Ques: What are the parts of small intestine and their functions? (3 Marks)
Ans: The small intestine is a part of the human digestive system. It is a coiled-like structure that ranges from 6-7.5 m long. This is a site for the process of absorption of nutrients like proteins, fats, etc. This is a narrow tube with a large internal surface area. It has three main parts. They are given below.
- Duodenum is the first part of the small intestine and occurs in between the small intestine and jejunum.
- The jejunum is the second portion of the small intestine. It is located between the duodenum and ileum.
- The ileum is the last portion of the small intestine that ends in the large intestine.
The step-by-step process of digestion takes place in the small intestine.
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