Large Intestine: Structure and Contribution in Digestion, Practice Problems and FAQs

We have a variety of food. But have you ever wondered what happens to the food components that are not digested? Yes. You might have heard about the process of defecation.

The food we eat gets digested and absorbed in the various parts of the alimentary canal and provides us energy after assimilation to the different parts of the body. But, all the food we eat is not digested. The remaining undigested food is converted into stool or faeces in the large intestine.

GIF: Defecation

The large intestine is the last part of the digestive system. The stool forms in the large intestine and gets stored in the rectum until it pushes out of the body. You all know if this process didn’t happen properly, then you will get abdominal discomfort and pain. Let’s understand more about the large intestine in this article.

Table of contents:

Structure of large intestine
Digestion in large intestine
Absorption in large intestine
Practice Problems
FAQs

Structure of large Intestine

Large intestine is known because of its large diameter. It is a tubular structure which is about 1.5 m long. Large intestine is divided into three parts such as caecum, colon and rectum.

Caecum

Caecum has a characteristic pouch-like structure into which ileum opens. It is host to symbiotic microbes. There is a large blind sac that opens into the colon. Caecum has a vermiform appendix that is a narrow finger-like projection. It is considered as a vestigial organ. It is a residual part from ancestors with no known function. Caecum is well- developed in herbivores like horse, rabbit etc. Inflammation of the appendix is known as appendicitis.

Fig: Caecum

Colon

Colon is responsible for the reabsorption of fluids. It processes waste products and prepares for elimination from the body. Colon is composed of the following four parts:

Ascending colon

This part of the colon passes upwards on the right side of the abdominal cavity.

Transverse colon

This part of the colon bends to the left and runs across the abdominal cavity.

Descending colon

This part of the colon extends downwards on the left side of the abdominal cavity.

Sigmoid colon

This part is also called the pelvic colon. It turns to the right and joins with the rectum part of the large intestine.

Fig: Colon

Taeniae coli and haustra

The taeniae coli are the three separate longitudinal ribbons of smooth muscles present on the outside of the ascending, transverse, descending and sigmoid colon part of the large intestine. Haustra are the small pouches present on the colon part of the large intestine. This gives the large intestine a segmented appearance.

Rectum

Rectum is considered as the last part of the digestive tract. It ends in anal canal that opens through anus. It is about 15 - 20 cm in diameter. Haemorrhoids or piles are normally seen in this region. The bowel movement of anus is controlled by two sphincters:

Internal anal sphincter has involuntary and smooth muscle fibre
External anal sphincter has voluntary and striped muscle fibre

Fig: Rectum

Digestion in large intestine

The process of mechanical and chemical breakdown of food into simple molecules is called digestion. Digestion is done through two processes. These are as follows:

Mechanical digestion
Chemical digestion

Mechanical digestion

It changes the physical structure of food molecules. In this process, enzymes are not involved in breaking.

Chemical digestion

It changes the chemical structure of the food molecules. Enzymes are involved in this process.

The digestive system has three major regions where mechanical and chemical digestion occurs. These are the oral cavity, stomach and small intestine. Therefore, the large intestine plays a little role in digestion of food. Majorly, it helps in the absorption of food molecules. In the large intestine, more water is absorbed from the gastrointestinal tract to the bloodstream. Symbiotic bacteria present in the large intestine that help in breaking down remaining nutrients. As a result, vitamin K and B complexes are formed.

Absorption in large intestine

Large intestine helps in the absorption of digested products. The colon part of the large intestine helps in the absorption of water and electrolytes. The large intestine changes the absorbed undigested food into stool. Most of the water is reabsorbed by the process of osmosis. The rectum stores stool for some time until it pushes out through anus. Peristalsis is responsible for pushing the stool into the rectum. Therefore, the faecal matter is not completely solid.

Fig: Absorption in large intestine

Practice Problems

Q1. Large intestine is classified into how many parts?

A. Two
B. Three
C. Four
D. Five

Solution: Large intestine is divided into three parts: caecum, colon and rectum.

Caecum has a characteristic pouch-like structure into which ileum opens. It is host to symbiotic microbes. This opens into the colon.
Colon is responsible for the reabsorption of fluids. It processes waste products and prepares for elimination from the body.
Rectum is considered as the last part of the digestive tract. It ends in anal canal that opens through anus.

Hence, the correct option is b.

Q2. Match column A with column B and choose the correct option:

Column A	Column B
Proteases	i. Nucleic acids
Lipases	ii. Carbohydrates
Carbohydrases	iii. Proteins
Nucleases	iv. Fats

a. 1 - iii, 2 - iv, 3 - ii, 4 - i
b. 1 - iv, 2 - i, 3 - ii, 4 - iii
c. 1 - iii, 2 - i, 3 - iv, 4 - ii
d. 1 - ii, 2 - iv, 3 - i, 4 - iii

Solution: Proteases catalyse the hydrolysis or breakdown of proteins or polypeptides into amino acids. Lipases are enzymes that catalyse the hydrolysis or breakdown of fats into fatty acids and glycerol. Carbohydrases catalyse the breakdown or hydrolysis of polysaccharides into monosaccharides. Nucleases are the set of enzymes that catalyse the hydrolysis or breakdown of nucleic acids like RNA and DNA into its sugars and bases. Hence, the correct option is a.

Column A	Column B
Proteases	iii. Proteins
Lipases	iv. Fats
Carbohydrases	ii. Carbohydrates
Nucleases	i. Nucleic acids

Q3. Determine the statement which is not associated with the large intestine.

A. Absorption of water
B. Absorption of vitamins
C. Digestion of food
D. Elimination of solid waste

Solution: Water absorption, mineral absorption from food, and solid waste removal are all activities of the large intestine. Symbiotic bacteria prevalent in the large intestine that produce considerable amounts of vitamin K and vitamin B complex. In the large intestine part of the alimentary canal, these vitamins are absorbed. The large intestine does not produce any digestive enzymes and hence does very little digestion. The small intestine is the site where food digestion normally ends.

Q4. What function does the large intestine play in the process of digestion?
Answer: Large intestine helps in the absorption of digested products. The colon part of the large intestine helps in the absorption of water and electrolytes. It changes the unabsorbed undigested food into stool.

Q5. How can the large intestine be differentiated from the small intestine?
Answer: Large intestine is known because of its large diameter. It is a tubular structure which is about 1.5 m long. Large intestine is divided into three parts such as caecum, colon and rectum.

On the contrary, the small intestine is a tubular structure and has a small diameter. It is divided into duodenum, jejunum and ileum.

FAQs

Q1. Which type of cells is present in the large intestine?
Answer: In the large intestine, simple columnar epithelial cells are present. Goblet cells are also present in the large intestine.

Q2. What causes pain in the large intestine?
Answer: The pain that arises in the large intestine or colon is due to constipation, diarrhoea, colitis, and colorectal disease.

Q3. How much faeces is present in the large intestine of a healthy adult?
Answer: The large intestine of a healthy male produces one pound of faeces and the large intestine of a female produces 14 ounces of faeces in a day.

Q4. Which type of bacteria are present in a large intestine?
Answer: In the large intestine, obligate anaerobes are commonly found. The most abundant bacteria are the members of the genus Eubacterium, Lactobacillus and Clostridium.

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