Large Intestine- Structure and Functions

Introduction

The large intestine or the large bowel is the last part of the digestive system responsible for the final absorption of food. It eliminates all the waste out from the body as fecal matter. The large intestine is 5 feet long (approximately) and is shaped like an inverted or an upside-down ‘U’. The large intestine is shorter when compared to a small intestine but is referred to as large due to its larger lumen. It has a larger diameter of around 3 inches. The main purpose of the large intestine is to absorb the water and salts and store the stool till it is excreted from the body through the anus. This process is known as defecation.

Length of the Large Intestine in Humans

The large intestine is approximately 5 feet long (or 1.5 meters) and about 3 inches wide tubular structure which forms the distal part of the gastrointestinal tract. The length of the large intestine in humans is 1/5th of the whole length of the digestive tract starting from the mouth running till the anus. 

The length of the large intestine in humans is shorter when compared to the small intestine which is approximately 22 feet long (or 7 meters) but it is much broader than the small intestine. The first part, the cecum, is around 2 inches long, the major section of the large intestine is the colon which is the longest and the rectum is about 7 inches in length. Anus; is the last part of the intestine and is only about 1 inch long.

Large Intestine Function in the Digestive System

The large intestine runs from the end of the small intestine to the anus, making it the last part of the digestive tract. The large intestine has 4 parts, namely: cecum, colon, rectum, and anus. The large intestine is also called the colon, as the colon forms the maximum of the large intestine. The large intestine function in the digestive system is to get rid of the waste which is left from the food. 

The majority of the digestion process and absorption is completed in the small intestine. When the food reaches the large intestine, it is generally the undigested fibers remaining from plants, salts and bile pigments. From the small intestine, food in liquid form reaches the large intestine, where it is converted to a semi-solid state by absorbing the water and any essential nutrients which can be useful for the body. This solid waste is eliminated from the body through rectum and finally is out via the anal canal. The large intestine function in the digestive system is to absorb water as per the requirement in the body. 

Other than water and electrolyte absorption, the large intestine also provides an environment for thriving of good microbes and synthesis of vitamins by good gut microflora. Vitamin B complex and vitamin K are synthesised in the colon, which helps in metabolism and boosts immunity. The large intestine function in the digestive system is also to store the waste material or the feces in the rectum temporarily for about a day before it is expelled from the body.

The intestines (both small and large, take up most of the area of the abdominal cavity)

The large intestine is located just below the waist in the right pelvic region of the body.

 The large intestine starts at the cecum which is the first part of the large intestine situated in the lower right abdomen. The last part of the small intestine known as ileum connects to the cecum, which is a pouch-like structure and is the widest. A small vestigial organ, known as the vermiform appendix, is an extension from the cecum and is connected to the cecum. Appendix serves as the home for gut bacteria and helps in the restoration of gut microflora. The cecum is the first to receive the digested food material from the small intestine and takes over the digestion process from thereon. The cecum continues the process of absorption of water and salts from the partially digested food. 

The cecum takes the food to the colon. The colon is the longest part of the large intestine. The large intestine, the colon has 4 parts, namely: 

• Ascending colon 
• Transverse colon
• Descending colon 
• Sigmoid colon

The ascending colon travels up the right side of the abdomen for about 8 inches, the transverse colon runs across the abdomen and the descending colon travels down to the left of the abdomen forming an upside-down U shape. The sigmoid colon depicted in the diagram of the large intestine is an S-shaped structure that connects to the rectum. The large intestine has smooth inner walls lined with mucous membrane, which secretes mucus and lubricates the undigested waste material. The reabsorption of water and other electrolytes mainly takes place in ascending and transverse colon. The descending colon mainly stores the feces till it reaches the rectum. The contraction of segmented muscles (churning and peristaltic movement) of the intestine helps in the forward movement of waste material. The muscles in the sigmoid colon contract to increase pressure and move the feces from the descending colon into the rectum.

The rectum is the last part of the large intestine and opens into the anus. The feces accumulate in the rectum just before the discharge. Due to the pressure on the walls of the rectum, the nerves of the rectal cavity sends signal to the anal canal and brain, developing the urge to eliminate the feces.

The rectum ends at the anal canal, which opens as the anus to the body’s exterior.  Anal canal is the extreme segment of the alimentary canal. The muscles of the anal canal relax to ease the process of elimination of faecal matter from the body. Anus helps in the process of defecation. Defecation is the process of elimination of feces, which is the undigested food in a semi-solid state from the digestive process.

Purpose of the Large Intestine 

The purpose of the large intestine in humans is to complete the process of digestion, which starts at the mouth. The food moves through the gastrointestinal tract via the stomach, small intestine and large intestine to reach the anal canal. The main purpose of the large intestine is the formation of feces from the undigested material which is left after the digestion is completed by the small intestine. 

The peristaltic movement in the intestine helps in pushing the food towards the distal part of the alimentary canal and aids in the completion of digestion.

Three functions of the large intestine:

• Absorption of water and nutrients: The large intestine absorbs most of the water from the food (known as chyme), which reaches the colon in a liquid state. Reabsorption of water and essential nutrients converts the food to a semi-solid form. The muscular contractions help food to move towards the rectum.
• Formation of feces: Undigested food mainly contains fiber from plant sources and form the feces. Feces contain the old cells from the lining of the intestine, waste from undigested food and water, fats and proteins. It also contains secretions like bile, which gives the brown colour to the feces. The mucus secreted from the lining of the large intestine facilitates smooth bowel movements.
• Production of vitamins: As there are no digestive enzymes secreted in the large intestine, digestion mainly takes place due to the action of bacteria present in the colon. The large intestine acts as a nourishing environment for gut microflora. These bacteria also protect the gut from harmful microbes and form infections. These microbes protect our gut lining and help synthesise a few essential vitamins like Vitamin B complex, and Vitamin K. These vitamins are then readily absorbed by the blood. 

Conclusion

The large intestine is the final site of digestion in the human digestive system. This organ is responsible for the absorption of water and minerals from undigested food. It helps to get rid of the waste from the body. Large intestine assists in maintaining the water and salt balance in the body. The large intestine function in the digestive system is to eliminate all the waste, toxins and undigested food from the body as faecal matter.