Large intestine: Explanatory overview of anatomy and physiology

What is Large Intestine ?

The large intestine, also known as the large bowel, is the last part of the gastrointestinal (GI) tract which plays a central role in digestion. It is where undigested food received from the small intestine is processed and any water and remaining nutrients are absorbed before the indigestible matter is excreted from the body as feces (stool).

It is approximately 3 inches in diameter compared to the small intestine which is closer to 1 inch. In total, the large intestine is around 6 feet long, while the small intestine is approximately 21 feet long.

The large intestine also plays a central role in your health by influencing the enteric nervous system which not only directs digestion and gut motility (movement) but is also involved in stimulating hormones and the immune response.

Cecum-

The first part of the large intestine is a small pouch called the cecum which connects to the last part of the small intestine, called the ileum. The section of the large intestine, measuring only about 2.5 inches long, receives and holds partially digested food (chyme) so that gut bacteria can break down an indigestible fiber called cellulose.

The cecum also absorbs salts and electrolytes from chyme and secretes mucus that lubricates the solid waste so that it passes more easily into the colon.

Colon

The colon is the largest part of the large intestine. It is tasked with extracting key nutrients and any remaining water from the indigestible material to form stool. From start to finish, the colon is between 60 and 65 inches long.

The colon consists of four sections:

• Ascending colon: This is the first part of the colon that starts in the lower right abdomen and runs upward toward the diaphragm.
• Transverse colon: This is the second part which runs horizontally across the top of the abdomen from the right to the left,
• Descending colon: This is the third part which moves downward along the left side of the abdomen.
• Sigmoid colon: This is the final S-shaped section situated in the lower left abdomen that connects to the rectum.

Structure of the Large Intestine-

The colon is comprised of four layers of tissues, similar to other regions of the digestive tract. These include:

• Mucosa: This innermost layer secretes mucus to lubricate stools, protect against abrasive particles, and prevent parasites, viruses, or bacteria from penetrating intestinal tissues.
• Submucosa: This underlying layer contains blood vessels, nerves, and connective tissue that supports the other tissue layers. 
• Muscularis: This is the layer of smooth muscles that contract involuntarily to move food through the digestive tract (otherwise known as peristalsis).
• Serosa: This is a thin layer on the outside of the intestine that secretes a lubricating fluid that helps it glide against other internal organs.

Rectum-

The rectum is the final straight portion of the large intestine measuring about 5 inches long. It acts as a temporary storage site for feces until it is ready to be passed (defecated).

At the top and bottom of the rectum are two circular muscles, called the internal and external sphincters, that open and close to move feces in and out of the rectum.

The internal sphincter next to the sigmoid colon is involuntary and opens and closes when needed. The external sphincter next to the anus is voluntary and can be held shut to keep stool from leaking out (referred to as fecal incontinence).

As the walls of the rectum expand with feces, nerve signals are sent to the brain that trigger contractions that initiate defecation.

Anus-

The anus is the external passageway through which feces exit the body. Running about two inches, the anus is where mucosal membranes in the digestive tract change to normal skin. When you are ready to defecate, the internal sphincter helps push stool out of the body.

The anal canal is composed of inner grooves, called anal sinuses, that secrete mucus to aid with the smooth passage of stool.

The final grooved section (where mucosal tissues turn to skin) is called the anal valve. This is the area populated with nerves that can sense pain, itch, heat, and cold. The rest of the large intestine lacks sensory nerves.

Function of the Large Intestine-

As part of the gastrointestinal tract, the large intestine has four key functions that enable digestion:

• It breaks down indigestible fiber so that it can pass more easily through the intestine.
• It absorbs water from chyme to form healthy stools.
• It secretes mucus to make the passage of stool smoother.
• It absorbs any remaining nutrients in chyme, including minerals and vitamins like thiamine and riboflavin.

NOTE-

Importance of Gut Bacteria-

The large intestine houses over 700 bacteria that contribute not only to digestion but other vital functions as well. Some researchers regard gut bacteria as an organ system as they influence the interaction between the gut and brain, called the enteric nervous system (ENS).