The Large Intestine

Figure 1 - General Diagram of the Large Intestine

The ileum enters into the large intestine through the ileocecal valve. Chyme the enters through the ascending colon, transverse colon, descending colon, sigmoid colon, and finally the rectum.

Ileocecal Sphincter

Located between ileum and the ascending colon and has the function of preventing the backflow of fecal matter and gut flora into the ileum. It is able to regulate movement of waste material since if waste material stays in the ileum for long periods of time, the body will start absorbing waste.

Caecum

The caecum is a pouch at the beginning of the large intestine. It primarily absorbs salts and electrolytes and because of its thick mucus membrane, lubricates fecal matter to move through the large intestine.

Figure 2 - Caecum's Position

Appendix

Originally considered a vestigial organ (not vital for life), it does have a function. Due to its distance away from the flow of food and containing immune tissue, it is a safe haven for beneficial gut flora as a reservation for when illness wipes out the original gut flora. Therefore, it can help "reboot" the large intestine. It is located at McBurney's point, the distance two thirds from the belly button the the right hip bone.

Figure 3 - Appendix's Position

Taenia coli

On the large intestine are 3 bands of longitudinal smooth muscles that form a triple helix structure from the appendix to the sigmoid colon. It assists in the propulsion of gut contents by peristalsis. Because the taenia coli are actually shorter than the large intestines itself, this produces sacculation, creating bulges also known as haustra.

Appendices epiploicae

On the taenia coli are pouches of peritoneum filled with fat that are absent in the rectum. They are mostly appended to the transverse and sigmodi colon but have an unknown function.

The colon itself has no villi or plicae circulare, instead its main function is the absorption of any left over chyme from the small intestine, water, electrolytes, and vitamin B12. They still have crypts of Lieberkühn and an abundance of goblet cells to produce mucus to protect the large intestine against gut flora. Notably, crypts of Lieberkühn don't contain paneth cells. The absorption of water and electrolytes is through a chain cycle of sodium creating an electrochemical gradient for chloride to pass as well which creates an osmotic gradient. Osmosis is when water moves from low concentration to high concentration through a semipermeable membrane that is able to block the solute.

Figure 4 - Diagram of Osmosis

Anal Canal

The anal canal is the final portion of the large intestine and is where the 3 taenia coli converge. It has the highest concentration of goblet cells throughout the large intestine. Its function is the temporary storage of fecal matter. Rectal walls have stretch receptors to stimulate desire to defecate. The puborectalis has a resting contraction to prevent incontinence. As the rectum is distended, the muscles relax and the angle between the rectum and anus is straightened for fecal matter to be excreted. The rectum, before the anal canal, also has valves of houston that prevent the urge to defecate because of its alternating zigzag structure.

Page updated

Report abuse