12 Large Intestine
Large Intestine
We’ve now reached the final stage of our digestion journey. The large intestine is a large n-shaped organ spanning about 1.5 meters and can divided into 4 large sections:
Table 13 Structural components of the large intestine
| # | Name: |
| 1 | Cecum |
| 2 | Colon |
| 3 | Rectum |
| 4 | Anal canal |
The primary function of the large intestine is for water absorption and secretion of mucus to lubricate passage of colonic contents. Think of it as the final filter, where the body determines what will be reabsorbed or eliminated.
The cecum is the most proximal end and connects the ileum of the small intestine to the large intestine through the ileocecal sphincter (valve). Attached to the cecum is the narrow twister-shaped organ known as the appendix. The cecum quickly leads into the colon which can be divided into the:
Table 14 Structural components of the colon from proximal to distal
| # | Name: |
| 1 | Ascending colon |
| 2 | Transverse colon |
| 3 | Descending colon |
| 4 | Sigmoid colon |
The ascending colon positioned on the right side of the abdomen is separated from the transverse colon through the right colic (hepatic) flexure. The transverse colon beneath the spleen passes downwards to the descending colon from the left colic (splenic) flexure. The descending colon follows into the s-shaped sigmoid colon which projects towards the midline (medially) to the rectum.
The colon itself is identifiable through pouches depicted in fig. X known as haustra. These pouches are bordered by thickened bands of longitudinal muscles called teniae coli.
Figure 21 Anterior view of the large intestine
Rectum and Anal Canal
After travelling through the colon, the rectum is the last 20 cm of the large intestine which ends in the anal canal.
The anal canal is arranged in longitudinal folds called anal columns lined with blood vessels leading to the anus. The anus is guarded by the internal anal canal (made of involuntary smooth muscle) and external anal canal (made of voluntary skeletal muscle).
Water Absorption
One of the coolest features about the colon is that it regulates water absorption, like a filter. However, this process can go haywire, resulting in two extremes:
- Over-absorption of water can dehydrate the stool leading to constipation. When the colon holds feces for prolonged periods of time, water absorption can become excessive as depicted with constipation.
- Under-absorption of water can overhydrate the stool leading to diarrhea. Frequent diarrhea can result in dehydration or electrolyte imbalance.
Check out this meme!
Hint: The large intestines main responsibility is reabsorbing water leaving behind feces, hence the depiction of the pink character holding back water from the poop. For a reminder of the colon’s water absorbing properties click here: