12 Colon, Rectum, and Anal Canal
Large Intestine:
The large intestine is a large n-shaped organ spanning about 1.5 meters and can divided into 4 large sections:
- Cecum
- Colon
- Rectum
- Anal canal
The primary function of the large intestine is to prioritize water absorption and secretion of mucus to lubricate passage of colonic contents. The following paragraphs outline the different segments of the large intestine.
Cecum:
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:
- Ascending colon
- Transverse colon
- Descending colon
- 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 the following figure known as haustra. These pouches are bordered by thickened bands of longitudinal muscles called teniae coli.
The rectum is the last 20cm 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).
Colon Water Regulation:
Interesting fact about the colon is that while it regulates water absorption this regulation can often be disrupted in two varying 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.