Final answer:
The rectum features rectal valves that assist in the separation of feces and gas, whereas the has two sphincters with different control mechanisms, with internal being involuntary and external being voluntary. These components are critical for the defecation process.
Step-by-step explanation:
The rectum and the play crucial roles in the process of defecation. Food residue leaving the sigmoid colon enters the rectum in the pelvis. The rectum has three internal transverse folds known as rectal valves, which facilitate the separation of feces from gas. On the other hand, the includes two sphincters, the internal and external sphincters. The internal sphincter is made of smooth muscle and operates involuntarily, while the external sphincter is made of skeletal muscle and is under voluntary control.
These anatomical features have different functions during the defecation process. When feces move from the colon into the rectum, it triggers the defecation reflex mediated by the spinal cord. The sigmoid colon and rectum contract, the internal sphincter relaxes, and the external sphincter initially contracts, but it can be voluntarily relaxed to allow defecation.