ACTIVITIES OF THE LARGE INTESTINE
The large intestine is so named because its diameter is greater than that of the small intestine though it is, in fact, much the shorter of the two. Fluid, con-taining the unabsorbed products of digestion, directly enters from the small intestine at a junction that is also the site of the vestigial cecum and appen-dix (Figure 11.22). Absorption of Na+ and water occurs over the surface of the large intestine, which also acts as a reservoir for material resistant to diges-tion by GIT enzymes. However, bacterial action on this material releases some nutrients from food, for example certain vitamins as well as about 200–2000 cm3 of gas in 10–14 episodes per day. The final waste together with bacteria forms the feces, which passes to the last section of the GIT, the rectum, and is eliminated through the anus. Two sphincter muscles control elimination: the first of smooth muscle opens involuntarily in response to pressure within the rectum; the second is controlled voluntarily and allows for a conscious deci-sion to defecate.