The intestine is that portion of the digestive tract, that runs between the stomach and the anus. It is divided into two parts - the small intestine and the large intestine. The small intestine forms a major part of the human intestine with a length of around six meters. It can be found in the center of the abdominal cavity. The large intestine (large bowel) starts at the point, where the small intestine ends. As compared to the small intestine, the large bowel is wider; but is only 1.5 meters in length, which is around one-fifth of the length of the intestinal canal. The large intestine consists of the cecum, ascending colon, transverse colon, descending colon, and sigmoid colon.
Large Intestine Anatomy and Physiology
As mentioned earlier, the large bowel starts from the point, where the small intestine ends. To be more precise, it starts from the right iliac region of the pelvis, which is located at the right waist or just below it. It starts with the cecum (where the end of the small intestine opens into the large intestine) and travels upwards; and runs across the abdominal cavity; again turns down to end with the sigmoid canal; which is followed by the rectum, anal canal and the anus.
The portion of the large intestine that comes after the cecum and runs upwards is called the ascending colon, and that travels across the abdomen is called the transverse colon. The descending colon follows the transverse colon, and turns downward to end in the sigmoid colon, which is followed by the rectum.
The large intestine is almost like an arch, which surrounds the coiled small intestine in the abdominal cavity. As a major part of the digestive process happens in the small intestine, the large bowel is left with the function of absorbing water and some nutrients. It is mainly responsible for storing the fecal matter, solidifying it by absorbing water and expelling it through rhythmic contractions (peristaltic movements) of the intestinal muscles.
The large intestine starts with the cecum, which is like a pouch in structure, and connects the ileum (the last part of the small intestine) to the ascending colon. It is separated from the ileum by the ileocecal valve or Bauhin's valve. The cecocolic junction separates it from the ascending colon. It is about six centimeters in length and the vermiform appendix hangs from the cecum.
Ascending colon comes after the cecum and travels upwards, till it reaches the hepatic flexure or right colic flexure (located near the liver), where it turns sidewards. In other words, hepatic flexure is the bend between the ascending colon and the transverse colon. The ascending colon bends to form the hepatic flexure, which is followed by the transverse colon, which travels across the abdominal cavity.
The transverse colon starts from the right hepatic flexure, and is the longest and the movable part of the large intestine. It is slightly curved downwards with a sharp upward rise near its end; where it bends downwards to form the left colic flexure or splenic flexure, which is located near the spleen. It is from this left colic flexure, the descending colon starts. The transverse colon is connected to the stomach with a band of tissues, which is known as the greater omentum. The posterior side of the transverse colon is attached to the posterior abdominal wall by peritoneum (the membrane that lines the abdominal cavity), and this attachment is called transverse mesocolon.
Descending Colon and Sigmoid Colon:
The descending colon starts from the splenic flexure and ends at the beginning of the sigmoid colon. It is more deeply placed, as compared to the ascending colon, and has some parts of the small intestine in front of it. It ends with the sigmoid colon, which is the last part of the large intestine. The sigmoid colon ends at the point, where the rectum starts. Sigmoid colon is an S-shaped structure, which contains muscles, that contract to create a pressure in the colon; in order to expel the fecal matter and move it to the rectum.
The main functions of the large bowel is to absorb water, store waste, absorb nutrients like vitamin K, solidify and expel the fecal matter, etc. The large intestine houses around 700 species of bacteria, which aid in fermentation of the fibers in the food. These bacteria produce large amounts of vitamins, like vitamin K and Biotin (a B vitamin), which are absorbed into the blood.