The duodenum, the jejunum, and the ileum create the organ where digestion concludes and nutrients are absorbed into the blood stream. The small intestine is positioned between the pyloric sphincter that belongs to the stomach and the ileocecal valve which begins the large intestine. It can be located in the central lower area of the abdominal cavity and is structurally supported by the mesentery, except for the first segment. The small intestinal wall is coated with distinctive plicae circulares, ontestinl villi, and microvilli. The mesentery is an excellent support for the small due to its fan shape, which permits free motion of the small intestine but nearly prohibits the possibility of kinking or twisting. The mesentery also houses the necessary blood vessels, lymphatic vessels, and nerves which are able to provide the small intestines with the appropriate supply.
The small intestine is a 1 inch in diameter tubular organs that reaches about 12 feet in length. When the body dies, the muscles relax and the small intestine becomes nearly twice the normal size. It is still considered small when compared to the diameter of the large intestine. The small intestine serves as the body’s most primary digestive organ and is the primary site of nutrient absorption.
The superior mesenteric plexus is responsible for innervating the small intestine. Sensory fibers along with postganglionic sympathetic fibers and preganglionic parasympathetic fibers reach the small intestine through these various branches.
Branches of the celiac trunk and inferior mesenteric artery along with a direct line from the superior mesenteric artery are responsible for providing the small intestine with ample blood supply. The superior mesenteric vein provides the appropriate venous drainage, which then creates a unified vein with the splenic vein to create the hepatic portal vein. The hepatic portal vein then travels to the liver, where the nutrient laden blood supply can be utilized by the body.
Structural Modifications of the Small Intestine
The intestinal mucosa contains an epithelial lining that absorbs the nutrients from the molecular breakdown of food. While the entire small intestine has this lining, and there is absorption taking place within all three segments of the small intestine, the primary location of absorption takes place in the jejunum. Four highly specialized adaptations create rapid absorption. These adaptations create a larger surface area of the small intestine. This includes the length of the small intestines, the macroscopic folds of the mucosa known as plicae circulares, the macroscopic folds of the mucosa which line the lumen in a fingerlike fashion known as the intestinal villi, and the microvilli which are the microscopic projections created by the folds of the epithelial cell membrane.
When viewed under a light microscope, the microvilli can appear to blurr or brush the borders of the columnar epithelium. When discussing the small intestine, it is common to refer to the microvilli as brush border, referring to the way they create the blurred border edges of the epithelium. Columnar epithelial cells coat the intestinal villi, which are accompanied by interspersed goblet cells that secrete protective mucous. The core of every intestinal is created by connective tissue. The lamina propria forms this core. These cores contain large quantities of lacteal, which are lymphatic vessels, along with blood capillaries and lymphocytes. Lacteals are able to provide absorption for the fatty acids and cholesterol while the blood capillaries absorb the monosaccharides and amino acids. Microvilli can easily be considered the functional structure of the entire intestinal tract because of the impact that nutrient absorption has on the human body.
The intestinal villi cells are chronically completing a cycle, as they are each replaced by new cells as newer cells are created at the base of the villi, forcing their way up. Throughout various places along the intestinal epithelium, the intestinal villi chart downward and create pouches that lead to the opening of the pores into the intestinal lumen. These unique structures are known as either intestinal crypts or the crypts of Lieberkühn.
The Sections of the Small Intestine
The small intestine is segregated into three basic regions, based entirely on functional and structural bias. The first ten inches from the pyloric sphincter of the stomach is known as the duodenum. It remains in the basic shape of a C, and is considered to be in a fixed position. The duodenum is retroperitoneal, with a small exception of the first few inches in the beginning that draw from the stomach. The inside if the C, the concave side, receives two ducts, one from the pancreas and the other from the liver and gall bladder. These ducts feed the small intestine the necessary bile and pancreatic secretions which help in the digestive process. These two ducts unite to create one single entry point into the small intestine, known as the hepatopancreatic ampulla or the ampulla of Vater. This entry point allows the duct to enter the duodenum and drain the necessary fluids from an elevated point called the duodenal papilla, where bile and pancreatic fluid can then enter the small intestine freely.
The sphincter of ampulla, or the sphincter of Oddi, retains the ability to either open or close the duodenal papilla to maintain the appropriate amount of fluids at any given time. Brunner’s glands, also known as duodenum glands, that are embedded in the mucosa are one of the primary individualistic characteristics of this section of the small intestine. Mucous is secreted from these compound tubuloalveolar glands which are the most plentiful along the superior portion of the duodenum.
The second segment of the small intestine is the jejunum. Measuring about 3 feet in length, this section can be traced from the duodenum to the ileum. The structure of the ileum and the jejunum is nearly identical, except for the enhanced lumen and the larger percentage of internal folds.
The final 6 or 7 feet of the small intestine is known as the ileum. This is often confused with the ilium of the os coxae. The walls of the ileum are lined with lymph nodules known as mesenteric patches, or Peyer’s patches. The medial section of the cecum receives the terminal section of the ileum. The ileocecal valve is the defining border for each organ.