Small Intestine

Human anatomy > Digestive System > Small Intestine

The duodenum, the jejenum and the ileum form the organ that signifies the end of digestion and the start of nutrient and food absorption. The small intestine is located in between the pyloric sphincter (that is part of the stomach) and the ileocecal valve (which is part of the large intestine). It is specifically found in the central lower portion of the abdominal cavity, supported (in structure) by the mesentery (excluding the first segment). The small intestine wall is lined with ontestini villi, microvilli and plicae circulares. Movement of the small intestine is allowed by the mesentery, primarily due to its fan shape, but prevents twisting or kinking. The mesentery also supplies the small intestines with the chemicals carried by the blood vessels, lymphatic vessels and the nerves.

The small intestine measures 1 inch in diameter and 12 feet in length. When a person dies, the small intestine doubles in size because it relaxes. When compared to the large intestine, though, the small intestine is indeed small. It is the body’s primary digestive organ, in charge of the absorption of nutrients.

Small intestine Anatomy

The small intestine is innervated by the superior mesenteric plexus. Sensory fibers reach the small intestine through the postganglionic sympathetic fibers and the preganglionic parasympathetic fibers.

Blood supply to the small intestine is provided by the celiac trunk and the inferior mesenteric artery, helped along by a direct route from the superior mesenteric artery. Venous drainage that leads to the meeting with the splenic vein is performed by the superior mesenteric vein, creating the hepatic portal vein. The hepatic portal vein connects to the liver, bringing the nutrient-laden blood supply which can be used by the body.

The Sections of the Small Intestine

The small intestine can be divided into three regions, depending on the functions and the structures. The duodendum covers the first ten inches that extend from the pyloric sphincter of the stomach. It has a C-shape, and stays in a fixed position. It’s also retriperitoneal, except for the first few inches that come off the stomach. The concave side (the interior side of the C-shape) has two ducts: one from the pancreas, the other from the liver and the gall bladder. These ducts are necessary to the small intestine as they are the portals through which bile and pancreatic secretions that help in the digestive process pass. The two ducts combine to create one entrance into the small intestine known as the ampulla of Vater or the hepatopancreatic ampulla. This admission point makes it possible for the duodenum to empty the bile and pancreatic fluid from the duodenal papilla into the small intestine.

The amount of fluids admitted into the small intestine is regulated by the sphincter of ampulla (also called the sphincter of Oddi). It does that by opening or closing the duodenal papilla, whenever necessary. This portion of the small intestine features the duodenal glands, also known as the Brunner’s glands, which are placed along the mucosa. The tubualveolar glands, found in the major part of the duodenum, secrete mucous.

The small intestine’s second segment is called the jejenum. It measures 3 feet in lenght, and it extends from the duodenum to the ileum. The jejenum and the ileum are almost the same (structure-wise) if it’s not for the differences in the enhanced lumen and the availability of internal folds.

The ileum is the remaining 6 or 7 feet of the small intestine. This part of the small intestine is often mistaken for the ilium found in the os coxae. The ileum walls contain lymph nodules termed as Peyer’s patches or mesenteric patches. It’s ending section is received by the medial segment of the cecum. The boundary that separates the one from the other is known as the ileocecal valve.

Structural Modifications of the Small Intestine

Food that has been broken down to its molecular components is absorbed by the epithelial lining of the intestinal mucosa. The small intestine has this, and all three major segment of the organ are capable of absorbing food but the area that is mainly concerned with the absorption of nutrients is the jejenum. Fast absorption depends on four specialized adaptations. They make up a bigger surface area of the small intestine. This bigger surface area refers to the following: the macroscopic folds of the mucosa called as plicae circulares, the length of the small intestines, the macroscopic folds of the mucosa that are found along the lumen in a finger-like fashion which are called the intestinal vilii, and the microscopic projections formed by the folds of the epithelial cell membrane known as the microvilli.

The microvilli either appears blurred or brushing the sides of the columnar epithelium when it’s placed under a microscope. In fact, when the small intestine is discussed, the microvilli is commonly referred to as brush border, describing the manner with which they form a blurred image of the edges of the epithelium. The intestinal villi is coated with columnar epithelial cells, along with goblet cells that produce protective mucous. Every intestine’s core is made up of connective tissue. It is formed by lamina propria and it contains a great amount of lacteal (lymphatic vessels) blood capillaries and lymphocytes. Lacteal supply nutrient-absorption for fatty acids and cholesterol, and helps the blood vessels take in the amino acids and monosaccharides. Microvilli is considered as the basic foundation of the whole intestinal tract due to the role that it plays when it comes to the absorption of nutrients.

The intestinal villi is characterized by the creation of new cells because the villi continually create new cells, which move the older cells all the way up. The intestinal villi take a downward path along the intestinal epithelium and form pouches that open up the intestinal lumen through its pores to it. These pouches are known as crypts of Lieberkühn or intestinal crypts. 

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