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Organs Small Intestine Wall

Villi

These are small, finger-like projections that extend into the lumen of the small intestine. Each villus has many microvilli projecting from the enterocytes of its epithelium which collectively form the striated or brush border. Each of these microvilli are much smaller than a single villus. The intestinal villi are much smaller than any of the circular folds in the intestine. Villi increase the internal surface area of the intestinal walls making available a greater surface area for absorption. An increased absorptive area is useful because digested nutrients (including monosaccharide and amino acids) pass into the semipermeable villi through diffusion, which is effective only at short distances. In other words, increased surface area (in contact with the fluid in the lumen) decreases the average distance travelled by nutrient molecules, so effectiveness of diffusion increases. The villi are connected to the blood vessels so the circulating blood then carries these nutrients away.

 

Submucosal Vein

The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa. In the gastrointestinal tract, the submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle (fibers that run circularly within a layer of longitudinal muscle). The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa. The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa.       Source

 

Lymphatic Vessel

These are thin-walled vessels structured like blood vessels, that carry lymph. As part of the lymphatic system, lymph vessels are complementary to the cardiovascular system. Lymph vessels are lined by endothelial cells, and have a thin layer of smooth muscle, and adventitia that bind the lymph vessels to the surrounding tissue. Lymph vessels are devoted to the propulsion of the lymph from the lymph capillaries, which are mainly concerned with absorption of interstitial fluid from the tissues. Lymph vessels act as reservoirs for plasma and other substances including cells that have leaked from the vascular system and transport lymph fluid back from the tissues to the circulatory system. Without functioning lymph vessels, lymph cannot be effectively drained and edema (an abnormal accumulation of fluid in certain tissues within the body) typically will result.

 

Submucosal Artery

The submucosa is a dense, irregular layer of connective tissue with large blood vessels, lymphatics, and nerves that supports the mucosa. In the gastrointestinal tract, the submucosa is the layer of dense, irregular connective tissue or loose connective tissue that supports the mucosa, as well as joins the mucosa to the bulk of underlying smooth muscle (fibers that run circularly within a layer of longitudinal muscle). The submucosa is relatively thick, highly vascular, and serves the mucosa. The absorbed elements that pass through the mucosa are picked up from the blood vessels of the submucosa.     Source

 

Circular Muscle

The circular muscle layer is thicker than the longitudinal layer and more powerful in exerting contractile forces on the contents of the lumen. The long axis of the muscle fibers of circular muscle is oriented in the circumferential direction.

 

Lacteal

A lacteal is a lymphatic capillary that absorbs dietary fats in the villi of the small intestine. Lacteal, A minute blind-ended lymph vessel that occurs in each villus of the small intestine. Digested fats are absorbed into the lacteals and transported to the bloodstream through the thoracic duct.      Source

 

Mucosa

The mucosa is the innermost layer of the gastrointestinal tract. that is surrounding the lumen, or open space within the tube. This layer comes in direct contact with digested food (chyme). The mucosa is made up of 3 layers. The Epithelium – innermost layer. Responsible for most digestive, absorptive and secretory processes. The Lamina propria – a layer of connective tissue. Unusually cellular compared to most connective tissue. And The Muscularis mucosae – a thin layer of smooth muscle that aids the passing of material and enhances the interaction between the epithelial layer and the contents of the lumen by agitation and peristalsis. The mucosae are highly specialized in each organ of the gastrointestinal tract to deal with the different conditions. The most variation is seen in the epithelium.

 

Lymphoid Nodule

Lymph nodule, small, localized collection of lymphoid tissue, usually located in the loose connective tissue beneath wet epithelial (covering or lining) membranes, as in the digestive system, respiratory system, and urinary bladder. Lymph nodules form in regions of frequent exposure to microorganisms or foreign materials and contribute to the defense against them. The nodule differs from a lymph node in that it is much smaller and does not have a well-defined connective-tissue capsule as a boundary. It also does not function as a filter, because it is not located along a lymphatic vessel. Lymph nodules frequently contain germinal centres—sites for localized production of lymphocytes. In the small intestine, collections of lymph nodules are called Peyer’s patches. The tonsils are also local regions where the nodules have merged together.     Source

 

Muscularis Mucosae

This is a thin layer (lamina) of muscle of the gastrointestinal tract, located outside the lamina propria and separating it from the submucosa. It is present in a continuous fashion from the esophagus to the upper rectum (the exact nomenclature of the rectum’s muscle layers is still being debated). A discontinuous muscularis mucosae–like muscle layer is present in the urinary tract, from the renal pelvis to the bladder, as it is discontinuous, it should not be regarded as a true muscularis. The muscularis mucosae is composed of several thin layers of smooth muscle fibers oriented in different ways which keep the mucosal surface and underlying glands in a constant state of gentle agitation to expel contents of glandular crypts and enhance contact between epithelium and the contents of the lumen.

 

Mucosa

The mucosa, composed of simple epithelium cells, is the innermost layer of the gastrointestinal (GI) tract. It is the absorptive and secretory layer of the GI tract. The mucosa surrounds the lumen, or open space within the digestive tube. This layer comes in direct contact with digested food (chyme). The epithelium of the mucosa is particularly specialized, depending on the portion of the digestive system. The mucosa is the innermost layer, and functions in absorption and secretion. It is composed of epithelium cells and a thin connective tissue. The mucosa contains specialized goblet cells that secrete sticky mucus throughout the GI tract. On the mucosa layer, small finger-like projections called villi and microvilli help to increase surface area for nutrient absorption.      Source

 

Submucosa Plexus

This lies in the submucosa of the intestinal wall. The nerves of this plexus are derived from the myenteric plexus which itself is derived from the plexuses of parasympathetic nerves around the superior mesenteric artery. Branches from the myenteric plexus perforate the circular muscle fibers to form the submucous plexus. Ganglia from the plexus extend into the muscularis mucosae and to the mucous membrane. The nerve bundles of the submucous plexus are finer than those of the myenteric plexus. Its function is to innervate cells in the epithelial layer and the smooth muscle of the muscularis mucosae. 14% of submucosal plexus neurons are sensory neurons – Dogiel type II, also known as enteric primary afferent neurons or intrinsic primary afferent neurons.

 

Longitudinal Muscle

Muscles running lengthwise rather than across. Longitudinal muscle fibres run lengthwise along the body, and the circular fibres encircle it. The body contents are liquids or tissues that can be deformed into different shapes, but they maintain a constant volume. If longitudinal muscles contract and the body shortens, it must widen to accommodate its volume. if the circular muscles contract and the body thins, it must lengthen.       Source

 

Serosa

This is a smooth tissue membrane consisting of two layers of mesothelium, which secrete serous fluid. The inner layer that covers organs (viscera) in body cavities is called the visceral membrane. A second layer of epithelial cells of the serous membrane, called the parietal layer, lines the body wall. Between the two layers is a potential space, mostly empty except for a few milliliters of lubricating serous fluid that is secreted by the two serous membranes. It is one of the thin membranes that cover the walls and some organs of the thoracic and abdominopelvic cavities. Serous membranes have two layers. The parietal layers of the membranes line the walls of the body cavity (pariet- refers to a cavity wall). The visceral layer of the membrane covers the organs (the viscera). Between the parietal and visceral layers is a very thin, fluid-filled serous space, or cavity.

 

Myenteric Plexus

This provides motor innervation to both layers of the muscular layer of the gut, having both parasympathetic and sympathetic input (although present ganglionar cell bodies belong to parasympathetic innervation, fibers from sympathetic innervation also reach the plexus), whereas the submucous plexus has only parasympathetic fibers and provides secretomotor innervation to the mucosa nearest the lumen of the gut. The myenteric plexus functions as a part of the enteric nervous system (digestive system). The enteric nervous system can and does function autonomously, but normal digestive function requires communication links between this intrinsic system and the central nervous system. The ENS contains sensory receptors, primary afferent neurons, interneurons, and motor neurons. The events that are controlled, at least in part, by the ENS are multiple and include motor activity, secretion, absorption, blood flow, and interaction with other organs such as the gallbladder or pancreas. These links take the form of parasympathetic and sympathetic fibers that connect either the central and enteric nervous systems or connect the central nervous system directly with the digestive tract. Through these cross connections, the gut can provide sensory information to the CNS, and the CNS can affect gastrointestinal function. Connection to the central nervous system also means that signals from outside of the digestive system can be relayed to the digestive system.