The primary function of the small intestine is the absorption of nutrients and minerals found in food.

Intestinal villus : An image of a simplified structure of the villus. The thin surface layer appear above the capillaries that are connected to a blood vessel. The lacteal is surrounded by the capillaries. Digested nutrients pass into the blood vessels in the wall of the intestine through a process of diffusion.

The inner wall, or mucosa, of the small intestine is lined with simple columnar epithelial tissue. Structurally, the mucosa is covered in wrinkles or folds called plicae circulares—these are permanent features in the wall of the organ.

They are distinct from the rugae, which are non-permanent features that allow for distention and contraction. From the plicae circulares project microscopic finger-like pieces of tissue called villi Latin for shaggy hair. The individual epithelial cells also have finger-like projections known as microvilli.

The function of the plicae circulares, the villi, and the microvilli is to increase the amount of surface area available for the absorption of nutrients. Each villus has a network of capillaries and fine lymphatic vessels called lacteals close to its surface. The epithelial cells of the villi transport nutrients from the lumen of the intestine into these capillaries amino acids and carbohydrates and lacteals lipids.

The absorbed substances are transported via the blood vessels to different organs of the body where they are used to build complex substances, such as the proteins required by our body. The food that remains undigested and unabsorbed passes into the large intestine. Absorption of the majority of nutrients takes place in the jejunum, with the following notable exceptions:.

Section of duodenum : Section of duodenum with villi at the top layer. Search site Search Search. To sum up important information from the video: the structure of the small intestine the circular folds and villi increases surface area to maximize nutrient absorption.

Additionally carbohydrates, lipids, and proteins are absorbed differently, but most of the absorption for all three macronutrients happens in the small intestine.

Skip to main content. Search for:. Small Intestine The small intestine is the primary site of nutrient absorption. FYI: The pancreas is an accessory organ that produces a secretion called bicarbonate. When chyme leaves the stomach it is acidic which would harm the small intestine.

Bicarbonate is added to the chyme to neutralize its acidity prior to entering the small intestine. FYI: The liver is another accessory organ.

The ileum is the longest part of the small intestine, making up about three-fifths of its total length. It is thicker and more vascular than the jejunum, and the circular folds are less dense and more separated Keuchel et al, At the distal end, the ileum is separated from the large intestine by the ileocaecal valve, a sphincter formed by the circular muscle layers of the ileum and caecum, and controlled by nerves and hormones.

The ileocaecal valve prevents reflux of the bacteria-rich content from the large intestine into the small intestine. The ileum is rich in immune tissue lymphoid follicles. These are concentrated in the distal ileum and serve to keep bacteria from entering the bloodstream.

The duodenum accomplishes a good deal of chemical digestion, as well as a small amount of nutrient absorption see part 3 ; the main function of the jejunum and ileum is to finish chemical digestion enzymatic cleavage of nutrients and absorb these nutrients along with water and vitamins.

The brush border of the small intestine contains enzymes that complete the process of chemical digestion. Table 1 lists these enzymes and their roles. The rings of smooth muscle in the wall of the small intestine repeatedly contract and relax in a process called segmentation.

This moves intestinal contents back and forth. Segmentation distends the small intestine but does not drive chyme through the tract; instead, it mixes it with digestive juices and then pushes it against the mucosa to allow nutrient absorption.

The transport of nutrients across the membranes of the intestinal epithelial cells into the villi, and subsequently into blood capillaries and lacteals, occurs either passively or actively.

Passive transport requires no energy and involves the diffusion of simple molecules along a concentration gradient — movement from an area where they are in high concentration to one where they are in lower concentration — in this case, the blood. Water and some vitamins can cross the gut wall passively.

Active transport requires energy to pull molecules out of the intestinal lumen against a concentration gradient.

Digested carbohydrates enter the blood capillaries irrigating each villus. Glucose is actively absorbed via a co-transport mechanism using sodium ions as carriers. Other absorbable monosaccharides include galactose from milk and fructose from fruit.

Most products of protein digestion amino acids are also absorbed through an active co-transport mechanism with sodium ions and enter the blood capillary system of each villus. They then travel to the liver via the hepatic portal vein.

Digested fats mingle with bile salts, which ferry them to the mucosa where they are coated with lipoproteins and aggregated into small molecules called chylomicrons, which are taken into the central lacteals of the villi. They travel with lymph to the thoracic duct, where they enter the blood supply.

If there is malabsorption of fats, these pass into the large intestine, where they form pale, oily, foul-smelling stools steatorrhoea — see part 3. When that happens, certain fat-soluble vitamins A, D, E and K may also not be absorbed, potentially leading to deficiencies. The vitamin B complex encompasses eight water-soluble vitamins that are essential for key functions of the body, including red blood cell formation, maintenance of healthy hair and nails, and healthy functioning of the brain and heart.

These eight vitamins are: B1 thiamine , B2 riboflavin , B3 niacin , B5 pantothenic acid , B6 pyridoxine , B7 biotin , B9 folate and B12 cobalamin. Vitamin B1. Essential for metabolism, vitamin B1 also plays a role in healthy nerve conduction and muscle contraction.

It is found in fortified foods such as bread and cereals, but also in eggs, fish, nuts, legumes and certain meats Wiley and Gupta, Vitamin B1 deficiency is common in people who have a poor diet for example, homeless people and can cause a range of disorders including beriberi.

Vitamin B This vitamin is essential for red blood cell development, normal functioning of the nervous system, cell metabolism and DNA synthesis. The richest natural sources of vitamin B12 are liver and kidney, but it is also present in meat, fish, dairy products, eggs and shellfish.

Vitamin B12 is liberated from ingested food in the acid milieu of the stomach. In the duodenum, it binds with intrinsic factor produced by the gastric parietal cells see part 2 ; it is only in that bound form that it can be absorbed Moll and Davis, Absorption occurs in the terminal portion of the ileum, where vitamin B12 attaches to specific membrane receptors located on absorptive cells enterocytes at the bottom of the pits between the microvilli Schjønsby, To leave the enterocytes and enter the bloodstream, the vitamin must then bind to a carrier protein, transcobalamin II.

Food is ingested through the mouth; passed through the esophagus into the stomach; and filtered through the small intestine, pancreas and liver. Based on its particular function, each organ contributes in different ways to turn the food we eat into the nutrients we need.

The stomach allows large volumes of food to mix with digestive juices before it moves to the small intestine, which consists of the duodenum, jejunum and ileum. The intestinal walls absorb digested nutrients. Removing portions of the digestive tract can cause deficiencies in the nutrients those organs would normally absorb, as listed below.

It is important for anyone who has had a surgery that modifies any stage of the digestive process to discuss ways to retain these nutrients with their physician. You can use the following information from the National Institutes of Health NIH and Medscape to guide your conversation.

Vitamin A is a fat-soluble vitamin used to maintain normal vision and immune system functions. It also supports organs such as the heart, lungs and kidneys. This is highly relevant to GIST patients on long-term medication, as many drugs can lead to dehydration and eventual kidney impairment.

Stocking up on Vitamin A will help support the kidneys and prevent toxicity. Vitamin D is another fat-soluble vitamin used to promote calcium in the gastrointestinal tract, enable bones to grow and strengthen, and prevent osteoporosis. Vitamin E is a fat-soluble antioxidant that enhances immune functions, prevents cardiovascular disease and protects cognitive abilities.

It is particularly valuable for GIST patients, who often have weakened immune systems that can make them more susceptible to other illnesses and co-morbidities. Foods to promote Vitamins A, D and E include dark leafy greens, almonds, avocados, broccoli, canned tuna and salmon.

Water-soluble forms are a viable option if the ileum has been removed. Vitamin K is often associated with the process of blood clotting and even bone health. Deficiencies may occur if patients have GI disorders, therefore levels should be monitored regularly.

There are a number of dietary options for increasing Vitamin K levels to discuss with your doctor. The main enzyme is pancreatic amylase , which yields disaccharides from starch by digesting the alpha glycosidic bonds.

The disaccharides produced maltose, maltotriose, and α-dextrins are all converted to glucose by brush border enzymes.

Disaccharides occurring naturally in food do not require amylase to break them down. Brush border enzymes lactase, sucrase, trehalase hydrolyse these compounds into molecules of glucose, galactose and fructose.

Both glucose and galactose exit the cell via GLUT2 receptors across the basolateral membrane into the blood. Fructose enters the cell by facilitated diffusion via GLUT5 and is transported into the blood via GLUT2 receptors. Fig 1 — Sodium moves down its concentration gradient, bringing in glucose to the the cell.

Protein digestion begins in the stomach with the action of pepsin , which breaks protein into amino acids and oligopeptides. The process of digestion is completed in the small intestine with brush border and pancreatic enzymes.

They split the oligopeptides into amino acids, dipeptides and tripeptides. Amino acids are absorbed via a Sodium cotransporter, in a similar mechanism to the monosaccharides. They are then transported across the basolateral membrane via facilitated diffusion. Fig 2 — The sodium-amino acid transporter, which is nearly identical to the sodium-glucose transporter.

Lipids are hydrophobic, and thus are poorly soluble in the aqueous environment of the digestive tract. The remainder of the lipids are digested in the small intestine. Here, bile aids digestion by emulsifying the fat goblets into smaller chunks, called micelles, which have a much larger surface area.

Pancreatic lipase, phospholipase A2 and cholesterol ester hydrolase 3 major enzymes involved in lipid digestion hydrolyse the micelles, breaking them down into fatty acids, monoglycerides, cholesterol and lysolecithin. The products from digestion are released at the apical membrane and diffuse into the enterocyte.

Inside the cell, the products are re-esterified to form the original lipids, triglycerides, cholesterol and phospholipids. The lipids are then packaged inside apoproteins to form a chylomicron. The chylomicrons are too large to enter circulation, so they enter lymphatic system via lacteals.

Fig 3 — The action of bile acids. By enveloping the lipid, the bile enhances absorption. The average adult usually ingests L of water each day, but the fluid load to the small intestine is 9 to 10 L, 8 to 9 L being added by secretions of the GI system.

Most absorption of water and electrolytes occurs in the small intestine , with some water absorbed in the colon as well. Steatorrhea is due to a disruption to the normal absorption of lipids, leading to fat filled faeces.

There are numerous underlying causes for this such as pancreatitis, which prevents the correct secretion of pancreatic lipase and so lipids remain undigested. Another cause is gallstones which prevent bile from entering the duodenum and again prevents maximal absorption of lipids.

However, the absorption in the small intestine can be compromised, such as in inflammatory bowel diseases. To distinguish between the underlying causes of Steatorrhea, the small intestine and biliary tree must be visualised.

The small intestine can be visualised via endoscopy or radiography whilst the biliary tree can be visualised with e ndoscopic retrograde cholangiopancreatography.

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