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The Intestinal Villi Explained -- Absorption

Nutrient absorption in the villi -

Proteins are successively broken down into their amino acid components. A healthy diet limits lipid intake to 35 percent of total calorie intake. The most common dietary lipids are triglycerides, which are made up of a glycerol molecule bound to three fatty acid chains.

Small amounts of dietary cholesterol and phospholipids are also consumed. The three lipases responsible for lipid digestion are lingual lipase, gastric lipase, and pancreatic lipase.

However, because the pancreas is the only consequential source of lipase, virtually all lipid digestion occurs in the small intestine. Pancreatic lipase breaks down each triglyceride into two free fatty acids and a monoglyceride.

The fatty acids include both short-chain less than 10 to 12 carbons and long-chain fatty acids. The nucleic acids DNA and RNA are found in most of the foods you eat. Two types of pancreatic nuclease are responsible for their digestion: deoxyribonuclease , which digests DNA, and ribonuclease , which digests RNA.

The nucleotides produced by this digestion are further broken down by two intestinal brush border enzymes nucleosidase and phosphatase into pentoses, phosphates, and nitrogenous bases, which can be absorbed through the alimentary canal wall. The large food molecules that must be broken down into subunits are summarized in Table 2.

The mechanical and digestive processes have one goal: to convert food into molecules small enough to be absorbed by the epithelial cells of the intestinal villi. The absorptive capacity of the alimentary canal is almost endless. Each day, the alimentary canal processes up to 10 liters of food, liquids, and GI secretions, yet less than one liter enters the large intestine.

Almost all ingested food, 80 percent of electrolytes, and 90 percent of water are absorbed in the small intestine. Although the entire small intestine is involved in the absorption of water and lipids, most absorption of carbohydrates and proteins occurs in the jejunum.

Notably, bile salts and vitamin B 12 are absorbed in the terminal ileum. By the time chyme passes from the ileum into the large intestine, it is essentially indigestible food residue mainly plant fibers like cellulose , some water, and millions of bacteria.

Figure 5. Absorption is a complex process, in which nutrients from digested food are harvested. Absorption can occur through five mechanisms: 1 active transport, 2 passive diffusion, 3 facilitated diffusion, 4 co-transport or secondary active transport , and 5 endocytosis.

As you will recall from Chapter 3, active transport refers to the movement of a substance across a cell membrane going from an area of lower concentration to an area of higher concentration up the concentration gradient. Passive diffusion refers to the movement of substances from an area of higher concentration to an area of lower concentration, while facilitated diffusion refers to the movement of substances from an area of higher to an area of lower concentration using a carrier protein in the cell membrane.

Co-transport uses the movement of one molecule through the membrane from higher to lower concentration to power the movement of another from lower to higher. Finally, endocytosis is a transportation process in which the cell membrane engulfs material.

It requires energy, generally in the form of ATP. Moreover, substances cannot pass between the epithelial cells of the intestinal mucosa because these cells are bound together by tight junctions. Thus, substances can only enter blood capillaries by passing through the apical surfaces of epithelial cells and into the interstitial fluid.

Water-soluble nutrients enter the capillary blood in the villi and travel to the liver via the hepatic portal vein. In contrast to the water-soluble nutrients, lipid-soluble nutrients can diffuse through the plasma membrane.

Once inside the cell, they are packaged for transport via the base of the cell and then enter the lacteals of the villi to be transported by lymphatic vessels to the systemic circulation via the thoracic duct. The absorption of most nutrients through the mucosa of the intestinal villi requires active transport fueled by ATP.

The routes of absorption for each food category are summarized in Table 3. All carbohydrates are absorbed in the form of monosaccharides. The small intestine is highly efficient at this, absorbing monosaccharides at an estimated rate of grams per hour.

All normally digested dietary carbohydrates are absorbed; indigestible fibers are eliminated in the feces. The monosaccharides glucose and galactose are transported into the epithelial cells by common protein carriers via secondary active transport that is, co-transport with sodium ions.

The monosaccharides leave these cells via facilitated diffusion and enter the capillaries through intercellular clefts. The monosaccharide fructose which is in fruit is absorbed and transported by facilitated diffusion alone.

The monosaccharides combine with the transport proteins immediately after the disaccharides are broken down. Active transport mechanisms, primarily in the duodenum and jejunum, absorb most proteins as their breakdown products, amino acids.

Almost all 95 to 98 percent protein is digested and absorbed in the small intestine. The type of carrier that transports an amino acid varies.

Most carriers are linked to the active transport of sodium. Short chains of two amino acids dipeptides or three amino acids tripeptides are also transported actively.

However, after they enter the absorptive epithelial cells, they are broken down into their amino acids before leaving the cell and entering the capillary blood via diffusion.

About 95 percent of lipids are absorbed in the small intestine. Bile salts not only speed up lipid digestion, they are also essential to the absorption of the end products of lipid digestion. Short-chain fatty acids are relatively water soluble and can enter the absorptive cells enterocytes directly.

Despite being hydrophobic, the small size of short-chain fatty acids enables them to be absorbed by enterocytes via simple diffusion, and then take the same path as monosaccharides and amino acids into the blood capillary of a villus.

The large and hydrophobic long-chain fatty acids and monoacylglycerides are not so easily suspended in the watery intestinal chyme. However, bile salts and lecithin resolve this issue by enclosing them in a micelle , which is a tiny sphere with polar hydrophilic ends facing the watery environment and hydrophobic tails turned to the interior, creating a receptive environment for the long-chain fatty acids.

The core also includes cholesterol and fat-soluble vitamins. Without micelles, lipids would sit on the surface of chyme and never come in contact with the absorptive surfaces of the epithelial cells.

Micelles can easily squeeze between microvilli and get very near the luminal cell surface. At this point, lipid substances exit the micelle and are absorbed via simple diffusion. The free fatty acids and monoacylglycerides that enter the epithelial cells are reincorporated into triglycerides.

The triglycerides are mixed with phospholipids and cholesterol, and surrounded with a protein coat. This new complex, called a chylomicron , is a water-soluble lipoprotein.

After being processed by the Golgi apparatus, chylomicrons are released from the cell. Too big to pass through the basement membranes of blood capillaries, chylomicrons instead enter the large pores of lacteals. The lacteals come together to form the lymphatic vessels.

The chylomicrons are transported in the lymphatic vessels and empty through the thoracic duct into the subclavian vein of the circulatory system. Once in the bloodstream, the enzyme lipoprotein lipase breaks down the triglycerides of the chylomicrons into free fatty acids and glycerol.

These breakdown products then pass through capillary walls to be used for energy by cells or stored in adipose tissue as fat. Liver cells combine the remaining chylomicron remnants with proteins, forming lipoproteins that transport cholesterol in the blood.

Figure 6. Unlike amino acids and simple sugars, lipids are transformed as they are absorbed through epithelial cells. The products of nucleic acid digestion—pentose sugars, nitrogenous bases, and phosphate ions—are transported by carriers across the villus epithelium via active transport.

These products then enter the bloodstream. The electrolytes absorbed by the small intestine are from both GI secretions and ingested foods. Since electrolytes dissociate into ions in water, most are absorbed via active transport throughout the entire small intestine. During absorption, co-transport mechanisms result in the accumulation of sodium ions inside the cells, whereas anti-port mechanisms reduce the potassium ion concentration inside the cells.

To restore the sodium-potassium gradient across the cell membrane, a sodium-potassium pump requiring ATP pumps sodium out and potassium in. In general, all minerals that enter the intestine are absorbed, whether you need them or not.

Iron —The ionic iron needed for the production of hemoglobin is absorbed into mucosal cells via active transport. While individually the villi and crypts are small, together they provide a large surface area for nutrients to be absorbed into your bloodstream.

The surface becomes 6. Villi can be damaged because of digestive health conditions. Intestinal villi dysfunction, or villous atrophy , also can be caused by other health conditions such as HIV infection, as well as medication.

In celiac disease , consumption of the protein gluten found in the grains wheat, barley, and rye triggers your immune system to attack your intestinal villi and wear them down. Many people with celiac disease have vitamin and mineral deficiencies when they're first diagnosed because their intestinal villi are damaged.

Inflammatory bowel disease , which includes Crohn's disease, also can cause your villi to erode. Researchers think inflammatory disease may disrupt the relationship between the villi and crypts, leaving older cells in place longer in the intestinal lining.

Because these cells are more inefficient, they don't work as well in absorbing nutrients. Some studies have shown an association between certain types of lymphoma , a blood cancer, and damage to intestinal villi.

They include small intestinal T-cell lymphoma, and enteropathy-associated T-cell lymphoma. Enteropathy-associated T-cell lymphoma is closely linked to celiac disease. Separately, a rare condition that typically affects children can cause damage to the villi.

Primary intestinal lymphangiectasia PIL , also known as Waldmann disease, is usually diagnosed before age 3 although it can be later in life. The condition can be treated but researchers are finding an increasing amount of evidence that PIL is linked to a later diagnosis of lymphoma in both children and adults.

Intestinal villi damage can occur when you're taking certain medications. These include:. In these cases, discontinuing the medication should result in your villi growing back. Certain infections have been associated with intestinal villi damage.

They include:. Ongoing research in animal studies is finding other potential links between intestinal villi damage and infection, including toxoplasmosis. The parasitic infection is especially problematic during pregnancy or in people living with HIV. Intestinal villi dysfunction leads to malabsorption, so symptoms are typically consistent with nutrition deficiencies and digestive issues.

These may include:. Your healthcare provider can complete a physical exam, order blood tests, and consider other testing options to arrive at a diagnosis and discuss treatment with you.

Intestinal villi are tiny, but when there's damage to these structures the impacts can be significant because of their key role in assuring your body receives proper nutrition. Not all conditions that affect villi are diet-related, so if you have concerns about malabsorption, contact your healthcare provider.

Ensari A, Marsh MN. Exploring the villus. Gastroenterol Hepatol Bed Bench. Zuvarox T, Belletieri C. Malabsorption Syndromes. In: StatPearls [Internet]. Treasure Island FL : StatPearls Publishing; Jan—. PMID: Kai Y. Intestinal villus structure contributes to even shedding of epithelial cells.

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