Any remaining nutrients and some water are absorbed as peristaltic waves move the chyme into the ascending and transverse colons.
This dehydration, combined with peristaltic waves, helps compact the chyme. The solid waste formed is called feces. It continues to move through the descending and sigmoid colons. The large intestine temporarily stores the feces prior to elimination. The body expels waste products from digestion through the rectum and anus. This process, called defecation, involves contraction of rectal muscles, relaxation of the internal anal sphincter, and an initial contraction of the skeletal muscle of the external anal sphincter.
The defecation reflex is mostly involuntary, under the command of the autonomic nervous system. But the somatic nervous system also plays a role to control the timing of elimination. Download Digestive System Lab Manual.
See more from our free eBook library. Study: Immune system promotes digestive health from Science Daily. 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. Once inside mucosal cells, ionic iron binds to the protein ferritin, creating iron-ferritin complexes that store iron until needed. When the body has enough iron, most of the stored iron is lost when worn-out epithelial cells slough off.
When the body needs iron because, for example, it is lost during acute or chronic bleeding, there is increased uptake of iron from the intestine and accelerated release of iron into the bloodstream. Since women experience significant iron loss during menstruation, they have around four times as many iron transport proteins in their intestinal epithelial cells as do men.
Calcium —Blood levels of ionic calcium determine the absorption of dietary calcium. When blood levels of ionic calcium drop, parathyroid hormone PTH secreted by the parathyroid glands stimulates the release of calcium ions from bone matrices and increases the reabsorption of calcium by the kidneys.
PTH also upregulates the activation of vitamin D in the kidney, which then facilitates intestinal calcium ion absorption. The small intestine absorbs the vitamins that occur naturally in food and supplements. Fat-soluble vitamins A, D, E, and K are absorbed along with dietary lipids in micelles via simple diffusion. This is why you are advised to eat some fatty foods when you take fat-soluble vitamin supplements.
Most water-soluble vitamins including most B vitamins and vitamin C also are absorbed by simple diffusion. An exception is vitamin B 12 , which is a very large molecule.
Intrinsic factor secreted in the stomach binds to vitamin B 12 , preventing its digestion and creating a complex that binds to mucosal receptors in the terminal ileum, where it is taken up by endocytosis. Each day, about nine liters of fluid enter the small intestine.
About 2. About 90 percent of this water is absorbed in the small intestine. Water absorption is driven by the concentration gradient of the water: The concentration of water is higher in chyme than it is in epithelial cells.
After food is churned up in your stomach, a sphincter muscle at the end of your stomach opens to squirt small amounts of food into the top of your small intestine. This first section of the small intestine is called the duodenum. Your pancreas releases digestive juices through a duct into your duodenum.
This fluid is rich in enzymes that break down fats, proteins and carbohydrates. It also contains sodium bicarbonate which neutralises acid produced in your stomach. Your gall bladder squeezes out bile down a duct into your duodenum. Bile helps break down fats in your food. Digesting food is pushed through the small intestine by peristalsis.
Peristalsis is a muscular movement in which alternating waves of muscle contraction and relaxation cause food to be squeezed along the digestive tract. Most of the nutrients in the food you eat pass through the lining of your small intestine into your blood.
The lining of the small intestine is covered in tiny microvilli. These are microscopic, finger-like protrusions which give the lining of the small intestine a massive surface area for absorption of nutrients to occur across.
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