Digestion In The Small Intestine

Guilt Free Deserts

Guilt Free Desserts

Get Instant Access

When acidic chyme enters the duodenum, it triggers several events. The acid, along with short proteins called peptides and fatty acids in the chyme, causes cells at the beginning of the duodenum to secrete intestinal fluid. About 1 to 2 quarts of this digestive fluid is produced each day. The intestinal juice, which contains some mucus, is alkaline and helps to neutralize the acidic nature of chyme and protect the duodenum from the effects of the acid. The chyme also causes the release of two hormones from the duodenum. The hormone cholecystokinin (CCK) causes the gallbladder to constrict and pump bile into the small intestine. CCK also causes the pancreas to secrete digestive enzymes into the duodenum. The hormone secretin causes the pancreas to secrete large amounts of bicarbonate into the small intestine. The bicarbonate neutralizes most of the hydrochloric acid from the stomach. After the neutralization occurs, the small intestine contents are alkaline, creating the conditions needed for the digestive enzymes to work.

Digestive enzymes break starch, proteins, triglycerides, and nucleic acids into intermediate size pieces. Pancreatic amylase breaks down starch. The bicarbonate from the pancreas creates the alkaline conditions needed for amylase and other enzymes to function. Amylase does not break starch into glucose monosaccharide units, but into smaller pieces, including the disaccharide maltose. Thus far in the digestive process, the disaccharides, such as sucrose (from table sugar and fruits) and lactose (from milk, such as the chocolate shake in our example), that are ingested have not been broken down. Dietary triglycerides are broken apart by pancreatic lipase.

Proteins are broken into short peptide chains of amino acids by several pancreatic enzymes, including trypsin, chymotrypsin, elastase, and carboxypeptidase. The peptides formed as a result of the action of these enzymes are structurally very short and will be further broken down at the surface of villi and microvilli. Nucleic acids are broken into nucleotides that will also be further digested at the surface of the villi.

The final digestion of material occurs at the villi. Specific enzymes are derived from the epithelial cells of the villi that break disaccharides, peptides, and nucleotides into their most basic components for absorption. This process works very efficiently because the final breakdown of nutrients occurs at the exact site of nutrient absorption. This material passes through the lining cell membranes and into the blood capillaries of the villi almost immediately.

Lipid absorption is a more complicated process. Glycerol and short chain fatty acids from triglycerides are absorbed by simple diffusion across the cell membranes in the digestive lining. Cholesterol and long chain fatty acids cannot diffuse through the cell membranes and must be handled differently. Bile salts combine with these fats to form tiny spheres called micelles (Figure 6.4). These spheres can pass through the cell membranes and transport the fats into the cells. By the time the fats leave the lining cells, they have been combined with cell proteins that help carry them through lymph and blood. This combination of cholesterol, triglycerides, and lining cell proteins is called a chylomicron. The chylomicrons will circulate in the blood, dropping off triglycerides to body tissues. Eventually, the chylomicron remnants will be picked up by the liver.

In addition to the nutrients discussed, other materials are also absorbed through the small intestine, including electrolytes and trace elements such as sodium, bicarbonate, chloride, iodate, nitrate, calcium, iron, magnesium, and phosphates.

Small Intestine Fat Digestion

Figure 6.4 Because fats are not water soluble, they cannot easily diffuse across cell membranes. Thus, the body has found an alternate way to transport fat droplets into cells. First, the lipids combine with bile salts to form emulsion droplets. These droplets are then digested by enzymes called lipases and eventually form micelles, which can pass into the cell.

Figure 6.4 Because fats are not water soluble, they cannot easily diffuse across cell membranes. Thus, the body has found an alternate way to transport fat droplets into cells. First, the lipids combine with bile salts to form emulsion droplets. These droplets are then digested by enzymes called lipases and eventually form micelles, which can pass into the cell.

About 9 quarts of water enter the small intestine each day. Some water is ingested with food (like the chocolate shake), but most of it enters into the system at various points during the digestive process to aid in the breakdown of food. The source of these fluids is approximately as follows: 1 to 2 quarts with food, 1 quart of saliva, 2 quarts of stomach (gastric) juices, 1 quart of bile, 2 quarts of pancreatic juice, and 1 quart of intestinal juice. Most of the water is reabsorbed through the small intestinal walls.

Any leftover nutrients, waste, and some water leaves the small intestine and enters the large intestine. Absorption of nutrients is complete, but digestion is not. Some of the water is absorbed in the large intestines and very little is excreted in the feces.

Was this article helpful?

0 0
Lower Your Cholesterol In Just 33 Days

Lower Your Cholesterol In Just 33 Days

Discover secrets, myths, truths, lies and strategies for dealing effectively with cholesterol, now and forever! Uncover techniques, remedies and alternative for lowering your cholesterol quickly and significantly in just ONE MONTH! Find insights into the screenings, meanings and numbers involved in lowering cholesterol and the implications, consideration it has for your lifestyle and future!

Get My Free Ebook


Responses

Post a comment