Long chain fats (triglycerides) are insoluble in water, yet fats must be dissolved in water in order for them to leave the intestine and enter the absorptive cells. Lipases serve to reduce the length of these molecules and make them more soluble.
Figure 5: Fat absorption
Lipases work with bile salts to form micellar solutions of fat (figure 5).
Pancreatic lipase breaks triglicerides down into free fatty acids and monoglycerides (fatty acid + glycerol).
Figure 5 illustrates the cleaving of fatty acid chains from the triglyceride fat molecules. As these fats are reduced to monoglycerides, they reach the unstirred layer.
The unstirred layer is a mucous covering through which the lipids must pass in order to reach the cells that line the intestine.
The bile salts disassociate from the lipids, and diffuse back into the lumen of the small intestine. The monoglycerides are brought to a cellular structure called the endoplasmic reticulum, and enzyme acyltransferase synthesizes the monoglycerides back into triglycerides. The fats are then packed into membrane–bound structures by the golgi apparatus and enter the lymphatic system via the lacteal ducts and travel to the liver for storage until needed for metabolism.
Altheriosclerosis (coating and hardening of the arteries) occurs if too much cholesterol is allowed to enter the blood stream. When this happens, white blood cells are recruited, inflammation and plaques form and is then coated with calcium. Result can be a heart attack. If the plaque is located in the head, an ischemic stroke can occur.
Familial
hypercholesterolemia
is a defect in the low density lipoprotein receptor. In this disease, cholesterol blood level increases to three times higher than normal. The homozygous (two identical gene copies) condition of this disease results in a heart attacks at age 5!
High density lipoproteins help clear cholesterol from body by bringing cholesterol back to the liver for disposal. HDL also delivers cholesterol to steroid hormone producing locations, such as the ovaries, testes, and adrenal glands.
Pancreas
The pancreas is about 6" long, looks like a flat pink fish behind the stomach. Pancreatic juice is composed of fifteen digestive enzymes (including trypsin, chymotrypsin, lipase, amylase, and proteases) and a bicarbonate buffer for neutralizing stomach acid. These enzymes enter the duodenum through the pancreatic ducts. The pancreas also secretes the hormones insulin and glucagon into the blood stream for energy use and growth.
Insulin is a 51 amino acid hormone synthesized by specialized pancreatic cells called islet cells. Insulin is released when glucose levels in the blood rise as a result of eating. Likewise, when blood glucose levels decline, glucagon stimulates the liver to release glucose into the blood.
Insulin works by stimulating liver, muscle and fat cells to take up glucose. Insulin does this by binding to receptors on the surface of these cells. This causes these cells to activate glucose transporters, which results in the storage of glucose in the cell.
The Liver
The liver is the largest organ in body, weighing up to four pounds. It is located next to stomach on the right hand side of body, just below the rib cage. The liver receives and stores glucose as glucagon and converts it back to glucose when the body needs it. It stores vitamins and iron, filters blood, and processes toxic wastes. The liver makes bile, which is a substance we addressed when breaking down fat into smaller units that digest more easily.
This complicated outpocketing of digestive system performs hundreds of different chemical reactions. The blood that flows into the liver has glucose, fatty acids and amino acids dissolved in it.
Liver cells process glucose into glycogen for storage. If there is too much glucose available, the liver stores it as fat. The liver breaks down fatty acids for energy, or stores them as fat. Liver makes lipoproteins that transport fats to and from body cells. The liver also breaks down excess amino acids for energy and converts their nitrogen into waste urea.
Proteins and sugar go directly to the liver. As we have seen, fat enters the lymphatic system that can bypass the liver to direct the fat into the bloodstream if needed.
Homeostasis is maintained by the liver by processing a large volume of blood and controlling its chemical composition. It oversees the metabolism of carbohydrates, fats and proteins from food and also stores minerals and vitamins. It also destroys worn out red blood cells and recycles the iron (heme groups) contained within.
Hepatocytes (liver cells) make up to 1 liter of bile per day. Hepatocytes are organized into sesame seed–sized lobules in the liver.
The liver is an important site of vitamin storage. Stellate cells store up to 2 years' supply of vitamin A. Liver also stores fat soluble D, E, and K vitamins. It also stores the water soluable B
12
.
The liver gets 20% of its blood from a hepatic artery. The rest of the blood supply is oxygen–poor but nutrient rich blood carried from the small intestine in the portal vein.
The manufactured bile is stored in the gallbladder, which can be thought of as a small muscular bag. The bile emulsifies fat which allows the enzyme lipase a bigger surface area for chemical action. Bile salts are used in fat digestion in the duodenum.
The liver makes most of the proteins for blood plasma such as fibrinogen that is essential for blood clotting. It also manufactures albumen that maintains the water balance in the blood.
Appendix
The appendix is a structure that extends off the ascending (beginning) of the large intestine (colon). It may be used in digestion in some animals. It is a non–functional (vestigal) structure in modern humans. It must be removed if it becomes infected. It may have helped our survival in the past as a reservoir for beneficial bacteria lost to bouts of colon–voiding diarrhea.
Large Intestine
The large intestine receives watery, undigested material from the small intestine. The primary role of the large intestine is to remove water, sodium and chloride ions. The large intestine helps the body avoid dehydration while converting watery waste into solid feces.
The large intestine is about 6 feet long and about three inches wide. It has a smooth lining, with no villi as found in the small intestine. The small intestine joins the large intestine at a region named the caecum.
The large intestine goes up the right side (ascending), then across (transverse), then down and back (descending) to connect with the rectum. The rectum is a straight, five inch long tube that goes to anus.
Digested material passes through the large intestine over a five to thirty–six hour period. Remaining water, minerals and vitamins are absorbed here by specialized cells. Here bacteria (gut flora) break down undigested waste, make vitamins such as B complex and vitamin K, and generate gas such as odorless hydrogen, methane and carbon dioxide. They also generate odiferous hydrogen sulfide. The colon also helps the immune system by producing antibodies against pathogens.
Feces are water, undigested food, dead body cells from lining of gut. Dead bacteria can make up to 50% of the weight of feces.
