The Digestive System and the Liver - The liver, gallbladder, and pancreas
These three organs all share a common function—sending digestive substances to the duodenum—although, except in the case of the gallbladder, it is not their only function. Lying outside the GI tract proper, they nevertheless are indispensable in the processes of digestion and absorption. Digestive fluids from all three converge like tributaries of a river at the common bile duct, and their flow from there into the duodenum is controlled by a sphincter muscle-ring separating the duodenum and common bile duct.
From the liver, bile drips into the hepatic duct , which soon meets the cystic duct arriving from the gallbladder. Converging, they form one duct, the common bile duct , which meets the pancreatic duct , carrying enzymatic fluid from the pancreas. Like a smaller river meeting a larger one, the pancreatic duct loses its own name at this confluence and becomes part of the common bile duct, which empties on demand into the duodenum. When the sphincter of the bile duct is closed, bile from the liver is forced to back up into the cystic duct, and eventually into the gallbladder. There it is stored and concentrated until needed, when it flows back down the cystic duct.
Four pounds of highly efficient chemical-processing tissues, the liver is the largest solid organ in the body. You can locate it by placing your left hand over your right, lowermost ribs; your hand then just about covers the area of the liver. More than any other organ, the liver enables our bodies to benefit from the food we eat. Without it, digestion would be impossible, and the conversion of food into living cells and energy practically nonexistent. Insofar as they affect our body's handling of food—all the many processes that go by the collective name of nutrition—the liver's functions can be roughly divided into those that break down food molecules and those that build up or reconstitute these nutrients into a form that the body can use or store efficiently.
Breaking Down Food Molecules
Bile, as we have seen, assists in the destruction of large food molecules in the small intestine, enabling absorption of nutrients by the villi. Bile acts to increase alkalinity, breaking down big fat molecules; stimulates peristalsis; and prevents food from putrefying within the digestive tract. Unusable portions of the bile, destined to be eliminated as waste, include excess cholesterol, fats, and various components of dead disintegrated cells. Pigments from dead cells in bile give feces its normal, dark, yellow-brown color. Other cell fragments in bile, especially iron from disintegrated red blood cells, are reclaimed from the intestines and eventually make their way via the bloodstream to other parts of the body, where they are built into new cells.
Oddly enough, the liver rebuilds some of the proteins and carbohydrates that the bile has just so effectively helped to break down in the digestive tract. But this is really not so strange as it sounds. The types of proteins and carbohydrates that can be used by man for cell-rebuilding and energy are usually somewhat different in fine structure from those in food. Thus, the liver receives the basic building blocks of proteins and carbohydrates—amino acids and sugars—and with them builds up molecules and cells that can be utilized by the human body. The amino acids and sugars reach the liver through the portal vein , which is the great collection tube for nutrient-carrying blood returning from capillaries along the stomach and small intestine.
Glycogen and Glucose
In the liver, sugars from the small intestine are converted into a special substance called glycogen; amino acids are made available as needed for building new cells to replace the cells that are always naturally dying in a healthy normal body. Glycogen, simply speaking, is the liver's solution to a difficult space and storage problem. The form of carbohydrate the body can use best is a sugar called glucose , but the liver isn't large enough to store the necessary amount of glucose. The answer is glycogen, a tidy, compact sugar molecule that the liver can store in great quantities. When a call comes from any part of the body for glucose, the liver quickly converts some glycogen to glucose and releases it into the bloodstream. By this mechanism, healthy blood sugar levels are maintained.
The liver also builds up human fats from fatty acids and glycerol, packs them off to storage, then reverses the process when necessary by breaking down body fats into forms that can serve as fuel to be burned by the body for energy.
In addition to its functions closely related to digestion and nutrition, the liver also serves as a storehouse and processor of vitamins and minerals—it is, in fact, the manufacturer of vitamin A. It can remove many toxic substances from the blood and render their poisons harmless. It picks up spent red blood cells from the circulation and dismantles them; and it continually manufactures new blood elements.
The liver is also a manufacturing site for cholesterol , a substance belonging to the class of body chemicals called steroids. Above-normal levels of cholesterol in the blood have been linked to hardening of the arteries and heart disease; but cholesterol in the proper amounts is needed by almost every tissue in the body. Some brain and spinal tissues, for example, have cholesterol as one of their main structural components.
With all these vital chemical activities and more, the liver might be expected to be a most delicate and fragile organ. In a sense it is: minor liver damage from one cause or another is thought to be fairly common. But what saves our lives (and us) is that we have a great deal more of it than we need for a normal healthy life. Before symptoms of a liver deficiency appear, more than 50 percent of the liver cells may be destroyed. Furthermore, the liver has a great capacity for regeneration, rebuilding diseased tissues with new liver cells.
Bile stored in the gallbladder is much more concentrated and thicker than bile that is fresh from the liver. This allows the three-inch gallbladder to store a great deal of bile components. But the thickening process can also create problems in the form of extremely painful gallstones, which are dried, crystallized bile. Fortunately, the entire gallbladder can be removed with little or no lasting ill effect. All that is missing is a small storage sac for bile.
This manufacturer of powerful digestive enzymes, only six inches long, resembles a branchlet heavily laden with ripe berries. Its important role in digestion is often overshadowed by the fact that it also manufactures the hormone insulin . The pancreas cells that manufacture digestive enzymes are completely different from those that manufacture insulin. The latter are grouped into little clusters called the islets of Langerhans , which are discussed under The Endocrine Glands in this chapter.