Anatomy and Physiology: Running Along the Tract
Running Along the Tract
Oh, sure! It's easy to say that the food travels along the gastrointestinal tract, but how does it move? Through a cool process known as peristalsis. Remember the two layers of muscle in the muscularis layer of every digestive organ? The longitudinal layer shortens the length of the organ, while the circular layer constricts the lumen, and thus pushes the food along the tract, rather like pushing a present through a Christmas stocking. The alternation of these contractions propels the food forward.
In addition, the alternating contractions also help to mix the food in a process known as segmentation. Contraction at various points pinches the small intestine at different points, making it look a lot like a string of sausages—interesting, because the gastrointestinal tract of cows, pigs, sheep, and goats are still used today as a sausage casing! These segments, through repeated contractions, churn the food, allowing it to mix well with the various enzymes at work.
Accessorize with Accessory Organs!
These organs are all well and good, but they cannot function alone. They need help, and that help comes from the accessory organs. Some of you may take exception to having them called accessory, when their function seems so crucial. You couldn't survive, for instance, without a pancreas. Their accessory function merely implies that the food never passes through these organs.
|Primary Organ||Its Accessory Organ(s)|
|Small intestine||Liver, gallbladder, and pancreas|
Our Little Spit Factories
The Big Picture
Secretion of saliva is controlled by both parasympathetic nerve pathways and conscious thought. Placing food in your mouth will trigger receptors on the three of the cranial nerves (N VII, N IX, N X), but the mere thought of food can cause salivation. Thinking about the vinegar taste (an acid) in a pickle is enough to cause a lot of people to salivate, once again, to lower the pH!
The mouth may be the start of it all, but without the salivary glands, it wouldn't be a very fun ride! Have you ever chewed dry crackers when your mouth was already dry? Even worse, have you ever swallowed dry crackers when you had a dry, scratchy sore throat? SCRAAAAAPE!! Luckily, that is not a very common experience due to the wonders of spit, or should I say, saliva.
Saliva, from any of our three main salivary glands (parotid, sublingual, and submandibular), first and foremost provides lubrication for the chewed food, so that when it is swallowed, the spit and food combo (called a bolus) can slide easily along the esophagus. The other two functions are to start the digestion of starches by using salivary amylase (more about that later), and to raise the pH of the mouth. Saliva is slightly basic (pH up to 7.5). This serves two functions: to neutralize the acidic secretions of bacteria in the mouth (the scourge of dentists everywhere), and, under certain circumstances, to neutralize the effect of stomach acid after vomiting.
Not Just Good with Onions
While in the gallbladder, water is absorbed from the bile. Similar to the large intestine, the longer the bile remains, the more concentrated it becomes. During pregnancy, the elevated release of progesterone slows the contraction of smooth muscle, including in the gallbladder walls. The bile salts can ultimately crystallize, producing choleliths (gallstones). The release of cholecystokinin (CCK) can force the gallstones into the much smaller common bile duct, causing severe pain. Given the continuous, slow release of bile from the liver, one treatment is the removal of the gallbladder in a cholecystectomy.
The liver keeps very busy, as a quick glance at the following list shows. To start off, the fats we eat are often in clumps, especially given the way fats are stored in our bodies along connective tissue. These globs of fat have a very small surface area to volume ratio. Bile functions as an emulsifier, breaking these globs into smaller globules (just the way dish soap breaks up the fats on our dishes). These globules, with their smaller volume, have a much larger surface area to volume ratio, and it is on the surface area of the food that the enzymes do their work. More surface area means faster digestion, just the way a spoonful of sugar will dissolve faster than a sugar cube.
The bile is produced in the hepatocytes in each liver lobule. The bile is then collected in little tubes called bile canaliculi, and then collected in slightly larger bile ducts. These little bile ducts then leave the liver as the hepatic duct. The hepatic duct branches both to the gallbladder as the cystic duct (which comes from the gallbladder's other name, the cholecyst), and to the duodenum of the small intestine as the common bile duct. The common bile duct joins up with the pancreatic duct, to form the hepatopancreatic ampulla. The gallbladder stores the bile (up to 70 ml) until a hormonal signal from the duodenum (cholecystokinin or CCK) signals its release.
As I mentioned in my discussion of blood vessels, blood drained from the abdominal viscera (excluding the kidneys and the pelvic organs) is drained into the liver through the hepatic portal vein. An understanding of this blood flow helps you to understand some of the many functions of the liver, which include the following:
- Carbohydrate metabolism Excess glucose is converted into glycogen via glycogenesis, and, when needed, the glycogen is broken down via glycogenolysis.
- Lipid metabolism The amount of lipids in the bloodstream is regulated through both release and uptake in the liver. Since lipids are absorbed into lacteals and then returned to the blood only later, the lipid levels are regulated after they reach the liver via the hepatic artery.
- Amino acid metabolism The liver removes excess amino acids, for either protein synthesis or energy use.
- Waste removal Whenever amino acids are used for energy, through either breakdown or conversion, the amino group must be removed in a process called deanimation. These amino groups ultimately become the urea that is excreted by the kidneys.
- Vitamin and mineral storage Iron and fat-soluble vitamins are stored in the liver, to be released only when your diet doesn't have enough.
- Drug inactivation Drugs you ingest have a limited duration of effect because the liver inactivates the drug over time. One side effect of this is that the liver only has so much capacity to inactivate drugs. Dosage limits are crucial here, for the strain on the liver of exceeding a drug's recommended dosage can be fatal, as in the case of a gifted former anatomy student of mine who accidentally destroyed her liver by taking too much over-the-counter acetaminophen for pain over a short period of time; the only reason she is alive today is through the gift of life—she had a liver transplant.
Excerpted from The Complete Idiot's Guide to Anatomy and Physiology © 2004 by Michael J. Vieira Lazaroff. All rights reserved including the right of reproduction in whole or in part in any form. Used by arrangement with Alpha Books, a member of Penguin Group (USA) Inc.