The largest parts of the digestive system are located in the body between the diaphragm and the pelvis (abdominopelvic cavity).
The organs of the digestive system are divided into two segments: 1) the alimentary canal, and 2) the accessory digestive organs.
The alimentary canal, or GI tract, is essentially a digestive tube - mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
The cheeks are mainly muscle (the buccinator muscles - a good trivia fact to know). The cheeks and lips are mainly for keeping food in our mouths during chewing. The lips are translucent - their red color comes from local capillaries.
The teeth are pretty well-known so we won't spend much time on them. They are located in the mandible (lower jaw) and maxilla (upper jaw/face). As we chew, the tongue keeps the food between the teeth, which tear and grind it into smaller pieces - ready for its plunge into the perils of the stomach. Enough said about this important, but uninteresting body part.
The palate, or roof-of-the-mouth, consists of a hard, bony plate where the tongue essentially traps food during chewing; and transitions to a soft, muscular, mobile flap ending with the uvula (that piece of skin hanging down at the very back of the throat), which closes off the nasopharynx during swallowing - keeps food from going off toward the lungs, as mentioned in last month's discussion of the circulatory system.
The tongue is another mostly muscle body part, whose job is primarily to move food around during chewing - to keep it between the teeth. The tongue also functions as a kind of "churn;" mixing food and saliva to form a wad of mushed up food (formally, a bolus). When you swallow, the tongue helps move the wad into the pharynx. In addition, the tongue helps in speech, and provides the home for most taste buds.
The salivary glands secrete, you guessed it, saliva; which moistens the mouth, dissolves food chemicals so they can be tasted, and combines food into a bolus (a wad of chewed food). Enzymes in saliva start the digestion of starches. Saliva also contains a chemical that attempts to neutralize the acids in the mouth that cause tooth decay. Saliva also contains enzymes, antibodies, and a cyanide compound that, together, work to keep the mouth clean by killing bacteria.
Salivary glands are of two types: one provides a constant supply to keep the mouth moist, and the other is secreted only when we eat.
The small intestine consists of three parts: 1) the duodenum, 2) the jejunum, and 3) the ileum. The duodenum gets digestive enzymes from the pancreas, and bile from the liver and gall bladder - all for the more-or-less final digestive process completed in the small intestine. Most nutrient absorption occurs here, easily, because of the large surface area - you have no doubt seen pictures of this long tube with all its convolutions (folds) enabling it to fit into the lower abdomen. The trip that chyme takes through this maze (about 200 sq meters), takes enough time for complete nutrient absorption.
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Note 1 - Somewhere in an earlier Newsletter, macrophages were
discussed, but let's review them. They are generalized phagocytes,
which are immune cells. There are some specific cells that destroy
specific invaders, but macrophages are non-specific - they destroy
bacteria, foreign molecules, even dirt particles; they even get rid of
dead blod cells. They essentially eat foreign bodies.
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An interesting note - kind of: bile is the usual vehicle for ushering cholesterol out of the body, and bile salts keep the cholesterol dissolved within bile. Too few bile salts can lead to crystallization of cholesterol in the gallbladder - "gall stones."
The pancreas secretes two important hormones - insulin and glucagon, which lower and raise blood sugar levels in an effort to maintain homeostasis.
Q This question is a follow-up to my question
about pancreatitis, which appeared in the November 1995 issue of the
Newsletter. My Uncle R had the pancreatitis attack, and more recently
was found unconscious, was hospitalized, and sometime during this
ordeal, had convulsions, and was subsequently diagnosed as
diabetic. What is going on here?
M.N.F., Union City, CA
A At first I thought there was surely a connection between these two events because both are intimately tied to insulin, and maybe there is, but after looking into this question, that isn't really clear. As usual, only the doctors involved (if anyone) can give a definitive description of what actually happened, but I will provide some general information.
For me, part of the problem is that two opposite conditions can cause the same symptoms - unconsciousness and convulsions - hyperglycemia, and hypoglycemia.
First, what is diabetes? It is an incurable, but controllable, genetic disease. There are two types of diabetes: Type I and Type II.
Type I is the so-called "insulin-dependent" variety, and sufferers must take daily insulin injections to survive. Type II (once called "adult- onset" diabetes) can be controlled by diet and exercise. It typically requires careful monitoring of one's blood sugar level. As a genetic disease, it is carried by many, since before birth. For some, it is active from birth (Type I), but for many, it doesn't pop up until after about age 40 (Type II).
Some common causes (ultimately of diabetes) are starvation (don't skip meals, your body can't tell the difference between a fast and starvation), alcohol consumption, some drugs - like insulin or sulfonamides, and excess fat.
The normal problem with diabetes is too much blood sugar (glucose), a condition called hyperglycemia. The problem is that glucose isn't getting into the cells where it can be "burned" for energy. Consequently, the body starts to convert its own fat and muscle into fuel.
As the body starts to consume itself, excess ketones (products of incomplete fat metabolism that are created by the absence of carbohydrates) are produced. These ketones essentially poison the body, and can ultimately cause death. In many cases, enough glucose gets to its proper destination to prevent ketosis; but, down-the-line, serious problems can result (e.g., blindness, kidney failure, or gangrene of the feet). This all sounds pretty depressing, but it isn't inevitable. Diabetes can be managed, but it takes some effort.
In general, diabetics need lots of complex carbohydrates (the so-called starches), lots of fiber; and less sugar, fat, salt, and alcohol (use in moderation only).
Excess fat causes cells to become insulin resistant (they fail to recognize insulin). Obesity (defined as being 20% or more over one's ideal body weight; whatever that is), can also overwork the pancreas. As the cells resist insulin, more is produced, resulting in diabetes. Weight loss can often reverse diabetes.
Exercise is important to the diabetic, and the same amount at the same time is beneficial. It helps lower blood glucose levels, and, in general, helps with weight control Do something! Walking is good. Remember the Big Two: diet and exercise. And be sure to get a physician's approval of any exercise program.
Now back to hypo-/hyper- gly-cemia. Hypoglycemia is commonly associated with Type I diabetes, but Uncle R has Type II, so we could just concentrate on that. But, there are enough common symptoms that each type will be given some attention.
One thing that is clear is that weight loss (fat) is very often beneficial. It is estimated that 80 - 85% of diabetics are over-weight (fat). In the case of Type II diabetes, either insufficient amounts of insulin are produced to control blood glucose; or, for one reason or another, the cells targeted to receive glucose, don't respond to insulin in an appropriate way. It seems that sometimes one's fat, liver, or muscle cells don't respond to insulin even though these cells are stuffed with glucose. As a result, blood sugar levels rise (hyperglycemia), and the pancreas releases more insulin. Weight loss empties glucose storage depots, and the cells can then resume responding to insulin's signal to move glucose from the bloodstream into cells.
Hypoglycemia can cause unconsciousness, seizures, and brain damage. In essence, the brain is deprived of adequate glucose - the brain's energy source - and, consequently, it starts to shut down. Hypo- glycemia usually displays warning signs like headache, mood changes, irritability, lack of concentration, drowsiness, confusion, impaired judgment, or blurred vision; however, these symptoms may not be recognized as hypoglycemia. In that event, the more major symptoms, like convulsions or unconsciousness, may result.
It was once thought that diabetes was caused by too many sweets. You might remember the term "sugar diabetes." In reality, there is no connection between the two, unless you gain weight from too many sweets - sort of an indirect connection. It isn't the sweets themselves, but the resulting weight gain.
Low blood sugar levels are more common in the mornings (after the normal overnight fast), or anytime during the day when blood sugar may be low because of dietary conditions, like too long between meals. This relates more to Type I diabetics, but not exclusively. It is essential for diabetics to maintain glucose homeostasis (balance, equilibrium, stability). This can be done by keeping high-carb snacks (e.g., Lifesavers, glucose tablets, soft drinks - but not sugar-free) at hand for whenever mild symptoms occur.
Hyperglycemia can be caused by overeating, too little exercise, too little insulin, or illness (A cold? Pancreatitis?).
Just to complicate the issue, a third diabetic condition, in addition to hypo-/hyper- gly-cemia, is hyperosmolar nonketotic coma. This condition develops after blood glucose has gone very high. To get rid of excess glucose in the blood, it is eliminated through the urine, which leads to dehydration. The body fluid imbalances have symptoms - coma, for one - that are severe, and need to be treated in a hospital.
It is critical (literally) to do all you can to control diabetes because it is insidious. Left untreated, there are a plethora of potential problems down the line, and at the end - "death," and it doesn't get more serious than that.
Blood sugar levels can rise or fall from stimuli besides the vicissitudes of insulin levels (e.g., major infections like flu, a cold, or stomach upsets). Major surgery can cause hyperglycemia, and so can emotional tension.
On the other hand, hypo-glycemia (low blood sugar) can be caused by fasting (Watch it Andy!), or simply too little food, or strenuous exercise. Also, high doses of aspirin, blood- thinning drugs, alcohol on an empty stomach, or sulfonamides.
Low blood sugar can cause a diabetic to become irritable, uncoordinated, or even unconscious.
Be clear about this, diabetes can lower your life span; but that isn't inevitable. A program of exercise and diet that keeps your blood sugar level "normal" can put a diabetic on the same life span path as anyone else. The exercise. obviously, is to help control fat weight; and it may be that exercise increases the number of insulin receptors, which help get glucose from the blood to the cells where it is needed. Exercise also improves circulation and lowers blood fats (cholesterol and lipids) - both special problems for diabetics. [I apologize for this somewhat disjointed answer, but I hope you picked up the main points.]
Q In the June '96 Newsletter, you made reference
to a study that had selected a treadmill as the superior aerobics
machine. I just got an exercise bicycle. Should I trade it in?
H.G.M., San Jose CA
A Definitely not, if you like it better. If you remember, the question ended with something like _ _ _ and don't tell me, the one you will use. However, that is really the best answer. As reported in the July 1996, Penn State Sports Medicine Newsletter, there were only minor differences (slightly more or less calories burned) between the machines tested. They all work. So use the one that you like.
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