Bent Elmann Creative Corner Natural Science Section XII ( c)

The vehicles for the oxygen are the tiny red corpuscles of the blood. These are shaped like the sweets known as fruit gums and, although of microscopic size, their numbers are great enough to give the blood its characteristic colour. Apart from the red corpuscles the blood consists of a colourless fluid carrier medium and the so-called white corpuscles which serve a variety of purposes to be mentioned in due course.The left heart receives all the blood from the lungs and delivers this to arteries all over the body. The great artery by which it starts on its way is called the aorta, but this soon splits up into branches going to muscles everywhere.In parallel with the circuits from artery to vein through muscles are two other circuits which are very important. The so-called portal circulation leads blood to the digestive organs, where it is able to make good its deficiency of glucose. Only a proportion of the aortic blood-stream is dealt with in this way.The other parallel circuit is through the kidneys, where the blood yields up its surplus water. Here again only a proportion of the aortic blood is involved. Thus the blood is refurnished with the nutrimental principle and made free from surplus water on what engineers would call the bypass system. Oil from the lubrication circuit of  an engine is generally filtered and purified on the same plan. The entire circulation of the blood, so essential to the maintenance of the life process, is shown diagrammatically on this page.Each half of the heart is a two-stage pump. The intake portion, called the auricle, is thin walled, and when it contracts it delivers the blood through a non-return flap valve to the ventricle. Valves in the veins prevent any backflow of blood at this point. The ventricle is provided with very thick muscular walls, and for this reason it can deliver blood at a relatively high pressure-2 to 3 lb. per square inch. The outlet from each ventricle is provided with a non-return valve. The pulsation of the heart occur at a rate of about seventy a minute, and the capacity of the heart is sufficient to bring the blood round to itself again every half minute or so. However, the rate of circulation is governed by the use of the muscles, When very hard work is being done both heart and lungs must deal with the blood at a rate in excess of normal. If we run in a race, we shall be conscious that our heart is pounding and that our breathing is accelerated to the point of distress. We feel the need for more oxygen directly we exert ourselves, but the store of glucose in our blood is sufficient to last a considerable time. Carbon dioxide we get rid of throgh our lungs, and the rate of riddance is regulated by our breathing at the same time that this governs our oxygen intake. Normal <atmospheric air contains about 0,04 per cent of carbon dioxide (by volume) and air exhaled from our lungs contains about 4 1/2 per cent of this gas. The oxygen content drops frpm about 21 per cent to about 16 per cent; so that the net result of our breathing is to replace about a fifth of the atmospheric oxygen with carbon dioxide. Exercise results in the production of much water in our blood and the kidneys will not rid us of all of this. Some water is passed away in our exhaled breath-we can see it as a cloud on cold days -and some will pass away through our skins as perspiration. To complete the picture of vital processes in the human body we must describe the digestive system and the portal circulation it serves; we must also say a little more about the excretory system or kidneys,Food is the source of energy and tissue-building material, and digestion permits the assimilation into the blood-stream of what the body needs from the food. The digestive process begins in the mouth and is completed in stages throughout what is called the alimentary tract. This tract is a tube of variable width and great length which runs through the body without having direct communication with it at any point. Whatever passes from the alimentary tract into the body must do so through the walls of capillary vessels into the blood-stream. Soluble materials such as salt and sugar are ready for assimilation directly they are dissolved in water, and the saliva in the mouth is a digestive secretion that can begin at once to turn starch into sugar. However the bulk of the food proceeds by way of a tube called the oesophagus to the stomach, which is a bag-like swelling in the alimentary canal just below the diaphragm. The walls of the stomach secrete another fluid that is able to break down meat and allied foodstuffs into simpler substances soluble in water. Hydrochloric acid is a constituent of the stomach juices. The exit from the stomach is controlled by a valve called the pylorus, which opens periodically to permit the digested food to enter the duodenum, whence it passes to the small intestine. Food may stay in the stomach for several hours, and in this time it is subjected to a churning motion by muscular agitation of the stomach as well as to the chemical action of the gastric juice. In the duodenum it is joined by secretions from the liver, gall bladder and pancreas, but in the long, narrow intestine it gives back to the body such substances as are needed for the maintenance of the life processes. The walls of the intestine are interlaced with blood-vessels forming part of the portal circulation, and the dissolved nutriment from the food passes through the walls of these blood-vessels to become assimilated by the blood-stream. Glucose is the chief material absorbed in this way. From the small intestine the food residue passes into the large intestine, where it becomes more or less solid as a result of the assimilation of water bythe intestinal blood-vessels. From the terminal portion of the large intestine the food residue is periodically expelled. It was once believed that undue retention of this waste matter in the bowel resulted in progressive self-poisoning, so that drastic purges were habitually used to ensure a daily evacuation. The truth is that once matter has entered the large intestine it loses nothing to the blood-stream other than water, so that it can do little harm. The disorder resulting in headache, furred tongue and constipation has nothing to do with the lower bowel but rather with the digestive and assimilation processes higher up. The food consumed may be of a kind to overload the system with an excess of fat or sugar or some other substance. Or it may be that insufficient water has been imbibed. What we feel when we are indisposed may be the strain put on organs that have been given an excessive amount of work to do under difficult conditions. Quite naturally the orderly passage of digested matter through the intestines will be suspended if the food is of akind that requires exceptional activity of stomach glands or liver or pancreas. There may have to be a long wait before the necessary chemical trasnsformations can be wholly accomplished. If poisonous or indigestible matter is taken, the stomach may return it instead of letting it pass on by way of the pylorus. We vomit when the stomach is unable to reduce its contents to an assimilable condition. The same thing occurs when some inflammatory condition of the intestinal tract (e.g. a diseased appendix or a duodenal ulcer) makes it dangerous for the pylorus to open  and impose fresh work on weakened tissue. If a laxative or purging medicine be administered in such a case, the result to the patient may be a fatal rupture or perforation of the intestinal tract at the point of inflammation. Once intestinal matter escapes into the cavities of the body it will occasion the dangerous condition called perionitis. The passage of the food along the intestinal tract is promoted by rhythmical contractions of the latter known as peristalsis. The tube closes down on its contents at particular poinnts and then the points of contraction advance along the tube so that there is a pushing action on all contained matter. Before the peristaltic movement is over the tube recovers its normal diameter, but then the cycle begins all over again. The waves of contraction travel in only one direction under normal conditions; reversed peristalsis is a reaction to disease and can only lead to vomiting. Blood which has assimilated glucose and other substances from the intestines passes straight to the liver, a function of which seems to be to store this glucose in one form or another and deliver it to the main blood-stream in response to the body`s needs. The portal circulation includes the liver. When a muscle is used, glucose is used up and water is formed. The liver at once restores the missing glucose and the kidneys remove the excess water. The pancreas plays a part in this process by secreting a substance known as insulin into the blood-stream. If the pancreas is diseased or missing, the muscles do not utilise the glucose properly and the net result is a tendency for the glucose or sugar content of the blood to become excessive. The victim of this complaint is said to have diabetes, and the unequivocal symptom is the appearance of sugar in his urine, since the kidneys of the diabetic individual must rid the blood of excess sugar as well as excess water. Diabetes is successfully treated nowadays by introducing insulin into the blood artificially by means of injections. The kidneys are bean-shaped organs high up in the abdominal cavity behind the other organs. On its way through them the blood gives up excess water and any other substances it does not need for the proper performance of its functions. Blood assists in the process of tissue repair and renewal that is constantly going on, but it must also carry away the products of decay. Dead tissue is taken up by the blood with other soluble waste matter, and consequently the urine is never pure water even when the individual is perfectly healthy; It contains nitrogenous matter which gives it a characteristic colour and odour. The secreted urine passes drops by drop from the kidneys to a receptacle called the bladder, where it gradually accumulates. The tubes conveying it from the kidneys to the bladder are called ureters, and there is one for each kidney. When the bladder is unable to hold any more urine, discomfort is felt and its expulsion becones imperative. The passage from the bladder to the exterior is called the urethra. Common experience shows that much more water passes from the body by way of the kidneys at some times than at others. It is relatively greater, for instance, in cold weather, because then the loss by perspiration is decreased. Its amount is reduced in hot weather for the opposite reason, unless the individual partakes of more water or beer or tea to compensate for excessive perspiration. Being the vehicle for ridding the of body of much waste matter, urine becomes unduly thick and coloured if it is too scanty. For the preservation of good health it is desirable to drink plenty of water, particularly between meals.When the kidneys are discased or deprived of the chance to function properly, various waste products may not be completely eliminated from the body. A particularly painful complaint, called gout, is attributable to the presence of uric acid in the system. The cure is to avoid certain acid-forming articles of diet on the one hand and to assist the kindneys in their work of eliminating uric acid on the other hand. The healthy body is liable to become prey to micro-organisms, and such well-known diseases as measles, whooping-cough, tuberculosis, rheumatism and smallpox are the effect of microbic invasion. The body reacts to attack in certain well-defined ways. First the blood temperature increases. The excess of temperature deters many microbes because it renders them incapable of multiplying. When a fever is beneficial the doctor lets it run its course, knowing that only mischief could result from giving the patient cold baths or drugs to abate the blood temperature.