АНГЛЗАЧЕТ. Reveal striated поперечнополосатый
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The greater part of the muscular coat of the digestive tract consists of two layers: an internal layer with circular muscle fibres and an external layer with longitudinal muscle fibres. The wall of the pharynx and the superior part of the esophagus, and the tongue and the soft palate all contain striated muscle tissue. It is the muscular coat of the other parts of the digestive tract that consists of smooth muscle tissue. Contractions of the muscular coat move food along the digestive tract. The serous coat that covers the digestive organs in the abdominal cavity is called the peritoneum. The peritoneum has two layers, visceral and parietal. In the esophagus the serous layer is lacking and the outer coat is fibrous in nature. The digestive glands secrete digestive juices that contain enzymes and some other substances which take part in the chemical processes of digestion. In addition to the small glands in the mucous coat of the digestive tract, there are also large glands: the salivary glands, the liver and the pancreas. Though these glands are situated outside the digestive tract, they communicate with it through ducts. Any part of the digestive tract and the digestive glands are equipped with nerve fibres and their endings. The nerves of the digestive glands regulate the secretion of digestive juices. It is known that the nervous system not only regulates the activity of each organ, but also coordinates their activities. Urinary System 1. The uriuary system is the system which excretes the largest part of the waste products of the body. It consists of the kidneys, right and left, the ureters, a tube from each kidney which conveys the urine to the bladder, the urethra, a tube that leads from the bladder, along which the urine is passed out of the body. 2.The kidneys («renes» — Latin) are placed one on each side the lumbar region of the spine, on the posterior abdominal wall, at the level the twelfth thoracic and first-second lumbar vertebrae. A kidney weighs about 150 grams and is covered by membranes. The connective tissue membrane which directly adheres to the kidney is called the fibrous capsule. This capsule is surrounded by perirenal fat and is called the adipose capsule. The kidneys are two bean-shaped organs. The kidneys contain one million small tubes, which have to excrete products of metabolism and con- I rol the concentrations of most of the constituents of body fluids. These small tubules make up the parenchyma of the kidney. They are very fine and may be of various shape. Since dissolved (растворенные) wastes may be excreted by diffusion through the various cell membranes there is little evidence that such excretion occurs. 3. The inner margin of the kidney is known as the hilus. At the hilus (he ureter which conveys urine is a tube about 30 cm long. When the ureter leaves the hilus it descends along the posterior abdominal wall into the cavity of the pelvis where it perforates the wall of the bladder and opens into its cavity. As the muscular coat of the ureter contracts it has to perform peristaltic movements. 4. The bladder is a reservoir for urine. It is situated in the cavity of the pelvis. The bladder has three parts: the superior part or apex, the middle part or body, and the inferior part or fundus. The wall of the bladder consists of three coatings — mucous, muscular and connective tissue. The mucous membrane of the bladder forms numerous folds. If the bladder fills, the folds of the mucous coat will straighten out. The muscular coat consists of three layers of smooth muscles which are able to extend in different directions. It should be known that the capacity of the bladder of an adult is about 350—500 ml. The Kidneys Kidneys are a pair of glands which are situated close to the spine in the upper part of the abdomen. They are on a level with the last dorsal and upper two lumbar vertebrae. They are kept in this position by a quantity of fat, loose connective tissue, in which they are embedded, and the large vessels which have to supply them with blood. Structure. In size each kidney is about 4 inches long, 2.5 inches wide, 1.5 inches thick, and weighs over 4 ounces. The size, however, may vary a good deal. The left kidney is slightly longer and narrower, and lies a little higher in the abdomen than the right. Since the outer margin of the kidney is convex, the inner is concave. It presents a deep depression, which is known as the hilus, where the vessels enter its substance. At the hilus the renal vein lies in front of the renal artery, the former joins the inferior vena cava, and the latter springs from the aorta almost at a right angle. Vertical section through a kidney allows to disclose three concentric zones. The outer light-coloured zone is the renal cortex, within this is the darker renal medulla and within this again is a space - the renal sinus which is normally occupied by fibrous sac, the renal pelvis. The cortex extends inwards in a series of renal columns which divide the medulla into a number of renal pyramids. Within the cortex each minute artery presents a vascular knot, a glomerulus. Each glomerulus projects into the end of its corresponding renal tubule, which is separated by a thin layer of cells, glomerular (Bowman's) capsule; glomerulus plus capsule forms a renal (Malpighian) corpuscle. A renal corpuscle with tubules and blood vessels is called a renal unit, or nephron. Function. One chief function of the kidneys is to separate fluid and certain solids form the blood. The glomeruli are to filter from the blood the non-protein portion of the plasma. It is estimated that in 24 hours the total human glomeruli will be able to filter between 150 and 200 litres, 99 per cent of which is reabsorbed by the tubules. The kidneys are to be regarded as filters through which the whole blood of the body passes and which remove from the blood a substance, urea, together with other impurities, which together constitute the urine. The cleansed blood passes on in its vessels, and the urine drains into the ureters and finally into the bladder. If the kidneys cease to work the blood will become poisonous because of the accumulation of the waste matters. Leucocytes and Lymphocytes 1. Leucocytes. About 65 per cent of all white cells are leucocytes. Their protoplasm contains granules. Those leucocytes that stain neutral dyes - as the majority do - are called neutrophils. About 1.5 per cent of the total stain with acid dyes and are called eosinophils. And a still smaller number, 0.5 per cent, have granules that stain with basic dyes; these are called basophils. The percentage of oesinophils increases greatly when parasites invade the body. 2. One characteristic of leucocytes is the irregular, or lobed, appearance of the nucleus. The number of lobes is an index to the cell's age. Ordinarily, about 45 per cent of all leucocytes have a nucleus of three lobes. The life span1 of a leucocyte is short, from four to twelve days. 3. The function of leucocytes is primarily that of protection against infection. After the skin is pierced and the wound becomes infected, leucocytes from all the body are attracted to this place. Just what attracts them is not known - the process is called chemotaxis - probably some by-product of bacterial metabolism. When they arrive at the wound, they leave the blood stream. They wage war on the invaders, engulfing the bacteria within their own protoplasm, a process called phagocytosis (literally «cell-eating»). Before the infection is not too overwhelming, the victory usually goes to the leucocytes. 4. Lymphocytes. These cells, which comprise about 35 per cent of the white cells, have a nucleus which practically fills the cell. They are produced in lymph nodes scattered throughout the body; the tonsils are examples of lymph nodes. They live only a few hours after they leave the blood stream. They are incapable of movement and thus cannot pursue bacteria and have little cytoplasm so that phagocytosis is practically out of the question. Bacteria trapped in lymph nodes provoke the formation of them. Unfortunately if the infection is overwhelming, the lymph nodes themselves become infected. Thus in guinea pigs it has been possible to trace the route of tubercle bacilli from the intestines to the lungs, as the route is marked by successively infected lymph nodes. Nature оf the Heart Beat The fact that the heart, completely removed from the body, will go on to beat for a time shows that its beat is «automatic», i.e. does not require nerve impulses. The beat is rhythmic: it is not jerky; the ventricles relax fully before the next contraction. This is explained by a special property of cardiac muscle tissue. The period of time during which the muscle is not responsive to a stimulus is called the refractory period. It is characteristic of the heart muscle to have a long refractory period. When the heart muscle is stimulated, it will contract but will not respond again to that stimulus (though it may respond to a stronger one) until it has relaxed. This rest period is occupied by the heart filling with blood, in preparation for the next beat. Even the heart forced to beat rapidly maintains a perfectly rhythmic beat; although the beats come closer together, there is always that little rest period in between. The heart is a pump, but a double pump; the volume expelled by the right ventricle is the same as that expelled by the left. When exercise is suddenly undertaken, the «venous return», i.e. the blood returned to the heart through the veins, is suddenly increased. For a few beats the right ventricle does put out more blood than the left, but soon the additional blood has passed through the lungs and is entering the left ventricle. From then on, both put out the same amount. The Cardiac Output Cardiac output refers to the volume of blood which the left ventricle forces into the aorta per minute of time. It must be noted that this term refers to the output of the left ventricle only, and that the total output is twice as much. The reason that the output of the left ventricle is given this special name is that it supplies the entire body (except the lungs) with the blood. Another reason is that it does a much greater amount of work than does the right, and consequently is more likely to fail. Cardiac output is the product of two factors: heart rate (the number of beats per minute) and stroke volume (the volume expelled per beat). 1. The heart rate is normally controlled by a balance between impulses reaching it over the vagus and over the sympathetics. Thus, inhibition of the vagus centre speeds up the heart. And inhibition of the sympathetic centre slows down the heart. It seems that in the human most of the effect is achieved by inhibition of the vagal centre of the sympathetic region. 2. The second factor affecting cardiac output is the stroke volume, that is, the amount of blood which the left ventricle ejects per beat. The stroke volume depends upon the «venous return». The normal heart is capable of a considerable degree of enlargement; after the venous return is increased - as it is in exercise - the chambers of the heart are able to supply the additional blood. The walls of right atrium and the great veins are thin and stretch readily; therefore the heart rate is increased. The increased venous return in exercise is brought about in the following manner: 1) after muscles contract, they exert a «milking» effect on the blood vessels which they contain. With each contraction, blood is squeezed out1 into the veins; it cannot be squeezed back into the arteries because the arterial pressure is high - and with each relaxation the blood vessels of the muscle again fill up with blood; 2) in exercise, breathing becomes deeper. The heart lies within the thorax; when the thorax expands, blood is «sucked2 into» the heart. The two factors, working together, lead to the increase of the blood amount returned. First the right side of the heart, and within a few beats the left, are dilated and take bigger «bites» of blood. Thus the stroke volume is increased. Movements of Breathing Mechanism of Inhalation and Exhalation Respiration consists of rhythmically repeated inhalations and exhalations. Inhalation takes place as follows: the muscles participating in inhalation contract under the influence of nerve impulses. While contracting the diaphragm descends (flattens) increasing the vertical size of thoracic cavity. Contraction of the external intercostal and certain other muscles elevates the ribs increasing both the anteroposterior and transverse size of the thoracic cavity. Thus muscular contraction increases the capacity of the thorax. Since the pleural cavity contains no air and the pressure in it is negative both lungs expand simultaneously with the increase in capacity of the thorax. The lungs expanding, the air pressure in them drops and atmospheric air rushes into the lungs through the air passages. Hence an inhalation involves a contraction of muscles, an increase in the capacity of the thorax, an expansion of the lungs, and entrance of atmospheric air into the lungs through the air passages. 2. Inhalation is followed by exhalation. The muscles participating in inhalation relax, the diaphragm rising. The ribs drop as a result of contraction of the internal intercostal and other muscles and because of their own weight. The capacity of the thorax decreasing, the lungs become compressed, the pressure in them rises and the air rushes out through the air passages. 3. The respiratory movements are rhythmic. An adult at rest makes 16-20 respiratory movements per minute, children make more movements (a newborn child makes up to 60 movements a minute). Physical exertion, particularly in untrained people, is usually accompanied by faster respiration. Accelerated respiration is also observed in many diseases. Sleep is accompanied by a slowing of respiration. 4. Movements of breathing. Changes in the volume of air in the lungs are brought about by movements both of the diaphragm and ribs. Contraction of the diaphragm increases the length of the capacity of the chest, while the upward movement of the ribs increases the cross section of the chest. Inspiration is due to contraction of the diaphragm and of the muscles attached to the ribs. These contractions are induced by nervous impulses. Expiration is a less active process than inspiration, for when the muscles relax the elasticity of the lungs themselves tends to drive out the air previously inhaled. Any impediment to breathing due to pressure or constriction in the respiratory passages is especially noticeable during expiration, because it is usually of a passive character. When the volume of breathing is increased by physical exercise many assessory muscles are involved while producing deeper inspirations. Expiration also involves a vigorous action of the abdominal muscles. Regulation of Respiration. Nervous Control of Breathing The mechanism of regulation of respiration is very complex. Schematically it is as follows. In the medulla oblongata there is the respiratory centre. In the respiratory centre both excitation and inhibition continuously alternate. When excited it transmits impulses to the spinal cord and hence along nerves to the respiratory muscles; the latter contract and an inhalation takes place. When the respiratory centre is in a state of inhibition the transmission of impulses to the respiratory muscles ceases, the muscles relax and an exhalation results. The specific stimulus of the respiratory centre is carbon dioxide. As soon as the blood accumulates a certain amount of carbon dioxide, the respiratory centre becomes excited and an inhalation takes place. During inhalation the lungs expand, which stimulates the endings of the vagus nerve embedded in the tissue of the lungs. While arising in the receptors the excitation is transmitted along the vagus nerve to the respiratory centre and inhibits it, and an exhalation results. Thus respiration is automatically regulated; an inhalation stimulates an exhalation, and the exhalation brings about an accumulation of carbon dioxide which stimulates an inhalation. Respiration is subjected to the control of the cerebral cortex; this being demonstrated by the fact that a person can voluntarily hold his breath for a very short time or change both the rate and depth of respiration. Cortical regulation of respiration is also evident in the acceleration of respiration during emotional states. Protective acts, such as coughing and sneezing, are associated with respiration. Both of them are performed reflexly; the centres of the reflexes are situated in the medulla oblongata. Nervous control of breathing. The muscles of breathing have no independent or automatic rhythm, they contract only responding to impulses from the brain down the spinal cord. These impulses arise and are coordinated in a specialised area in the brain, the respiratory centre, which is in the medulla. The medulla is at the base of the brain and is a bulbous continuation of the spinal cord within the skull. The respiratory centre has to adjust the volume of air breathed and to maintain a uniform alkalinity of the blood; the centre effects the reciprocal alteration both of inspiration and expiration. The Digestive System and the Process of Digestion and Absorption The present text is given to explain the processes of digestion and absorption. The more we know about them, the better we shall understand how important these processes are. 2. Every cell of the human body requires certain chemical nutrients in the fluids that surround it. In order to supply these nutrients, the body must break down complex foods into molecules small enough to pass through tissues, enter the blood stream or lymphatic systems, and be delivered in a soluble form to the various body cells. This break of insoluble forms is known as digestion; the passage of such substances into the blood stream or lymph is known as absorption. |