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Pecularities of Renal Blood Flow

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Pecularities of Renal Blood Flow 4.3. Peculiarities of renal blood flow 263 and endoplasmic reticulum and secretory granules. physiologic capacity is variable, it varies about Some of these granules are considered to be specific 300 ml. The urinary bladder wall consists of three secretory granules containing renin the other gran- muscle coats, the lining of a superfiacial layer of flat ules do not contain renin, but it is suggested, that cells, and a deep layer of cuboid cells. In the region renin could be deposited in these cells in a non gran- of trigonum vesicae urinariae is an inner sfincter. ular form. The man urethra is divisible into three portions: In the juxtaglomerular triangle cells called lacis prostatic, membranous and cavernous. The female cells occur designated also as extraglomerular mesan- urethra is short. It’s lining consists of pavement ep- gial, or Goormaghtig’s cells. They have numerous ithelium. microvilli forming a fine network. The interstitium consists of cells and cell-free sub- stance. Interstitial cells resemble in their struc- ture to fibroblasts. They contain lipoid drops of prostaglandin precursors and a system of fibrils, 4.3 Peculiarities of renal probably identical with elastic fibrils. The thicker fibres cross the juxtaglomerular cells entering the blood flow Bowman’s capsule and the podocytes. 4.2.2 The urinary outflow tract Concerning the blood flow the kidneys are exep- Urine from collecting ducts is excreted into the renal tional organs. The peculiarity of renal haemody- pelvis, passing calyces renales minores et maiores. namics is a consequence of the fact, that the kid- From the renal pelvis is the urine transported into neys have 100 times greater blood flow than other the urinary bladder by the contractive activity of organs and tissues in human organism. The arteri- ureters. The wall of the urinary excretory system ovenous difference in blood oxygen content is low in has in all its segments almost the same structure. It renal blood vessels. In healthy adult man at rest the cosits of three coats: mucous, muscularis nad fibrous. renal blood flow is about 1200 ml per minute repre- The mucosa is lined by transitional epithelium. In senting 25 per cent of cardiac output. In fact, almost the proximal parts of urinary outflow tract 2-3 cellu- the whole blood flows through the glomeruli, only 5- lar layers in the more distal portion 5-7 layers can be 10 per cent of it courses trhough the paraglomerular found. Lamina propria mucosae lying beneath the anastomoses. epithelium consists of collagenous connective tissue. It was found that the blood pressure whitin the The middle coat of urinary tract excretory system glomerular capillaries is about 50-60 per cent of the consists of an inner longitudinal and an outer cir- blood pressure in systemic arteries – 10 kPa (80 torr). cular layer of smooth muscle. Calyces renales have The blood pressure in peritubular capillaries is about two muscle layers enabling contraction waves wich 1,9 kPa (15 torr) and in vena renalis about 0,8 kPa aid the urine transport into the renal pelvis. The re- (6 torr). The glomerular capillary network can be so nal pelvis has the same structure, consisting of two considered to be a high – pressure capillary network muscle layers. The distal third of ureters consists of in contrast to the low – pressure peritubular capillary three muscle coats – the thirth, outer coat is formed network. The striking difference in pressure between of longitudinally oriented smooth muscle layer. The the glomerular and peritubular capillaries is caused peristaltic waves of ureter occur 1-5/min. shifting by the high resistance within the vas efferens. the urine towards the urinary bladder. The stim- The high-pressure region of cortical glomerular ulus initiating these movements is not understood network resembles the arterial end of capillaries. In till now. It could be a certain filling pressure in re- the low-pressure peritubular capillary network pre- nal pelvis. The oblique ”entrance” of ureters into vails the retrograde diffusion of fluids according to the urinary bladder inhibits the urine reflux, though Starling’s law, thus the peritubular capillary bed ureters are not provided with sfincters at their ter- functions as the venous end of capillaries. Blood minal portion. flowing through the peritubular capillary network is Urinary bladder is the reservoir for urine. Its deprived of the water volume which has been filtered 264 Chapter 4. Pathophysiology of kidneys and urinary system ( I. Hul´ın) in glomeruli, thus its osmotic pressure is high, and wall of vas afferens, stimulated by tension changes thereby it has the ability to reabsorb the water. affecting the vessel wall. If this tension is dimin- The rate of blood flow within the perilobular ar- ished e.g. during reduction of blood flow through teries is relatively high, hence the erythrocytes move kidneys the renin secretion by these cells rises and mainly in the centre of this flow. Vasa afferentia are vice versa. The second type of receptors (chemore- derived from the aa. interlobulares in a nearly rectan- ceptors) is found in the wall of distal tubules in their gular direction, therefore more plasma than erythro- portion lying near to glomeruli: - the granular cells of cytes flow in them. Owing to this fact the cortical macula densa. They respond to changes in sodium glomeruli are supplied with blood containing more concentration of tubular fluid. The stimulation of erythrocytes than plasma. renin secretion occurs if the NaCl concentration in The arrangement of two successive capillary net- the vicinity of macula densa cells decreases. On the works is very important concerning urine production contrary, an augmented NaCl content in this area and its concentration. For understanding the disor- supresses renin release. Renin is releases from cells ders arising in kidneys during glomerular blood flow into the intersticium and from there it gets into the alterations is very important to know these facts. capillaries. Renin is a specific endopeptidase con- The blood distribution in kidneys is uneven. Al- verting angiotensinogen. A glycoprotein present in blood plasma - to angiotensin I which is converted by most 80 per cent of renal blood perfuses the outer converting enzyme to a substance with very strong cortical regions. This is why all changes in blood vasoconstrictive activity - the angiotensin II. flow will be reflected in alterations of this region. The blood flow in renal medulla does not depend of Renin-angiotensin system is one of the key sys- systemic arterial blood pressure. tems regulating the blood pressure. Despite of ex- Blood flow through the juxtamedullar glomeruli tensive research projects, all facts concerning the re- enables to maintain the blood flow in renal medulla lease and regulation of this very important factor are to the detriment of cortical glomeruli, especially not known in detail till now. The baroreceptor and when the blood supply of kidneys is diminished, or chemoreceptor mechanism explaining the triggering the systemic arterial pressure falls considerably. off this system is in principle accepted. The high blood flow through the renal cortex The mechanism of angiotensin action is not well responds sensitively to blood volume and pressure understood. Angiotensin II stimulates the calcium changes. An important fall of blood pressure or vol- uptake and the calcium release from celullar or- ume can elicit development of ischaemia, even of ganelles in target cells. Angiotensin II induces vaso- necrosis in outer cortical layers. Inflammatory al- constriction of renal vessels, above all in vas efferens. terations in the renal cortex can vice versa affect the In vas afferens is its vasoconstrictive action probably blood flow through the medulla renalis. masked by vasodilatation due to prostaglandins, syn- Oxygen consumption in cortical region is 9 ml per thesis of which is stimulated by angiotensin. During minute, it is about 20 times higher than in medullar decreased blood flow the constriction of vas efferens region (0,4 ml per minute). Thus, the renal blood ensures the glomerular filtration. This vas efferens flow supplies the organ with oxygen and nutriments constriction causes blood pressure fall in peritubular for its own metabolic processes and it ensures simul- capillary network leading further to improvement of taneously the processes of ultrafiltration of plasma fluid resorption in proximal tubule. The angiotensin and thereby of urine formation. Kidneys are there- receptors are situated also on mesangial cells within fore provided with several mechanisms enabling the the glomeruli. These cells may influence the per- adaptation of blood flow according the given require- meability of glomerular capillaries. Enhanced an- ments of organism. giotensin level suppresses by feedback mechanism the The autoregulation of renal blood flow is accom- renin release from juxtaglomerular apparatus. But plished by nerves and humoral factors. The most the suppression of renin production can be induced important regulation is accomplished by the renin- by an other pathway – by aldosterone release, sodium angiotensin system. There are two types of in- reabsorption and by its increased concentration in re- trarenal receptors recording stimuli for renin release: gion of macula densa. the first type are the baroreceptors localized on the The renin-angiotensin system is involved not only 4.3. Peculiarities of renal blood flow 265 in regulation of renal functions but it plays a very tion of vasoconstrictive substances is the result of en- important role in control of systemic arterial blood hanced sympathetic activity. In this situation have pressure. Angiotensin II by its strong vasoconstric- the prostaglandins a protective influence upon the tive effect increases the peripheral resistance of ves- renal circulation. sels and maintains so the blood pressure on appro- The natriuretic hormone is very probably sub- priate desired level. Apart from this, angiotensin II stance with low molecular weight, produced in brain, stimulates the sympathetic activity and facilitates mainly in hypothalamus.
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