On the other hand, even the biggest sized pores cannot normally allow molecules whose molecular weights are around 67.000

hydrostatic pressure of the Bowman's space, and can be expressed by the formula Effective fillration pressure [= IPg- [PB + COT)] where Pg = gtomerlar capillary blood pressure (normal value about 45 mm Hg), PB = hydrostatic pressure in Bow man's space (normal value about 10 mm Hg), COT = colloidal osmotic tension due to _plasma proteins of capilary blood plasma (normal value about 25 mm Hg). Putting the values, the effective filtration pressure thus becomes about 10 mm Hg Obviously, fall of effective filtration pressure will lead to fall or filration. (2) The permeabillity is another important factor for filtration to occur. Thinner the filtering membrane, greater is its pemmeability It has been she in chap: 1 of the section that because of anatomical (structural) reasons, the filtering membrane is exceedingly thin. Another reason of the high permeability is the presence of (hypothetical) pores in the filtering membrane [In chapter 1, sec VII, it has been mentioned that the capllary endothetial cells contain pores having diameters between 50 to 100 nm. But these pores are probably not important for the fitration because beyond them lies the basement membrane]. The hypothetical pores are in the basement membrane itself and probably are not stable anatomical structures. These pores are suspected to be channels in the basement membrane between collagen and proteogiycan molecules These (hypothetical) pores are of different diameters, some big, some small and others intermediate in size. Their sizes are such that molecules having molecular weights less than 5000 daltons can pass even through the smallest of them Such partides (e.g. glucose,Na + , K+, amino acids, urea and other crystalloids and the polysaccharide inulin) are therefore called freely filtrabte' substances. On the other hand, even the biggest sized pores cannot normally allow molecules whose molecular weights are around 67.000 daltons or more (for example plasma albumin, diameter 67 A, hemoglobin) However, many diseases can enlarge the pore size and in those conditions plasma alboumin can appear in the glomerular filtrate Molecular weight is not the only criterion, charge is another factor. A more negatively charged molecule ike plasma aiburmn is less readily filtable then hemoglobin whose mlecular Jar weight is very dose to that of albumin N B The capilary BP in the glomerulus has been stated above to be about 45 mm Hg This is in rats The corresponding figure in man in not defintely known, but probably close to it. In short. glomeruJar firiration depends on (i) effective filtration pressure and (ii) permeability of the filtering membrane GIornrnjlar filtration rate {GF R) The two kidneys together have about 2 4 million nephrons aii these nephron,s togeter, filter about 125 ml per minute or about 170 lters/24 hours. This amount, therefore, is the GFR. Long ago, it was believed that in man. normally, some giomerul remain quiescent (because their afferent arterioles remain in spasm) i e all glomeruli do not remain open, simuitaneously Drugs were believed to exist which could in crease the number of functional glomeruli (by convertining the quiescent glomerul into active ones). This idea changed later on and it became known, that this idea was probably erroneous Alltheglomeruin man remain concomitantly active it was Cushney's Idea that all the glornerull do not function concomrtantly A1 any given time, some of them function and others remain nonactive As stated above, the idea became unpopular later on However, probably Cushney's idea was not totally wrong Mesangial cells, which occupy the spaces between the capllary loops, are known to send protoplasmic processes between capiIlary endothelium and basement membrane Further, the mesangial cells can contract Therefore, when the mesangial cells contract, the capillaries become kinked and this results in non flowing of blood through the capilaries stoppage of filtration. The mesangial cells contract when they are subiected to the action of angiotensin and nor adrenalin. factors influencing the filtration (i) AlI factors affecting effective filtration pressure (renal arterial perfusion pressure, colloidal osmotic tension, hydrostatic pressure. In the glomerular space) can afect the GFR. (ii) Alteration of permeabilty of the filtering membrane can also affect the GFR (iii) A feed mechanism. called tubulogiomerular feed back (see later) strongly influences the GFR. Coeflicient of filtration, Kf. should be understood . GFR effective filtration pressure GFR'= Kf x effective filtration pressure. The value of Kf is normally about 14 ml/min/mm Hg Therefore, where the effective filtration pressure is 10 mm Hg, the GFR is (10 x 14) or 140 ml/min. The value of Kf in the same man can change, either due to pathological or due to physiological conditions. Therefore, where the Kf falls, the GFR falls inspite of the fact that the effective filtration pressure is styli same Kf value is an indication of efficiency of the permeable of the renal filtering membrane. Applied Physiology 1. it may be visualized that, fall of systemic arterial blood pressure (BP) fall of renal arterial perfusion ressure fall of glomerular BP fall of effective filtration pressure4 fall of renalfiltration.
Reverse should occur in high BP. In practice, however, the GFR is not affected by ordinary changes of the BP. The phenomenon is called renal autoregulation (for details, see later). Only when the arterial BP shows extreme changes, the autoregulation falls and the GFR is affected. 2. In glomerulonephrits, (a common disease of the kidney) the fitering membrane is thickened and its permeability reduced (= Kf value falls). so the GFR falls, 3. in obstruction of the ureter, (due to, say, stone) fluid accumulates tn the renal tubules hydrostatic pressure within the. Bowman's capsule rises GFR reduces Renal autoregulation, Autoregulation of the blood flow is remarkable in several organs, viz, heart, Kidney and brain. The general discussions on autoregulation has been made in chap 6, sec, V. In short autoregulation means that there is an arrangement by which the organ concerned can, to some extent, ward off the effects of changing perfusion pressure to itself, that is unless the perfusion pressure of blood changes extremely severely, the autoregulation ensures that the blood supply to the organ remains constant, this mechanism, is not mediated through nerves. Thus as stated in the previous paragraph, the GFR remains unaffected in the face of changing perfusion pressures in the renal artery, provided the charges are not extreme. Experiments have revealed that, between a mean arterial BP of SO to 180 mm Hg, the BP of the capillaries of the