58 By Dr. Abdel Aziz M. Hussein Lecturer of Medical Physiology Measurement of : 1.Reabsorption rate = filtered load excretion rate 2.Secretion rate = excretion rate filtered load 3.Excretion fraction (fraction excretion) (F ex ): It is the fraction of the filtered load of substance that's excreted Measurement of : 4. Clearance ratio; It is the ratio between clearance of a substance x and clearance of inulin 1.If Clearance ratio = 1 substance is not reabsorbed nor secreted i.e. substance reacts as inulin). 2.If the ratio is < 1 substance is partially reabsorbed as urea. 3.If ratio equals zero substance is filtered but completely reabsorbed e.g. glucose when P glucose < renal threshold. 4.If ratio is > 1 substance is secreted e.g. PAH. Clearance ratio is = ml/min 124 ml/min 1 ml/min Clearance ratio less than ml/min 124 ml/min 1 ml/min Clearance more than ml/min 124 ml/min 1 ml/min Clearance = zero 125 ml/min 124 ml/min 1 ml/min Transport Maximum (Tm Def., It is the maximal amount of substance transported by renal tubules per min Types: 2 types; 1.Reabsorptive Tm e.g. Tm for glucose (TmG) 2.Secretory Tm e.g. Tm for PAH (TmPAH) Normal Plasma Concentration Filtered load = 3 Normal Plasma Concentration Filtered load = 3 Reabsorption rate = 3 High Plasma Concentration Filtered load = 6 Filtered load = 6 Reabsorption rate = 4 (Tm) Excretion rate = 2 Transport Maximum (TmG) Def., It is the maximum amount of glucose reabsorbed by the renal tubules/min Types: It is 375 mg/min in male and 300 mg/min in female It occurs when plasma glucose concentration [P G ] 300 mg/dl. Def., It is the plasma concentration of substance above it; the filtered substance begins to appear in urine. Value: It is 180 mg/dl for glucose Cause: When PG is below 180 mg/dl, any filtered load of glucose is completely reabsorbed If PG increase above 180 mg/dl filtered load exceeds reabsorptive capacity of renal tubules glucose starts to appear in urine. Normal Plasma Concentration Filtered load = 3 Secretion rate = 4 Excretion Rate = 7 High Plasma Concentration Filtered load = 4 Secretion rate = 7 Excretion Rate = 11 Transport Maximum (TmPAH) Def., It is the maximum amount of PAH secreted by the renal tubules/min. Types: It is 80 mg/min corresponding to [P PAH ] of 20 mg/dl Below [P G ] 180 mg/dl: all filtered glucose will be reabsorbed and so, no glucose will appear in urine and hence the clearance of glucose = zero Increase P G above 180 mg/dl, glucose starts to appear in urine, and so its clearance starts. Further increase of P G above 180 mg/dl will be accompanied by increase in glucose clearance Excreted glucose = filtered glucose reabsorbed glucose U G. V o = GFR.P G T G By dividing the equation by [P G ] the result will be: Gradually decrease as long as [P G ] is increasing till theoretically reach very low value and so can be neglected and C G will equal C in. Excreted PAH = filtered PAH + secreted PAH U PAH. V o = GFR. P PAH + T PAH By dividing the equation by [P PAH ] the result will be gradually decrease with any increase in P PAH till by time it will be much decreased to the degree that can be neglected and C PAH will equal C in. 26 Plasma concentration Clearance (ml/min) CPAH CG Glomerular filtration rate would be increased by : a) constriction of the afferent arteriole b) a decrease in afferent arteriolar pressure c) compression of the renal capsule d) a decrease in the concentration of plasma protein e) a decrease in renal blood flow The volume of plasma needed each minute to supply a substance at the rate at which it is excreted in the urine is known as the : a) diffusion constant of the substance b) clearance of the substance c) extraction ratio of the substance d) tubular mass of the substance e) filtration rate of the substance. The glomerular filtration barrier is composed of all the following except : a) fenestrated capillary endothelium. b) macula densa. c) basement membrane. d) podocytes. e) mesangial cells. 1. Reabsorption: Transport of materials from tubular lumen (filtrate) to peritubular space or interstitial fluid to peritubular capillaries (PTC). 2. Secretion: Transport of materials from blood in PTC to interstitial fluid to tubular lumen. 1) Surface area Is very large since every tubule receives only 60 nl/min (SNGFR). 2) Flux is the rate of transport/unit time/unit surface area 1)Properties of the membrane of epithelial cells The difference between the apical and basolateral membranes properties account for the transepithelial transport of all solutes Apical membrane Basolateral membrane 2) Tight junctions Attach cells at their apical borders The tight junction may be; a. Loose tight junction (allows passage of solutes and water) or b. Tight junction (doesn't' allow transport or difficult transport). Tight junction Paracellular space 3) Establishment of transport maximum (Tm) & development of concentration gradient between the lumen and peritubular space This allow back diffusion of substance across the leaky tight junction. Both characters limit the transport process. 4) Metabolic state and vitality of the epithelium: Any transport whether active or passive needs energy. O 2 is needed for reabsorption of Na + which is responsible for reabsorption of other. So, O 2 lack affects tubular transport. 5) Hormonal & chemical substance as epinephrine, aldosterone, parathormone calcitonin, AII, ADH, ANP, PG and diuretics. 6) Removal of reabsorbed materials by PTC: The dynamics of reabsorption in PTC is determined by Starling forces: a) Capillary hydrostatic pressure (13 mmHg). b) Capillary oncotic pressure of plasma proteins (32 mmHg) c) Peritubular interstitial pressure (6 mmHg) d) Interstitial oncotic pressure (15 mmHg) If the algebraic sum of forces favoring reabsorption it will be faster. If not, back diffusion to tubular lumen will occur and reabsorption will be decreased. Net reabsorbing force: = (32+ 6 ) (13+ 15) = 10 mmHg 1.Cuboidal in shape. 2.Have two surfaces: luminal and basolateral surfaces: a.Luminal surface: has microvilli (making the brush border) that increases its surface area. Held together by leaky tight Junction b. Basolateral surface: Has many process that interdigitate each other Are separated by paracellular spaces 3.The cells have abundant mitochondria especially near basolateral border supplying energy for Na + - K + pump. 4. The cells are rich in intracellular and brush border carbonic anhydrous enzyme. 5. The proximal tubule is divided into two segments, convoluted and straighted segments; Convoluted segment (pars convolute) Straighted segment (pars recta) Represents segments 1 & 2.Segment 3. Early 2/3 which lies in the cortex. Late 1/3 which lies in cortex and outer medulla. Convoluted.Straighted. Have greater surface area due to numerous microvilli. Smaller surface area. Preferential reabsorption of essential nutrients as glucose, amino acids & HCO 3 -. Preferential Cl - reabsorption Transepithelial potential difference (TEPD) is -4 m.v. TEPD is +4 m.v. THANKS