1625915643-5d53d156c9525bd62bd0d3434ecdc231 (843955), страница 57
Текст из файла (страница 57)
At tonic levels of sympathetic nerve activity, the intrarenal systems will counteract this effect, to ensure the kidney vasculatureremains dilated, preserving GFR. During high sympathetic nerve activity (hemorrhage, strenuous exercise),sympathetic nerve activity overrides the intrarenal regulatory mechanisms and reduces renal blood flow andGFR.Intrarenal prostaglandins (PGE2 and prostacyclin [PGI2])are vasodilators and serve to counteract primarily angiotensin II–mediated vasoconstriction, acting at the levelof the arterioles and glomerular mesangial cells. Nonsteroidal anti-inflammatory drugs (NSAIDs) such as aspirinwill block prostaglandin synthesis and restrict the compensatory renal vasodilation.With blood loss from hemorrhage, the sympatheticnervous system (SNS) and hormone systems (RAAS,antidiuretic hormone [ADH], aldosterone) are activated to preserve systemic blood pressure, and prevent fluid loss.
If MAPfalls below 80 mm Hg, the high level of vasoconstriction willoverwhelm the intrarenal regulation of GFR, and GFR will drop.This can result in acute renal failure (GFR < 25 mL/min) ifblood volume is not restored quickly.208Renal PhysiologyCLINICAL CORRELATEAnalysis of Renal FunctionThis correlate will focus on the variety of calculations associatedwith renal function and give examples of their solution.A constant amount of inulin (in isotonic saline) was infused intravenously to a healthy 25-year-old male. After 3 hours, the manemptied his bladder completely, and then urine was collected afteranother 2 hours. A blood sample was obtained at the time of urinecollection.
Blood and urine were analyzed, with results shownbelow. Analyses of several parameters of renal function wereperformed.Inulin concentrationCreatinine concentrationPAH concentrationSodium concentrationUrine volume (UV) = 240 mLUrine collection time = 2 hoursHematocrit (HCT) = 0.42UrinePlasma1000 mg%1375 mg%300 mg%2.5 mEq/L20 mg%25 mg%1 mg%140 mEq/LThe following parameters can be calculated:.Urinary flow rate (V), the rate at which urine is produced.
Urineflow is dependent on general fluid homeostasis and fluid intake.Under normal circumstances, if fluid intake is increased, urineflow will increase. If a person ingests ∼3 L of fluid in food anddrink, the urinary losses will be slightly less, with the balance madeup by insensible losses (breathing, sweating)..V= urine volume/time= 240 mL/120 min= 2 mL/minGlomerular filtration rate (GFR), the volume of plasma filteredby the glomeruli per unit time. Normal GFR in an adult is∼100 mL/min, or ∼144 L/day.
The GFR in men is typically higherthan in women.GFR is determined by inulin clearance:.Cin= (Uin × V)/Pin= (1000 mg% × 2 mL/min)/20 mg%= 100 mL/minGFR can also be determined by creatinine clearance, which overestimates GFR by ∼10% because of creatinine secretion:.Ccr= (Ucr × V)/Pcr= (1375 mg% × 2 mL/min)/25 mg%= 110 mL/minEffective renal plasma flow (eRPF), the fraction of the renalplasma flow entering the glomeruli and available for filtration.eRPF is equated with the clearance of PAH:eRPF= CPAH= (300 mg% × 2 mL/min)/1 mg%= 600 mL/minEffective renal blood flow (eRBF), the fraction of renal blood flowentering the glomeruli.
It is usually ∼20% of cardiac output.eRBF= (eRPF)/(1 − HCT)= 600 mL/min/(1 − 0.42)= 1034 mL/min, or 1.034 L/minFiltration fraction (FF), the fraction of the renal plasma flow thatis filtered per unit time.FF= GFR/RPF= (100 mL/min)/(600 mL/min)= 0.17, or 17% of the RPF entering the kidney was filteredper minuteFiltered load of sodium (FLNa), the amount of plasma sodium thatis filtered per unit time.= Plasma Na × GFR= 140 mEq/L × 100 mL/min= 14 mEq/minUrinary excretion of sodium (UVNa or ENa)..UVNa = Urine concentration of Na × V= 2.5 mEq/L × 2 mL/min= 0.005 mEq/minFLNaReabsorbed sodium (RNa).RNa= FLNa − UVNa= 14 mEq/min − 0.005 mEq/min= 13.095 mEq/minFractional excretion of sodium (FENa), the fraction of filteredsodium that is excreted.
Usually 99+% of filtered sodium is reabsorbed, so less than 1% of the amount filtered is excreted.FENa= [(U/P)Na/(U/P)in] × 100= [(2.5/140)/(1000/20)] × 100= 0.035%Fractional reabsorption of sodium (FRNa), the fraction of filteredsodium that is reabsorbed back into the capillaries.FRNa= [1 − (ENa/FLNa)] × 100= [1 − (0.005/14)] × 100= 99.97%209CHAPTER17Renal Transport ProcessesGENERAL OVERVIEW OF RENAL TRANSPORTWhen the plasma filtered into Bowman’s space enters theproximal tubule, the process of reabsorption begins.
In general,nephrons reabsorb the majority of the fluid and solutes thatpass though them, with the proximal tubule having the greatest reabsorptive function, and the distal sites fine-tuning theprocess. In addition, there is secretion of select substancesfrom the peritubular capillaries into different segments of therenal tubule.The proximal tubule (PT) is the site of bulk reabsorption offluid and nutrients.
The proximal tubule is composed of threesegments, S1, S2, and S3, which differ in the depth of thebrush border and amount of mitochondria in the PT cells.This allows for a high capacity for reabsorption. From S1to S3 segments, the brush border becomes progressivelydeeper and the high concentration of cellular mitochondriaobserved in the S1 segment decreases. The high number ofmitochondria in the S1 is consistent with a high rate of activetransport in that segment. As the filtrate is reabsorbed, andless is present in the tubule in subsequent segments, the deeperbrush border increases surface area, which enhances continued reabsorption.trolyte homeostasis (Chapter 1). As seen with the intestinalabsorption of essential nutrients (see Section 6), sodium isalso a major driving force for the renal reabsorption of fluid,electrolytes, and a variety of nutrients.
As sodium transporterscarry sodium and other solutes, they generate the drivingforce for water reabsorption. When the water leaves the tubule,the concentration of additional electrolytes and solutes in thetubular fluid increases, providing gradients for their diffusioninto the cell.Approximately 65% to 70% of the water in tubular fluid isreabsorbed from the proximal tubule back into the peritubular capillaries, primarily following sodium reabsorption. Thefiltered load (FL) of sodium through the glomeruli is high(∼25,000 mEq/day), and to maintain body fluid homeostasis,greater than 99% of the FLNa must be reabsorbed back intothe blood. This is accomplished by apical secondary activetransport of sodium down a concentration gradient established by the basolateral Na+/K+ ATPase pumps.
Figure 17.1illustrates the primary sites and transporters for sodium reabsorption along different segments of the nephron.■SODIUM-DRIVEN SOLUTE TRANSPORTSodium, Chloride, and Water■Sodium is the major extracellular cation, and regulation of itslevels is necessary for maintenance of general fluid and elec-In general, of the total filtrate coming into the nephrons,the proximal tubule reabsorbs:■■■■■■65% to 70% of the Na+ and H2O80% to 85% of the K+65% of the Cl−75% to 80% of the phosphate100% of the glucose100% of the amino acidsFollowing this bulk reabsorption, “fine-tuning” of reabsorptionoccurs in subsequent segments of the nephron.■Proximal convoluted tubule (S1 and S2 segments): Bulkflow occurs by secondary active sodium cotransportwith several substances including glucose, amino acids,phosphate, and organic acids.
The proximal tubule alsohas Na+/H+ antiporters, which allow H+ secretion intothe proximal renal tubular fluid.Proximal straight tubule (S3 segment): Na+/H+ antiporters continue to reabsorb sodium and secrete H+ into thetubular fluid. The reabsorption of sodium and fluid alsoprovides the electrochemical gradient that facilitateschloride reabsorption. Cl− concentration increases alongthe proximal tubule segments as water is reabsorbed.Chloride enters the cells in the S3 segment down itselectrochemical gradient through antiporters, resultingin apical secretion of anions such as OH−, HCO3−, SO4−,and oxalate.
Cl− reabsorption also occurs paracellularly,or between the cells. (The whole PT reabsorbs ∼65% to70% of FLNa.)Thin descending limb of Henle (tDLH): This segment isimpermeable to sodium and most other solutes but is permeable to water in the presence of antidiuretic hormone(ADH), and thus concentrates the tubular fluid (more onthis in Chapter 18).210Renal PhysiologyThiazide-sensitive channelLumenNa+BloodLumenBlood3Na+Na+3Na+Cl–2K+ATPATP2K+XNa+Na+XH+X = GlucoseAmino acidsOrganicanionsPiCl–HCO3–K+CACa2+CO2 + H2ONa+3Na+Ca2+3Na+Na+ATPATP2K+H+2K+A–K+Cl–Cl–A– = OH–HCO2–Oxalate–HCO3–SO4–Cl–Na+2Cl–K+3Na+Na+Cl–ATP2K+H+CANa+K+Ca2+K+HCO3–CO2 + H2OFiltered Load Factors That Stimulate Factors That InhibitReabsorptionReabsorptionReabsorbed (%)Angiotensin IIDopamineProximal tubule67Sympathetic nervesLoop of Henle25Sympathetic nervesDistal tubule~4AldosteroneCollecting duct~3AldosteroneAtrial natriureticpeptide (ANP)Figure 17.1 Nephron Sites of Sodium Reabsorption Sodium reabsorption is critical for properfluid and electrolyte homeostasis.
![](https://s.studizba.com/z.php?f=/templates/si/i/noavatar.png&w=100&h=100&t=1"){kind=avatar}
