Renal Function in Pregnancy

Abstract

Although our understanding of many renal mechanisms has been advancing rapidly over the past few years, there is still a dearth of infoimation about changes that occur in renal function during pregnancy -surely one of the greatest physiological disturbances possible. It is difficult in many instances to find a description of the changes that occur quite apart from the underlying alterations in mechanisms that are responsible for them.Part of the difficulty has arisen because of the ethical and methodological problems that arise when women are used as 'experimental animals'; until recently there has not been a suitably documented animal model, though whether this was because of technical difficulties or lack of interest on the part of investigators is not known. Recently, however, it has become apparent that many changes in renal function in the rat are similar to those occurring in women and some aspects of renal function in the rat have been investigated in detail. Since not all aspects of renal function can be covered in this review we shall concentrate on three of the more important areas, namely haemodynamics, sodium and water handling and glucose excretion, and discuss how these are changed during pregnancy; comments on the possible mechanisms involved in these changes are presented where appropriate. HaemodvnamicsIn pregnant women cardiac output increases during the fust trimester and thereafter is maintained. The fall in cardiac output that has been noted towards the end of pregnancy [ 11 is now considered to be due primarily to the posture of the women while the measurements were made [2-41. The Correspondence: Professor R. Green, Department of Physiology, University of Manchester, Manchester M13 9PT, U.K. increased cardiac output is distributed primarily to the skin, uterus, kidneys, breasts and gut [S].The portion of the increased blood flow received by the kidney is out of proportion to its normal flow; cardiac output increases by about 30% but renal plasma flow (RPF) and glomerular fdtration rate (GFR) can increase by 50% or more. There is now good evidence that changes in GFR occur very early during pregnancy, a rise of 45% occurring within the first 9 weeks [6], and that they reach a maximum at the end of the fust trimester and then remain high until the end of pregnancy [7,8]. However, if GFR is measured in late pregnancy with the subject in the supine position [9,10] it is found to have decreased. The position is further confused in that when inulin or creatinine was infused to measure GFR, and the body fluid compartments were expanded with glucose solution, there was no change in GFR late in pregnancy; use of endogenous creatinine clearance, however, showed a fall in GFR [8]. One conflicting study shows evidence that posture has no effect on GFR [ 111, so the role of posture and its effect in late pregnapcy still requires some clarification.Changes in RPF are not as well documented as changes @ IjFR, presumably because they are more difficplt to measure, but the general pattern seems to be simila...

Sodium and water reabsorption in the proximal and distal nephron in conscious pregnant rats and third trimester women.

“…It is perhaps surprising, therefore, that although there are many demonstrations of increased whole-kidney salt and water reabsorption (see Atherton & Green, 1983, for references) there is little information on the nephron site(s) involved.…”

Pregnant Rats With 5/6 Nephrectomy Have Normal Volume Expansion Despite Lower Renin and Kallikrein

“…5,6 To achieve positive sodium balance and cumulative plasma volume expansion, important changes in renal function also occur during pregnancy. There is a rise in glomerular filtration rate (GFR) and in renal plasma flow, the pressure natriuretic response is markedly blunted, 7,8 and vasoactive hormones from renal origin, such as kallikrein and renin, are increased during pregnancy. 9 On the basis of these observations, we have speculated that normal renal function and the ability of the kidney to increase the production of vasoactive hormones are important for adequate plasma volume expansion during pregnancy.…”

Atrial Natriuretic Peptide in the First Gestational Trimester: A Longitudinal Study

“…Plasma volume, stroke volume, heart rate and car diac output are significantly increased by week 8 of ges tation [1,2]. Furthermore there is a decrease in systemic vascular resistance and blood pressure [1] together with a 40-50% increase in glomerular filtration rate and renal plasma flow [3]. Atrial natriuretic peptide (ANP), a pep tide stored within granules in atrial cardiocytes, is known to be intimately involved in the control of fluid volume homeostasis [4], Its release is predominantly stimulated by atrial distension [5] as for example caused by intravascular volume expansion [6], ANP is known to exhibit natriuretic and diuretic effects, along with vaso relaxant properties [4], To date only few, cross-sectional data have been pub lished with regard to measurements of plasma ANP lev els during the first trimester of pregnancy.…”

“…In view of the effects of this peptide some involvement in the early decrease in systemic vascular resistance and blood pres sure [ 1 ] or the increase in glomerular filtration rate and renal plasma flow [3] cannot be ruled out.…”