Fluid volume or arterial pressure, that is the question
1 Humboldt-University, Charité, Johannes-Müller Institut für Physiologie, Tucholskystr. 2, 10117 Berlin, Germany
How is blood pressure controlledNot few will say that long-term blood pressure is determined by the renal medulla (Mattson, 2003), which is also pivotal in volume homeostasis. Fluid volume and electrolyte concentration are of primary importance and it seems hardly possible for them to be entirely subordinated to the demands of arterial pressure.
, 百拇医药
A clue to understanding priorities the renal medulla must make is obtained by investigating renal medullary circulation: vasa recta are very long vessels that supply this region with blood and act together with the tubular network in adjusting renal medullary tonicity. The amount of fluids being retained from the renal collecting ducts heavily depends on the osmotic gradient between the collecting duct fluid and the medulla. If this osmotic gradient is actively controlled in accordance with the demands of arterial pressure, then this can be taken as an indication of the pivotal role of the renal medulla in long-term pressure control. On the other hand if renal circulation is adjusted in order to maintain the osmotic gradient, it seems as if volume electrolyte homeostasis prevails over the requirements of arterial pressure control.
, 百拇医药
In this issue of The Journal of Physiology, Dobrowolski & Sadowski (2005) provide important information with regard to the inter-relationship of renal medullary tonicity and medullary blood flow. Changes in medullary interstitial tonicity were induced by inhibiting Na+–K+–2Cl– cotransport in the ascending limb of the loop of Henle. In response to this intervention, medullary blood flow decreased in parallel to medullary ionic tonicity. A possible implication of this experiment is that the medulla itself can control its circulation in to match the demands of maintaining medullary tonicity. How does this come aboutIt is probably related to the renin–angiotensin system and to prostaglandins. The vasoconstrictory effect of angiotensin II (Ang II) is less pronounced in the renal medulla as compared to the renal cortex. Indeed, very recently, augmented medullary blood flow has been found in response to Ang II (Mattson & Meister, 2005). This is probably due to concurrently released vasodilators, such as NO (Cowley et al. 2003) and/or prostaglandins. Dobrowolski & Sadowski blocked prostaglandin synthesis by indomethacin. This manoeuvre not only reduced medullary blood flow but also significantly attenuated the decrease in renal medullary blood flow in response to reducing medullary tonicity. Thus, the authors conclude that the tonicity of the medulla is an important controller of medullary blood flow.
, 百拇医药
A new piece in the puzzle in the role of the medulla for arterial blood pressure and volume electrolyte homeostasis seems to have been added by Dobrowolski & Sadowski (2005). The demonstration that medullary tonicity is not simply a function of medullary blood flow, but that medullary blood flow can be adjusted according to the needs of medullary tonicity, extends our understanding considerably. It is becoming clearer that the renal medulla is a key regulator in volume and electrolyte homeostasis independently of arterial blood pressure.
, 百拇医药
References
Cowley AW Jr, Mori T, Mattson D & Zou AP (2003). Am J Physiol Regul Integr Comp Physiol 284, R1355–R1369.
Dobrowolski L & Sadowski J (2005). J Physiol 567, 613–620.
Mattson DL (2003). Am J Physiol Regul Integr Comp Physiol 284, R13–R27.
Mattson DL & Meister CJ (2005). Am J Physiol Regul Integr Comp Physiol; doi: 10.1152/ajpregu.00207.2005, http://www.100md.com(Pontus B. Persson)
How is blood pressure controlledNot few will say that long-term blood pressure is determined by the renal medulla (Mattson, 2003), which is also pivotal in volume homeostasis. Fluid volume and electrolyte concentration are of primary importance and it seems hardly possible for them to be entirely subordinated to the demands of arterial pressure.
, 百拇医药
A clue to understanding priorities the renal medulla must make is obtained by investigating renal medullary circulation: vasa recta are very long vessels that supply this region with blood and act together with the tubular network in adjusting renal medullary tonicity. The amount of fluids being retained from the renal collecting ducts heavily depends on the osmotic gradient between the collecting duct fluid and the medulla. If this osmotic gradient is actively controlled in accordance with the demands of arterial pressure, then this can be taken as an indication of the pivotal role of the renal medulla in long-term pressure control. On the other hand if renal circulation is adjusted in order to maintain the osmotic gradient, it seems as if volume electrolyte homeostasis prevails over the requirements of arterial pressure control.
, 百拇医药
In this issue of The Journal of Physiology, Dobrowolski & Sadowski (2005) provide important information with regard to the inter-relationship of renal medullary tonicity and medullary blood flow. Changes in medullary interstitial tonicity were induced by inhibiting Na+–K+–2Cl– cotransport in the ascending limb of the loop of Henle. In response to this intervention, medullary blood flow decreased in parallel to medullary ionic tonicity. A possible implication of this experiment is that the medulla itself can control its circulation in to match the demands of maintaining medullary tonicity. How does this come aboutIt is probably related to the renin–angiotensin system and to prostaglandins. The vasoconstrictory effect of angiotensin II (Ang II) is less pronounced in the renal medulla as compared to the renal cortex. Indeed, very recently, augmented medullary blood flow has been found in response to Ang II (Mattson & Meister, 2005). This is probably due to concurrently released vasodilators, such as NO (Cowley et al. 2003) and/or prostaglandins. Dobrowolski & Sadowski blocked prostaglandin synthesis by indomethacin. This manoeuvre not only reduced medullary blood flow but also significantly attenuated the decrease in renal medullary blood flow in response to reducing medullary tonicity. Thus, the authors conclude that the tonicity of the medulla is an important controller of medullary blood flow.
, 百拇医药
A new piece in the puzzle in the role of the medulla for arterial blood pressure and volume electrolyte homeostasis seems to have been added by Dobrowolski & Sadowski (2005). The demonstration that medullary tonicity is not simply a function of medullary blood flow, but that medullary blood flow can be adjusted according to the needs of medullary tonicity, extends our understanding considerably. It is becoming clearer that the renal medulla is a key regulator in volume and electrolyte homeostasis independently of arterial blood pressure.
, 百拇医药
References
Cowley AW Jr, Mori T, Mattson D & Zou AP (2003). Am J Physiol Regul Integr Comp Physiol 284, R1355–R1369.
Dobrowolski L & Sadowski J (2005). J Physiol 567, 613–620.
Mattson DL (2003). Am J Physiol Regul Integr Comp Physiol 284, R13–R27.
Mattson DL & Meister CJ (2005). Am J Physiol Regul Integr Comp Physiol; doi: 10.1152/ajpregu.00207.2005, http://www.100md.com(Pontus B. Persson)