Renal Blood Flow
Renal blood flow is the volume of blood delivered to the kidneys per unit time. It is large relative to the kidneys' mass because the organ filters plasma rather than meeting a high metabolic demand, and it is distributed unevenly between the well-perfused cortex and the more sparsely perfused medulla.
Definition
Renal blood flow is the rate at which blood perfuses the kidneys; renal plasma flow is its plasma fraction, and both are determined by the perfusion pressure gradient across the kidney divided by total renal vascular resistance.
Scope
This topic covers the magnitude and intrarenal distribution of blood flow, the series arrangement of afferent and efferent arterioles that frames glomerular pressure, the relationship between renal blood flow and glomerular filtration rate, and the principal mediators that set vascular resistance. It treats renal blood flow as a physiological quantity and does not give clinical advice.
Core questions
- How large is renal blood flow relative to the kidney's size and to cardiac output?
- How is blood flow distributed between the renal cortex and medulla?
- How do afferent and efferent arteriolar tone set glomerular capillary pressure and filtration?
- Which mediators raise or lower renal vascular resistance?
Key concepts
- Renal blood flow and renal plasma flow
- Renal vascular resistance
- Cortical versus medullary perfusion
- Afferent and efferent arterioles in series
- Glomerular capillary hydrostatic pressure
- Filtration fraction
Mechanisms
Blood enters the kidney through the renal artery and passes through interlobar, arcuate, and interlobular arteries to the afferent arterioles, then through the glomerular capillaries, the efferent arterioles, and the peritubular or vasa recta capillaries. Because the glomerular capillary network sits between two arterioles in series, the relative tone of the afferent and efferent vessels sets glomerular capillary hydrostatic pressure and thereby the driving force for filtration; constricting the afferent arteriole lowers both flow and glomerular pressure, whereas efferent constriction tends to raise glomerular pressure while reducing flow. Resistance and flow are modulated by vasodilators such as nitric oxide and prostaglandins and by vasoconstrictors such as angiotensin II, while intrinsic autoregulation keeps flow stable across a range of perfusion pressures.
Clinical relevance
The way renal blood flow and its distribution relate to filtration is part of how clinicians reason about renal perfusion and oxygenation, with the medulla described as relatively vulnerable to reduced perfusion. This entry is descriptive physiology and is not a basis for diagnosis or treatment decisions.
Evidence & guidelines
The quantitative and mechanistic statements here are drawn from standard physiology texts and from review articles on renal hemodynamics and its regulation by nitric oxide.
History
Estimates of renal blood flow and plasma flow were developed through clearance methods using substances such as para-aminohippurate, and subsequent work characterized the cortical-to-medullary gradient of perfusion and the mediators that adjust renal vascular tone.
Key figures
- Christopher Wilcox
- Chris Baylis
- William Arendshorst
Related topics
Seminal works
- carlstrom-2015
- baylis-1996
Frequently asked questions
- Why is renal blood flow so high relative to the kidney's mass?
- The kidneys filter large volumes of plasma to regulate body fluids and excrete wastes, so they require a high perfusion that far exceeds their own metabolic needs.
- How does renal blood flow relate to glomerular filtration?
- Glomerular filtration depends on the hydrostatic pressure in the glomerular capillaries, which is set by the balance of afferent and efferent arteriolar tone within the overall renal blood flow; filtration fraction expresses what proportion of plasma flow is filtered.