Renal blood flow

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Renal blood flow is normally 1200 ml per minute, just over 20% cardiac output. It is affected by various factors (Box 8.2).

There is autoregulation of renal blood flow between a MAP of 70-170 in normal subjects (autoregulation in hypertensive patients is shifted to the right, Figure 8.1). Resistance of the renal vascular bed plays a large part in autoregulation - the smooth muscle walls of the afferent and efferent arterioles in the cortex constrict and dilate according to changes in pressure. Renal artery perfusion depends on a critical perfusion pressure.

input pressure - output pressure

Renal blood flow = -

resistance of renal vascular bed that is, renal blood flow is maintained, within the limits of autoregulation, by changing the pressure gradient and resistance of the vascular bed.

The kidney is one of the most highly perfused organs in the body. In other organs, as blood flow falls, oxygen extraction increases but, in the kidney, oxygen consumption falls in parallel with blood flow. Initially the afferent (input) vessels dilate and the efferent (output) vessels constrict, to try and maintain an effective perfusion pressure to produce a glomerular filtrate. Total renal oxygen consumption is low except in the outer medulla where active sodium reabsorption occurs. Frusemide blocks the active sodium-potassium-chloride cotransport system in the thick ascending limb of the

CD C

Renal blooc

Renal blooc

Chronic hypertension

CD C

Glomerular filtration rate

Chronic hypertension

Mean renal artery pressure (mmHg)

Figure 8.1 Renal blood flow. The renal blood flow autoregulation curve is shifted to the right in chronic hypertension loop of Henle (Figure 8.2). This area is normally relatively hypoxic because of high tubular reabsorptive activity. In rats, frusemide abolishes the physiological outer medullary hypoxia despite a fall in blood flow but this has failed to translate into a benefit in clinical practice.

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Blood Pressure Health

Blood Pressure Health

Your heart pumps blood throughout your body using a network of tubing called arteries and capillaries which return the blood back to your heart via your veins. Blood pressure is the force of the blood pushing against the walls of your arteries as your heart beats.Learn more...

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