Essentials of Clinical Nephrology (Shorouk Press, Cairo, 2000, ISBN ...

REGULATION OF THE RENAL BLOOD FLOW (RBF) AND GLOMERULAR
FILTRATION RATE (GFR).
Normally the kidney receives 20-25% of the cardiac output. Extrarenal
(prerenal) factors including blood pressure and circulating blood volume will
affect RBF and GFR. When blood pressure or circulating blood volume
decreases RBF and GFR decrease and vice versa.
Autoregulations of renal haemodynamics:
There are intrarenal mechanisms which control RBF and GFR to keep
it within normal when there is a decrease in the renal perfusion pressure (e.g.
hypotension). Of these are the Juxta-glomerular apparatus and the tone of the
afferent and efferent arterioles. With hypotension or hypovolaemia renal blood
flow and renal perfusion pressure decrease. This stimulates Juxta glomerular
apparatus to secrete Renin which changes a circulating protein called
Angiotensinogen into Angiotensin-I. This is changed by a converting enzyme
into Angiotensin II, which causes spasm in the efferent arteriole, which will
increase the intraglomerular pressure (filtration pressure), this maintains the
GFR. Such renal autoregulation mechanisms keep satisfactory renal
hemodynamics and GFR down to a mean renal perfusion pressure about 70
mmHg below which renal hemodynamics and GFR fail and renal functions
are impaired.
THE NEPHRON:
Is the functional unit of the kidney (Fig. 1.6). Each kidney contains
approximately one million nephrons. The first part of the nephron is the
glomerulus (renal corpuscle) which lies mainly in the renal cortex, followed by
proximal convoluted tubule which also lies mainly in the renal cortex. This is
followed by a loop of Henle which is partly in the cortex and partly extends
deep into the medulla. Loop of Henle is composed of a thin part and a thick
part. This is followed by the distal convoluted tubule which lies in the renal
cortex. Part of the distal convoluted tubule comes into contact with the hilum
of the glomerulus and afferent arteriole. Cells in the hilum of the glomerulus
and those in distal convoluted tubule and afferent arteriole are modified to
form the Juxta glomerular apparatus (Fig. 1.7). Distal convoluted tubule ends
into the collecting duct which lies partly in the cortex and partly in the medulla.
In the medulla, collecting ducts descend in the pyramids, at the renal papillae
collecting ducts unite together to form ducts of Bertini which discharge urine
into renal pelvis.