Renal physiology:
structure, renal blood flow and GFR
Nephrons is the functional unit of the
kidney. Each kidney has 1 million nephron.
Nephron consists of glomerulus and
collecting tubules.
1- Glomerulus: consist of
a. Glomerular capillary: receive blood by
afferent arteriole and drained by efferent arteriole. Lined by endothelium.
b. Bowman's capsule: surround glomerular
capillary. Inner surface is lined by podocytes (epithelial cells) that surround
endothelial cells of glomerular capillary. Together they form blood-urine
barrier.
2- Collecting tubules: drains glomerulus
and consists of proximal convoluted tubule (PCT), loop of Henle (LH), distal
convoluted tubule (DCT) and collecting duct (CD).
Nephron
Function of nephron: ultrafiltrate of
plasma within the lumen of Bowman's capsule, driven by Starling forces across
the glomerular capillaries.
Function of collecting tubules:
Reabsorption of salt and water, secretion of substances and metabolism of
certain substances.
Renal blood flow (RBF)
RBF is defined as the pressure difference
between the renal artery and renal vein divided by the renal vascular
resistance.
The kidneys represent < 0.5% of body
weight, but they receive 25% of cardiac output (1300ml/min through both
kidneys; 650ml/min per kidney) (cardiac output is more than 5 liters/min).
So, combined blood flow in the two renal
arteries is 1300ml/min and combined blood flow in the two renal veins is 1299ml/min,
The difference in flow rates represents
the urine production rate (i.e. ~1ml/min).
RBF is autoregulated (remains essentially
constant over a range of perfusion pressures (80 - 180mmHg)). Increase
perfusion pressure causes contraction of afferent arteriole and stimulate
macula densa of JGA to secret angiotensin II that contracts afferent and
efferent arterioles.
Clearance
Clearance is the volume of plasma that is
completely cleared of solute by the kidney per minute.
Clearance of a substance from the plasma
can be expressed mathematically as:
Clearance = U X V / P
(Where U is the concentration of a given
substance in urine, P is its concentration in plasma, and V is the urine volume).
handling of substances in renal unit
Glomerular filtration rate (GFR) is the volume of plasma filtered by the
kidney per minute. Equals clearance for any substance which is freely filtered
and is neither reabsorbed, nor secreted by the kidney
Normally about 1/5 of the plasma that
flows through the glomerular capillaries is filtered (120ml/min) (total plasma
flow is 600ml/min).
GFR is directly related to renal blood
flow (RBF).
Lab Measurement of GFR (by 24hr clearance)
A substance that is freely filtered at
the glomerulus and neither secreted nor reabsorbed by the renal tubules,
clearance is equivalent to GFR. e.g, Aniline (experimentally most accurate but
not practical).
A substance is both filtered at the
glomerulus and secreted by the renal tubules, its clearance will be greater
than GFR. e.g, Creatinine (90% filtration and 10% secretion) (less accurate than
aniline but practical as it is an endogenous substance).
A substance is filtered at the
glomerulus, but reabsorbed by the renal tubules, its clearance will be less
than GFR.
So, Clinically, GFR is estimated using
creatinine, and is ~125ml/min.
Methods of
measurement of GFR: radioisotope renal scan, aniline clearance, creatinine
clearance, serum creatinine and BUN (last 2 reflects GFR changes).
: structure, renal blood flow and GFR){kind=link}