Tuesday, February 12, 2013

Renal physiology (part 1): structure, renal blood flow and GFR

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).
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 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).



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