One of the things the kidney does is maintain a proper balance of water and electrolytes in the body. It will eliminate the excess and to withhold if they are needed. It will also eliminate nitrogen containing substances like creatinine and urea which are basically toxic byproducts of normal metabolism.
To do this in a proper efficient way the kidney needs 2 very important things:
- BLOOD FLOW: Blood needs to flow through the glomerulus so it can properly be filtrated. This explains why hypovolemia causes renal failure.
- PRESSURE: Blood in the tuft needs to flow at an appropriate pressure. This explains why hypotension will also cause failure. This also explains why vasopressors increase diuresis and may initially improve filtration and creatinine leves.
The way flow and pressure is maintained in the glomerular tuft is by the regulation of dilation and constriction of the arterioles that are before and after it. Normally the afferent art. will be dilated providing a proper blood flow. If this afferent arteriole constricts too much, blood flow will be affected and renal function will be impaired. To keep a good amount of blood flowing at an appropriate pressure, the efferent arteriole will stay constricted but if anything dilates it excessively, flor and pressure will be reduced in the tuft and filtration will decrease.
The following illustrations explain the physiologic filtration and illustrate which factors may cause imbalance and filtration impairment:
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-The Plague Doctor