Answer:
Altering the radii of the afferent and efferent arterioles provides for glomerular filtration rate homeostasis and glomerular hydrostatic pressure homeostasis.
Explanation:
The afferent arteriole carries blood to the glomerulus and the efferent arteriole transports blood away from the glomerulus.
Both the the arteriole and the glomerulus are involved in the process it takes for the kidney to purify the blood and get rid of excess water resulting in a by product we all know as urine.
Filtration by the glomerulus is the first step in making urine. It is the process that your kidneys use to remove excess fluid and waste products out of the blood into the urine collecting tubules of the kidney, so they may be eliminated from your body.
Normally the diameter of the afferent arteriole is larger than the efferent. This means there is high resistance as the blood is forced from a wider vessel to a narrower one and this enhances filtration. If the arterial blood pressure remains constant then contracting either vessel reduces blood flow as it increases resistance. However contracting either has opposite effects on the filtration pressure. If you contract the afferent arteriole there will be less of a pressure difference between the afferent and efferent arteriole so there will be reduced filtration pressure.
However if you constrict the efferent arteriole you are increasing the pressure difference between the two and filtration pressure increases.
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