Questions on Constriction and Dialation of Afferent and Efferent Arterioles and EXPLANATION

Question 1: Which of the following causes an increase in glomerular filtration rate but a decrease in renal blood flow?

A- Efferent arteriole constriction
B- Afferent arteriole constriction
C- Efferent arteriole dilation
D- Afferent arteriole dilation

Question2: Dilation of the afferent arteriole results in which of the following?

A-Increased pressure within Bowman’s capillary but a decreased GFR.
B-Decreased pressure of the ultrafiltrate.
C-Decreased pressure within Bowman's capillary.
D-Decreased glomerular filtration rate.
E-Increased pressure of the ultrafiltrate.

Question3: Dilation of the efferent arteriole will cause an increase in which parameter?

A-GFR.
B-Renal blood flow.
C-Net filtration.
D-Renal tubular flow.
E-Ultrafrltration pressure.

ANSWERS 1(A) 2(D) 3(B)

For these questions it is important to understand the concept-

The blood flows through as: Afferent arteriole -> Glomerular capillaries(ULTRAFILTRATION) -> Efferent arteriole

Here it be kept in mind:

Vasoconstriction always decreases the blood flow in a line by increasing resistance.

More pressure in glomerular capillaries means more GLOMERULAR FILTRATION RATE - GFR.

A VERY USEFUL CONCEPT IS THAT THERE IS MORE PRESSURE UPSTREAM TO A REGION OF HIGH RESISTANCE SO FLUID CAN IN A STATE OF EQUILIBRIUM FLOW THROUGH BUT PRESSURE DECREASES DOWNSTREAM FOR EG: IF YOU CLOSE A PIPE IE. INCREASE THE RESISTANCE YOU WILL HAVE LESS PRESSURE DOWNSTREAM TO IT.

So constriction of afferent arteriole:

Increase resistance of afterent arteriole
Decreases blood in glomerular capillaries
Decreases pressure in glomerular capillaries
Decreases the GFR.

OR ANOTHER EXPLANATION COULD BE THIS

Increased resistance reduces pressure in the downstream region.
So Decreased resistance increases pressure in the downstream region. so GFR WILL INCREASE.

But vasoconstriction of efferent arteriole, increases the resistance of efferent arteriole so it makes it more attractive for fluid to get filtered in Glomerulus instead of reaching the efferent arteriole. This can also be understood in terms of pressure, if you dam a river, the pressure just before the dam increases, similarly pressure in Glomerular capillaries increases and GFR is also increased.

Note: It must be noted that constriction of Efferent arteriole only in physiological range causes increased GFR, too much constriction can cause the blood flow to decrease too much and reduce GFR.

So constriction of Efferent arteriole:

Decreases blood in glomerular capillaries.
Decreases pressure in glomerular capillaries.
Increases the GFR.

To summarise:

Constriction of afferent arteriole:

Decreases blood in glomerular capillaries
Decreases pressure in glomerular capillaries
Decreases the GFR.

Dialation of afferent arteriole:

Increases blood in glomerular capillaries
Increases pressure in glomerular capillaries
Increases the GFR.

Constriction of Efferent arteriole:

Decreases blood in glomerular capillaries.
Decreases pressure in glomerular capillaries.
Increases the GFR.

Dialation of Efferent arteriole:

Increases blood in glomerular capillaries.
Increases pressure in glomerular capillaries.
Decreases the GFR.

Note: 

-It must be noted that constriction of Efferent arteriole only in physiological range causes increased GFR, too much constriction can cause the blood flow to decrease too much and reduce GFR.

-These concepts will be of use when you study pharmacology.

-Syed Moris

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