Renal Blood Flow and Glomerular Filtration II

Question 1

How does increased afferent resistance affect GFR and RPF?

Answer 1

decrease both

Question 2

How does decreased afferent resistance affect GFR and RPF?

Answer 2

increase both

Question 3

How does increase efferent resistance affect GFR and RPF?

Answer 3

decrease RPF, increase GFR

Question 4

How does decreased efferent resistance affect GFR and RPF?

Answer 4

increase RPF, decrease GFR

Question 5

How does increase afferent and efferent resistance affect GFR and RPF?

Answer 5

decrease both

Question 6

How does decreased afferent and efferent resistance affect GFR and RPF?

Answer 6

increase both

Question 7

What is the eqn. for net filtration pressure?

Answer 7

Glomerular hydrostatic pressure and BS oncotic pressure minus [glomerular oncotic pressure + BS hydrostatic pressure]

Question 8

What determines BS hydrostatic pressure?

Answer 8

BS hydrostatic pressure is determined by the rate of urine flow.

For example, frequent emptying of bladder can reduce BS hydrostatic pressure. And therefore increased net filtration pressure and GFR.

Question 9

What things can increase BS hydrostatic pressure (via decreased urine flow)?

Answer 9

The rate of urine flow may be reduced by any obstruction in the lower urinary tract.

For example, kidney stone, tumors, or hypertrophic prostate in elderly men. Reduced urinary flow leads to elevated BS hydrostatic pressure.

Therefore the result is reduced net filtration pressure and reduced GFR.

Question 10

What causes glomerular oncotic pressure?

Answer 10

the protein concentration in the plasma

Question 11

Protein concentration in the capillary may be increased if ___ is increased.

Answer 11

FF. If the RPF is reduced due to some reason assuming that GFR is auto regulated the FF increases leading to increased oncotic pressure and therefore reduced net filttration pressure and reduced GFR.

Question 12

T or F. The net hydrostatic pressure is fairly constant from the afferent end to efferent end of glomerular capillary.

Answer 12

T.

Question 13

How does oncotic pressure change within the glomerular capillary?

Answer 13

The oncotic pressure is lowest at the afferent end, and increase gradually toward the efferent end. The net filtration pressure is the difference between these two pressures.

The net filtration pressure is highest at the afferent end and decrease gradually toward the efferent end.

Question 14

What happens if the renal plasma flow is reduced due to some factor (assuming auto regulatory mechanisms try to maintain a normal GFR)?

Answer 14

The glomerular oncotic pressure rises above the normal level and matches the net hydrostatic pressure in the middle of glomerular artery.

So, the net filtration pressure declines faster than normal, and therefore, the GFR is reduced (this would be a condition when auto regulatory mechanism cannot keep up with increasing oncotic pressure.

Question 15

What is auto regulation of GFR?

Answer 15

Under physiologic conditions a constant GFR is maintained in the face of changes in arterial pressure, venous pressure or any other obstructions. There are physiologic regulatory mechanisms that maintain a constant GFR in the kidney.

In all conditions I mentioned about how GFR can be altered by changes in different pressures, I was referring to conditions where these auto regulatory mechanisms are overwhelmed or suppressed, that would be a pathophysiologic situation.

So, when the physiologic regulatory mechanisms are overridden to change GFR we call it a pathophysiologic condition. That is the distinction between physiology and pathophysiology.

Question 16

T or F. The auto regulation of GFR is independent of systemic influences

Answer 16

T, because this property can be demonstrated in an isolated kidney.

So, systemic neurohormonal regulation is not involved in the mechanism of auto regulation of GFR.

Question 17

Describe auto regulation in a graph plotting the flow rate of renal plasma (y) and glomerular filtrate against increasing mean renal arterial blood pressure (x).

Answer 17

Concave down.

As the renal arterial pressure increases the GFR increases. However, you can see the increase the GFR does not change above 70 mm Hg of renal arterial pressure due to auto regulation. This was correlated with similar changes in RPF. Increase in renal arterial pressure increases RPF, but after 60-80 mm Hg pressure, RPF does not change.

Question 18

How does the kidney respond to high renal arterial pressures?

Answer 18

One factor associated with this process is a change in renal vascular resistance. The renal vascular resistance is not altered by renal arterial pressure up to 60-80 mm Hg.

But, a further increase in renal arterial pressure shows a proportional increase in renal arterial resistance. This increase in renal vascular resistance prevents an increase in RPF and therefore prevents increase in GFR.

This is the auto regulation of GFR.

Question 19

What is the impact of auto regulation?

Answer 19

So, your mean arterial blood pressure may vary under normal physiologic conditions, such as exercise and diuranal variations, but the GFR is regulated at a (fairly) constant normal level.

Question 20

What is normal daily GFR? Fluid reabsorption?

Answer 20

Normal GFR is 180 L/day, fluid reabsorption in nephron is 178.5 L/day and therefore urine excretion is 1.5 L/day.

Question 21

If the autoregulatory mechanism does not exist, a __ increase in (diastolic) arterial blood pressure will increase GFR.

Answer 21

25%. If the reabsorption is remained constant, there will be an increase of urine to 46.5 L/day.

This can reduce the blood volume to a dangerously low level. So, auto regulation is an important physiologic regulatory mechanism in maintenance of homeostasis.

Question 22

What is the mechanism involved in auto regulation of GFR?

Answer 22

One important factor associated with auto regulation of GFR is change in afferent arteriolar resistance.

Increased oncotic pressure and uretral obstruction both reduces afferent arteriolar resistance, which increases RBF and therefore increased GFR, that counteracts on the reduced GFR due to change in net filtration pressure.

Increase in systemic arterial pressure increases renal vascular resistance and therefore reduced RBF and GFR. So, first point to take into consideration in the mechanism of auto regulation of GFR is afferent arteriolar resistance.

But the autoregulation can be defined as the intrinsic adjustment of vascular resistance that counter balance any extrinsic factor that would change flow by other than direct influence on renal vascular resistance itself.

Of course, Now the question is how these factors regulate the resistance of afferent arteriole.

Question 23

What are the two theories of auto regulation of GFR?

Answer 23

• myogenic mechanism

- tubuloglomerular feedback mechanism

Question 24

What is the myogenic mechanism?

Answer 24

Involves direct stimulation of arteriolar smooth muscle

When the arterial blood pressure increases it directly stimulates smooth muscle cells in arteriole resulting in contraction and control of the blood flow. And this mechanism likely plays a role in renal vascular blood flow to some extent. But, sometimes GFR is auto regulated at the expense of constant RBF.

Therefore, there has to be additional mechanisms to explain the efficient autoregulation of GFR

Question 25

Autoregulation

Answer 25

IF the ECV decreases, GFR will drop slightly, PCT Na and CL reabsorption will increase to maintain blood volume and thus less will be delivered to the thick AL. This will cause two things:

1) renin release- causes ald and angioII production that increase Na reuptake and efferent arteriole constriction, respectively.
2) ATP release that prevents vasoconstriction of the afferent arterioles

Question 26

If blood pressure drops too low due to excessive fluid loss, then the sympathetic nervous system will override renal autoregulation. Sympathetic nerves innervate the afferent arteriole, causing smooth muscle contraction. The sequence of events is as follows: loss of ECF volume (due to hemorrhage, diarrhea or dehydration) causes a drop in mean arterial pressure (MAP). Decreased MAP is detected by arterial baroreceptors, which leads to sympathetic nervous system activation, afferent arteriole constriction, and decreased GFR.

Answer 26

If blood pressure drops too low due to excessive fluid loss, then the sympathetic nervous system will override renal autoregulation. Sympathetic nerves innervate the afferent arteriole, causing smooth muscle contraction. The sequence of events is as follows: loss of ECF volume (due to hemorrhage, diarrhea or dehydration) causes a drop in mean arterial pressure (MAP). Decreased MAP is detected by arterial baroreceptors, which leads to sympathetic nervous system activation, afferent arteriole constriction, and decreased GFR.