Glomerular Filtration & Renal Blood Flow

Question 1

What are podocytes?

Answer 1

Podocytes (or visceral epithelial cells) are cells in the Bowman’s capsule in the kidneys that wrap around capillaries of the glomerulus

Question 2

What 3 things make up the tricompartmental filtration barrier?

Answer 2

1. endothelium (fenestrated)
2. basal lamina
3. podocytes

Question 3

What is the glomerular filtration rate? What is a normal value for it?

Answer 3

The rate of transudation of fluid across the glomerular filtration barrier.

Normal GFR = 90-125 ml/min

Question 4

____ forces determine GFR

Answer 4

starling forces

Question 5

What is the equation for the net filtration pressure (starling forces)?

*know this formula. it will be tested*

Answer 5

The net filtration pressure is equal to the pressure in the glomerular capillaries minus the back-pressure in the tubules minus the oncotic pressure in the glomerular capillaries. The oncotic pressure in the tubules is always zero so we can negate that.

NFP = ( P_gc - P_t ) - ( π_gc - π_t )

NFP = ( P_gc - P_t ) - ( π_gc )

*know this formula. it will be tested*

Question 6

Under normal circumstances, why is there a net ultrafiltration due to starling forces?

Answer 6

The pressure in the glomerular capillaries far exceeds the back-pressure in the tubules and the oncotic pressure in the glomerular capillaries. This means there is a strong net flow out of the glomerular capillaries and into the tubules causing ultrafiltration.

Question 7

True or False: The filtration pressure is constant along the length of a glomerulus

Answer 7

False. As blood travels through the glomerular capillary, there is transudation of fluid and as you get more distal in the capillary, the oncotic pressure goes up because there is less plasma water. The net filtration eventually reaches 0 mmHg.

Question 8

What is the K constant in when calculating GFR?

Answer 8

K = pA

K is a product of hydraulic conductivity (p) and surface area (A)

The hydraulic conductivity is a value that is intrinsic to the glomerular filtration barrier. This is constant in a normal patient.

If a patient (or mouse) has a normal net filtration pressure but their GFR is abnormal, then there must be a problem with K which has to do with the filtration barrier itself (area or permeability)

Question 9

What is a normal GFR value?

Answer 9

125 ml/min

Question 10

What is a normal RPF value?

Answer 10

625 ml/min

(1/5 of cardiac output)

Question 11

How do you calculate filtration fraction and what is a normal value for it?

Answer 11

FF = GFR / RPF

FF = 125/625 = 0.2

meaning… of the 625 ml/min of plasma that enters the glomerular capillaries, 20% of it enters the bowman’s capsule. The other 80% enters the efferent arteriole.

Question 12

Would urinary obstruction increase or decrease GFR?

Answer 12

GFR falls in urinary obstruction because of increased back-pressure from the tubules (P_t)

Question 13

Does GFR fall or increase during severe hypovolemia?

Answer 13

GFR falls during severe hypovolemia because of decreased RPF (via increase in filtration fraction causing increased oncotic pressure in the glomerular capillaries).

Question 14

Does K increase or decrease in glomerular diseases (e.g. diabetes, lupus, etc)? Does this cause GFR to increase or decrease?

Answer 14

In glomerular diseases, K decreases which means GFR decreases

Question 15

How do you calculate GFR?

Answer 15

GFR = K ( P_gc - P_t ) - ( π_gc - π_t )

Question 16

What is the myogenic response?

*know this for the exam*

Answer 16

The myogenic response can maintain RBF and GFR during normal swings in systemic blood pressure.

It is an autoregulatory response that is intrinsic to the vascular smooth muscle cells of the afferent arterioles. An increase of systemic mean arterial pressure (MAP) can be stopped at the level of the afferent arterioles by vasoconstriction of the myogenic response and a decrease in systemic MAP can be helped at the level of the afferent arterioles by vasodilation of the myogenic response.

*know this for the exam*

Question 17

What happens to afferent and efferent vasculature (dilate or constrict) when there is hypovolemia?

Answer 17

Afferent and efferent vasculature will vasoconstrict.

Notice that this is the opposite of what would happen in a myogenic response. The myogenic response is during normal changes in blood pressure. Outside of the normal range, the opposite effect occurs. In severe hypovolemia, baroreceptors detect low blood pressure which causes a sympathetic output to the smooth muscles in the afferent and efferent vessels causing vasoconstriction (mostly in the afferent).

Question 18

Does angiotensin II increase tone at afferent or efferent renal arterioles?

Answer 18

Mainly efferent

Question 19

What do prostaglandins do in the renal arterioles?

Answer 19

Prostaglandins will dilate at the level of the afferent arterioles. This counteracts the constriction that’s caused by sympathetic output from baroreceptors or from the renin-angiotensin axis.

Question 20

What medications control the muscle tone of the afferent arterioles? Efferent arterioles?

Answer 20

NSAIDS control muscle tone of the afferent arterioles through regulation of prostaglandins.

ACE-inhibitors and ARBS control muscle tone of the efferent arterioles through regulation of the production and reception of Angiotensin II.

Question 21

True or False: Kidney docs don’t really like NSAIDS, ACEinhibitors, and ARBs because they can lower GFR causing renal failure.

Answer 21

True

Basically, to have filtration, you have to have a narrower efferent arteriole than afferent arteriole (you have to have less fluid leaving the efferent arteriole than coming in through the afferent arteriole. This can cause a hydrostatic pressure to push water across the glomerular capillary and into the tubule

Question 22

Does autoregulation or hypovolemic response respond to rapid, normal fluctuations (up or down) in MAP within the normal range?

Answer 22

Autoregulation

Question 23

Does autoregulation or hypovolemic response respond to chronic, significant drops in MAP below normal range

Answer 23

Hypovolemic response

Question 24

What is the purpose of autoregulation? What is the purpose of the hypovolemic response?

Answer 24

The purpose of autoregulation is to maintain GFR and RBF constant under normal conditions.

The purpose of the hypovolemic response is to maintain GFR around constant at the cost of having a reduced RBF (for circulatory emergencies)

Question 25

Are afferent or efferent arterioles involved in autoregulation? How about for hypovolemic response?

Answer 25

Autoregulation: afferent arterioles

Hypovolemic response: afferent and efferent arterioles

Question 26

What is the mechanism of arteriolar change for autoregulation?

How about for hypovolemic response?

Answer 26

Autoregulation: myogenic

Hypovolemic response: baroreceptor mediated (direct neural + hormonal)

Question 27

Is autoregulation intrinsic to the kindney or extrarenal? How about hypovolemic response?

Answer 27

Autoregulation: intrinsic to the kidney

Hypovolemic response: intrinsic and extrarenal