L2 Renal Blood Flow And GFR

Question 1

Kidneys receive _______ of cardiac output

Answer 1

1/4th

High flow not needed for metabolism

Needed to support filtration (20% of plasma is filtered)

Question 2

_________ filtrate is formed each day

Answer 2

180 L

Question 3

Entire plasma volume filtered _____ times per day

Answer 3

65

If everything filtered were lost, you would lose entire plasma volume in <30 min

Question 4

How is the filtration fraction (FF) calculated?

Answer 4

FF = Glomerular Filtration Rate/Renal Plasma Flow
= (GFR/RPF)

Renal plasma flow (RPF) = (1 - Hct)(Renal Blood Flow RBF)

If RBF = 1.1 L/min, RPF = (1-0.45)(1.1L/min) = 605 ml/min
Then FF = 125/605 = 0.2 or 20%

Question 5

Vasoconstrictors that help regulate RBF

Answer 5

Sympathetic nerves: vasoconstriction via alpha-1 receptors —> decreased RBF and GFR

Angiotensin II, ADH, ATP, endothelin
—> decrease RBF and GFR
ATII constrictors both the afferent and efferent arterioles but the EFFERENT arteriole is more sensitive at lower concentrations of ATII

Question 6

Vasodilators that help regulate RBF

Answer 6

Atrial natriuretic peptide (ANP), glucocorticoids, NO, prostaglandins (ie PGE2 and PGI2)

All increase RBF and GFR

Question 7

____________ allows for constant blood flow and DFR at different arterial pressures

Answer 7

Auto regulation

True for pressures between 80-180 mmHg

Can be overridden by large increases in sympathetic tone

Question 8

During severe blood loss, _____ and _____ can decrease due to hypotension

Answer 8

RBF and GFR

Question 9

The two mechanisms for auto regulation of RBF and GFR

Answer 9

Myogenic mechanism - intrinsic to VSMC, contract in response to stretch

Tubuloglomerular feedback (“flow-dependent”) - increasing GFR increases NaCl delivery to LOH, sensed by the macula densa which causes the resistance of the afferent arteriole to increase thereby decreasing RBF & GFR

The signal affects RBF and GFR mainly by changing the resistance of the afferent arteriole

Question 10

Feedback mechanism for maintaining constancy of salt load delivered to distal tubule

Answer 10

Tubuloglomerular feedback

An increase in GFR increases NaCl delivery to LOH, sensed by the macula densa and signals the afferent arteriole resistance to increase, thereby decreasing RBF and GFR

Question 11

_________ does not normally pass the filter in significant quantities

Answer 11

Protein

All small-MW solutes that are not protein-bound appear in the filtrate in the same concentrations as in blood plasma

Question 12

The fluid in Bowman’s capsule is essentially a …

Answer 12

Protein-free filtrate of blood plasma

Question 13

Route of filtrate

Answer 13

Fenestrae —> basal lamina —> filtration slits between pedicels (foot processes) of podocytes. Filtration slits are bridged by diaphragms

Question 14

Substances in filtrate are separated by __________ and ___________.

Answer 14

Size (MW 7000 yes, 70,000 no) and electrical charge.

Basal lamina and slits are coated with negative charges. Proteins usually have (-) charge and are repelled.

Question 15

________ and ________ are main barriers to proteins

Answer 15

Basal lamina and filtration slits

Question 16

Equation that describes the glomerular filtration rate (GFR)

Answer 16

Starling Equation

GFR = Kf[(Pgc-Pbc) - (πgc - πbc)]
Where Kf = filtration coefficient and (Pgc-Pbc) - (πgc - πbc) = the net filtration pressure (NFP)

Note that πbc is approximately zero (no proteins in Bowman’s capsule)

Question 17

The capillary filtration coefficient (Kf) is …

Answer 17

The filtration rate (ml/min/mmHg) produced by each mmHg of NET filtration pressure.

Glomerular capillary Kf may be 7-15 ml/min/mmHg

Question 18

Kf for glomerular capillaries is ____________ than that for capillaries in the skin/muscle.

Answer 18

50-100 times greater

Question 19

Kf can be altered by

Answer 19

Mesangial cells (angiotensin II can reduce Kf)

Question 20

GFR = Kf x NFP = …

Answer 20

15 ml/min/mmHg x 8 mmHg = 120 ml/min

Question 21

Determining Net Filtration Pressure (NFP)

Answer 21

(Pgc-Pbc) - (πgc - πbc)

Pgc = blood pressure in glomerular capillary (driving force for GFR)

Pbc = back pressure in Bowman’s capsule (diminishes GFR, but approx 0)

πgc = oncotic pressure of glomerular capillary blood

Question 22

If Pgc = 45 mmHg, πgc = 26 mmHg, Pbc = 10 mmHg and Kf = 14 ml/min/mmHg

What is the GFR?

Answer 22

NFP = (45mmHg-10mmHg)-(26mmHg-0) = 9mmHg

GFR = (14 ml/min/mmHg)(9mmHg) = 126 ml/min

Question 23

When afferent (Ra) and efferent (Re) arteriole resistances both increase…

Answer 23

There is no effect on Pgc but there is a dramatic decrease in RBF

Question 24

When afferent (Ra) arteriole resistance increases but Re remains constant…

Answer 24

Both Pgc and RBF decrease

Question 25

When efferent arteriole resistance (Re) increases but Ra remains constant…

Answer 25

Pgc increases and RBF increases

Question 26

NSAIDs block the effects of ___________

Answer 26

Prostaglandins

PGE2 and PGI2 are produced locally in the kidney in response to SNS stimulation of vascular smooth muscle in afferent arterioles and high levels of circulating angiotensin II.

Both PGE2 and PGI2 VASODILATE the afferent arterioles, reducing vasoconstriction produced by SNS stimulation to maintain blood flow.

NSAIDS inhibit the synthesis of prostaglandins by inhibiting the activity of COX-1 and COX-2

Question 27

Increased glomerular surface area because of relaxation of glomerular mesangial cells will…

Answer 27

Result in increased GFR

Question 28

Increased renal arterial pressure, decreased afferent-arteriolar resistance or increased efferent-arteriolar resistance will …

Answer 28

Result in increased GFR

Question 29

Increased intratubular pressure because of obstruction of tubule or extrarenal urinary system will …

Answer 29

Result in decreased GFR

Question 30

Increased systemic-plasma oncotic pressure or decreased renal plasma flow will …

Answer 30

Result in decreased GFR