Renal Physiology 2 - Checked and Complete

Question 1

What are typical values of Renal Blood Flow, Renal Plasma Flow, Urine Flow Rate and Glomerular Filtration Rate?

Answer 1

RBF ~1.1 L/min

RPF ~.625 L/min

GFR ~ .125 L/min

Urine ~ .001L/min

Question 2

Define Filtration Fraction

Answer 2

GFR/RPF ~ .2

**Means about 20% of all freely filtered solutes (Na, K, glucose, etc.) present in the plasma enter
Bowman’s space **

Question 3

How are glomerular capillaries unique in comparison to other capillary beds?

Answer 3

Common capillary beds have net filtration at arterial end and net reabsorption at venous end

HOWEVER

Glomerular capillary beds have net filtration ENTIRE length

Question 4

Why do glomerular capillaries have such different properties? (Net filtration throughout, changing oncotic pressure, higher hydrostatic pressure?)

Answer 4

Pressure in glomerular capillaries is higher ~55mmHg because afferent arteriole diameter greater than efferent arteriole diameter

Glomerular capillaries have low resistance across whole length (leading to constant hydrostatic pressure) because of many parallel capillary loops

Colloid Oncontic Pressure Increases as plasma water filters out and solutes become more concentrated (this decreases filtration but hydrostatic pressure is still great enough to overcome this force)

Question 5

Define net filtration pressure in an equation

Answer 5

Net filtration pressure =

glomerular capillary hydrostatic pressure - (plasma oncotic pressure + pressure in bowman’s space)

**NFP = P_GC – (π_GC + P_BS) **

Question 6

Define GFR in an equation

Answer 6

GFR = K_f x NFP

GFR influenced by (surface area x permeability of capillaries to water) x net filtration

Question 7

Why is the filtration coefficient (K_f) larger in Glomerular Capillaries in comparison to other capillary beds?

Answer 7

K_f defined by surface area and permeability to water

Fenestrated capillaries are more permeable

Glomerular capillaries have significant looping and branching

Question 8

Where does reabsoprtion occur in the kidneys if the glomerular capillaries don’t perform this function?

Answer 8

Peritubular capillaries

Question 9

Describe some clinical factors that could influence GFR

Answer 9

Urinary Tract Obstruction (i.e. kidney stone) - increases P_BSand opposes filtration - decrease NFP, decrease GFR

Increase in MAP - increase P_GC, increase NFP, increases GFR - HOWEVER, often autoregulated and countered by body

Renal Artery Stenosis - decrease P_GC, decrease NFP, decrease GFR

**Reduction in number of functional nephrons **- reduce surface area, reduce K_f,reduce GFR

**Sympathetic Activation **- vasoconstriction, reduced P_CG AND reduced K_f(via closed loops decreasing surface area) makes decreased GFR

**Loss of plasma proteins **-(i.e. starvation or renal disease) increase oncotic pressure, decrease NFP, decrease GFR

Question 10

Describe renal autoregulation

Answer 10

Works both intrinsically (isolated kidney) and extrinsically (nervous system, hormonal influences)

MAP can range from 80-180 and RBF and GFR remain relatively constant

Question 11

Describe the two basic mechanisms of intrinsic renal autoregulation.

Answer 11

**1) myogenic mechanism (least important): arterioles when stretched tend to contract **

This increases resistance to flow; thought to be caused by depolarization of smooth muscles when wall stretched, hyperpolarization when not stretched

**2) tubuloglomerular feedback mechanism (most important): **negative feedback of JGA

macula densa cells release local vasocontrictor (maybe ATP or Adenosine) when tubular flow past them increases

Question 12

What is unknown about the intrinsic renal regulation?

Answer 12

Not sure how macula densa cells sense increased GFR - via increased NaCl uptake?

Not sure how macula densa cells communicate vasocontriction to afferent arterioles

Question 13

What percent of water, salt, bicarb, and glucose is generally reabsorbed in kidneys?

Answer 13

99%

Question 14

How is renal plasma flow calculated?

Answer 14

Renal Blood Flow x (1-Hematorcrit)

Question 15

Define renal clearance

Answer 15

Volume of plasma cleared of a substance by the
kidneys per minute

NOT the amount of the substance removed, but instead the volume of plasma from which it was removed

Question 16

Define renal clearance in mathematical terms.

Answer 16

**P_X x C_X = U_X x V **

amount of substance X cleared from the plasma is equal to the amount of substance X excreted in the urine

C_x= (U_X x V) / P_x

Question 17

Why is calculating renal clearance an important tool?

Answer 17

Clearance of certain substances helps to calculate GFR and RPF

Question 18

Why is inulin clearance the gold standard for calculating GFR?

Answer 18

Inulin is not reabsorbed or secreted. Excretion of inulin into the urine directly equals renal filtration.

Inulin clearance = GFR

Question 19

Under what circumstance could clearance of a substance be greater than GFR (filtration through the kidneys)?

Answer 19

If a substance is actively secreted into the urine

PAH (para-aminohippurate) is one example - it’s clearance is about 90% (as opposed to normal 20% filtration)

Question 20

Since PAH clearance is so close to 100%, what can it be used to determine?

Answer 20

RPF or EffectiveRPF

Question 21

When is PAH _not _an effective way to measure RPF?

Answer 21

When PAH concentration is high and transporters that aid in secretion are Maxed out (reach T_M)

Question 22

Describe Creatinine Clearance

Answer 22

Some creatinine is secreted (which makes estimate of filtration less ideal) so it overestimates GFR 10-20%

Can measure clearance of creatinine to approximately determine kidney GFR and health

May also just measure plasma creatinine because GFR and creatinine are inversely correlated - high creatinine means low GFR usually

High BUN in plasma may also indicate low GFR and failing kidneys