Renal Plasma Flow

Question 1

Define RPF. What can it be used to estimate?

Answer 1

Renal plasma flow - amount of plasma that perfuses the kidneys per unit time.
Important determinant of GFR (greater RPF = greater filtrate formed)
estimate RBF
Both indicators of renal health

Question 2

What is PAH and what is it’s use?

Answer 2

Para-aminohippuric acid
to estimate RPF
freely filtered and secreted

Question 3

Equation for RPF?

Answer 3

RPF = (U x V)/P

of PAH

Question 4

Define fraction filtration. What is it usually?

Answer 4

proportion of plasma that forms filtrate
usually 20%
FF = GFR/RPF

Question 5

If ff increases, what filtration force is increased? Does reabsorption or secretion increase as a result?

Answer 5

Colloid osmotic pressure increases. So tubular reabsorption increases.

Question 6

What is the normal packed cell volume (PCV)? Difference between haematocrit and PCV?

Answer 6

normal PCV = 0.4

fraction of blood volume occupied by RBCs

Question 7

RBF equation?

Answer 7

Can be derived from RPF and PCV

RBF = RPF/ (1-haematocrit)

Question 8

RBF and GFR are constant over what range of pressures?

Answer 8

Over arterial blood pressure of ~90-180 mmHg

Question 9

What is the mehcanism by which the renal function is constant regardless of arterial pressure?

Answer 9

Because of autoregulation

Involves changes in afferent arteriolar resistance.

Question 10

What are the 2 mechanisms of autoregulation? Explain them both.

Answer 10

Myogenic mechanism: stretch on an artery = opens Ca channels = contraction of vessel to conteract increased pressure.
Tubuloglomerular feedback: NaCl sensed by macula densa or juxtaglomeurular apperatus = signals for contraction

Question 11

What 2 things make up the juxtaglomerular apparatus?

Answer 11

macula densa + granular cells

Question 12

What is the important protein channel involved there?

Answer 12

they transport more cells into the macula densa cells

Question 13

Explain how increased salt in blood leads to vasoconstriction. What’s the key molecule?

Answer 13

more salt = more salt in filtrate= more salt in macula densa (part od DCT) = more salt exported out to granuar cells outside the afferent arteriole = more adenosine = more vasoconstriction
GFR decreases

Question 14

If more NaCl reaches the kidneys, how does GFR change?

Answer 14

NaCl - triggers adenosine release = vasoconstricts
GFR decreases
increased bp would increas NaCl

Question 15

What are the 2 things that can change RBF and GFR

Answer 15

Vasoconstrictors decreases RBF and GFR (sympathetic nerves and angiotensin II)
Vasodilators increase RBF and GFR (prostaglandins, PGE2 and PGI2)

Question 16

Why shouldn’t you give NSAIDs in case of reduced RBF?

Answer 16

block synthesis of prostaglandins and so intefere with preservation of RBF. May lead to renal tubular necrosis.

Question 17

At low angiotensin II conc, which arteriole is preferentially constricted?

Answer 17

Efferent constriction predominates - helps maintain GFR in presence of hypotension. Both constrict at high concentration.

Question 18

In which part of the nephron is glucose reabsorbed?

Answer 18

Proximal convoluted tubule

Question 19

Give examples of when glycosuria would occur

Answer 19

Untreated diabetes mellitus
hyperthyroidism
fanconi syndrome
familial renal glucosuria 
pregnancy 
drugs

Question 20

Give equations for
Filtered glucose rate
Glucose excretion rate
Glucose reabsorbed rate

Answer 20

Filtered glucose rate: GFR x plasma glucose conc
Glucose excretion rate: urine flow x urine glucose conc
Glucose reabsorbed: glucose filtered - glucose excreted