Renal Plasma Flow; Measurment And Control; Filtartion Fraction

Question 0

Filtration fraction

Answer 0

Proportion of plasma that forms filtrate
=GFR/RPF
About 120/600=0.2
Increased FF->increased colloid osmotic-> increased forces for tubular reabsorption
Proteins and cells left behind-> increase colloid osmotic pressure
If anymore blood was filtered the remaining blood would be too thick for the kidney

Question 1

Why measure RPF

Answer 1

Amount of plasma that perfumes the kidneys
Determinant of GFR
Used to estimate renal blood flow
Use an indicator that is freely filtered and secreted
So stick to low concs 0.83mmol/l
Underestimate by 10% as some renal blood supplied non secreting parts of kidney
RPF=Upah x V/Ppah

Question 2

Renal blood flow

Answer 2

Derived from RPF and packed cell volume 
Whole blood:
Cellular fraction=40% 
Plasma fraction=60% 
Haematocrit (PCV)= fraction of blood volume occupied by RBCs=0.4
RBF= RPF/1-PCV 
600/1-0.4= 1l/min 
CO=5l/min
Kidneys receive 20-25% =1300ml blood
600ml plasma/min

Question 3

Factors influencing renal blood flow

Answer 3

RBF and GFR increase with arterial blood pressure
Constant at BP 90-180 then increase again
Relative maintenance of fluid and solute excretion
Brought about by adjustments of afferent arteriolar calibre:
Myogenic-> contraction in response to pressure and stretch-> increased Ca
Tubule glomerular feedback-> via macula densa

Question 4

Macula densa mechanism

Answer 4

Macula densa cells have NaCl co transporters in the membrane (Na,Cl,K)
Movement in to cells-> increased conc in granular cells/Juxtaglomerular cells-> andenosine/ATP production-> vasoconstriction in afferent arterioles-> decreased hydrostatic pressure-> decreased GFR and RBF
Decreased NaCl-> increased GFR

Question 5

Factors effecting RBF and GFR

Answer 5

Auto regulation absent bellow 90mmHg and above 180mmHg
Increased afferent resistance-> decreased hydrostatic pressure-> decreased RPF and GFR
Increased efferent resistance-> increased glomerular capillary hydrostatic pressure-> decreased RPF and GFR
Vasoconstrictors-> increased vascular activity-> decreased RPF, increased GFR
Sympathetic nerves:
Afferent and efferent arterioles innervated
Activated by decreased BP or strong emotional stimuli

Question 6

Factors effecting RPF and GFR 2

Answer 6

Angiotensin 2:
Release activated by decreased BP
Juxtaglomerular cells release renin
Efferent more sensitive than afferent->at low concs-> efferent constriction-> helps maintain GFR in hypotension
Vasodilators:
Decrease vascular resistance-> increased RBF and GFR
Prostaglandins:
Produced locally in kidney.
Afferent and efferent vasodilation
Counter excessive vasoconstrictive effects of sympathetic nerves, ag2 and preserve RPF and GFR
NSAIDS and cox inhibitors block synthesis of prostaglandins and so interfere with preservation of RBF

Question 7

Relationship between plasma conc and clearance of insulin, PAH, and glucose

Answer 7

PAH As plasma conc increases-> RPF decreases 
Transporters become saturated 
No effect on inulin
Glucoses increases slightly 
Clearance>inulin indicates secretion 
Clearance<inulin, indicates reabsorption
Clearance=inulin, only filtered

Question 8

GFR

Answer 8

Ui/c x V/ Pi/c

Question 9

RBF

Answer 9

UpahxV/Ppah

Question 10

RBF

Answer 10

RPF/1-PCV

Question 11

FF

Answer 11

GFR/RPF

Question 12

Filtered load

Answer 12

GFR x plasma conc

Question 13

Excreted

Answer 13

Urine flow x urine conc

Question 14

Reabsorbed

Answer 14

Filtered-excreted