Week 1: intro to renal physiology

Question 1

What are some disturbances of renal function?

Answer 1

• most common causes of renal failure are chronic: diabetic nephropathy, hypertension
• primary inherited renal disease: ADPKD autosomal dominant polycystic kidney disease
• overmedication, CHF, glomerulonephritis, acid base disturbances

Question 2

Define filtration, reabsorption, secretion, excretion

Answer 2

• filtration: From glomerular capillaries into renal tubular lumen. occurs in glomerulus.
• reabsorption: transport from tubular lumen fluid back into systemic circulation
• secretion: transport from non glomerular capillaries or interstitial fluid into tubular lumen fluid
• excretion: removal of a material from body fluids into urine

Question 3

What is the equation for excretion of material x?

Answer 3

Excretion(x)=filtration(x)+secretion(x)-reabsorption(x)

excretion: amount in urine x urine flow rate
filtration: amount in plasma x plasma filtration rate
secretion: Amt added across tubule/time
reabsorption: Amt removed form tubule/time

Question 4

What is the glomerular filtration rate (GFR)?

Answer 4

• rate (ml/min) of production of ultra filtrate across glomerular capillaries into Bowman’s space
• filtrate if blood and protein free
• sum of filtration across all glomeruli in the two kidneys
• normal: GFR is about 120ml/min=180L/day
• 20% of plasma filtered out of the blood while 80% of plasma continues past the capillaries and enters peritubular capillary network

Question 5

What is the filtration fraction?

Answer 5

FF=GFR/RPF
RPF: renal plasma flow
in normal: =120/600=.20
20% of plasma filtered out of the blood

Question 6

What is clearance? renal clearance?

Answer 6

• rate of removal of a substance from a volume of blood or plasma. blood cleared of the substance in volume/time
• renal clearance: rate the blood or plasma is cleared of a substance by excretion into urine over time

Question 7

How is GFR estimated using inulin?

Answer 7

-Inulin is a polyfructuse that is freely filtered at glomerulus and not reabsorbed, secreted, synthesized, or metabolized.
P(In) X GFR = U(In) x V
plasma [inulin] x GFR=Urine[Inulin] X urine flow rate
-inulin clearance=GFR =(U x V)/P

Question 8

How is GFR estimated from clinical data, i.e. creatinine?

Answer 8

• creatinine is a by product of skeletal muscle creatine metabolism
• is produced at constant rate and normally excreted in urine at same rate
• P[creatinine] x GRF= U[creatinine] X V
• excretion rate=U x V
• means that P x GRF is a constant
• if GFR falls, P[Cr] rises

Question 9

What are the limitations of estimating GFR from plasma creatinine?

Answer 9

• Cr levels may be high from high meat diet, low in malnourished patients, high after marathon
• Cr production and GFR may be low in cirrhotic patients

Question 10

Describe PAH secretion.

Answer 10

• PAH is an organic acid, its excretion is greater than rate of PAH filtration
• PAH enters cell across basolateral membrane by PAH/aketoglutarate anti port mechanism
• aketoglutarate concentrates in the cell, concentration driven by downhill cotransport with Na+

Question 11

How can renal blood flow be measured by determining the clearance of PAH?

Answer 11

-PAH is infused at low level so that 100% will be removed from renal circulation, urine and plasma concentrations, and urine flow rate are measured
RPF= U[PAH] x V/ Plasma[PAH]
Partery x RPFartery -[Pvein x RPFvein]=U x V
[Pvein x RPFvein]=0
P[PAH]artery x RPFartery=U[PAH] X V
C=UV/P=RPF
-Renal blood flow=RPF corrected for hematocrit

Question 12

What is that transport rate?

Answer 12

Transport rate =filtration rate-excretion rate
filtration rate=GFR x P[x]
excretion rate=U[x] x V