Equations (Renal)

Question 1

What is the equation for water and ion balance?

What are types of water inputs and outputs?

For water, what is required for steady state, negative balance and positive balance?

Answer 1

Balance = Total in - Total out

Water inputs:

• Beverages
• Food
• Metabolic

Water outputs:

• Insensible (evaporation, not aware of)
• Sweat
• Fecal
• Urinary (only output that can be regulated to ensure balance)

If urine flow…
= balance = steady state
> balance = negative balance
< balance = postive balance

Question 2

What does the filtration fraction represent?

What is its equation?

Answer 2

The kidney receives 25% of CO. % of CO needed to support filtration = filtration fraction (FF) = 20% CO

FF = GFR / RPF

GFR = Glomerular Filtration Rate

RPF = Renal Plasma Flow = (1-HCT)*RBF

RBF = Renal Blood Flow = 1.1L/min

Question 3

What is Net Filtration Pressure (NFP)?

What is its equation?

What other equation uses NFP?

Answer 3

NFP comprises the different pressures effecting filtration:

P_GC = blood pressure in glomerular capillary
– driving force for GFR
P_BC = back pressure in Bowman’s capsule
– retards GFR
π_GC = oncotic pressure of glomerular capillary blood
– due to proteins unable to cross barrier; retards GFR; increases as plasma diverted into BC

NFP = P_GC - P_BC - π_GC

NFP is also used in the Glomerular filtration rate (GFR) equation

Question 4

What is the equation for Glomerular Filtration Rate (GFR)?

What equation(s) uses GFR?

Answer 4

GFR = (K_f)(NFP)

NFP = Net Filtration Pressure

K_f = capillary filtration coefficient

GFR is used in:

FF = GFR/RPF

Question 5

What is the general Clearance Equation (C_x)?

What is the free water clearance Equation (C_H2O)?

Answer 5

C_x = (U_x)*(V)/P_x

X = ion, creatinine, inulin, osmolar

U = Concentration of x in urine

V = rate of urine production

P = Concentration of x in plasma

C_H2O = V - C_osm

Question 6

What is the equation and what does it represent?

Answer 6

C_x/C_inulin

It compares the clearance of any substance to that of inulin

ratio = 1 = GFR marker, filtered and not reabsorbed

ratio < 1 : Substance is not filtered or is filtered and then reabsorbed

ratio > 1 : Substance is filtered and secreted

Question 7

How can renal plasma flow (RPF) be estimated?

Answer 7

C_PAH can be used to estimate RPF because all PAH is cleared from plasma and excreted.

C_PAH = (U_PAH)*(V)/P_PAH

U = urine concentration

V = rate of urine production

P = plasma concentration

PAH = Para-aminohippurate

Question 8

What does the Tubular FLuid to Plasma ratio represent?

Answer 8

TF/P represents..