First Aid - Renal Physiology I

Question 1

What is the total water by liters and body weight by compartment in the human body?

Answer 1

Intracellular ~ 27 L, 2/3 total fluid, 40% body weight. Extracellular ~ 15 L, 1/3 total fluid, 20% body weight.
Extracellular ~ 1/4 plasma volume, 5% body weight
Extracellular ~ 3/4 interstitial volume, 15% body weight

Question 2

How are the plasma and interstitial volumes measured, and what is the normal osmolarity?

Answer 2

Plasma volume is measured by radio labeled albumin. Interstitial volume is measured by inulin. Typical osmolarity is 290 mOsm/L

Question 3

What are the cell layers and types that make up the glomerular filtration barrier, and what molecule adds significant charge to the layer?

Answer 3

Fenestrated capillary endothelial cells, Epithelial layer composed of podocytes, fused basement membrane with strong negative charge from heparin sulfate.

Question 4

What part of the glomerular filtration barrier is lost in nephrotic syndrome and what conditions does this result in?

Answer 4

The charge barrier is lost in nephrotic syndrome. This results in albuminuria, hypoproteinemia, generalized edema, and hyperlipidemia.

Question 5

How is the renal clearance of compound A calculated?

Answer 5

Clearance(a) = (Urine [a] x Urine flow rate)/Plasma [a]
Ca = (Ua x V)/Pa, (ml/min) = ((mg/ml) x (ml/min))/(mg/ml)

Question 6

What is the significance of Cx greater than GFR, Cx less than GFR, and Cx = GFR?

Answer 6

Cx greater than GFR: net tubular secretion of x, Cx less than GFR: net tubular reabsorption of x, and Cx = GFR: no net secretion or reabsorption of x.

Question 7

What is measured to calculate the glomerular filtration rate, does it over or underestimate the GFR?

Answer 7

Inulin gives the most accurate assessment of GFR, however creatinine is most easily determined and is used clinically more often. Creatinine slightly overestimates the GFR because it is moderately secreted in tubules. Still use Cx = UxV/Px because Cx = GFR here.

Question 8

What is the normal GFR?

Answer 8

100 ml/min

Question 9

What is the equation of Starling forces that calculates GFR?

Answer 9

GFR = Kf [(Pgc - Pbs) - (Pigc - Pibs)] P = hydrstatic pressure, Pi = osmotic pressure, Pibs = Osmotic pressure of Bowmans Space and is usually 0. Kf is a constant.

Question 10

How is the Effective Renal Plasma Flow calculated?

Answer 10

The ERPF is estimated using para-aminohippuric acid (PAH) because it is both filtered and secreted in proximal tubule resulting in nearly 100% clearance on first pass. ERPF = Cpah = (Upah x V)/Ppah

Question 11

How can RPF be calculated from the RBF and HCT?

Answer 11

RPF = RBF(1-HCT), that is, the Renal Plasma Flow is equal to the portion of Renal Blood Flow that is not Hematocrit.

Question 12

How is the Filtration Fraction calculated?

Answer 12

Filtration Fraction (FF) = GFR/RPF, where GFR = Creatinine clearance and RPF = PAH clearance.

Question 13

What are two factors that can increase both the GFR and FF?

Answer 13

Efferent arteriole constriction and decreased plasma protein concentration.

Question 14

What are two factors that can decrease both the GFR and FF?

Answer 14

Increased plasma protein concentration and constriction (obstruction) of ureters.

Question 15

What is one factor that decreases the GFR but has no effect on the FF?

Answer 15

Constriction of the afferent arteriole.

Question 16

How is secretion of a substance calculated?

Answer 16

Secretion = excreted - filtered (excreted amount is greater than filtered amount because compound is secreted into the tubules) Excreted = V x Ux; Filtered = GFR x Px

Question 17

How is the reabsorption of a substance calculated?

Answer 17

Reabsorption = filtered - excreted (filtered amount is greater than excreted because compound is reabsorbed in the tubules) Filtered = GFR x Px; Excreted = V x Ux

Question 18

How is glucose reabsorbed and at what levels is it excreted?

Answer 18

At normal plasma levels, glucose is completely reabsorbed in the proximal tubule by Na+/glu co-transporters (strong Na+ gradient into epithelial cells). Glucose excretion begins at plasma glucose of ~200mg/dL and all cotrasporters are saturated at ~375mg/dL.

Question 19

How are amino acids reabsorbed?

Answer 19

AA are reabsorbed in the proximal tubule via Na+/AA co-transporters (Strong Na+ gradient into epithelial cells).

Question 20

What is Hartnup disease?

Answer 20

Hartnup disease is an autosomal recessive disorder that results in a deficiency of neutral AA transporters in proximal tubule. Results in neutral aminoaciduria and dec. absorption from the gut, pellagra-like symptoms, treatment is high protein diet and nicotinic acid.