Renal Phys Flashcards
Renal Blood FLow
1000 ml/min (~ 20% of cardiac output!)
Kidneys receive highest percentage of cardiac output per gram of tissue
Renal plasma flow (RPF)
Renal plasma flow (RPF) = (1 – Hct) * RBF = 600 ml/min
Assuming hematocrit (Hct) of 40% or 0.4
Glomerular filtration rate (GFR)
Volume of plasma filtered by the kidney per unit time ~ 125 mL/min
Filtration fraction (FF)
Filtration fraction (FF) = GFR / RPF = ~ 20%
Fraction of RPF that is filtered by the kidney
What happens to larger Mw molecules and rest of plasma not filtered by the kidneys
The remaining 4/5th’s of the plasma continues to flow through the renal microvasculature (i.e., efferent arteriole → peritubular capillaries → renal vein)
Plasma osmolarity (Posm) is estimated as:
Posm = 2(Na+) + Glucose/18 + BUN/2.8
- The numbers 18 and 2.8, are adjustments to convert lab values (mg/dL) to (mmol/L).
Osmolar gap
The difference in the measured osmolality vs. the estimated or calculated osmolality
Normal osmolar gap is < ± 10 mOsm/kg water
Do NaCl and mannitol cross cell membranes?
No
Decrease in ECF volume
Volume contraction
Hypoosmotic volume expansion
I.e. SIADH
- ECF osmolarity decreases
- Water shifts from ECF to ICF
- ECF and ICF volumes increase
- Plasma protein concentration decreases
- Hct remains unchanged
Increase in ECF volume
Volume expansion
Hypoosmotic volume contraction
i.e. adrenocortical insufficiency (loss of NaCl)
Examples of Isoosmotic volume contraction
i.e. diarrhea; burns
Examples of hyperosmotic volume contraction
i.e. sweating, fever, DM insipidus
Example of hyperosmotic volume expansion
High NaCl intake
What does an infusion of isotonic NaCl cause?
Isoosmotic volume expansion
Composition of glomerular filtration barrier
- Glomerular fenestrated capillaries
- Basement membrane (contains large negatively charged proteoglycans)
- Podocytes
What glomerular structure is critical in restricting the filtration of albumin?
Podocytes
Podocyte injury or dysfunction underlies most, if not all, proteinuric kidney diseases associated with albuminuria.
Mesangial cells
Support glomerular capillary loops
they secrete extracellular matrix and are contractile cells that can alter GFR by
changing the capillary surface area available for filtration (overall, a minor role in regulating GFR)
How to calculate GFR from ultrafiltration pressure
Why is the hydrostatic pressure of glomerular capillaries higher than all other capillary beds?
PGC is ~ 50 mmHg along its entire length becuase of distal resistance (efferent arteriole)
In normally function kidneys, what is the oncotic pressure in Bowman’s space?
usually 0 becuase of the low filterability of proteins
How does the oncotic pressure within glomerular capillaries change going from afferent to efferent?
Increases as fluid is filtered across into Bowman’s space
In the kidney’s circulation system, where are there a high density of alpha-1 receptors?
Afferent arterioles
Thus, an ↑ in sympathetic nerve activity will ↑ RA and ↓ both RBF and GFR and vice versa.