Renal Hemodynamics, Blood Flow Flashcards
Glomerular filtration rate
An index of renal function
NOTE: Kidneys can control GFR and renal blood flow through neurohumoral and local instrinsic mechanisms
How is renal blood flow calculated?
RBF= ΔP/R
ΔP= difference between renal artery pressure and renal vein pressure
R= total renal vascular resistance
NOTE: The total renal vascular resistance is the sum of all resistances in kidney vasculature
What is the typical renal blood flow?
Approximately 1100 mL/min
Kidneys receive between ______ and ______% of cardiac output.
20-22%
NOTE: Oxygen and nutrients delivered to kidneys normally greatly exceeds their metabolic needs.
A large fraction of renal oxygen consumption is related to ___________.
Renal tubular sodium reabsorption
NOTE: If we are able to dictate movement of Na, we are able to move large amounts of water and of course other electrolytes
How is the net filtration pressure calculated?
(Glomerular hydrostatic pressure)- (Bowman’s capsule pressure)- (Glomerular oncotic pressure)
What is the normal glomerular capillary hydrostatic pressure?
60 mmHg
*This is force that favors filtration
NOTE: The glomerular hydrostatic pressure does not really change much. This is important for maintaining proper filtration
Which forces oppose filtration? What are the normal values for each?
- Glomerular colloid osmotic pressure- 32 mmHg
- Bowman’s capsule pressure- 18 mmHg
What would cause an increase in the glomerular colloid osmotic pressure?
An increase in protein concentration within the capillary, as filtered substance leaves the gloemrular capillary
What happens to glomerular capillary hydrostatic pressure, if resistance is increased in the efferent arterioles?
- It would initially cause a transient increase in glomerular capillary hydrostatic pressure
- This transient increase is however short-lived because resistance in either the afferent or efferent, decreases renal blood flow.
What affect would the blockage of prostagladin synthesis have on GFR?
Decrease
NOTE: This is usually i,portant only when there are other disturbances that are already tending to lower GFR
Endothelin is a potent _________ (vasoconstrictor/vasodilator).
Vasoconstrictor
*Release of endothelian will result in a decrease in GFR
What affect does angiotensin II have on the renal flow and GFR?
Decreases renal flow and maintains GFR
Myogenic Mechanism to Autoregulation
- INcrease in volume at afferent arterioles
- Stretch sensed by vascular smooth muscle of afferent arteriole
- Activation of L-type Ca++ channels
- Vasoconstriction
Flow of renal plasm
Afferent arteriole-> Efferent arteriole-> Peritubular capillaries-> Renal vein
Normal GFR
125 ml/min
NOTE: The glomerular filtrate composition is about the same as plasma, except for large proteins
Filtration fraction
GFR/ renal plasma flow
NOTE: The normal filtration fraction is 0.2
Which factors would place a patient at risk for renal disease?
Hypertension
Diabetes
Pregnancy
What does a standard urinary dipstick test for?
Proteins in urine
Microalbuminuria
Urine excretion of >30 but <150 mg of albumin per day
What are some causes of microalbuminuria?
Early diabetes
Hypertension
Glomerular hyperfiltration
NOTE: Diabetic patients with microalbuminuria are 10-20 fold more likely to develop persistent proteinuria
Normal Kf
12.5 mL/min per mmHg
Kf
Hydraulic conductivity x surface area
*Kf is not normally highly variable
Which diseases can reduce Kf and GFR?
- Chronic hypertension
- Obesity/ diabetes mellitus
- Glomerulonephritis
True or false. Bowman’s capsule hydrostatic pressure normally changes as a function of GFR, not a physiological regulator of GFR.
True
What conditions increase bowman’s capsule hydrostatic pressure?
- Tubular obstruction
- Kidney stones
- Tubular necrosis
- Urinary tract obstruction
- Prostate hypertrophy/cancer
How is the filtration fraction related to the glomerular oncotic pressure?
Directly related.
Net filration pressure _________ (decreases/increases) along the glomerulus because of increasing glomerular colloid osmotic pressure.
Decreases
What factors influence glomerular hydrostatic pressure?
- Arterial pressure (effect is buffered by autoregulation)
- Afferent arteriolar resistance
- Efferent arteriorlar resistance
True or false. Autoregulation is regards to renal blood flow and GFR but not urine flow.
True
NOTE: With an increase in mean arterial pressure, urine flow increases. However, with an increase in mean arterial pressure renal blood flow and glomerular filtration rate will remain the same, if autoregulation comes into play.
Summary of determinants of GFR
When is angiotensin II released to maintain GFR?
Low sodium diet
Volume depletion
*The main point of angiotensin is to prevent a decrease in GFR
What affect do prostaglandins have on GFR?
Prostaglandins decrease resistance and therefore increase GFR and RBF
NOTE: Blockage of prostaglandin synthesis will lead to a decrease in GFR but this is only important when there are other disturbances that are already tending to lower GFR. So nonsteroidal antiinflammatory drugs should be given to volume depleted patients or patients with heart failure, cirrhosis.
What affect does endothelial-derived nitric oxide have on GFR?
Endothelial- derived nitric oxide causes a decrease in resistance, so an increase in GFR.
NOTE: This substance protects against excessive vasoconstriction. Patients with endothelial dysfunction, such as atherosclerosis, may have greater risk for excessive decrease in GFR in response to stimuli such as volume depletion.
What affect does endothelin have on GFR?
Endothelin cause an increase in resistance and henceforth causes a decrease in GFR.
In what conditions will endothelin antagonists be useful?
- Hepatorenal syndrome
- Acute renal failure
- Hypertensive patients with chronic renal failure
Summary of neurohumoral control of GFR and renal blood flow
What factors are important in maintaining urine volume?
Good autoregulation
Adaptive increase in tubular reabsorption
Myogenic mechanism
- Increase in arterial pressure
- Increase in stretch of blood vessel
- Increase in cell Ca++ entry
- Increase in intracellular Ca++
- Increase vascular resistance
- Decrease in blood flow and GFR
How do the afferent and efferent arterioles respond to prerenal AKI respectively?
Afferent arterioles
- Response to decrease in renal blood flow by increase in vasodilating prostaglandins
- Blunted by NSAIDS that inhibit prostaglandin prostaglandin production
Efferent arterioles
- Response to decrease in renal blood flow by preferential constriction of efferent arteriole by Angiotensin II
- Blunted by ACE inhibitors/ ARBs that inhibit Angiotensin II production
