Lecture 31 Flashcards
What is filtration?
Movement of plasma from the glomerular capillaries into the glomerular capsule
What does filtration create?
Plasma-like filtrate
What determines glomerular filtration?
Filtration barrier, renal blood flow and driving forces
What is filtered?
Small substances with low molecular masses
What isn’t filtered?
Large substances with high molecular masses
What happens to the substances that aren’t filtered?
They remain in the capillaries, exiting the glomerular capillaries via the efferent arteriole and then traveling to the peritubular capillaries
What is renal blood flow?
The amount of blood that flows through the kidneys per unit time
How much cardiac output do the kidneys reciece?
20% at rest
What is the numerical value for healthy renal blood flow?
1100-1200 mL of blood / minute
What is the high blood flow through the kidneys for?
Filtration (not metabolism)
What are the two glomerular driving forces
Hydrostatic pressures and colloid osmotic pressures
What is the hydrostatic pressure due to?
The volume of fluid present
What does the hydrostatic pressure driving force cause in the glomerulus?
A push of fluid away from the glomerular capillaries
What is the colloid osmotic pressure due to?
Proteins - like a magnet
What does the colloid osmotic pressure driving force cause in the glomerulus?
A pull of fluid into the glomerular capillaries
What is a positive pressure?
A pressure favouring filtration
What is a negative pressure?
A pressure opposing filtration
What are the two hydrostatic forces in the glomerulus?
Glomerular hydrostatic pressure and capsular hydrostatic pressure
What is the glomerular hydrostatic pressure due to?
Blood pressure
What is the value for glomerular hydrostatic pressure?
+ 50 mmHg
What is the capsular hydrostatic pressure due to?
Pressure of filtrate already present
What is the value for capsular hydrostatic pressure?
-15 mmHg
What are the two colloid osmotic pressures?
Blood colloid osmotic pressure and capsular colloid osmotic pressure
What is the blood colloid osmotic pressure due to?
Albumin
What is the value for blood colloid osmotic pressure?
-25 mmHg
What is the capsular colloid osmotic pressure due to?
There is none because there are no proteins in the capsular space
What is the value for capsular colloid osmotic pressure?
0 mmHg
What is the value of net filtration pressure?
+ 10 mmHg
What is filtration fraction?
The proportion/percentage of the kidneys plasma flow that is filtered
What is the value for filtration fraction?
20% of plasma entering the kidneys
What is the glomerular filtration rate (GFR)?
The amount of plasma filtered per unit time
What is the value for healthy glomerular filtration rate?
125mL / min
How do you find glomerular filtration rate?
Renal plasma flow x filtration fraction
How much plasma is filtered per day?
Normally 180 L
How much urine is produced per day
~1.5L
What percentage of plasma-like filtrate is reabsorbed?
> 99%
What happens to glomerular filtrate rate as you age?
It slowly declines
What is renal filtered load?
How much of a substance in the plasma is filtered per unit of time
What is the filtered load equation?
glomerular filtration rate x solute plasma concentration
What is renal clearance?
How much plasma is cleared of a substance per unit time
What are the filtration characteristics of creatinine?
Freely filtered, not secreted or reabsorbed so all that is filtered is excreted in urine
What is the renal clearance of creatinine?
Glomerular filtration rate
What are the filtration characteristics of PAH (medications and toxins)?
Freely filtered, entirely secreted and none reabsorbed so all that flows through kidneys is excreted in urine
What is the renal clearance of PAH (medications and toxins)?
Volume of plasma flowing through the kidneys per minute (RPF)
What are the filtration characteristics of glucose?
Freely filtered, not secreted but all is reabsorbed so none is in urine
What is the renal clearance of glucose?
0
What are the filtration characteristics of Sodium?
Freely filtered, not secreted and nearly entirely reabsorbed so very little is excreted in urine
What is the renal clearance of sodium?
A very small volume of plasma per minute
What is Ux?
Concentration of x in urine (mmol/L)
What is V?
Volume of urine produced per unit time (mL/min)
What is Px?
Concentration of x in plasma (mmol/L)
How can we estimate glomerular filtration rate?
Using renal clearance
What are the requirements of a substance to be able to measure GFR?
To be freely filtered, not reabsorbed nor secreted
What are the two main substances that meet the requirements to measure GFR?
Inulin and creatinine
What is inulin?
A polysaccharide that is not metabolised or found in the body. It must be injected
What is creatinine?
A waste product produced by muscles therefore it is already in the body and most commonly used clinically
What is your GFR if you have two healthy, working kidneys?
125 mL/min
What is your GFR if you have one healthy, working kidney?
60 mL/min
What happens when your GFR is < 25 mL/min
Plasma creatinine concentration increases as the kidneys ability to clear waste product is reduced
What happens when your GFR is <15 mL/min?
Kidney failure, the kidney’s ability to clear waste products from the blood is greatly reduced
What is the value for renal plasma flow (RPF)?
625 mL/min
What is the renal clearance of sodium?
<1 mL/min
What is the renal clearance of glucose?
0 mL/min
What is the renal clearance of medications and toxins?
625 mL/min
What is the renal clearance of creatinine and inulin?
125 mL/min
