Lecture 6: Elements of Renal Function Flashcards
What are the 2 populations of nephron and location?
1) Cortical (superficial)
2) Juxtamedullary (deep)
Each nephron has how many arteioles and capillaries?
- Two arterioles
- Two sets of capillaries
Which arteriole delivers blood to the glomerular capillaries?
Afferent arteriole
Arterial blood leaves the glomerulus through which arterial, which quickly subdivides into?
Efferent arteriole, which quickly subdivides into second set of capillaries, the peritubular capillaries (vasa recta).
What do the peritubular capillaries supply?
The renal tissue with blood
Where is blood flow rate highest and lowest in the kidney?
- Highest in cortex (4-5 mL/min) = filtration is slow
- Low in the medulla: helps maintain hyperosmolar enviornment in this region = filtration is high
What are some of the differences between the cortical and juxtamedullary nephrons?
Cortical: surrounded by extensive network of peritubular capillaries, rate of filtration is slow, larger diameter efferent arterioles
Juxta: longer loop of henle, longer thin loop, lower renin, different tubular permeability, high rate of filtration, and long (small diameter) efferent arterioles
Main function of the cortical nephron?
Excretion of waste products in the urine
Main function of the juxtamedullary nephron?
Concentration of urine
How many liters of fluid filters into the Bowman’s capsule each day at about what mOsM?
180 L at about 300 mOsM
What is the osmolarity of the filtrate leaving the proximal tubule as it enters the loop of henle, how about at the end of the loop of henle?
Start of loop: 300 mOsM
End of loop: 100 mOsM
What is the approximate osmolarity at the end of the collecting duct; why the variability?
- 50-1200 mOsM
- Depends on the body’s need to conserve or excrete water and solute
Where does filtration happen as opposed to absorption and secretion?
- Filtration occurs in renal corpuscle as fluid moves from the glomerular capillaries into Bowman’s capsule
- Reabsorption/secretion occur along remainder of tubule, material is transferred between the lumen and the peritubular capillaries
What happens to filtrate that remains in the lumen at the end of the nephron?
Excreted as urine.
What is the GFR is stages 1-5 of CKD?
Stage 1: >90 mL/min
Stage 2: 60-89 mL/min
Stage 3: 30-59 mL/min
Stage 4: 15-29 mL/min
Stage 5: <15 mL/min
The volume of filtrate formed by both kidneys per minute is the?
GFR
What percentage of the blood enters the kidney to be filtered per minute = how much filtrate per minute/per day in men vs women (GFR)?
Heart pumps 5L of blood per min. and 20% (1 liters) enters the kidney per minute.
Men: 125 mL/min of filtrate = 180 L/day
Women: 105 mL/min of filtrate = 150 L/day
What percent of the filtrate is reabsorbed and returned to circulation?
99%
Renal blood flow =
CO x % blood flow to kidney (usually 20%)
Renal plasma flow =
Renal blood flow x (1-hematocrit)
*Hematocrit is typically 45 (.45) and 1-0.45 = 0.55
GFR =
Renal plasma flow x % entering capsule (usually 19%)
Urine per minute =
GFR x fraction of filtrate not reabsorbed (.008)
What are the 3 possible outcomes of a substance as its filtered through kidneys
1) Substance is filtered and secreted, but NOT reabsorbed
2) Substance is filtered, and a fraction is reabsorbed
3) Substance is filtered and completely reabsorbed
GFR will always be in what units of measurement?
mL/min
The plasma clearance of any substance is defined as; importance
The volume of plasma completely cleared of that substance by the kidneys per minute; expresses the kidneys effectivness in removing various substances, which is more related to how they are functioning
What is the equation that can be used which allows us to assess renal function using only analysis of the urine and the blood?
C(x) x P(x) = U(x) x V
C(x) = clearance rate of substance x (mL/min)
P(x) = plasma concentration of substance x
U(x) = urine concentration of substance x
V = urine flow rate (mL/min)
What is the rearranged equation for renal clearance?
What is the best marker for measuring GFR; what’s the issue with it?
- Inulin
- Is not made endogenously, must be administered by continous IV, expensive
Filtered load =
GFR x Plasma concentration (P(x))
Excretion rate =
Urine concentration (Ux) x Urine flow rate (V)
How do the Cx and GFR relate in terms of absorbtion and excretion?
If C(x) < GFR = net tubular reabsorption of substance
If C(x) > GFR = net secretion of substance
If C(x) = GFR = no net secretion of reabsorption
C(x) = excretion rate
GFR = filtration rate
GFR is routinely measured as the clearance of?
Endogenous Creatinine
Problem with using creatinine as a marker?
There is typically an overestimation
Normal serum creatinine is considered to be?
< 1.2 mg/dL = 0.012 mg/mL
Why is serum creatinine concentration a good reflection of GFR?
Production of creatinine is typically constant over short periods.
Why is PAH a good way to measure RPF?
Tubular secretory system for PAH is so efficient at low plasma concentrations it removes 90% + of PAH from thr plasma as it flows through the kidneys.
Equation for RPF using PAH?
How can we get RBF if we have RPF?
RBF = RPF/(1-Hct)
What is Prerenal failure?
- Failure of perfusion of the kidney
- GFR falls because of volume depletion, decreased CO, or vasodilation
- Increase BUN and creatinine (20:1 BUN/creatinine)
- Decreased urinary output and urine sodium
- Increased urine osmolality
- Increased specific gravity
What is Intrarenal failure?
Damage to kidney itself (traumatic, injury, burn, ect.)
- Elevated BUN and creatinine
- Decreased output,
- increased sodium, and increased osmolarity
What is Postrenal kidney failure?
- Kidneys work, but obstruction distal to kidney
- Increased pressure = decreased GFR
- increased BUN and creatinine
- Decreased output, decreased sodium
- Normal sed rate, variable osmolarity, variable gravity
