Glomerular Filtration and GFR

Question 1

How much of the CO is bloodflow to the kidney?

Answer 1

20% (1L/min)

Question 2

What type of water does filtration of the blood occur in?

Answer 2

Plasma water only

Question 3

What are the components of the glomerular filtration barrier?

Answer 3

1. Endothelial cells of glomerular capillaries
2. Glomerular basement membrane
3. Epthelial cell foot processes including proteins of the podocytes and filtration slits

Question 4

2/3 of what ions are reabs in the PT?

Answer 4

Na, K+, Cl-, H20

Question 5

80% of what is reabs in the PT?

Answer 5

Bicarb and phosphate

Question 6

What is almost all reabs in the PT?

Answer 6

Glucose and amino acids

Question 7

What is the first barrier to filtration?

Answer 7

Endothelium

Question 8

What are capillary fenestrations filled with?

Answer 8

Glycocalyx with various negatively charged proteoglycans that restrict filtration across endothelial layer

–>restrict albumin

Question 9

What driving pressures push fluid out of the glomerulus capillary? (ie favor filtration)

Answer 9

Capillary hydrostatic pressure (constant in a normal kidney) and bowman’s oncotic pressure

Question 10

What driving pressures oppose ultrafiltration?

Answer 10

Bowman’s hydrostatic pressure and capillary oncotic pressure (these are low in a normal kidney)

Question 11

Describe Bowman’s space hydrostatic pressure

Answer 11

• Opposes filtration
• Low
• Entire capillary sees the same low pressure along its entire length
• Fxn of rate of urine formation and flow of urine into the PT

Question 12

Describe Bowman’s oncotic pressure

Answer 12

• Favors filtration

- But is very small except under pathologic conditions since normally proteins are only minimally filtered

Question 13

Describe glomerular capillary hydrostatic pressure

Answer 13

Nearly constant along the length of the glomerular capillaries due to regulation of resistances of afferent and efferent arterioles (which determine this pressure)

Question 14

Describe glomerular capillary oncotic pressure

Answer 14

• Opposes filtration across the capillary
• Rises during transit along cap bed because of progressive extraction of water by ultrafiltration across the filtration barrier

Question 15

How do we calculate the net driving force favoring ultrafiltration?

Answer 15

Difference between forces favoring filtration and those opposing

Question 16

What is the formula for filtration fraction

Answer 16

GFR/renal plasma flow

Question 17

Angio II effect on arteriolar tone?

Answer 17

Vasoconstriction (efferent>afferent)

Question 18

Catecholamines effect on arteriolar tone?

Answer 18

Vasoconstriction (afferent>efferent)

Question 19

Endothelin effect on arteriolar tone?

Answer 19

Vasoconstriction, both

Question 20

Prostaglandins effect on arteriolar tone?

Answer 20

Vasodilation on efferent (NSAIDs inhibit these)

Question 21

Describe autoregulation of GFR

Answer 21

Mostly a fxn of changes in afferent arteriolar tone/resistance

Maintained by a myogenic response, when afferent art are stretched, they respond with contraction and by tubulo-glomerular feedback system)

Question 22

Where is the juxtaglomerular apparatus located?

Answer 22

Thick ascending limb of the loop of henle and the glomerulus between the afferent and efferent vessels

Question 23

What are the cells of the thick ascending limb of the loop at the JGA?

Answer 23

macula densa cells adj to the hilum of the glomerulus

Question 24

What do macula densa cells do?

Answer 24

Sense sodium and cause a signaling cascade that modulates afferent arteriolar resistance by causing juxtaglomerular cells in the walls of the afferent arteriole to synthesize renin

Question 25

Tubulo-glomerular feedback

Answer 25

1. Uptake via Na-K-Cl cotransporter on luminal membrane of the MD cells is driven by an increase in Cl concentration
2. Increased production of adenosine which
3. Triggers an increase in cytosolic calcium in the extraglomerular mesangial cells
4. Granular cells containing renin and smooth muscle cells of afferent arterioles vasoconstrict and decrease renin production

—ie increase cl tells us that there is too much volume, pee stuff out please

Question 26

How many liters per day of plasma water do the kidney’s filter?

Answer 26

180L/d

Question 27

What is a normal GFR

Answer 27

120-125ml/min

Question 28

What is clearance?

Answer 28

Urinary excretion rate (U*V)/Plasma concentration (P)=clearance

• net of filtration, reabsorption and secretion
• Not about how much is removed, but the volume of the plasma it was removed from

Question 29

What are the characteristics of an ideal GFR marker?

Answer 29

• Inert/non-toxic
• Freely filtered
• Not metabolized or synthesized in the kidneys
• Not secreted

Question 30

What is the equation for GFR?

Answer 30

(UV)/P of inulin or creatinine …but really we use BUN/Creatinine ratio to estimate the GFR because creatinine overestimates GFR and urea underestimates GFR

Question 31

What happens to creat concentration in the urine as GFR decreases?

Answer 31

More is found in the urine. It is derived from creatinine secreted by the tubules rather than the glomerular filtrate

–>so it becomes less of a good measure of GFR at lower levels. At normal GFR, secretion accounts for 10-15% of creatinine in urine. At low GFRs it raises to 45%

–>organic cation transporters excrete creatinine

Question 32

Drugs that inhibit creatinine secretion

Answer 32

inhibit the organic cation transporters that excrete it

• Cimetidine
• Trimethoprim (bactrim, septra)
• Probencid

Question 33

Plasma creatinine does what as GFR declines?

Answer 33

Increases

Question 34

What is a possible future substitute for creatinine measuring GFR?

Answer 34

Cystatin C

Question 35

What is a fractional excretion?

Answer 35

Amt filtered/Amt in urine (UV/P*GFR) if freely filtered