urine formation -- Lecture 29

glomerular filtration rate (GFR)

• the volume of filtrate formed by both kidneys per minute (ml/min)

  • this is NOT = to amount of urine produced per minute

what is cardiac output and how does that relate to kidneys

• cardiac output (ml of blood pumped by the heart/min) is ~5000ml

  • kidneys receive ~20% of that (~1000ml), 55% (550ml) of which is plasma

  • normal GFR is >1000ml/min → volume of plasma filtrate being moved from glomerulus to Bowman’s capsule per minute

changes to GFR

• pressure

• diameter of arterioles

• proteins in blood (pigc)

NFP equation

• P_gc-P_bc)-(pi_gc-pi_bc)

constrict afferent

• decrease Pgc

• decrease GFR

constrict efferent

• increase Pgc

• inccrease GFR

blockage downstream

• inc Pbc

• dec GFR

increase in blood protein

• inc pigc (oncotic — holding on to water more)

• dec GFR

excretion =

filtration - reabsorption + secretion

• filtration

• reabsorption 

• secretion

• excretion

GFR

volume of fluid filtered per unit time

how do we measure GFR?

• need to measure the amount of something that is filtered per unit of time that is NOT reabsorbed and NOT secreted

• option 1: inulin → req. IV admin

• option 2: creatinine

  • produced at constant rate

  • freely filtered

  • NOT reabsorbed

what is estimated GFR - creatine clearance`

• clearance_Cr = urine flow rate x urine [Cr] / plasma [creatinine]

  • = mL/min x mg/dL

    • = mL/min

BP fluctuates, but GFR

stays pretty constant

regulation of GFR types

• intrinsic

  • myogenic response

  • tubuloglomerular feedback

• extrinsic

  • sympathetic NS

  • hormones

    • renin-angiotensin-aldosterone system

juxtaglomerular apparatus

1. macula densa

2. granular cells

3. mesangial cells

macula densa

• area of specialized cells in thick ascending limb

• monitor NaCl content in filtrate

granular cells

• specialized cells in wall of afferent arteriole

• secrete renin (enzyme)

• a lot of SNS innervation

mesangial cells

• resemble smooth muscle cells

tubuloglomerular feedback

• ^ blood pressure = ^Pgc = ^ GFR

1. ^ NaCl delivery to macula densa

2. NKCC2 ^ NaCl absorption into macula densa cells, not enough ATPases to get rid of extra Na+

3. macula densa cells swell

4. ATP/AMP are released into interstitium

5. ATP/AMP broken down into ADO (adenosine) and bind A1 receptors on mesangial cells

   1. = ^ cystolic Ca2+

6. Ca2+ acts on VSMC (smooth muscle) and GCs (granular cells)

   1. = vasoconstriction and dec renin release

7. dec GFP