Ch. 39: Glomerular Filtration Rate, Renal Clearance, Intrarenal Homeostasis

What is Glomerular Filtration?

The movement of fluid and solutes from glomerular capillaries (blood vessels) across a barrier into Bowman’s Space

What causes Glomerular Filtration?

• Starling Forces

Glomerular Filtrate is…

the Net glomerular filtration pressure that initiates urine formation

• Due to Starling forces

Glomerular Filtrate:

• Pushes cell-free and protein-free filtrate of plasma

• Driven by Starling Forces

• Fluid (filtrate) is pushed out of golmeruli into Bowman’s space

• from tubule to renal pelvis

Glomerular Filtration is a ____

• passive process

• hydrostatic pressures force fluid & solute through membrane (glomerular filtration barrier)

Glomerular Capillaries in Kidney:

• More efficient filter than any other capillary beds in body

Glomerular Filtration Membrane:

• Large surface area & very permeable to water and solutes

Glomerlar capillary pressure is very ____

high (50 mmHg)

Filtration of plasma out glomerular capillaries (blood vessel) into Bowman’s capsule (tubule)…

defines Glomerular Filtration

Ions having the same concentration as plasma, no cells, no proteins…

defines Glomerular Filtrate

In Glomerular filtration, fluid passes through tubules, composition changes, & there’s a movement of substances…

into and out of peritubular capillaries & vasa recta capillaries

Proteinuria:

• Common finding during diabetes & hypertension kidney damage occurred, if untreated may progress to end stage renal disease & renal failure

Glomerular Filtration Rate (GFR):

• Volume of fluid filtered into Bowman’s Space per unit time

• Measured clinically to determine how efficiently kidneys filter wastes from blood

• Regulated by neural & hormonal input to afferent & efferent arteriole vascular smooth muscle cells

• 125 mL/min = 180 L/day

Plasma Volume (PV) =

3 L = filtered 60x/day

ECFV =

14 L = filtered 10x/day

Urine Flow Rate =

V = 2 L/day

Filtered Load of Substance A:

• Amount of Substance A present in Bowman’s Space per unit time (formula in slides)

Excretion Rate of Substance A:

• Amount of Substance A present in Urine per unit time

Afferent Arteriole & Efferent Arteriole contraction & relaxation is controlled by…

extrinsic + intrinsic factors (SNS, hormonal, local)

Afferent Arteriole & Efferent Arteriole contraction & relaxation…

• Alters renal blood flow

• Alters glomerular filtration rate

Decreased Glomerular Filtration Rate

= AA Vasoconstriction & EA vasodilation

Increased Glomerular Filtration Rate

= EA vasoconstriction & AA vasodilation

Renal Clearance is the clearances of…

certain substances used to measure GFR.

Renal Clearance: Inulin

• Freely filtered, not reabsorbed,

Renal Clearance: Creatinine

• Freely filtered, not reabsorbed, secreted in small amounts

All the Inulin that is filtered…

is excreted

Quantifying Renal Function:

Clearance = volume of plasma (ml) cleared of substance in 1 minute, mL/min

• Cl = UV/P

U =

Concentration of substance in urine, mg/mL

V =

Flow rate of urine formation mL/min

P =

Concentration of substance in plasma, mg/mL

Inulin =

Cl(IN) = GFR

Renal Clearance of Inulin = Filtration Rate

Creatinine =

Cl(Cr) ~ GFR

Renal Clearance Creatinine: Estimated GFR

Rate of Production = Rate of Excretion

• 1 g/day = 1 g/day

Renal Clearance of Creatinine Cl(Cr) is inversely related to…

P(Cr) plasma creatinine

As GFR decreases,

P(Cr) increases