First Aid Renal Physiology Part 1

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60-40-20 rule (% of body weight)

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Fluid Compartment Measurement: 

1) plasma volume = radiolabeled albumin

"Measurement of plasma volume requires a tracer which is mostly limited to this compartment and this is achieved by using a tracer which binds to albumin.The tracers used are the azo dye known as Evan’s blue (or T1824) which binds avidly to albumin, or radio-iodine labelled serum albumin (RISA)."

2) ECF = inulin

"Tracers used fall into 2 groups:

• Ionics (eg 82Br, ³⁵SO4, chloride isotopes)

• Crystalloids (eg Inulin, mannitol)"

3) Interstitial Fluid (no tracer) = ECF - Plasma Volume

4) Intracellular Fluid (no tracer) = Total body water - ECF

Is potassium higher intracellular or extracellularly? 

HIKIN': HIigh K INintracellular

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Glomerular Filtration Barrier (3 layers) 

1) fenestrated capillary endothelium

2) Fused glomerular basement membrane with heparan sulfate (negative charge barrier)

3) Epithelial layer w/ podocytes foot process and slit diaphragms

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What happens to the glomerular filtration barrier in nephrotic syndrome?

the charge barrier is lost --> albuminuria, hypoproteinemia, generalized edema, hyperlipidemia

Renal Clearance and Glomerular Filtration Rate: 

Renal Clearance (Cx)= UxVx

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Px

This is the volume of plasma from which the substance is completed cleared per unit time.

Cx = clearance of X in mL/min
Ux = urine concentration of X
Px = plasma concentration of X
V = urine flow rate

GFR does not equate excretion but only filtration. Therefore, if GFR is greater than clearance there is a net reabsorption of a substance.

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What do incremental reductions in GFR denote? stages of chronic kidney disease

Effective Renal Plasma Flow

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Effective renal plasma flow (ERPF) = U_pah x V/ Ppah = Cpah

Renal Blood Flow = RPF/ (1-HCT)

Determinants of Glomerular Filtration Rate (GFR)

Explain why a decrease in plasma protein concentration causes an increase in GFR?

The primary driving force for GFR is blood pressure opposed by fluid pressure in Bowman's capsule and colloid osmotic pressure due to plasma proteins. With fewer plasma proteins, the plasma has lower than normal colloid osmotic pressure. With less colloid osmotic pressure opposing GFR, GFR increases.

Calculation of reabsorption and secretion rate

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Filtered load = GFR X Px

Excretion rate = V X Ux

Reabsorption = filtered - excreted

Secretion = excreted - filtered

Glucose Clearance

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Filtration Fraction (FF) = GFR/RPF

Filtered Load = GFR x plasma concentration 

What is affected in renal artery stenosis?  the afferent artery; renal artery stenosis decreases GFR and FF.

Plasma protein concentration relationship with RPF, GFR, and FF.

 Plasma protein concentration does not affect renal plasma flow. RPF is only changed by afferent/efferent arteriole constriction/dilatation. However increased plasma protein increases plasma oncotic pressure, leading to a decreased filtration fraction. Thus GFR is decreased but RPF is constant. Therefore, there is a decrease overall Filtration Fraction (GFR/RPF).

Glucose Clearance

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SGLT2: High Capacity Low Affinity
SGLT1: Low Capacity High Affinity

Glucosuria: the excretion of glucose into the urine. Ordinarily, urine contains no glucose because the kidneys are able to reclaim all of the filtered glucose back into the bloodstream. 

~ 160 mg/dL of plasma glucose, glucosuria begins
350 mg/dL, transporters are saturated (Tm)

Clinical Relevance for Glucosuria: Diabetes Mellitus

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Amino Acid Clearance

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Sodium Dependent Transporters in PCT reabsorb amino acids.

Clinical Relevance: Hartnup's Disease = an autosomal recessive defect results from impaired transport of neutral amino acids across (i.e. tryptophan) epithelial cells in renal proximal tubules and intestinal mucosa. Symptoms include transient manifestations of pellagra (rashes), cerebellar ataxia and psychosis.

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