Renal Physiology - Measurement of Renal Function

Measurement of Renal Function

A simple means of estimating renal function is to measure pH, blood urea nitrogen, creatinine, and basic electrolytes (including sodium, potassium, chloride, and bicarbonate). As the kidney is the most important organ in controlling these values, any derangement in these values could suggest renal impairment.

There are several more formal tests and ratios involved in estimating renal function:

Measurement Calculation Details
renal plasma flow Volume of blood plasma delivered to the kidney per unit time. PAH clearance is a renal analysis method used to provide an estimate. Approximately 625 ml/min.
renal blood flow (HCT is hematocrit) Volume of blood delivered to the kidney per unit time. In humans, the kidneys together receive roughly 20% of cardiac output, amounting to 1 L/min in a 70-kg adult male.
glomerular filtration rate (estimation using creatinine clearance) Volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time. Estimated using inulin. Usually a creatinine clearance test is performed but other markers, such as the plant polysaccharide inulin or radiolabelled EDTA, may be used as well.
filtration fraction Measures portion of renal plasma that is filtered.
anion gap AG = − ( + ) Cations minus anions. Excludes K+ (usually), Ca2+, H2PO4−. Aids in the differential diagnosis of metabolic acidosis
Clearance (other than water) where U = concentration, V =urine volume / time, U*V = urinary excretion, and P = plasma concentration Rate of removal
free water clearance or The volume of blood plasma that is cleared of solute-free water per unit time.
Net acid excretion Net amount of acid excreted in the urine per unit time
Urinary system physiology: renal physiology and acid-base physiology
Filtration
  • Renal blood flow
  • Ultrafiltration
  • Countercurrent exchange
  • Filtration fraction
Hormones affecting filtration
  • Antidiuretic hormone (ADH)
  • Aldosterone
  • Atrial natriuretic peptide
Secretion/clearance
  • Pharmacokinetics
  • Clearance of medications
  • Urine flow rate
Reabsorption
  • Solvent drag
  • Na+
  • Cl-
  • urea
  • glucose
  • oligopeptides
  • protein
Endocrine
  • Renin
  • Erythropoietin (EPO)
  • Calcitriol (Active vitamin D)
  • Prostaglandins
Assessing Renal function/
Measures of dialysis
  • Glomerular filtration rate
  • Creatinine clearance
  • Renal clearance ratio
  • Urea reduction ratio
  • Kt/V
  • Standardized Kt/V
  • Hemodialysis product
  • PAH clearance (Effective renal plasma flow
  • Extraction ratio)
Acid-base physiology
  • Fluid balance
  • Darrow Yannet diagram

Body water: Intracellular fluid/Cytosol

  • Extracellular fluid
  • (Interstitial fluid
  • Plasma
  • Transcellular fluid)
  • Base excess
  • Davenport diagram
  • Anion gap
  • Arterial blood gas
  • Winter's formula
Buffering/compensation
  • Bicarbonate buffering system
  • Respiratory compensation
  • Renal compensation
Other
  • Fractional sodium excretion
  • BUN-to-creatinine ratio
  • Tubuloglomerular feedback
  • Natriuresis
  • Urine

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