Renal Function Tests: Anatomy and GFR

Renal Function Tests (I)

• Learning outcomes include familiarizing with kidney structure and function, and understanding the importance of glomerular filtration rate (GFR) and plasma creatinine as markers of kidney dysfunction.

Anatomy of Kidneys

• Kidneys contain a vascular component: afferent arterioles (blood to glomerulus), glomerulus (capillary tuft filtering blood), and efferent arterioles (blood away). Peritubular capillaries supply renal tissue.
• The tubular component begins with Bowman's capsule collecting filtrate. Proximal tubules reabsorb/secrete substances. Loops of Henle, in juxtamedullary nephrons, establish osmotic gradients. Distal tubules and collecting ducts have controlled reabsorption/secretion of sodium, water, potassium, and hydrogen ions, ultimately forming urine.
• The juxtaglomerular apparatus regulates kidney functions.
• Approximately 20% of plasma entering the glomerulus is filtered.

Structure of the Glomerulus

• Kidney functions include extracellular fluid volume regulation, H+ homeostasis, urea excretion, erythropoietin production, and vitamin D metabolism.
• Glomerulus components are the basement membrane, capillary lumen, and urinary space. They contain podocytes and endothelial cells. Mesangial cells are contractile and phagocytic.

Aldosterone

• Aldosterone, secreted from the adrenal cortex, acts on principal (P) cells in the distal tubule and collecting duct.
• It increases sodium reabsorption, modifies sodium channels, and promotes new channel and ATPase expression.

Vasopressin (ADH)

• Vasopressin (ADH), released from the posterior pituitary, acts on collecting duct cells.
• It promotes insertion of aquaporin-2 water channels into the luminal membrane.
• This increases water permeability and subsequent water reabsorption.

Glomerular Filtration Rate (GFR)

• GFR assesses glomerular function. Creatinine, a muscle metabolism byproduct, is used in GFR measurements.
• Plasma creatinine concentration is dependent on muscle mass. Lower values are seen in wasted individuals or children; pregnancy and vigorous exercise can slightly elevate levels.
• Units for GFR measurements are ml/min. Calculating GFR involves multiplying urine creatinine concentration by urine flow rate, then dividing by plasma creatinine concentration. Collection of urine, timed correctly, is essential for accurate GFR results. Incorrect urine collection is a major source of error.

GFR or Plasma Creatinine

• GFR changes significantly before detectable plasma creatinine change. GFR is more sensitive to early renal damage.
• Plasma creatinine measurements are more precise and easier than GFR measurements.
• Plasma creatinine measurements are adequate for monitoring the progression of renal disease and transplantation rejection.

Causes of Elevated Plasma Creatinine

• Impaired renal perfusion (due to decreased blood pressure, blood volume reduction, or narrowed renal artery).
• Loss of functioning nephrons (e.g., glomerulonephritis with antibody-mediated damage, deposition of immune complexes, increased pressure on the tubule, kidney stones, and prostatic enlargement).

Interpretation of Plasma Urea Measurements

• Normal plasma urea range is 2.5-6.6mmol/L.
• High protein intake or increased protein breakdown after trauma will increase plasma urea.
• Low protein intake, liver failure, and water retention will decrease plasma urea.
• Decreased renal perfusion, renal disease, and urinary outflow obstruction can elevate plasma urea.

Summary

• Kidneys are crucial for pH balance and toxin removal.
• Renal function is important for fluid balance.
• GFR and plasma creatinine are used to assess kidney function, but have limitations. Urine dipsticks can be part of initial/routine diagnostics.

This quiz focuses on the anatomy of the kidneys and the function of renal tests, including the significance of glomerular filtration rate (GFR) and plasma creatinine. It covers kidney structure, including its vascular and tubular components, as well as urine formation processes. Test your understanding of these crucial concepts in renal physiology.