Kidney Functions and Hormones Overview

Kidney Functions

• Kidneys regulate water and electrolytes.
• They maintain acid-base balance.
• They remove waste products from protein and nucleic acid metabolism (e.g., urea, creatinine, uric acid).
• They produce hormones.

Kidney Control by Hormones

• Arginine vasopressin (AVP, also known as ADH) influences water balance.
• Aldosterone promotes sodium reabsorption in the nephron.
• Parathyroid hormone promotes tubular reabsorption of calcium and phosphate.
• Excretion and synthesis of 1,25-dihydroxycholecalciferol (active vitamin D) occur.
• Renin, produced by juxtaglomerular cells, catalyzes the formation of angiotensin I and ultimately aldosterone synthesis.

Renal Function Tests

• Blood urea and blood urea nitrogen (BUN)

• Serum creatinine

• Glomerular filtration rate (GFR)

• Substances with nitrogen are classified as protein nitrogen and non-protein nitrogen (NPN). Urea accounts for 75% of NPN.

• Urea is the major nitrogenous waste product of protein catabolism.

• More than 90% of urea is excreted through the kidneys.

Uremia and Azotemia

• Uremia: high urea nitrogen in blood
• Azotemia: significant increase in plasma urea and creatinine in kidney insufficiency.
• Pre-renal uremia is caused by extra-renal factors like high protein diets, increased protein catabolism, dehydration, and heart failure.
• Renal uremia is caused by diminished glomerular filtration, as in acute or chronic kidney diseases like glomerulonephritis and polycystic kidney disease.
• Post-renal uremia is usually due to obstruction of urine flow (e.g., malignancies, stones, prostate enlargement).

Serum Creatinine

• Creatinine is a breakdown product of creatine phosphate.
• It is produced in the kidneys, liver, and pancreas.
• It's transported by blood to muscle and brain where it's phosphorylated into phosphocreatine.
• Muscle creatine phosphate is used as an immediate energy source in muscle contraction.
• Creatinine levels are related to muscle mass.
• Creatinine is freely filtered by glomeruli, with a small percentage secreted by renal tubules.
• Creatinine excretion is relatively constant in the urine.
• In renal diseases, creatinine excretion is impaired and reflected by increased creatinine in the blood.
• Serum creatinine levels are higher in males than in females.
• High-protein diets and catabolic states affect plasma creatinine concentration (less than that of urea).

Glomerular Filtration Rate (GFR)

• GFR is the amount of glomerular filtrate formed per unit of time in all nephrons of both kidneys.
• Inulin, cystatin C, and creatinine are used to measure GFR.
• GFR measurement aids in detecting renal insufficiency.
• GFR adjustment helps in calculating medication dosages.
  • GFR = (140 - age in years) * weight (kg) * 0.85 (if female) / serum creatinine (mg/dL) * 72.

Uric Acid

• Nucleic acids contain purines (adenine and guanine) and pyrimidines.
• Purine breakdown yields uric acid.
• Uric acid is mainly present in plasma as sodium urate.
• Hyperuricemia is abnormally high uric acid in the blood.
• Gout is a medical condition caused by uric acid precipitation in aqueous solutions (e.g., synovial fluid).
• Uric acid is formed through de novo synthesis, endogenous DNA/RNA metabolism, and dietary nucleic acid breakdown.
• Most (70%) uric acid is excreted by the kidneys.
• Other (30%) uric acid is excreted in the gut, broken down by bacteria.
• Increased production or decreased excretion of uric acid can lead to gout.

Proteinuria

• Proteinuria is the presence of excess protein in the urine.
• Glomerular proteinuria arises from glomerular damage.
• Overflow proteinuria occurs if plasma concentrations of low molecular weight proteins increase.
• Tubular proteinuria arises from decreased tubular capacity to reabsorb proteins.
• Creatinine/protein ratio help assess if proteinuria is benign or pathological for any glomerular disease.
• Microalbuminuria is an early indicator of nephropathy.