Kidney Physiology Functions Study Notes

Module Overview

  • Focus on abbreviations related to renal disease.
  • Importance of understanding normal kidney function for comprehension of kidney disease alterations.

Normal Kidney Physiology and Functions

Kidney Anatomy

  • The kidneys contain a complex capillary network and an array of tubules.
  • Tubules perform regulatory and metabolic functions.
  • Familiarity with anatomy from previous coursework is beneficial.

Nephrons

  • Over a million nephrons exist in each kidney.
  • Nephrons are the functional units of the kidney, composed of:
    • Glomerulus: A mass of capillaries that filters blood and produces ultrafiltrate.
    • Located between two arterioles (afferent and efferent).
    • Provides a barrier against the loss of proteins and red blood cells during filtration.
  • The process: Blood enters the glomerulus; ultrafiltrate is produced, which ultimately is processed into urine.

Ultrafiltrate Composition

  • The composition of ultrafiltrate is similar to that of blood.
  • Daily ultrafiltrate production is significant but primarily reabsorbed back into circulation:
    • Approximately 65% of filtered sodium and water are reabsorbed via active transport in the proximal tubule.
    • Controlled by aldosterone and vasopressin:
    • Aldosterone: Directs sodium reabsorption.
    • Vasopressin: Controls water reabsorption and urine concentration based on physiological needs (fluid balance, plasma volume, blood pressure).

Kidney Functions

Excretory Functions

  • The kidneys excrete waste products including:
    • Urea: Byproduct of protein metabolism.
    • Excess water-soluble vitamins and minerals.
    • Metabolites of medications and poisons.
  • Overall ultrafiltrate production is about 200 liters, while actual urine excretion is only 1-2 liters per day.

Blood Pressure Regulation

  • The kidneys maintain blood pressure through the Renin-Angiotensin System:
    1. Low blood pressure (hypotension) triggers the glomerulus to secrete renin.
    2. Renin converts angiotensin I to angiotensin II.
    3. Angiotensin II functions:
    • Vasoconstriction: Increases blood pressure.
    • Stimulates aldosterone and vasopressin: Increases sodium and fluid retention, which raises blood volume and pressure.

Red Blood Cell Production

  • Kidneys produce erythropoietin, stimulating red blood cell production in the bone marrow.
  • Feedback loop: Enhanced circulation of erythrocytes signals the kidneys to reduce erythropoietin production.

Bone Health Maintenance

  • The kidneys assist in maintaining calcium and phosphorus homeostasis by producing 125 dihydroxyvitamin D3, enhancing calcium absorption and eliminating excess minerals in urine.

Acid-Base Balance

  • There is maintenance of blood pH (7.35 - 7.45) through a buffering system:
    • Bicarbonate carries hydrogen ions, either excreted in the tubules or returned to blood.
  • Kidneys regulate phosphate levels, which help buffer intracellular fluid.

Stress Response and Blood Pressure Maintenance

  • In emergencies (e.g., hemorrhage), kidneys prioritize blood pressure maintenance:
    • Aldosterone and vasopressin are released to retain sodium and water, increasing blood pressure for tissue perfusion.

Lab Values for Assessing Kidney Function

Urinalysis

  • Urine sample evaluated for:
    • Microalbuminuria: Presence of albumin indicates glomerular damage and potential renal insufficiency.

Serum Lab Values

  • Key lab values include:
    • Glomerular Filtration Rate (GFR): Calculated from creatinine, BUN, age, and race. Commonly drawn in metabolic panels.
    • Cystatin C: Protein consistently filtered by kidneys; abnormal levels indicate renal dysfunction.

Signs and Symptoms of Renal Insufficiency

Alterations in Kidney Functions and Associated Symptoms

  • Reduced sodium regulation => sodium retention => hypertension and edema.
  • Impaired hydrogen ion secretion => metabolic acidosis.
  • Anemia due to inadequate erythropoietin.
  • Inability to excrete waste => nitrogenous waste buildup (uremia/azotemia).
  • Inability to excrete potassium => hyperkalemia.
  • Secondary hyperparathyroidism due to altered calcium and phosphorus regulation.
  • Prolonged renal disease implications => renal osteodystrophy due to poor calcium and phosphorus regulation.

Conclusion

  • Understanding kidney physiology clarifies the pathophysiology in renal disease and correlates symptoms with underlying dysfunction. This knowledge is crucial for developing effective treatment strategies for patient care.