Aldosterone: Increases sodium reabsorption, leading indirectly to water reabsorption and increased blood pressure.
Anti-diuretic Hormone (ADH): Pulls more water from the filtrate, increases blood pressure and decreases blood osmolality.
Atrial Naturiuretic Peptide (ANP): Made in the heart, decreases blood pressure by inhibiting sodium reabsorption and ADH production, leading to increased urinary volume.
Mechanism for Urine Concentration
Constant filtrate formation (~180L/day).
Reabsorption of essential elements before final urine formation.
Urine volume regulation via hormone-mediated adjustments (
Aldosterone, ADH etc.).
Renin-Angiotensin-Aldosterone System (RAAS)
Trigger: Low blood pressure.
Mechanism: Renin release -> Angiotensinogen (from liver) -> Angiotensin I -> ACE enzyme (primarily in lung endothelium) -> Angiotensin II -> Aldosterone & ADH secretion, thirst stimulation.
Effects: Vasoconstriction, sodium reabsorption, increased blood pressure and volume.
Aldosterone
Secreted due to Angiotensin II or high potassium levels.
Enhances sodium reabsorption in the distal convoluted tubule and collecting ducts, increasing blood volume and pressure.
ADH (Anti-diuretic Hormone)
Made in hypothalamus, stored and released by posterior pituitary.
Triggered by increased blood osmolality or low blood pressure.
Increases water reabsorption in the collecting ducts via aquaporins.
Can lead to decreased urine output and more concentrated urine.
Deficiency leads to Diabetes Insipidus (large urine volume, low glucose).
ANP (Atrial Natriuretic Peptide)
Secreted by the heart’s atria in response to high blood volume and pressure.
Inhibits sodium reabsorption and ADH secretion, promoting fluid loss in urine, reducing blood volume and pressure.
Physiology of Micturition
Ureters: Transport urine from kidneys to bladder.
Bladder: Stores urine, can hold ~1L. Lined with transitional epithelium and smooth muscle (detrusor muscle).
Urethra: Male urethra longer, also part of reproductive system; female urethra shorter.
Sphincters: Internal (elastic tissue in males to prevent semen from entering bladder during ejaculation), external (skeletal muscle, voluntary control).
Urinary Flow Mechanics
Hydrostatic pressure of 10mm Hg forces fluid through nephron.
Peristalsis moves urine down ureters.
Bladder compresses ureters, preventing backflow.
Micturition reflex: Stretch receptors in bladder detected stretch -> Parasympathetic signals contract bladder muscles -> Relaxation of external sphincter.
Conditions and Diseases
Aging Effects: Kidneys decrease in size and nephrons; reduced blood flow, filtration, reabsorption, and secretion efficiency; reduced responsiveness to ADH and aldosterone.
Glomerulonephritis: Inflammation of the glomerulus, can be acute (e.g. post-strep infection) or chronic; may lead to renal failure.
Pyelonephritis: Bacterial infection (often E. coli) of renal pelvis, affects nephrons.
Renal Failure: Acute or chronic, leads to waste accumulation in blood. Monitored by Glomerular Filtration Rate (GFR).
Testing Kidney Function
Plasma Clearance: How much of a substance is removed from plasma per minute.
Glomerular Filtration Rate (GFR): Indicator of kidney health; creatinine or inulin used for measurement.
Renal Plasma Flow: Use of substances like para-aminohippuric acid (PAH) to examine secretion function.
Renal Threshold: Maximum reabsorption capacity; glucose used as general example (diabetes mellitus).