In-Depth Notes on the Urinary System

Major Functions of the Urinary System

  • Filtration of Blood:

    • Daily filter of approximately 200 liters of blood to remove toxins, metabolic waste, and excess ions.

  • Regulation of Blood Composition:

    • Maintains balance of water, salts, acids, and bases.

  • Gluconeogenesis:

    • Liver (60%) and kidneys (40%) contribute during fasting.

  • Hormonal Production:

    • Production of renin (regulates blood pressure) and erythropoietin (stimulates red blood cell formation).

  • Activation of Vitamin D:

    • Converts inactive Vitamin D to its active form.

Organs of the Urinary System

  • Kidneys:

    • Primary organs involved in filtering blood and producing urine.

  • Ureters:

    • Tubes that transport urine from kidneys to bladder.

  • Bladder:

    • Stores urine until it is excreted.

  • Urethra:

    • Conducts urine out of the body.

Composition Comparison

  • Blood Plasma vs. Filtrate vs. Urine:

    • Blood plasma contains cells and larger proteins.

    • Filtrate is blood plasma without blood cells and proteins; mostly water and small solutes.

    • Urine consists of waste products, certain ions, and water.

Nephron Structure

  • Components:

    • Glomerulus, proximal convoluted tubule (PCT), loop of Henle (descending and ascending limbs), distal convoluted tubule (DCT), and collecting ducts.

  • Functionality:

    • Glomerulus: Site of filtration.

    • PCT: Reabsorption of glucose, amino acids, and bicarbonate.

    • Loop of Henle: Water reabsorption (descending) and sodium chloride reabsorption (ascending).

    • DCT: Further NaCl reabsorption.

    • Collecting Ducts: Final adjustments before urine formation.

Urine Formation Processes

  1. Filtration:

    • Occurs in the glomerulus; BP forces water and small solutes into Bowman’s capsule.

  2. Reabsorption:

    • Essential nutrients and water reabsorbed back into blood, primarily in PCT and loop of Henle.

  3. Secretion:

    • Additional waste ions and H+ ions secreted from blood into renal tubules.

Glomerular Filtration Rate (GFR)

  • Definition:

    • Volume of filtrate produced per minute (normal ~125 mL/min).

  • Factors Affecting GFR:

    • Surface area available for filtration, permeability of the filtration membrane, glomerular blood pressure, and net filtration pressure.

  • Mechanisms Controlling GFR:

    • Neural Control:

    • Sympathetic nervous system can decrease GFR by constricting afferent arterioles.

    • Hormonal Control:

    • Renin-angiotensin system regulates blood pressure and GFR.

    • Intrinsic Regulation:

    • Autoregulation keeps GFR relatively constant despite BP fluctuations.

Urine Composition

  • Normal Urine:

    • Mostly water (95%) with urea, creatinine, uric acid, electrolytes, and small proteins.

  • Abnormal Constituents:

    • Presence of blood, glucose, proteins, or ketones may indicate disease.

Aging Effects on the Urinary System

  • Decreased Size and Function:

    • Kidney size and function decline with age.

  • Reduced Blood Flow:

    • Gradual decrease in renal blood flow and glomeruli number.

  • Impaired Urine Concentration:

    • Decreased responsiveness to ADH and aldosterone may lead to urine concentration issues.

Developmental Aspects of the Urinary System

  • Formation:

    • Arises from urogenital ridges in the intermediate mesoderm during the 4th week of pregnancy.

  • Kidney Development:

    • Initial three sets of kidneys form; typically, only the metanephros persists.

  • Childhood to Elderly Changes:

    • Control of micturition matures by age 2; aging brings decline in kidney function and increased urinary incontinence.