PHYSIOLOGY OF THE URINARY SYSTEM

• Micturition - the emptying of the urinary bladder
• 3 factors of blood composition:

  • Diet
  • Cellular metabolism
  • Urine output
• Roles of kidneys in maintaining blood composition:

  • Excreting nitrogenous wastes
  • Maintaining water balance in blood
  • Maintaining electrolyte balance in blood
  • Ensuring proper pH
• Normal amount of water in the human body:

  • Babies: 75%
  • Young adult males: 60%
  • Young adult females: 50%
  • Elderly: 45%
• 3 main fluid compartments of water:

  • Intracellular fluid (ICF) - inside cells; 2/3 of body fluid; 40% body weight; about 25 L
  • Extracellular fluid (ECF) - outside cells; includes plasma (20% of ECF), interstitial fluid (80% of ECF), lymph, and transcellular fluid; 20% body weight
  • Plasma - is ECF, but accounts for 3 L of total body water; links internal and external environments
• Electrolytes - ions that conduct electrical current in an aqueous solution

  Examples: sodium, potassium, and calcium ions
• Water intake must equal water output to hydrate the body:

  Water input:

  • Beverages/Fluids - 60%
  • Foods - 30%
  • Metabolic processes - 10%

  Water output:

  • Urine - 60%
  • Insensible losses - 28%
  • Sweat - 8%
  • Feces - 4%
• Osmoreceptors - cells in the hypothalamus that become more active in reaction to small changes in plasma solute concentration; reinforces thirst mechanism
• Antidiuretic hormone (ADH) - prevents excessive water loss in urine and increases water reabsorption; targets collecting ducts
• Aldosterone - helps regulate blood composition and blood volume; acts on kidney
• Renin-angiotensin mechanism - most important trigger for aldosterone release; mediated by the juxtaglomerular (JG) apparatus of the renal tubules
• Renin - produces Angiotensin II

  Angiotensin II - causes vasoconstriction and aldosterone relase
• Acidosis - blood pH below 7.35
• Alkalosis - blood pH above 7.45
• Physiological acidosis - blood pH between 7.0 and 7.35
• Acid-base controlling systems:

  • Blood buffers
  • Respiration
• Blood buffers:

  • Acids - proton (H+) donors
  • Bases - proton (H+) acceptors
• 3 major chemical buffer systems:

  • Bicarbonate buffer system
  • Phosphate buffer system
  • Protein buffer system
• Bicarbonate buffer system - mixture of carbonic acid (weak acid) and sodium bicarbonate (weak base)
• Respiratory mechanisms for acid-base balance:

  • retains CO2 to decrease blood pH, removes CO2 to increase blood pH
• Urine pH varies from 4.5 to 8.0
• Control of voluntary urethral sphincter does not start until 18 months of age and complete nighttime control may not occur until 4 years of age.
• Escherichia coli - a bacterium that accounts for 80% of UTIs