Urinary System

• Functions of the Kidneys

  • Excreting metabolic wastes, toxins, drugs
  • Regulating total water volume and total solute concentration in water
  • Regulating ion concentrations in extracellular fluid (ECF)
  • Ensuring long-term acid-base balance
  • Producing erythropoietin (regulates RBC production) and renin (regulates blood pressure)
  • Activating vitamin D
  • Carrying out gluconeogenesis, if needed
• Nephrons

  Functional units of kidney, each contains ~1 million, various parts
• Renal Corpuscle

  • Glomerulus: highly porous capillaries, forms filtrate
  • Glomerular (Bowmans) capsule: surrounds glomerulus allows filtrate to enter
• Renal tubule

  • Proximal convoluted tubule: closest to corpuscle
  • Nephron loop (loop of henle): descends into and out of the medulla
  • Distal convoluted tubule: furthers from corpuscle
• Collecting duct

  Distal convoluted tubule drains into
• 180 L of fluid processed daily, but only 1.5 L of urine is formed
• Kidneys filter body's entire plasma volume 60 times each day
• Kidneys consume 20–25% of oxygen used by body at rest
• Filtrate

  Produced by glomerular filtration, basically blood plasma minus proteins
• Urine

  Produced from filtrate
• Physiology of Kidney

  1. Glomerular filtration: produces filtrate

  2. Tubular reabsorption: selectively returns 99% of substances from filtrate to blood

  3. Tubular secretion: selectively moves substances from blood to filtrate
• Glomerular Filtration

  • Passive process, requires no metabolic energy
  • Hydrostatic pressure forces fluids and solutes through filtration membrane
  • Allows water and solutes smaller than plasma proteins to pass (normally no cells pass)
  • No reabsorption into capillaries of glomerulus occurs
• Hydrostatic pressure in glomerular capillaries (HPgc)


  Essentially glomerular blood pressure, chief force pushing water, solutes out of blood, quite high: 55 mm Hg

  
• Hydrostatic pressure in capsular space (HPcs)


  Filtrate pressure in capsule, 15 mm Hg
• Colloid osmotic pressure in capillaries (OPgc)


  "Pull" of proteins in blood, 30 mm Hg
• Net filtration pressure (NFP)


  Sum of forces
• Glomerular Filtration Rate (GFR)


  Volume of filtrate formed per minute by both kidneys (normal = 120–125 ml/min)
• GFR


  Affects systemic blood pressure
• Intrinsic controls (renal autoregulation)


  • Maintain GFR in kidney
• Extrinsic controls


  • Maintain systemic blood pressure, nervous system and endocrine mechanisms are main extrinsic controls
• Myogenic mechanism


  1. Decreased BP detected through decreased stretch in afferent arterioles, leads to vasodilation to increase filtration

  2. Increased BP detected through increased stretch, leads to constriction of afferent arterioles, restricts blood flow into glomerulus, protects from damage
• Tubuloglomerular feedback mechanism


  1. Respond to filtrate's NaCl concentration

  2. If GFR decreases, filtrate flow rate decreases, increased reabsorption time, causing lower NaCl levels in filtrate, change in NaCl concentration detected, feedback mechanism causes vasodilation, increased GFR

  3. Opposite mechanism for increased GFR
• Sympathetic nervous system


  Under abnormal conditions, such as extremely low blood pressure, epinephrine and norepinephrine released, causing systemic vasoconstriction, which increases blood pressure, constriction of afferent arterioles, which decreases GFR
• Tubular reabsorption


  Quickly reclaims most of tubular contents and returns them to blood

  Includes active and passive tubular reabsorption
• Transcellular route


  • Solute enters apical membrane of tubule cells, travels through cytosol of tubule cells, exits basolateral membrane of tubule cells, enters blood through endothelium of peritubular capillaries
• Paracellular route


  • Between tubule cells, limited by tight junctions, but leaky in proximal nephron, water, Ca2+, Mg2+, K+, and some Na+ in the PCT move via this route
• Na+ reabsorption


  Involves active transport out of tubule cells, Na+-K+ ATPase pumps Na+ into interstitial space, Na+ is then swept by bulk flow into peritubular capillaries, provides energy and means for reabsorbing almost every other substance
• Electrochemical gradient created by Na+ pumps


  Gives "push" needed for transport of other solutes, organic nutrients reabsorbed by secondary active transport are cotransported with Na+, movement of Na+ and other solutes creates osmotic gradient for water and is transported through osmosis by aquaporins
• Proximal convoluted tubule


  • Site of most reabsorption, all nutrients reabsorbed, 65% of Na+ and water reabsorbed, many ions, about half of urea (later secreted back into filtrate)

  
• Nephron loop


  • Descending limb: H2O can leave, solutes cannot, Ascending limb: H2O cannot leave, solutes can
• Distal convoluted tubule and collecting duct


  • Hormonally regulated in these areas, Antidiuretic hormone (ADH) causes principal cells of collecting ducts to insert aquaporins in membranes, increasing water reabsorption, Aldosterone increases Na+ reabsorption, K+ secretion
• Tubular secretion


  Reabsorption in reverse, most occurs in PCT, substances moved from peritublar capillaries through tubule cells out into filtrate (K+, H+, NH4+, creatinine, organic acids and bases, substances synthesized in tubule cells also are secreted)
• Descending limb


  • H2O can leave, solutes cannot
• Ascending limb


  • H2O cannot leave, solutes can
• Reabsorptive Capabilities of Renal Tubules and Collecting Ducts

  • Distal convoluted tubule and collecting duct
  • Hormonally regulated in these areas
• Antidiuretic hormone (ADH)


  • Causes principal cells of collecting ducts to insert aquaporins in membranes, increasing water reabsorption
  • Increased ADH levels cause an increase in water reabsorption
  • Also increases urea reabsorption
• Aldosterone


  • Increases in Na+ reabsorption, K+ secretion
  • As a result, little Na+ leaves body
  • Cl- follows Na+
  • Functions: increase blood pressure and decrease K+ levels

  
• Tubular secretion


  1. Reabsorption in reverse

  2. Most occurs in PCT

  3. Substances moved from peritublar capillaries through tubule cells out into filtrate

  4. K+, H+, NH4+, creatinine, organic acids and bases

  5. Substances synthesized in tubule cells also are secreted (example: HCO3–)
• Tubular secretion


  • Important for disposing of substances, such as drugs or metabolites
  • Eliminating undesirable substances that were passively reabsorbed (example: urea)
  • Ridding body of excess K+ (aldosterone effect)
  • Controlling blood pH by altering amounts of H+ or HCO3– in urine

  
• Role of Kidneys in Acid-Base Balance

  1. Lungs eliminate carbonic acid by eliminating CO2

  2. Kidneys regulate acid-base balance by adjusting amount of bicarbonate and H+

  3. Renal regulation of acid-base balance depends on kidney's ability to secrete or retain H+

  4. To reabsorb bicarbonate, kidney must secrete H+

  5. To excrete excess bicarbonate, kidney must retain (not secrete) H+