• 29th June
  • 29

Comprehensive Nephron/Kidney Review for MCAT

Excretory System

·      Roles:

  • o   Remove Wastes
  • o   Balance body Water content
  • o   Balance body Ionic content
  •   §  want to take up all ions, kidney must get rid of excess or uptake ions if we don’t get enough
  • o   Balance Acid-Base balance  (respiratory adjust gases)
  • ·      Kidney also Regulates RBC’s and secretes Renin as part of regulation of blood pressure

·      Under normal hydration: 20% of Plasma is filtered (rest (80% of plasma à Efferent Arteriole) is still filtered in future rounds) @ the Glomerulus Capsule 

Materials Filtered into Bowman’s capsule**

  • ·      Water
  • ·      Ions (Na+, Cl-, K+, ect.)
  • ·      Glucose, Amino Acids
  • ·      Wastes (NH3, urea, etc.)
  •    o   Ammonia, Urea, Uric acid
  • ·      A few Plasma Proteins
  •    o  Smallest one – Albumin
  •    o   most are too big to get through pore unless pores have been damaged by High BP

·      Tubular Reabsorption involves transport of molecules in filtrate back into the blood. (Energetically Expensive)

  1. 1. Passive Diffusion
  2. 2.   Active Transport
  3. 3. Pinocytosis ( Reabsorption of proteins)

Cross epithelial —> get through intestinal stage —> plasma

Tubular Reabsorption

  • ·      Glucose (co-transport with Na+; active)
  •     o   Also seen in Small Intestine
  • ·      Amino Acids (co-transport with Na+; active)
  •     o   Also seen in small intestine
  • ·      Na+ (Active—about 67% reabsorbed in proximal tubule)
  • ·      All other Positive ions (Ca2+, K+, etc) Active
  • ·      Some Negative ions (sulfate, phosphate)Active
  • ·      Cl- Passive
  • ·      Water-passive (by osmosis) following movement of other molecules
  •     o   take up 2/3 of water in filtrate
  • ·      Proteins = Pinocytosis
  •     o   wall of membrane of tubules in-fold on protein and break it down and reabsorb amino acids
  • ·      Wastes = some urea diffuses back into blood
  •    o   half of this is filtered back into blood —> okay because blood is continually circulating

At the End of the Proximal Tubule:

·      All glucose, amino acids, Many ions except some of the Na+, Cl-, Almost all protein, 65% of water, 50% urea —> Have been reabsorbed back into blood

·      Remaining in Filtrate:   about 35% of water, wastes, the rest of Na and Cl, excesses of any ions, toxins, bilirubin & drugs have no reabsorption transporters

Loop of Henle (Medulla)

  • ·      Sole purpose is to Conserve Water
  •      o   allows us to concentrate our urine in an environment not dependent on water
  • ·      Depends on an extracellular Gradient of Na and Cl concentration
  •     o   gradient gives us the ability to absorb more water than we could if we didn’t have loop of henle

Descending Loop – Permeable to Water

Ascending Loop- Not Permeable to Water

  • ·      Taking solute molecules from a fixed volume of solvent
  • ·      water volume does Not change
  • ·      absorb NaCl back into blood
  • ·      When we reach top – we have same water volume but low osmotic pressure because we have taken out a lot of NaCl

Distal tubule

  • ·      Active secretion of K+ if necessary
  • ·      Active secretion of H+ if necessary
  • ·      CO2 + H20 —> H2CO3(carbonic anhydrase) —> H+ + HCO3-
  • ·      Where kidney takes part in pH regulation
  •     o   Carbonic anhydrase – blood and stomach also
  •     o   CO2 too high in blood we actively transport H+ out of blood to increase pH
  • ·      Have another shot to get rid of potassium if we need to
  •     o   We filtered some K+ in filtrate, had some carriers to actively transport K+, here we have the option to getting rid of even more K+ from blood to the urine

Collecting Duct 

·      Going down the Collecting tubule- last place to regulate things if we have too much or too little of anything

    o   Critical to maintain those homeostatic regulated variables

·      Collecting duct is going through same osmotic pressure gradient as loop of henle

·      If Vasopressin is Present: walls of the collecting tubule are more permeable to water

·      If No vasopressin —> no more water absorbed

·      System is Variable! Hypothalamus determine how much vasopressin the Posterior Pituitary releases

Effect of Alcohol on Vasopressin Secretion

·      Causes hypothalamus to shutdown vasopressin secretion

·      start producing large volume of urine that is Not regulated – causing dehydration – excrete more volume of urine than volume of liquid taken in


·      Renin-angiotensin system helps Vasopressin CONSERVE Water, if necessary, for Regulation of Body Water and Blood Pressure

Angiotensin II also causes: ****

·      INCREASE Aldosterone Release from Adrenal Gland —>
INCREASE Na+ Uptake from urine  —> INCREASE Water Uptake from urine —>

·      INCREASE Blood Volume  —> INCREASE Venous Return  —> INCREASE Stroke Volume —> INCREASE Cardiac Output

    o   Net Effect of INCREASE Angiotensin II  —> INCREASE BP

·      Effect on Collecting Duct, if vasopressin is present will also increase water absorption (with NaCl)

·      Need to maintain Blood Volume to maintain BP

·      Aldosterone cause more water absorption than just vasopressin alone

·      If No vasopressin water present (No Water absorption) But Aldosterone present = only Regulation of Sodium absorption

·      If Vasopressin present and Aldosterone present Regulation of water and sodium uptake!

Additional Na+ Regulation

·      DECREASE Blood Na+ —> Adrenal Cortex Gland(sensor and Integrator: Mineral-corticoid —> INCREASE Aldosterone secretion —>

INCREASE Active Uptake of Na+ from filtrate in collecting duct —>


·      Just for Sodium content regulation (separate from water regu

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    ich nix kapito.
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    This is literally the only thing I enjoyed learning about in physiology. I don’t know why.
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