Occurs in Three Stages:

  • Glomerular Filtration – Movement of fluids from the blood INTO the Bowmans capsule
  • Reabsorption – Transfer of essential solutes and water from the nephron tubules BACK into the blood
  • Secretion – Movement of materials from the blood into the nephron

Filtration

  • Blood moves from the:
  • AORTA to the RENAL ARTERY to the AFFERENT ARTERIOLE and into the GLOMERULUS
  • The glomerulus has a capillary bed that is highly pressurized
  • Dissolved solutes move from areas of high pressure to areas of low pressure through a selectively permeable membrane.
  • Therefore, dissolved solutes pass through the walls of the glomerulus into the Bowman’s capsule

Solute Filtration: Reabsorption

  • Every minute, 120 mL of dissolved substances are filtered out of the blood into the nephron tubules
  • Cannot lose this much water and maintain homeostasis
  • Fortunately, about 119 mL is reabsorbed
  • Depending on the substances transported, reabsorption may be active or passive.
  • Reabsorption occurs until the threshold level of a substance is reached
  • Glucose is entirely reabsorbed (through active transport)
  • However, if way too much is present in the blood, some may end up in the urine
  • Sodium ions are partially reabsorbed (most through active transport)
  • Depends on how much is present in the system

  • Na+ is actively transported out of the nephron, causing negative ions (Cl & HCO3) to follow by charge attraction.
  • Glucose and amino acids are actively transported out of the proximal tubule.
  • As the solutes are removed from the urine, an osmotic gradient is formed
  • Water leaves passively through osmosis as it flows through the descending loop of Henle
  • If you lost all of the water that has been removed by the nephron at this point then you would need to consume 1 litre of water every 10 minutes to replace it
  • The protein which remained in the blood at the glomerulus draws the water into the capillaries from the interstitial fluid
  • As water is reabsorbed, the remaining solutes become more concentrated

Secretion

  • Secretion is the process where waste substances move into the distal and collecting tubules from the blood
  • Substances typically move through active transport
  • Nitrogen containing waste and excess H+ and K+ are some of the substances secreted
  • The filtrate is now officially called URINE

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