Occurs in Three Stages:
- Glomerular Filtration – Movement of fluids from the blood INTO the Bowman’s 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
