Body Systems: Urine Formation

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