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