The Kidneys

Question 1

Describe the process that occurs in the Loop of Henle.

Answer 1

1. The Na ions in the filtrate move to the medulla by active transport (low-high) in the ascending limb. Water molecules stay in the filtrate because its only permeable to the Na ions. This increases the ion concentration in the medulla and lowers the water potential in the medulla.
2. In the descending limb there is a higher water concentration than in the medulla. So water molecules move from the filtrate into the medulla by osmosis (high-low), this increases the ion concentration in the filtrate. The water in the medulla gets reabsorbed back into the capillaries to the blood.
3. On the top of the ascending limb there’s a higher concentration of ions than in the medulla. So Na ions move from the filtrate into the medulla by diffusion (high-low). Water molecules stay in the filtrate because the ascending limb is not permeable to water.
   This again increases the ion concentration in the medulla and decreases the water potential.
4. In the DCT, water molecules move to the medulla back into the blood by osmosis (high-low).
5. Because steps 1-3 increase the ion concentration in the medulla, that means it lowers the water concentration. In the collecting duct and DCT, water gets reabsorbed back into the blood by osmosis.

Question 2

Which Hormone regulates water potential in the blood?

Answer 2

ADH (antidiuretic hormone)

Question 3

Where are most substances reabsorbed in the kidney?

Answer 3

The PCT (Proximal convoluted tube)