Which part of the loop of Henle actively transports sodium ions out into the tissue fluid of the renal medulla and is impermeable to water?

The part of the loop of Henle that actively transports sodium ions out into the tissue fluid of the renal medulla and is impermeable to water is the ascending limb. Here are the steps explaining this process:

1. The loop of Henle is a part of nephron in the kidney, which is responsible for the concentration of urine. It has two limbs - the descending limb and the ascending limb.

2. The descending limb is permeable to water but not to ions. As the filtrate moves down this limb, water is drawn out into the surrounding tissue by osmosis, concentrating the filtrate.

3. The ascending limb is impermeable to water but actively transports ions, including sodium ions, out into the tissue fluid of the renal medulla. This is done through active transport, which requires energy.

4. The active transport of sodium ions out of the ascending limb creates a high concentration of ions in the tissue fluid of the renal medulla. This high concentration of ions in the medulla is crucial for the reabsorption of water from the filtrate in the collecting duct, which follows the loop of Henle in the nephron.

5. Therefore, the ascending limb of the loop of Henle is the part that actively transports sodium ions out into the tissue fluid of the renal medulla and is impermeable to water.