Descending limb of Henle's loop, hypertonic urine, reabsorption of water, ascending limb of Henle's loop

Answer

### Step-by-Step Solution

1. **Understanding Henle's Loop**:
- Henle's loop is a U-shaped portion of the nephron tubule found in the kidneys of reptiles, birds, and mammals. It plays a crucial role in the concentration of urine.

2. **Descending Limb of Henle's Loop**:
- The descending limb is permeable to water but impermeable to electrolytes (such as sodium and chloride ions). As the filtrate moves down this limb, water is reabsorbed into the surrounding medullary fluid, concentrating the filtrate.

3. **Ascending Limb of Henle's Loop**:
- The ascending limb is impermeable to water but allows the active or passive transport of electrolytes. As the concentrated filtrate ascends, electrolytes are transported out into the medullary fluid, leading to dilution of the filtrate.

4. **Hypertonic Urine**:
- Hypertonic urine is produced when water moves out from the urine, resulting in a concentrated urine solution. This occurs primarily due to the actions in the descending limb and the reabsorption processes in the nephron. Mammals produce hypertonic urine to conserve water.

5. **Reabsorption of Water**:
- Reabsorption of water, also known as tubular reabsorption, is the process where the nephron removes water and solutes from the tubular fluid (pre-urine) and returns them to the bloodstream. This process helps to concentrate the glomerular filtrate, which is essential for urine formation.