Glomercular filtration, antidiuretic hormone, hypertonic urine, collecting duct.

Answer

### Step-by-Step Solution:

1. **Understanding Glomerular Filtration**:
- Glomerular filtration is the first step in urine formation, occurring in the nephron, specifically in the glomerulus.
- The glomerulus is a network of capillaries with high hydrostatic pressure, which facilitates the filtration of blood.
- Blood enters the glomerulus through the afferent arteriole and exits through the efferent arteriole. The efferent arteriole is narrower than the afferent arteriole, creating high pressure that aids in filtration.

2. **Role of Antidiuretic Hormone (ADH)**:
- Antidiuretic hormone (ADH), also known as vasopressin, plays a crucial role in regulating water balance in the body.
- ADH acts primarily on the distal convoluted tubule (DCT) and the collecting duct of the nephron.
- It increases the permeability of these structures to water, allowing more water to be reabsorbed back into the bloodstream, which results in concentrated urine.

3. **Understanding Hypertonic Urine**:
- Hypertonic urine is produced when the urine has a higher concentration of solutes compared to the surrounding fluid.
- This condition occurs when more water is reabsorbed in the DCT and collecting duct, leading to the formation of concentrated urine.

4. **Function of the Collecting Duct**:
- The collecting duct is a crucial part of the nephron that lies adjacent to the DCT and runs parallel to the loop of Henle.
- Its primary function is to reabsorb sodium ions (Na+) and water, further concentrating the urine.
- The action of ADH on the collecting duct is essential for the formation of hypertonic urine.

### Summary:
- Glomerular filtration occurs in the glomerulus due to high hydrostatic pressure.
- ADH increases water reabsorption in the DCT and collecting duct, leading to concentrated (hypertonic) urine.
- The collecting duct plays a vital role in the final concentration of urine by reabsorbing sodium and water.