Which structure contains the lowest concentration of urea?

The following diagram represents a mammalian kidney tubule (nephron) and its blood supply.

Parts indicated by the guidelines 1 to 8 are as follows :

1. Afferent arteriole from the renal artery;

2. Efferent arteriole

3. Bowman's capsule,

4. Glomerulus ;

5. Proximal convoluted tubule with blood capillaries;

6. Distal convoluted tubule with blood capillaries;

7. Collecting tubule;

8. U-shaped loop of Henle.

Study the diagram and answer the questions that follow;

(a) Where does ultrafiltration take place?

(b) Which structure contains the lowest concentration of urea?

(c) Which structure contains the highest concentration of urea?

(d) Which structure (normally) contains the lowest concentration of glucose?

(e) Where is most water reabsorbed?

Name the blood vessel that has the lowest concentration of urea.