Leukocytes in urine are not a dangerous condition, but they are a sign that you are suffering from an infection or damage. Leukocytes are white blood cells (WBC) that work with the immune system to defend against infectious disease. Damage to the kidneys, ureters, urethra or bladder can cause leukocytes to appear.
Empty vessels is a metaphor for a very shallow person whose thoughts and actions have no significant meaning. Such people are constantly talking or otherwise making noise in an attempt to get attention from other people. The more the person is ignored because he has no real input to give, the louder he will become.
Wrong. It only provokes more wanderlust in the speaker. The cry of the cuckoo, a sign of warmer weather, makes our speaker feel downright down in the dumps. It tells him it's time for yet another journey. The Seafarer's spirit leaps out of his chest and soars all over the world, then returns to him unsatisfied. He knows the world's riches will not last, since everyone dies and you can't take your possessions with you.