Where does filtration exclusively occur in the kidney? what occurs when the plasma concentration of a substance exceeds the kidney's capacity for transporting that substance?

1) Filtration occurs in the renal corpuscle.

Blood that is going to be filtered enters the first part of the nephron, the glomerulus, which is a tuft of capillary vessels. The glomerulus is inside a "sac" called a glomerular capsule.Together, the glomerulus and the glomerular capsule form the renal corpuscle, which is the filtering unit.

2) the excess substance is not reabsorbed and will be excreted in the urine.
 The kidneys are able to control the amount of that substance that is needed. So, the needed amount will be reabsorbed and the rest that is in excess, will be wasted to form urine.