Which substances are filtered by the kidneys?

Phosphate, Pottassium, Hydrogen.

The ions and large molecules pass from the blood to the filtrate by the process of secretion. Potassium, phosphate, hydrogen, and ammonium ions and many organic acids enter the filtrate through secretion. Secretion is the process of transporting solutes into the renal tubule so that they can be excreted in the urine. It allows substances such as hydrogen ions to be eliminated at a rate that exceeds glomerular filtration.

Both reaborption and secretion are controlled by the selective permeability of different areas of the renal tubule to water, sodium, and urea (a by-product of protein metabolism) and the response of the distal collecting tubules in the kidney to hormones such as aldosterone, antidiuretic hormone (adh), and parathyroid hormone (pth).