Fluid extracted by the excretory system from the blood or body cavity. The excretory system produces urine from the filtrate after extracting valuable solutes from it and concentrating it. filtration The first stage of kidney function; blood plasma is forced, under pressure, out of the glomerular capillaries into Bowman's capsule, through which it enters the renal tubule. first law of thermodynamics( thur-moh-dy- nam-iks) The principle of conservation of energy. Energy can be transferred and transformed, but it cannot be created or destroyed.