Key functions of excretory systems include:

• Filtration.
  Water and solutes are forced across the membranes of capillaries to produce a filtrate.

• Reabsorption.
  Valuable solutes such as glucose and amino acids are reclaimed from the filtrate by active transport.

• Secretion.
  Toxins and excess ions are extracted from body fluids to the filtrate by active transport.

• Excretion.
  The filtrate leaves the system.