The primary excretory system in humans is the urinary system. The skin also acts as an organ of excretion by removing water and small amounts of urea and salts (as sweat). The urinary system includes a pair of bean-shaped kidneys located in the back of the abdominal cavity. Excretion is a process by which metabolic waste is eliminated from an organism. Invertebrates this is primarily carried out by the lungs, kidneys and skin. Excretion is an essential process in all forms of life. For example, in mammals, urine is expelled through the urethra, which is part of the excretory system.
Excretion occurs in plants and animals as they both have waste products to be removed from their bodies.
Oxygen is a waste product of photosynthesis, and carbon dioxide is a waste product of aerobic respiration. Resins, gums, latex are some other waste products.