Excretory System

Some one-celled and simple multicellular aquatic organisms have no excretory organs; nitrogenous wastes simply diffuse across the cell membrane into the aqueous environment. In others, Paramecium for example, a specialized organelle, the contractile vacuole, aids in excretion by expelling excess water and some nitrogenous waste. In fresh water, the inside of cells has a higher concentration of salt than the surrounding water, and water constantly enters the cell by osmosis. Radiating canals in Paramecium collect excess water from the cell and deposit it in the contractile vacuole, which squeezes it out through a pore on the surface. This process requires energy supplied by ATP produced in the cell's mitochondria.

Saltwater-dwelling animals must survive in water that is has a higher salt concentration than in cells and body fluids. These animals run the risk of losing too much body water by osmosis or taking in too much salt by diffusion. Several adaptations protect them. The skin and scales of marine fish are relatively impermeable to salt water. In addition, the salts in the water they continually drink are excreted by special cells in their gills. In fact, marine fish excrete most of their nitrogenous waste as ammonia through the gills and only a little as urea, which conserves water. Sharks and other cartilaginous fish, on the other hand, store large amounts of urea in their blood. As a result, the concentration of their blood is slightly greater than the surrounding sea water, and water does not enter by osmosis. Special cells in the rectal gland of these fish excrete whatever excess salt does enter the system.