Excretory System

As in aquatic animals, the excretory system in land animals removes nitrogenous waste and helps establish a balance between salt and water in the body. Terrestrial animals, however, also run the risk of drying out by evaporation from the body surface and the lungs. The elimination of feces and the excretion of urine also bring about water loss. Drinking, foods containing large amounts of water, and producing water during cellular respiration help overcome the loss. Animals that produce uric acid need less water than those excreting urine. Flame cells in flatworms, the nephridia in segmented worms, Malpighian tubules in insects, and kidneys in vertebrates are all examples of excretory systems.

Planarians and other flatworms have an excretory system that consists of two or more longitudinal branching tubules that run the length of the body. The tubules open to the outside of the animal through holes or pores in the surface. The tubules end in flame cells, bulb-shaped cells that contain cilia. The cilia create currents that move water and wastes through the canals and out the pores. Flatworms lack a circulatory system, so their flame cells excretory system picks up wastes directly from the body tissues.

The cells of segmented worms, such as earthworms, produce urea that is excreted through long tubules called nephridia, that work in conjunction with the earthworm's circulatory. Almost every segment of the earthworm's body contains a pair of nephridia. A nephridium consists of a ciliated funnel, a coiled tubule, an enlarged bladder, and a pore. The ciliated funnel collects wastes from the tissue fluid. The wastes travel from the funnel through the coiled tubule. Additional wastes from blood in the earthworm's capillaries enter the coiled tubule through its walls. Some of the water in the tubule is reabsorbed into the earthworm's blood. A bladder at the end of the nephridium stores the collected wastes. Finally the bladder expels the nitrogenous wastes through the pore to the outside.

Malpighian tubules are excretory organs that operate in association with the open circulatory system of grasshoppers and other insects. They consist of outpocketings of the digestive system where the midgut attaches to the hindgut. Blood in the open sinuses of the grasshop pers' body surrounds the Malpighian tubules. The ends of the tubules absorb fluid from the blood. As the fluid moves through the tubules, uric acid is precipitated. A lot of the water and other salts are reabsorbed into the grasshopper's blood. The remaining fluid plus uric acid passes out of the Malpighian tubule and enters the gut. Water is reabsorbed from the digestive tract. Finally, the uric acid is eliminated from the rectum as a dry mass.

The vertebrate excretory system works with circulatory system to remove wastes and water from blood, and convert them to urine. The urine is stored in a urinary bladder before it is expelled from the body. Kidneys are the main organs of excretion in vertebrates. Within the kidneys, working units called nephrons take in the liquid portion of the blood, filter out impurities, and return necessary substances to the blood stream. The remaining waste-containing portion is converted to urine and expelled from the body.