Adult frogs have a closed circulatory system with a three chambered heart. Circulation is accomplished by the ventricle pumping blood to the body to deliver oxygen and nutrients, and pick up wastes, and back to the lungs and skin to obtain oxygen and remove carbon dioxide. Tadpoles have a loop circulatory system and use gills to breathe dissolved oxygen from the water. It is absorbed and circulated by the bloodstream. Their skin is also used for respiration. Reproduction is external both for fertilization and development, which occurs in the pond. The frog has a similar digestive system to humans, with the digestive process beginning in the mouth, then, food travels to the esophagus, stomach, small and large intestine. They have an opening called the cloaca to remove wastes and to release sperm or eggs depending on their sex. It also is for the removal of urea. Frogs have two kidneys to filter urea and ammonia from the blood and to excrete it. They have a well-developed nervous system with a brain, spinal nerve cord and nerves throughout the body. Their sensory organs include eyes, tympanic membrane for hearing, nostrils, etc. Their skeletal muscular system is adapted for life on land as well as in water. Their very muscular hind legs are adapted for jumping as well as for swimming. The frog has hormones that regulate metamorphosis from juvenile tadpole stage to the adult stage. Basically, frogs are very complex organisms.