The excretory system regulates homeostasis by removing metabolic, liquid, and gaseous waste from the body. The kidneys filter blood and produce urine by removing wastes and excess water. Urine contains urea, uric acid, ammonia, hormones, dead blood cells, proteins, salts, and toxins. The kidneys filter blood through nephrons, which collect the filtrate and modify it into urine by selectively reabsorbing necessary substances and secreting others. Urine is stored in the bladder and expelled through the urethra.