The kidneys perform several crucial functions in the human body, primarily related to homeostasis and regulating the composition of blood. They filter waste products and excess substances from the bloodstream, forming urine for excretion. Kidney functions include maintaining electrolyte balance, regulating blood pressure, and managing acid-base balance. Additionally, they produce hormones like erythropoietin, which stimulates red blood cell production. Overall, the kidneys are essential for ensuring physiological balance and health.