Final answer:
The kidneys are the structures that filter the blood to regulate its composition, utilizing millions of nephrons to perform this vital function.
Step-by-step explanation:
The structure that filters the blood to regulate its composition is the kidneys. The kidneys are essential components of the urinary system, which also includes the ureters, urinary bladder, and urethra. They are made up of about 2 to 3 million nephrons, each containing a glomerulus that is responsible for filtering the blood. This intricate process removes various substances from the blood, excluding larger elements such as blood cells and proteins while allowing solutes like ions, amino acids, vitamins, and wastes to pass through, forming a filtrate that eventually becomes urine. Notably, the kidneys filter all the blood in the human body around 60 times a day.
The ureters are responsible for carrying urine away from the kidneys to the urinary bladder, which serves to store urine. When the bladder is full, urine is expelled from the body via the urethra. Therefore, while the kidneys play the critical role of blood filtration, the other parts of the urinary system are responsible for transporting, storing, and excreting the urine formed.