What continuously forms urine as an ultra filtrate of plasma?

The kidneys are responsible for the continuous formation of urine as an ultra filtrate of plasma. This process begins in the nephron, the functional unit of the kidney, where blood is filtered through a structure called the glomerulus. During this filtration, water, ions, and small molecules pass through the glomerular membrane, leaving larger molecules and blood cells in the bloodstream. This filtrate then travels through various segments of the nephron, where substances such as sodium, potassium, and water are reabsorbed or secreted as needed, ultimately forming urine.

The constant production of urine is crucial for maintaining homeostasis in the body by regulating fluid balance, electrolytes, and waste elimination. The other options, while important organs in the body, do not play a direct role in urine formation. The liver is primarily involved in metabolism and detoxification, the heart functions as the pump for blood circulation, and the spleen is part of the immune system and aids in filtering blood, but none are responsible for urine production. Thus, the kidneys are the correct answer as they are uniquely designed for this vital function.