Which of the following substances is filtered out of blood in the kidneys?

The correct choice is ions because the kidneys play a crucial role in filtering the blood and maintaining homeostasis in the body. During the filtration process in the nephrons of the kidneys, small substances such as ions (including sodium, potassium, calcium, and chloride) are easily filtered out of the blood into the renal tubules. This process is vital for regulating electrolyte balance, blood pressure, and overall fluid homeostasis.

Ions are small, charged molecules that can pass through the filtration membrane of the glomerulus, whereas larger substances like hormones and larger proteins are typically retained in the bloodstream. This retention is important to maintain proper physiological functions, as hormones need to remain active in circulation to exert their effects, and a high concentration of large proteins in the blood is crucial for managing osmotic pressure. Additionally, red blood cells are also too large to pass through the filtration barrier and remain in circulation, which is essential for delivering oxygen throughout the body. Thus, the filtering mechanism is selective, allowing only small molecules, such as ions and waste products, to be excreted while preserving larger, functional elements in the blood.