You are presented with the question "what contains a system of blood vessels and tubules" and the following answer choices: 
- clouds
 - kidneys
 - organ
 - density
 - lungs
 - complex
 - blood cells
 - alveoli

Now knowing that nephrons are the structural and functional units of the kidneys and each nephron contains a system of blood vessels and tubules, choose the best answer.


 

  kidneys       