a pH imbalance a pressure difference a temperature increase a concentration gradient
I am hesitating between 4 options to answer the following question, which option should I choose?
Question: In the human kidney, urea from the blood is filtered through the glomerular membrane into a nephron. The movement of urea across this membrane occurs without an input of energy. Which factor is the most likely reason urea absorption does not require energy?
Possibilities:
- a pH imbalance
- a pressure difference
- a temperature increase
- a concentration gradient

a concentration gradient