instruction:
Given a scientific question, generate a correct answer to it.
question:
While in the lungs, blood gives up carbon dioxide and picks up what element before returning to the heart?
answer:
oxygen

question:
What is continually released during childbirth through a positive feedback mechanism and prompts uterine contractions to fetal head toward the cervix?
answer:
oxytocin

question:
What can be classified as monoprotic or polyprotic based on the number of acidic hydrogens they contain?
answer:
acids