• Why do you sweat when you are hot?
  Sweating helps to cool you down. When the body becomes hot, sweat ...

• How often does the heart beat?
  A child's heart usually beats about 80 times a minute, a bit faster ...

• How big is the heart?
  The heart is about the same size as your clenched fist. It lies ...

• What is the heart made of?
  The heart is made of a special kind of muscle, called cardiac (heart) ...

• What do white blood cells do?
  White blood cells surround and destroy germs and other intruders that ...

• What is plasma?
  Just over half the blood is a yellowish liquid called plasma. It is ...

• How much blood do you have?
  An average man has between II and 13 pints (5 and 6 liters) of blood; ...

• How much air do your lungs hold?
  An adult's lungs hold about 13 1/2 pints (6 liters) of air, while a ...

• Why do the lungs have so many alveoli?
  In order to provide a huge surface across which oxygen and carbon ...

• Why does your breath sometimes look misty?
  The air you breathe out contains water vapor. On a cold day this ...