Of all the oxygen that a human breathes, only twenty percent goes to the brain. From the age of thirty, humans gradually begin to shrink in size. Gases that build up in your large intestine cause flatulence. It usually takes about 30 to 45 minutes for these gases to pass through your system. Hair and fingernails are both made from the same substance, keratin. Humans inhale and exhale approximately one liter of air in ten seconds.