THE SEMI-COMPLETE USER'S GUIDE TO...YOUR LUNGS

THEY POWER A WORKOUT, FILTER OUT INVADERS AND WORK FOR US 24/7-UNLESS GERMS, POLLUTANTS, BAD LUCK OR OUR OWN NEGLECT GETS IN THE WAY. TAKE A DEEP BREATH AND DISCOVER HOW TO HELP THEM OUT.

- KEVIN DUPZYK

WHAT THEY DO

YOU KNOW FROM high school that your lungs bring in oxygen and move it to the blood (inhale) while taking carbon dioxide out of the blood (exhale). Not as well-known: Your lungs are also "immunologically the biggest filter ever," says Dr. Ana Zamora Martinez, a critical-care pulmonologist at the Mayo Clinic. Their branched structure's tiny passages for blood and air naturally restrict pollution, germs, and even blood clots. And the lungs help regulate blood pressure by processing a hormone called angiotensin that's created in the kidneys.

HOW THEY DO IT ALL

THERE ARE ROUGHLY 8.4 million passageways (aka airways) a breath can travel through to get from your trachea to your bloodstream. The smallest is less than a millimetre in diameter. After air goes in your trachea, it travels through your bronchi to your lungs, then passes into progressively smaller tubes called bronchioles. It eventually reaches alveoli, super-small air sacs surrounded by capillaries. That's where the key hand-off happens: oxygen diffuses into the blood and CO2 diffuses out. The surface area of all of the alveoli is roughly the size of a tennis court and allows tons of oxygen to be exchanged quickly. That's especially handy when sprinting for a plane or across a finish line.

WHAT CAN GO WRONG

THE MOST COMMON LUNG PROBLEMS HAPPEN WHEN OXYGEN HAS TROUBLE REACHING YOUR ALVEOLI.