Hemoglobin is a special protein inside your red blood cells that carries oxygen throughout your body. Think of it as a tiny delivery truck that picks up oxygen in your lungs and drops it off at every cell that needs it, from your brain to your toes.

Here's how it works: when you breathe in, oxygen enters your lungs and attaches to hemoglobin molecules in your blood. Your heart then pumps this oxygen-rich blood through your arteries to every part of your body. After hemoglobin releases its oxygen cargo, it picks up carbon dioxide (a waste product your cells produce) and carries it back to your lungs, where you breathe it out.

Hemoglobin contains iron, which is why you need iron in your diet. The iron gives hemoglobin its ability to grab onto oxygen, and it's also what makes your blood red. When hemoglobin is carrying lots of oxygen, blood looks bright red. When it has less oxygen, blood appears darker, almost purplish.

Without enough hemoglobin, a person can develop anemia, which causes tiredness and weakness because cells aren't getting the oxygen they need. Doctors can measure hemoglobin levels with a simple blood test. This remarkable molecule is working right now in your body, helping your red blood cells carry oxygen every moment to keep you alive and active.