Basic Science

Oxygen’s surprisingly complex journey through your body

You breathe in about 17,000 times per day. It’s a process you rarely think about, but behind the scenes, a huge coordinated effort is playing out. Your vital organs, the gut, brain, bones, lungs, blood, and heart work together to sustain your life by delivering oxygen to tissues throughout your body. Most of our cells need oxygen because it’s one of the key ingredients of aerobic respiration. That’s the process that produces a molecule called ATP, which our cells use to power their many incredible functions.

But getting oxygen throughout our bodies is a surprisingly difficult task. Gas enters cells by diffusing in from their surroundings. And that only happens efficiently over tiny distances. So for oxygen to reach the cells within our bodies, it needs a transportation network. This is where our 20 trillion red blood cells come in. Each one contains about 270 million oxygen-binding molecules of hemoglobin, which is what gives blood its scarlet hue.

To make these cells, the body uses raw materials that become available from the food we eat. So in some ways, you could say that oxygen’s journey through the body really begins in the gut. Here, in an amazing display of mechanical and chemical digestion, food gets broken down into its smallest elements, like iron, the building block of hemoglobin. Iron is carried through the cardiovascular system to the body’s hematopoietic tissue. This tissue is the birthplace of red blood cells, and it can be found enclosed within our bone marrow cavities.

The kidneys regulate our levels of red blood cells through the release of erythropoietin, a hormone which causes marrow to increase production. Our bodies churn out roughly 2.5 million red blood cells per second, a number equivalent to the entire population of Paris, so that oxygen that makes it to the lungs will have ample transportation. But before oxygen can even reach the lungs, the brain needs to get involved. The brainstem initiates breathing by sending a message through your nervous system, all the way to muscles of the diaphragm and ribs.

This causes them to contract, thus increasing the space inside the rib cage, which allows the lungs to expand. That expansion drops your lungs internal air pressure, making air rush in. It’s tempting to think of our lungs as two big balloons, but they’re actually a lot more complicated than that. Here’s why. The red blood cells in the vessels within your lungs can only pick up oxygen molecules that are very close to them.

If our lungs were shaped like balloons, air that was not in direct contact with the balloon’s inner surface couldn’t diffuse through. Luckily, our lungs’ architecture ensures that very little oxygen is wasted. Their interior is divided into hundreds of millions of miniature balloon-like projections called alveoli that dramatically increase the contact area to somewhere around 100 square meters. The alveolar walls are made of extremely thin flat cells that are surrounded by capillaries. Together, the alveolar wall and capillaries make a two-cell thick membrane that brings blood and oxygen close enough for diffusion.

These oxygen-enriched cells are then carried from the lungs through the cardiovascular network, a massive collection of blood vessels that reaches every cell in the body. If we laid this system out end to end in a straight line, the vessels would wrap around the Earth several times. Propelling red blood cells through this extensive network requires a pretty powerful pump, and that’s where your heart comes in.

The human heart pumps an average of about 100,000 times per day, and it’s the powerhouse that ultimately gets oxygen where it needs to go, completing the body’s team effort. Just think – this entire complex system is built around the delivery of tiny molecules of oxygen. If just one part malfunctioned, so would we. Breathe in. Your gut, brain, bones, lungs, blood, and heart are continuing their incredible act of coordination that keeps you alive. Breathe out.

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