Giraffes? Giraffes!

"Mrs. Dursley was thin and blonde and had nearly twice the usual amount of neck, which came in very useful as she spent so much of her time craning over garden fences, spying on the neighbours." (Harry Potter and the Philosopher’s Stone)

A question came up in physiology lab: What happens to animals like giraffes, trumpeter swans, and Petunia Dursley who have a lot of anatomic dead space due to long necks?

So for those who don't have a lung physiology background, gas exchange (to put oxygen into your blood and remove carbon dioxide) occurs at the boundary of alveoli (air sacs) and capillaries (blood vessels) in the lungs. Dead space refers to parts of the respiratory system where there is ventilation (air coming in) but no perfusion (no blood). Everyone has anatomic dead space - this refers to things like your nose, mouth, trachea (windpipe). You ventilate these structures when you breathe, but there are no alveoli or gas exchange capillary beds there. Dead space represents air that you work to breathe in, which does not participate in oxygenating your blood. Giraffes have a lot of dead space because of their long necks - they must work to bring in air, but air sitting in the neck does not contribute to gas exchange.

Well, it turns out that giraffes compensate for increased dead space by having incredibly large lungs - the San Diego Zoo website says giraffe lungs can hold 55 liters of air (our lungs hold maybe 6 liters of air). The giraffe also breathes really slowly, allowing it to move lots of air in and out with each breath (large tidal volumes). It turns out that the dead space/tidal volume ratio is 0.34, comparable to humans. So giraffes, despite having ridiculously long necks, are able to efficiently take in oxygen and get rid of carbon dioxide.

For those thinking about comparative anatomy, you might also wonder how giraffe hearts can pump blood all the way up to their brains. Interesting stuff.