the act of breathing -- 2/15/18
Today's encore selection - from Extreme Medicine by Kevin Fong, M.D. The human body constantly amazes us with its intricate, delicate and resilient organs. Here's a brief description of how oxygen comes in contact with the three million alveoli in our lighter-than-sponge lungs:
"The desire to breathe is among our most primitive urges. We're designed to draw air into our lungs, to exchange fresh oxygen for the waste gas of carbon dioxide. Our lives depend upon this perpetual to and fro of gases ...
"When we describe the path that oxygen takes from the outside world to its final destination in our mitochondria, we do so as though it has agency of its own. We talk of molecules of oxygen moving into our bodies, diffusing across membranes, arriving at mitochondria, almost as though they know where they want to go. But of course oxygen has no free will of its own. In the act of living, your body must solve the problem of how to grab molecules of this gas from the atmosphere and bundle them into cells in sufficient concentration that they can do the stuff of life.
"The first part of that performance is the act of breathing. Your ribs are attached to your breastbone at the front and the bony column that is your spine at the rear. At the end of each exhalation, they slope steeply downward toward the ground. Contracting the muscles in the chest wall that do the work of breathing lifts the ribs up, to a nearly horizontal position, increasing the volume of the chest. At the same time your diaphragm, the large dome-shaped muscle that separates the chest from the contents of your abdomen, contracts and drops down, further increasing the volume of the cavity inside your chest.
"Your lungs sit inside the cage formed by your ribs, adherent to the chest wall. As the chest moves, your lungs move with them. As the volume in your chest cavity increases, so too does that inside your lungs. The increase in volume leads to a decrease in pressure in your chest. That in turn produces suction, in exactly the same way as separating the handles on a bellows does, and air begins to flow.
"That air passes through your upper airways, the larynx, and the trachea, and then down into your bronchial tree. I always thought of that branching network of airways as inverted sprigs of broccoli rather than trees. In terms of morphology, that's not far off. There's a hollow central trunk that sprouts branches of ever decreasing caliber, at the very end of which are saclike structures called alveoli: the buds, if you like, at the end of that sprig of broccoli. The cadaveric lung, formalin-soaked in the medical school's dissecting rooms, is solid and heavy; its airspaces are occupied by pungent preservative fluid. But in life, air-filled lungs are lighter than sponge, light enough to float on water. ...
"That fine structure exists to provide a massive surface area over which air can be brought into contact with blood. The alveoli, those tiny air sacs at the end of the bronchial tree, are each no more than a fraction of a millimeter in diameter, but each lung holds one and a half million. If you were to unfurl them and lay them out flat, they would form a mat of tissue half the size of a tennis court at Wimbledon. That vast area is required to bring enough air into contact with enough blood to keep you alive.
"Over the surfaces of those alveoli runs a spiderlike network of capillaries, vessels with walls a single cell thick, providing just enough structure to confine the blood cells squeezing through them, while offering the minimum obstruction to the molecules of oxygen diffusing through their walls.
"This is the most delicate interface in your body. Nowhere else is the point of contact between your body and the material from the outside world more insubstantial or delicate. That is why it is buried deep in your chest and protected with a formidable cage of ribs."