I’m breathing, and so are you. Our bodies need to take oxygen molecules from the air and send carbon dioxide back out. Each one of us walks around in our own personal life-support system, a body with an inside and an outside. Our insides can do all sorts of things, but only when supplied with stuff from the outside: energy, water, and the right molecular building blocks. Breathing is just one of the supply routes. It’s ingenious: Expand your ribcage, increase the volume of your lungs, and the mob of tiny jostling air molecules close to your mouth is pushed down your windpipe by the air farther away. Take a deeper breath and your chest expands even more, making space for more of the atmosphere to rush in and touch the smallest structures in your lungs. Then, as you relax the muscles around your ribcage, your ribs are pulled downward by the Earth, pushing the air molecules in your lungs closer together until they jostle each other back to rejoin the outside. Oxygen isn’t the only molecule pulled into your lungs that your body can use. As the air passes by the sensors inside the top of your nose, some of the billions of molecules bumping into the walls will happen to collide with a larger molecule attached to the wall, one they temporarily fit into like a key into a lock. The underlying cell senses the molecular click as they fit together. That’s the start of our sense of smell—a few floating molecules of the right type bumping into the right place. Inside now has some information about what’s outside.
A human body is a vast coordinated collection of cells, about 37 trillion of them last time anyone tried to count, each one a tiny factory. Every single cell needs supplies, but it also needs a safe environment, with the right temperature, pH, and moisture level. As you walk through the world, your body is constantly adjusting to adapt to the conditions around it. If you spend too long in a warm room, the molecules close to your skin surface vibrate faster because they’ve got more energy. If those vibrations were passed deeper into your body, they could start to disrupt the workings of your cells. So as you’re sitting in the warm room, you need to give away energy. That sounds easy; water molecules evaporate easily in the warm, taking energy with them. You’ve got lots of water in you that could evaporate. But the water is stuck inside because you’re waterproof. You need to sweat.
Your skin has a very thin layer of fat molecules just underneath the outermost skin cells, a barrier preventing any liquid moving between inside and outside. But while you’re sitting in the warm room, your skin will open up tunnels through the barrier: your pores. Sweat seeps through the pores, penetrating the waterproof layer, and reaches the outside. Individual water molecules bump into each other and the warm skin surface until the most energetic are traveling fast enough to escape. One by one, they float away, leaving your skin cooler. When you are cool enough, the pores close, and you are waterproof again. Your skin isn’t waterproof just to keep water out. It’s also waterproof to keep water in, because the internal supply of water is limited. Water is transported around your body in your blood, the internal supply system that lets your body share out its resources. This supply system must run continuously to keep your cells alive. And we can check that it’s working: We all have a pulse.
Our pulse is a three-dimensional disturbance, a traveling pressure wave that provides clues about blood flow. Our hearts are constantly squeezing the blood in their chambers, raising the fluid pressure and so forcing the blood out into our arteries. It’s a powerful push, and as it comes to an end, the fluid pressure in the heart chambers drops. The forces on the blood have now been reversed, and the recently expelled blood would rush back in if it weren’t for the one-way valves guarding the exit. The sudden rush of fluid backward closes the valves, and the liquid thumps against the valve tissues as it’s stopped. This thump is so strong that it pushes outward on the tissues around it, and they push on the tissues beyond, and a pressure wave travels through the body, slightly compressing the muscle and bone in its path as it travels. It takes this pressure wave about six milliseconds to reach the outside of the body, and if you put a stethoscope or your ear up against someone’s body, you can hear it. This is your heartbeat. If waves didn’t travel through our tissues, we wouldn’t ever be able to hear our hearts. In fact it’s a double beat, with two pulses: “lub-dub,” because the heart has four valves and they close in pairs, one pair just after the other. This accidental combination of physics and physiology broadcasts the most significant sign of life throughout our bodies.
After sweating, your blood carries fewer water molecules than before. Now, your body needs to replenish itself from outside. In order for you to take a simple drink of water, your cells need to coordinate their activities. Decisions, and the actions required to coordinate the necessary body parts to carry them out, are made first subconsciously and then consciously in the brain.
One brain cell is no use by itself. It only works because it’s connected to others, and the network of connections seems to be as important as the brain cells themselves. As a decision about finding a drink emerges from the connections, the brain cells need to connect with other cells farther afield. The vehicle for this internal communication is a nerve fiber, a thin strand of cell that is the body’s equivalent of an electrical wire. By shunting electrically charged particles across a membrane at one end of the nerve fiber, the brain cell starts an electrical signal which ripples along the nerve fiber by playing electrical dominoes. At the end of the first nerve fiber, there’s another one. The dance of the electrically charged particles sends the message across the gap, and then more electrical dominoes carry the message onward. The message is relayed from cell to cell for the fraction of a second that it takes to reach one of the muscles in your leg. At around the same time, the messages from other nerve fibers carrying coordinated signals to other leg muscles also arrive, and your leg muscles contract to lift you off the sofa. The feel of the floor beneath your feet and the temperature change on your skin as the movement generates a slight breeze are conveyed back to your brain via yet more electrical signals.