Good Reading : September 2010
26 goodreading ı SEPTEMBER 2010 by Gavin Pretor-Pinney the wavewatcher’s companion BOOKBITE EXTRACT: Waves are quite literally at the heart of human existence. They are the very means by which blood courses around our bodies. For your heart to pump the 4300 gallons it does in any twenty-four-hour period -- cycling oxygen-rich blood through your arteries, veins and organs -- it has to beat 100 000 times. Each and every one of those beats takes the for m of a wave. Muscular contractions of the heart seem so different from undulations travelling over the surface of the water that you might wonder how it can even make sense to describe both as waves. What can the beat of your heart possibly share with the ripples that spread across the surface of your bath as the soap slips from your hand and drops into the water? Both are for ms of travelling oscillations, or vibrations. As one region seesaws between different states, it causes the region next door to start, so that the pattern of movement spreads. In your bath, the falling bar of soap disturbs the surface of the water, causing it to oscillate between dipped and raised levels, and this disturbance spreads in expanding rings. In your heart, the spreading oscillations are of the muscle cells contracting and expanding. Like the changing surface of the water, these contractions spread from one region of the heart tissue to another, though they do so in a very different way. Tiny electric currents drive the waves of movement that form the beating of your heart. Each cell within the muscle tissue contracts when stimulated by an electric pulse. But in order for the heart to pump your blood efficiently, these contractions need to pass rapidly down through the walls of the heart in a coordinated fashion.The cur rent itself is initiated by a clump of 'pacemaker cells' at the top of your heart, which produce a small electric 'shock'. This electrical activity spreads down through the muscle, with each cell contracting and passing on the electrical cur rent to its neighbours. After each cell has fired, it becomes momentarily unable to do so again -- as if it is exhausted, and having a rest. Known as the 'refractory period', this delay in the cell excitability, which lasts between one- tenth and one-fifth of a second, elegantly ensures that the wave can spread only once through the muscle tissue. Until, that is, the pacemaker cells spontaneously fire up again, starting the wave of your heartbeat once more. The sterling work done each day by your 'household divinity' -- as the seventeenth-century physician William Harvey described the heart -- is equivalent to the effort required to lift a 1kg weight about twice the height of Mount Everest. (And without the need for a team of Sherpas.) To perform such a feat, timing is crucial. For the heart's four chambers each to fill with blood and pump it the right way around the system, they have to contract and expand in a very synchronised, coordinated manner.The two on the right of your heart pump blood through your lungs to oxygenate it.The chambers on the left of your heart then pump the oxygenated blood around the rest of your body. And this timing depends critically on the electrical signals spreading through the muscle tissue being the right shape of wave: one that starts at the closed end of the chamber and progresses evenly across the muscular tissue towards the valve that blood needs to be pumped through. Yet heartbeats are just one of the many muscular waves that are constantly passing through your body.These may not be the types of wave to get well-toned surfers excited, but they should, for they are the waves on which our very lives depend. Which is, presumably, why these muscular contractions are involuntary -- and therefore why most of us don't even realise we are producing them. You don’t have to go to the seaside to experience waves. GAVIN PRETOR-PINNEY, author of the bestelling The Cloudspotter’s Guide, outlines how waves can be found everywhere, from the rhythmic beating of our hearts and the electrical signals that fire through our brains, to microwaves, Mexican waves and shockwaves from explosions. The author shows how our world is filled with more of these undulating pulses of energy than we ever realised.