7. THE HEART AND BLOOD Stopped. -- LAST WORD OF THE BR ITISH SURGEON AND ANATOMIST JOSEPH HENRY GREEN (1791- 1863) WHILE FEELING HIS OWN PULSE I THE HEART IS the most misperceived of our organs. For a start, it looks nothing like the traditional symbol associated with Valentine''s Day and lovers'' initials carved into tree trunks and the like. (That symbol First appeared, as if from out of nowhere, in paintings from northern Italy in the early fourteenth century, but no one knows what inspired it.) Nor is the heart where we place our right hand during patriotic moments; it is more centrally located in the chest than that. Most curious of all, perhaps, is that we make it the emotional seat of our being, as when we declare that we love someone with all our heart or profess a broken heart when they abandon us. Don''t misunderstand me.

The heart is a wondrous organ and fully deserving of our praise and gratitude, but it is not invested even slightly in our emotional well-being. That''s a good thing. The heart has no time for distractions. It is the most single-minded thing within you. It has just one job to do, and it does it supremely well: it beats. Slightly more than once every second, about 100,000 times a day, as many as 3.5 billion times in a lifetime, it rhythmically pulses to push blood through your body--and these aren''t gentle thrusts. They are jolts powerful enough to send blood spurting up to three meters if the aorta is severed.

With such an unrelenting work rate, it is a miracle that most hearts last as long as they do. Every hour your heart dispenses around 70 gallons of blood. That''s 1,680 gallons in a day--more gallons pushed through you in a day than you are likely to put in your car in a year. The heart must pump with enough force not merely to send blood to your outermost extremities but to help bring it all the way back again. If you are standing, your heart is roughly four feet above your feet, so there''s a lot of gravity to overcome on the return trip. Imagine squeezing a pump the size of a grapefruit with enough force to move a uid four feet up a tube. Now do that again once every second or so, around the clock, unceasingly, for decades, and see if you don''t feel a bit tired. It has been calculated (and goodness knows how, it must be said) that during the course of a lifetime the heart does an amount of work sufFicient to lift a one-ton object 150 miles into the air.

It is a truly remarkable implement. It just doesn''t care about your love life. For all it does, the heart is a surprisingly modest thing. It weighs less than a pound and is divided into four simple chambers: two atria and two ventricles. Blood enters through the atria (Latin for "entry rooms") and exits via the ventricles (from another Latin word for "chambers"). The heart is not really one pump but two: one that sends blood to the lungs and one that sends it around the body. The output of the two must be in balance, every single time, for it all to work correctly. Of all the blood pumped out of your heart, the brain takes 15 percent, but actually the greatest amount, 20 percent, goes to the kidneys.

The journey of blood around your body takes about Fifty seconds to complete. Curiously, the blood passing through the chambers of the heart does nothing for the heart itself. The oxygen that nourishes it arrives via the coronary arteries, in exactly the way oxygen reaches other organs. The two phases of a heartbeat are known as the systole (when the heart contracts and pushes blood out into the body) and diastole (when it relaxes and reFills). The difference between these two is your blood pressure. The two numbers in a blood pressure reading--let''s say 120/80, or "120 over 80" when spoken--simply measure the highest and lowest pressures your blood vessels experience with each heart-beat. The First, higher number is the systolic pressure; the second, the diastolic. The numbers speciFically measure how many millimeters of mercury is pushed up a calibrated tube.

Keeping every part of the body supplied with sufFicient quantities of blood at all times is a tricky business. Every time you stand up, roughly a pint and a half of your blood tries to drain downward, and your body has to somehow overcome the dead pull of gravity. To manage this, your veins contain valves that stop blood from ow-ing backward, and the muscles in your legs act as pumps when they contract, helping blood in the lower body get back to the heart. To contract, however, they need to be in motion. That''s why it''s important to get up and move around regularly. On the whole, the body manages these challenges pretty well. "For healthy people there is a less than 20 percent difference between blood pressure at the shoulder and at the ankle," Siobhan Loughna, a lecturer in anatomy at the University of Nottingham Medical School, told me one day. "It''s really quite remarkable how the body sorts that out.

" As you may gather from this, blood pressure isn''t a Fixed Figure, but changes from one part of the body to another, and across the body as a whole throughout the day. It tends to be highest during the day when we are active (or ought to be active) and to fall at night, reaching its lowest point in the small hours. It has long been known that heart attacks are more common in the dead of night, and some authorities think the nightly change in blood pressure may somehow act as a trigger. Much of the early research on blood pressure was done in a series of decidedly gruesome experiments on animals conducted by the Reverend Stephen Hales, an Anglican curate of Teddington, Middlesex, near London, in the early eighteenth century. In one experiment, Hales tied down an aged horse and attached a nine-foot-long glass tube to its carotid artery by means of a brass cannula. Then he opened the artery and measured how high blood shot up the tube with each dying pulse. He killed quite a number of helpless creatures in his pursuit of physiological knowledge and was roundly condemned for it--the poet Alexander Pope, who lived locally, was especially vocal on the matter--but among the scientiFic community his achievements were celebrated. Hales thus had the double distinction of advancing science while at the same time giving it a bad name.

Though Hales was denounced by animal lovers, the Royal Society awarded him its very highest honor, the Copley Medal, and for a century or so Hales''s book Haemastaticks was the last word on blood pressure in animals and man. Well into the twentieth century, many medical authorities believed that high blood pressure was a good thing because it indicated vigor-ous ow. We now know, of course, that chronically elevated blood pressure very seriously raises the risk of a heart attack or stroke. A more difFicult question is, What exactly constitutes high blood pres-sure? For a long time, a reading of 140/90 was generally considered the baseline for hypertension, but in 2017 the American Heart Association surprised nearly everyone by abruptly pushing the number downward to 130/80. That small reduction tripled the number of men and doubled the number of women aged forty-Five or under who were deemed to have high blood pressure and lifted practically all people over sixty- Five into the danger zone. Almost half of all American adults-- 103 million people--are on the wrong side of the new blood pressure threshold, up from 72 million previously. At least 50 million Americans, it is thought, are not receiving appropriate medical atten-tion for the condition. Heart health has been one of the success stories of modern medi-cine.

The death rate from heart diseases has fallen from almost 600 per 100,000 in 1950 to just 168 per 100,000 today. As recently as 2000, it was 257.6 per 100,000. But it is still the leading cause of death. In the United States alone, more than eighty million people suffer from cardiovascular disease, and the cost to the nation of treating heart disease has been put as high as $300 billion a year. There are lots of ways the heart can falter. It can skip a beat, or more usually have an extra beat, because an electrical impulse misFires. Some people can have as many as ten thousand of these palpitations a day without being aware of it.

For others, an arrhythmic heart is an endless discomforting ordeal. When the heart''s rhythm is too slow, the condition is called bradycardia; when too fast, it is tachycardia. A heart attack and a cardiac arrest, though usually confused by most of us, are in fact two different things. A heart attack occurs when oxygenated blood can''t get to heart muscle because of a blockage in a coronary artery. Heart attacks are often sudden--that''s why they are called attacks--whereas other forms of heart failure are often (though not always) more gradual. When heart muscle downstream of a block-age is deprived of oxygen, it begins to die, usually within about sixty minutes. Any heart muscle we lose in this way is gone forever, which is a bit galling when you consider that other creatures much simpler than we are--zebra Fish, for instance--can regrow damaged heart tissue. Why evolution deprived us of this useful facility is yet another of the body''s many imponderables.

Cardiac arrest is when the heart stops pumping altogether, usu-ally because of a failure in electrical signaling. When the heart stops pumping, the brain is deprived of oxygen and unconsciousness swiftly follows, with death not far behind unless treatment is quickly applied. A heart attack will often lead to cardiac arrest, but you can suffer cardiac arrest without having a.