Saturday, November 15, 2008

The Wonders of Blood

The Wonders of Blood
You’re born with a little over a pint of it, by adulthood you’re up to
four or five quarts, and if at any point you suddenly shed more than a
third of your share, you must either get a transfusion or prepare to
meet your mortician.
Taste for Blood (October 21, 2008)
Human cultures have long recognized that blood is essential to lifeand have ascribed to it a vast array of magical powers andmetaphorical subroutines. Blood poultices and blood beverages weresaid to cure blindness, headaches, gout, goiter, worms and gray hair.The Bible mentions blood more than 400 times, William Shakespeareclose to 700. It’s “all in the blood,” your temperament, your fate. Areyou a blue-blooded Mesopotamian princess or a red-bloodedAmerican male?Yet to scientists who study blood, even the most extravagant bloodlore pales in comparison to the biochemical, evolutionary andengineering marvels of the genuine article.The fluid tissue we call blood not only feeds us and cleans us,delivering fresh oxygen and other nutrients to all 100 trillion cells ofthe body and flushing out carbon dioxide, ammonia and othermetabolic trash. It not only houses the immune system that defendsus against the world.Our blood is the foundation of our very existence as multicellularanimals, said Andrew Schafer, a professor at Weill Cornell MedicalCollege and the outgoing president of the American Society ofHematology. Blood is the one tissue that comes into contact withevery other tissue of the body, and it is through blood that ourdisparate parts communicate, through blood that our organscooperate. Without a circulatory system, there would be no internalcivilization, no means of ensuring orderly devotion to the commoncause that is us.“It’s an enormous communications network,” Dr. Schafer said — theoriginal cellphone system, if you will, 100 trillion users strong.Blood can also be thought of as a private ocean, a recapitulation ofwhat life was like for all the years we spent drifting as microscopic,single-celled organisms, “taking up nutrients from sea water andthen eliminating waste products back into sea water,” Dr. Schafersaid. Not only is blood mostly water, but the watery portion of blood,the plasma, has a concentration of salt and other ions that isremarkably similar to sea water.Of course, we can’t rely on wind and weather to keep our hiddenseas salubriously churned and aerated, so we have evolved an activerespirator and pumping mechanism, the lungs and heart. Our eightpints of blood circulate through the powerhouse duet maybe 60times an hour, absorbing recently inhaled oxygen from thehoneycombed fabric of the lungs and proceeding into the thicklymuscled heart, which then shoots the enriched fluid outward.Oxygen allocation is the task of the red blood cells, whichhematology researchers refer to with a mix of affection and awe.“Red cells have enormous capabilities,” said Stanley Schrier ofStanford University’s School of Medicine. They give up so much tomake room for their hemoglobin, the proteins that can latch ontooxygen and that give blood its brilliant grenadine sheen. Aloneamong body cells, red cells at maturity jettison their nucleus andDNA to accommodate their cargo.And oh how roughly they are treated. A red cell at rest looks like aplump bialy and measures about 8 microns, or .0003 inches, across.Yet to reach every far-flung, oxygen-hungry customer, the cells mustsqueeze through capillaries less than half their width, which theyaccomplish by squashing down into threads that then crawl in singlefile along the capillary wall, pulling themselves forward, Dr. Schriersaid, like tank treads gripping the road.Blood is also a genius, able to sustain two contradictory stateswithout going mad. To ceaselessly shuttle along the body’s 60,000miles of arteries, veins and capillaries, blood must be fluid, our trustysouvenir sea.Yet even though we constantly replace components of our blood,directing the aged and the battered to the spleen and liver — the“graveyards for blood cells,” Dr. Schafer said — and replenishingthem with fresh blood cells forged in the bone marrow, the turnovercycle is gradual and we can’t afford to lose everything in one biggush wrought by a predator’s gash. Blood, then, departs from seawater, or, for that matter, from breast milk, another prized bodyfluid, in one outstanding way: it is always poised to clot, to relinquishliquidity and assume solidity.In deciding whether to flow or clot, blood takes its cues from itssurroundings. As blood glides through the bulk of its tubularcircuitry, the comparatively heavy red cells are driven toward thecenter of the swirl, said James N. George, a hematologist at theUniversity of Oklahoma Health Sciences Center, while two other,lighter characters are pushed out to the periphery: the white bloodcells that operate as immune warriors, and the platelets, tiny cellsthat have been called the Band-Aids of the body. Theirmarginalization is no accident. “They’re surveillance cells,” Dr.George said. “It’s almost like they’re scouting for trouble.”White blood cells look for signs of invasive microbes, while plateletsscan for leaks. As long as the platelets detect the Teflon-like surfaceof unbroken endothelium, the tissue with which blood vessels arelined, they keep moving.But even the tiniest cut or gap in the smooth vessel wall will exposesome of the fibrous strands beneath, and the platelets are primed toinstantly detect the imperfection. A passing platelet will stick to theraggedy strand and change shape, from round to octopoid, which inturn attracts other platelets, forming a little clump. “If the cut issmall, that’s all you need,” Dr. George said. If not, the next phase offlood control begins. Signals from the platelets arouse the blood’sclotting factors, free-floating proteins that can cross-link togetherinto bigger, better Band-Aids.“Platelets and clotting factors,” Dr. Schrier said. “It’s a marriagemade in heaven.”Up to a point. Just as our immune cells can go awry and beginattacking our own body tissue, so an overzealous clot response canhave dire consequences. Should a clot happen to cut off blood flow toa vital organ like the heart or brain, the only one playing the harpwill be you.

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