TÀI LIÊU Y HỌC CẦN THAM KHẢO
To the Mesopotamians, the liver was the body’s premier organ, the seat of the human soul and emotions. The ancient Greeks linked the liver to pleasure: The words hepatic and hedonic are thought to share the same root.
The Elizabethans referred to their monarch not as the head of state but as its liver, and woe to any people saddled with a lily-livered leader, whose bloodless cowardice would surely prove their undoing.
Yet even the most ardent liverati of history may have underestimated the scope and complexity of the organ. Its powers are so profound that the old toss-away line, “What am I, chopped liver?” can be seen as a kind of humblebrag.
After all, a healthy liver is the one organ in the adult body that, if chopped down to a fraction of its initial size, will rapidly regenerate and perform as if brand-new. Which is a lucky thing, for the liver’s to-do list is second only to that of the brain and numbers well over 300 items, including systematically reworking the food we eat into usable building blocks for our cells; neutralizing the many potentially harmful substances that we incidentally or deliberately ingest; generating a vast pharmacopoeia of hormones, enzymes, clotting factors and immune molecules; controlling blood chemistry; and really, we’re just getting started.
“But if your liver fails, there’s no machine to replace all its different functions, and the best you can hope for is a transplant.”
And while scientists admit it hardly seems possible, the closer they look, the longer the liver’s inventory of talents and tasks becomes.
In one recent study, researchers were astonished to discover that the liver grows and shrinks by up to 40 percent every 24 hours, while the organs around it barely budge.
Others have found that signals from the liver may help dictate our dietary choices, particularly our cravings for sweets, like a ripe peach or a tall glass of Newman’s Own Virgin Limeade — which our local supermarket chain has, to our personal devastation, suddenly stopped selling, so please, liver, get a grip.
Scientists have also discovered that hepatocytes, the metabolically active cells that constitute 80 percent of the liver, possess traits not seen in any other normal cells of the body. For example, whereas most cells have two sets of chromosomes — two sets of genetic instructions on how a cell should behave — hepatocytes can enfold and deftly manipulate up to eight sets of chromosomes, and all without falling apart or turning cancerous.
That sort of composed chromosomal excess, said Dr. Markus Grompe, who studies the phenomenon at Oregon Health and Science University, is “superunique,” and most likely helps account for the liver’s regenerative prowess.
Scientists hope that the new insights into liver development and performance will yield novel therapies for the more than 100 disorders that afflict the organ, many of which are on the rise worldwide, in concert with soaring rates of obesity and diabetes.
“It’s a funny thing,” said Valerie Gouon-Evans, a liver specialist at the Mount Sinai School of Medicine. “The liver is not a very sexy organ. It doesn’t look important. It just looks like a big blob.
“But it is quietly vital, the control tower of the body,” and the hepatocytes that it is composed of “are astonishing.”
The liver is our largest internal organ, weighing three and a half-pounds and measuring six inches long. The reddish-brown mass of four unevenly sized lobes sprawls like a beached sea lion across the upper right side of the abdominal cavity, beneath the diaphragm and atop the stomach.
The organ is always flush with blood, holding about 13 percent of the body’s supply at any given time. Many of the liver’s unusual features are linked to its intimate association with blood.
During fetal development, blood cells are born in the liver, and though that task later migrates to the bone marrow, the liver never loses its taste for the bodywide biochemical gossip that only the circulatory system can bring.
Most organs have a single source of blood. The liver alone has two blood supplies, the hepatic artery conveying oxygen-rich blood from the heart, the hepatic portal vein dropping off blood drained from the intestines and spleen. That portal blood delivers semi-processed foodstuffs in need of hepatic massaging, conversion, detoxification, storage, secretion, elimination.
“Everything you put in your mouth must go through the liver before it does anything useful elsewhere in the body,” Dr. Lok said.
The liver likes its bloodlines leaky. In contrast to the well-sealed vessels that prevent direct contact between blood and most tissues of the body, the arteries and veins that snake through the liver are stippled with holes, which means they drizzle blood right onto the hepatocytes.
The liver cells in turn are covered with microvilli — fingerlike protrusions that “massively enlarge” the cell surface area in contact with blood, said Dr. Markus Heim, a liver researcher at the University of Basel.