Blood

• 1. Introduction

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  Blood is the body fluid that performs a paradox: it is bright red when its hemoglobin is oxygenated, and a darker shade of red when it is not. That single molecule, hemoglobin, gives blood its color and increases the total oxygen capacity of the body seventyfold. Yet color is only the surface of a far stranger story. The average adult carries roughly 5 liters of it, about 1.3 gallons, weighing in at 7% of body weight. Its density, around 1060 kilograms per cubic meter, sits remarkably close to that of pure water. How does a fluid this ordinary carry oxygen, fight infection, seal its own leaks, and regulate the heat of the entire body? Why did ancient Greeks believe it was made from food, and why does Leviticus 17:11 declare that the life of a creature is in the blood? The answers run from the bone marrow to the battlefield of religion, from a jumping spider's legs to a sculptor casting his own head in frozen blood.

• 2. Cells And Plasma

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  Fifty-five percent of blood is plasma, a straw-yellow fluid that is 92% water by volume. The rest is cells suspended within it: red blood cells, white blood cells, and, in mammals, platelets. By volume the red blood cells make up about 45% of whole blood, the plasma about 54.3%, and the white cells just 0.7%. One microliter of blood holds between 4.7 and 6.1 million erythrocytes in males, and 4.2 to 5.4 million in females. The proportion of blood occupied by red blood cells is called the hematocrit, normally about 45%. Mature red blood cells in mammals lack a nucleus and organelles entirely. Their combined surface area would be roughly 2,000 times as great as the body's exterior surface. White blood cells number 4,000 to 11,000 per microliter, destroying old or aberrant cells and attacking pathogens; their cancer is called leukemia. Platelets, numbering 200,000 to 500,000, take part in clotting, where fibrin from the coagulation cascade creates a mesh over the platelet plug. Plasma carries more than cells. It holds serum albumin, blood-clotting factors, immunoglobulins, lipoprotein particles, and electrolytes that are mainly sodium and chloride. Remove the clotting proteins from plasma and what remains is called serum, mostly albumin and immunoglobulins. Whole blood, plasma and cells together, behaves as a non-Newtonian fluid.

• 3. The Narrow Range

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  Blood pH is held within the range of 7.35 to 7.45, making it slightly basic. Drift below 7.35 and the blood is too acidic; rise above 7.45 and it is too basic. A pH below 6.9 or above 7.8 is usually lethal. Keeping the fluid inside this band is the work of the respiratory and urinary systems, which adjust the balance through a process called compensation. An arterial blood gas test measures pH alongside the partial pressures of oxygen and carbon dioxide and the level of bicarbonate. About 70% of carbon dioxide is converted to bicarbonate ions by the enzyme carbonic anhydrase inside the red blood cells. Roughly 7% dissolves in the plasma, and about 23% binds to hemoglobin as carbamino compounds, attaching not where oxygen binds but to the N-terminal groups on the four globin chains. These chemistries are linked. When oxygen levels rise, carbon dioxide binding falls, an interaction known as the Haldane effect. When carbon dioxide rises or pH drops, oxygen is offloaded from hemoglobin, the Bohr effect. Deoxyhemoglobin, meanwhile, binds most of the hydrogen ions, since it has a far greater affinity for them than oxyhemoglobin does.

• 4. Carrying Oxygen

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  98.5% of the oxygen in arterial blood is chemically combined with hemoglobin, while only about 1.5% is physically dissolved in the blood's other liquids. Each hemoglobin molecule carries four heme groups, and its oxygen binding capacity runs between 1.36 and 1.40 milliliters of oxygen per gram. In adults at rest, hemoglobin leaving the lungs is about 98 to 99% saturated, delivering between 950 and 1150 milliliters of oxygen per minute. The body at rest consumes roughly 200 to 250 milliliters per minute, so blood returning to the lungs is still about 75% saturated. Sustained exercise changes those numbers sharply. In a trained athlete, the oxygen saturation of venous blood can fall below 15%, and even arterial saturation may drop to 95% or less. Sustained hypoxia below 90% is dangerous to health, and severe hypoxia below 30% may be rapidly fatal. A fetus faces a problem an adult never does. Receiving oxygen through the placenta, it lives at about 21% of the oxygen pressure found in an adult's lungs. To survive there, it produces a different form of hemoglobin with a much higher affinity for oxygen, hemoglobin F.

• 5. The Circuit

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  William Harvey described the circulation of blood in 1628. The strong left ventricle pumps blood through arteries to peripheral tissues, where it returns to the right atrium through veins, passes into the right ventricle, and is driven through the pulmonary artery to the lungs. From the lungs it returns through the pulmonary veins to the left atrium and into the left ventricle to begin again. There is one telling exception to the usual rule. Arteries carry oxygenated blood, except the pulmonary arteries, which carry the most deoxygenated blood in the body, while the pulmonary veins carry oxygenated blood. The skeletal muscles assist this circuit, compressing veins and pushing blood through their valves toward the right atrium. The cells of blood are manufactured in the bone marrow in a process called hematopoiesis. During childhood almost every bone produces red blood cells, but in adults production narrows to the larger bones: the vertebrae, the sternum, the ribcage, the pelvic bones, and the bones of the upper arms and legs. A healthy erythrocyte lives about 120 days before the spleen and the Kupffer cells in the liver break it down. The liver also produces most of the protein component of blood, while the kidney secretes waste products into the urine.

• 6. Heat And Pressure

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  Blood circulation transports heat throughout the body, and adjusting that flow is central to thermoregulation. In warm weather or strenuous exercise, more blood reaches the surface, warming the skin and speeding heat loss. When the outside temperature is low, flow to the extremities and skin is cut back, and blood is sent preferentially to the important organs. Flow is not evenly shared among organs. The liver has the most abundant supply at roughly 1350 milliliters per minute, followed by the kidney at 1100 and the brain at about 700. Measured per 100 grams of tissue, the order shifts, with the kidney, adrenal gland, and thyroid ranking first, second, and third. Blood can also act as a fluid under pressure. Restricting its flow in specialized tissues causes engorgement and erection, as in the erectile tissue of the penis and clitoris. The jumping spider offers a more dramatic case. Blood forced into its legs under pressure straightens them for a powerful jump, sparing the animal the need for bulky muscular legs.

• 7. When Blood Fails

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  A healthy adult can lose almost 20% of blood volume, about 1 liter, before the first symptom, restlessness, appears, and 40%, about 2 liters, before shock sets in. Shock is the ineffective perfusion of tissues, and it can follow blood loss, infection, or poor cardiac output. Atherosclerosis narrows arteries as atheroma lines them, a process worsened by smoking, hypertension, excess circulating lipids, and diabetes mellitus. When tissue is inadequately perfused, the result is ischemia; when the supply is blocked entirely, the tissue dies, a process called infarction. Anemia is insufficient red cell mass, confirmed by a blood test when hemoglobin falls below 13.5 grams per deciliter in men or 12.0 in women. It can stem from bleeding, from disorders like thalassemia, or from nutritional deficiencies, and may require transfusion. Hemophilia, a genetic illness, disrupts one of the clotting mechanisms, often producing hemarthrosis, bleeding into joint spaces that can be crippling. Blood is also a vector of infection. HIV, the virus that causes AIDS, is transmitted through blood, semen, or other secretions, and hepatitis B and C spread primarily through blood contact. Bacterial infection of the blood is called bacteremia or sepsis, and bloodstained objects are treated as a biohazard.

• 8. Symbol And Sacrament

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  Robin Fahraeus, the Swedish physician who devised the erythrocyte sedimentation rate, suggested that the ancient Greek theory of the four humors arose from watching blood clot in a transparent container. Left undisturbed for about an hour, drawn blood separates into four layers: a dark clot at the bottom read as black bile, red blood cells above it as blood, a whitish layer of white cells as phlegm, and clear yellow serum on top as yellow bile. Greek thinkers believed blood was made from food, a view traceable to Homer's Iliad. Plato thought fire in the belly transformed food into blood; Aristotle placed that transformation in the heart. The science of blood types came much later. Karl Landsteiner discovered the ABO blood group system in 1900, and Jan Jansky classified blood into the four types A, B, AB, and O in 1907. The Rhesus factor was discovered in 1937, and the first non-direct transfusion was performed on the 27th of March 1914. Across religions, blood carries weight far beyond medicine. Leviticus 17:11 states that the life of a creature is in the blood, grounding Jewish, Christian, and Islamic dietary law. Among the Germanic tribes, blood from sacrifices called Blots was sprinkled on walls, statues, and participants; the Old English word for this act, bloedsian, was borrowed by the Roman Catholic Church and became to bless. The Ancient Greeks believed the blood of the gods, ichor, was poisonous to mortals. The same fluid that draws lines of kinship, expressed in phrases like blood is thicker than water, also fed the Eastern European Nosferatu legend of a human who drinks the blood of others for immortality.