by Bill Schutt
Most historical accounts of mammalian circulation emphasize the work of William Harvey, who, in the early seventeenth century, used scientific methodology to prove that blood did not ebb and flow like the tides. Instead, the heart pumped it around the body in a pair of loops, one to the lungs and back (the pulmonary circuit), and one to supply the body and its tissues (the systemic circuit).‡53 Before Harvey’s discovery, the general belief was that disease-laden “bad blood” had a tendency to pool in the extremities where it would stagnate. Bloodletting, therefore, was a way to eliminate the bad blood. Unfortunately for George Washington (and countless other patients), physicians mistakenly thought that the often copious amounts of blood they drained would be replaced within a very few hours by new, healthy stuff. As Dr. Craik and his colleagues would never learn, this just wasn’t the case.
Beginning in the seventeenth century, bloodletting was not generally undertaken by physicians or surgeons (the top two rungs of the medical practitioner’s ladder, respectively). Procedures such as therapeutic phlebotomy, leeching, and even minor surgeries were usually carried out by a lower class of medical personnel, the barber-surgeons (who were themselves a rung or two higher than midwives). Barber-surgeons were the descendants of the bath men who toiled in medieval bathhouses. Both had duties that included shaving, cutting hair, bleeding patients, administering enemas, and changing wound dressings. During wars, some barber-surgeons traveled with their respective armies—treating fractures and probing for bullets. They became the first military surgeons. Back home, they advertised their talents with a striped barber pole outside their establishment—the red stripes signifying blood, blue stripes were veins, and white stripes represented the gauze bandages they used to stem the bleeding. The pole itself was a symbol of the stick that patients would grip tightly as they were being bled and the ball atop the pole signified the blood collection basin (and the container they used to hold leeches).
Barber-surgeons “breathed veins” to treat all serious maladies and any number of lesser complaints, from asthma and bone fractures to drunkenness and pneumonia. Women were bled to reduce menstrual flow and “lunatics” were drained to treat mental illness. Even drowning victims were bled!
In modern times, with the remarkable medical advances that we see on an almost daily basis, it’s easy to overlook the fact that in many ways medical research was relatively stagnant from the time of the ancient Greeks until the first decades of the twentieth century.
One of those responsible for attempting to revive experimental medicine was Andreas Vesalius. Born into a family of Belgian physicians, Vesalius received his doctorate in 1537 from the University of Padua, where he soon became the chair of surgery and anatomy. There, as in the other early medical schools, Galen’s massive literary output served as the basis for all relevant courses and their syllabi. But rather than blindly accepting Galen’s well-worn teachings, Vesalius took a new and dangerous approach. He employed dissection in his classroom and preached a hands-on approach to his students. Fortuitously, a sympathetic judge gave Vesalius access to the corpses of executed criminals. The young anatomist not only studied their anatomy but also produced a set of remarkable and highly detailed anatomical diagrams, which were included in his seven-volume On the Fabric of the Human Body. It was his masterwork and it hammered Galen’s inaccurate and erroneous views on anatomy into the ground like so many tent pegs. Using cadavers, Vesalius disproved Galen’s concept of invisible pores in the heart. He also demonstrated that the human heart had four chambers (not three) and that half of the body’s major blood vessels did not originate in the liver (as described by Galen). Additionally, Vesalius clearly showed that the liver itself was not the five-lobed organ that Galen claimed it was.
Understandably, Vesalius (who was not yet thirty) upset many of the Galen faithful by dismantling so many of their master’s long-held claims. One outraged Galenite went so far as to publish a paper in which he asserted that the work of Vesalius didn’t prove Galen wrong, it simply indicated that the human body had changed since Galen’s time.
Vesalius died in 1564, after his ship was wrecked returning from a pilgrimage to the Holy Land. A long-held rumor that he had fled to the Mideast to escape the Inquisition (after dissecting a “corpse” whose heart suddenly started beating) has been discredited.
Blood is a very special juice.
—Mephistopheles, speaking to Faust
5.
THE RED STUFF
One day while I was rummaging around in a tidal flat on the South Shore of Long Island, a stray bit of metal ribbon sliced a neat new crease in my wrist. I was around ten years old at the time and I remember staring in silent fascination as the blood welled up and then began to run out of the half-inch cut and down my arm.
My mother, who had been standing nearby having a smoke with my aunt Rose, must have come up behind me to see what I was doing. (I guess I hadn’t moved from where I was squatting for nearly a minute, and in my mother’s mind this was clearly reason enough for alarm.)
“Are you out of your friggin’ mind?” she screamed, nearly sending me sprawling into the mud.
“Oops,” I said.
Realizing immediately that I’d have to defuse the situation, I pointed to my wounded wrist. “It doesn’t hurt, Mom,” I remarked cheerfully.
“You are out of your friggin’ mind,” my mother shrieked, grabbing me by the nearest nonbleeding appendage and hauling me toward Aunt Rose.
This is not going to be pretty, I thought.
I should explain that when I was a kid, I had something like eight Aunt Roses. As you can imagine, telling them apart was something of a dilemma (“That’s Aunt Rose DiMango, not Aunt Rose DiDonato!”). So, inspired by the Peterson Field Guide series, I approached the problem by developing a set of identifiable field characteristics based on traits like “total body length” and “facial mole placement” (an original creation of mine). Within the Aunt Rose group, individuals ranged in height from a kid-friendly four feet eight inches on up to a towering sixty inches. This one was a midsized Aunt Rose, easily identifiable however, by her miniature poodle, Fifi, as well as her unique talent for combining body language and swearing into a kind of interpretive Italian dance. And as my mother dragged me toward the sidewalk, bleeding and ear tweaked, I couldn’t help noticing that my aunt had already initiated some preemptive hand gestures (holding an index finger to her temple, thumb extended skyward).
I don’t remember too much about the cleanup—although I do recall getting blood on one of Aunt Rose’s bathroom statues. (“Gracie, the sa-na-va-bitch is bleeding all over the Virgin Mother!”)
Many people have been and continue to be intrigued by blood, while others (like my mom and at least five of my aunt Roses) are—how should I put this?—somewhat less than intrigued by it. But whether you were fascinated by blood or repelled by it, as a child in the 1960s we were all starting to see quite a bit more of the red stuff. The assassinations of President Kennedy and his brother Senator Robert Kennedy, the student deaths at Kent State, and the Manson murders were beamed right into our living rooms, their visual horror intact. Color news footage from the Vietnam War seemed to take a hard turn toward graphic around the time that Sam Peckinpah’s Wild Bunch and Arthur Penn’s glamorized versions of Bonnie and Clyde were going down in slow motion—in a blaze of explosive Technicolor squibs.
Forty years ago, the sight of all that red was shocking. Today, some of us are still repelled by blood. Others are titillated by it (think of the mountains of money brutal gore-fests like the film Saw have taken in). Still others have become acclimated to it. Similar to the way our bodies learn not to respond to inconsequential stimuli (you don’t feel your socks once you pull them on, do you?), we have adapted to the sight of blood with a corresponding decrease in sensitivity.
So what is blood, exactly? One answer is that it’s food for the creatures inhabiting this book. Because of that, I don’t feel badly about taking a few detours to explore the substance a
bit.
An anatomist might start by describing blood as a connective tissue—just like bone, cartilage, tendons, and ligaments. Confused? Well, not for long, once you figure out what it takes to be considered a card-carrying connective tissue. Tissues are accumulations of different types of cells (along with their matrix, which is the noncellular medium that surrounds them). At an organizational level, they’re one rung on a kind of hierarchical ladder that characterizes all living things. In this regard, tissues are a rung above cells and a rung below organs, which are structures that are composed of several different tissues. Taking this hierarchy a bit further, several organs, working together, form an organ system, and organ systems combine to form an organism. Going in the opposite direction on our ladder, cells are made up of subunits called organelles (such as the oft-memorized mitochondria), and organelles are composed of biochemical units like proteins and lipids. And so on.
Okay, back to tissues.
The next requirement for a tissue is that its cells work together in some particular function. For example, nervous tissue is composed of neurons and a support team of glial cells—each contributing in specific ways to the functioning of the nervous system.*54
Connective tissue is characterized by being composed of relatively few cells surrounded by a significant amount of noncellular matrix. As a result, connective tissue cells are generally not in contact with each other. Think of them as bricks in a wall, with the matrix being the mortar that surrounds them and binds them to each other. The matrix is also what gives connective tissues their physical properties. For example, the hardness of bone comes from calcified bone matrix, not from the bone cells (osteocytes) themselves.†55
In blood, the matrix is called blood plasma and it’s neither solid nor gel—it’s a liquid (composed primarily of water)—and this property is just as important functionally as hardness, flexibility, and strength are for the other types of connective tissue. The reason is that plasma acts as the transport medium for blood cells, as well as tiny cell fragments called platelets that are involved in blood clotting. Additionally, many other important substances are carried within the plasma in a dissolved state, including nutrients, vitamins, hormones, waste products, gases, and ions.
Pumped out of the heart by the muscle-bound left ventricle, the force exerted by the blood on the inside of the vessels it passes through is known as blood pressure. When the left ventricle contracts, expelling its arterial blood, the blood pressure increases. This produces the higher number (known as the systolic pressure) in a typical blood pressure measurement. As the left ventricle empties, relaxes, and begins to fill again, the blood pressure drops, producing the lower diastolic pressure.*56
As the arterial blood nears its destination, it moves from arteries to smaller arterioles and finally to miles and miles of microscopically tiny (and thin) capillaries. These mini-vessels form dense, netlike beds around organs and other structures. During this passage to smaller and smaller blood vessels, the blood pressure drops significantly. To understand how this pressure drop takes place, visualize the water traveling through a garden hose. Now imagine that the far end of that hose begins to split into smaller and smaller tubes, each of those tubes splitting again and again—until the end of the hose has been divided into a million tiny branches. The pressure of the water in any one of these branches would be far less than the original pressure. Basically, this is because of the tremendous increase in the total combined area inside the millions of branching tubes (as compared to the area inside the original hose). This is also the reason why the water pressure drops when everybody decides to take a shower at the same time.
But not only are these low-pressure capillaries incredibly small, their walls are so thin that once the blood gets to its destination, nutrients and oxygen contained within the blood plasma simply diffuse through the capillary walls to supply the surrounding tissues and their cells.*57
Metabolic waste products and carbon dioxide move in the exact opposite direction (from the tissues into the blood) via the same process. Once within the capillaries, they start their journey back to the heart, passing through increasingly larger vessels (venules leading into veins) before entering the heart’s right atrium. Unfortunately, this low-pressure blood sometimes has a hard time returning from places like the legs and feet—since it must overcome the considerable force of gravity. Under normal conditions, venous return is aided by a series of one-way valves and something called the skeletal muscular pump. In places like the calf muscles, involuntary muscular contractions squeeze the veins traveling through them. The low-pressure blood within the compressed veins shoots upward against gravity and back toward the heart, while the valves prevent backflow of the blood toward the feet. You can actually see this process in action by sitting cross-legged and watching your calf muscles. The irregular twitching you see is the skeletal muscular pump in operation.
Unlike large complex creatures (like us), microscopic organisms don’t have blood or a wildly intricate circulatory system (and they don’t have organs or organ systems either). For the majority of species on this planet, things are generally much simpler.
Picture how easy it would be for a single-celled amoeba to exchange gases with its environment. With only one cell to supply, oxygen and other incoming substances are obtained directly from the environment. Although some of this transport requires energy expenditure, in many instances, incoming and outgoing material and gases simply follow concentration gradients across the organism’s thin cell membrane. Now imagine a creature shaped like a ball—composed of millions of cells. How would the cells toward the center of the ball get their nutrients and oxygen or rid themselves of waste? Although you might be able to think up several ways that this might occur—the solution that evolved for many creatures here on earth consists of a muscular pump (the heart), a remarkably intricate vascular transportation system, and a unique and versatile tissue carried within it: blood.
Before we get too carried away with just how cool our circulatory systems are, you should know that there are some relatively large organisms that get by just fine without elaborate circulatory systems. Insects, for example have low-pressure systems that are termed open circulatory systems because they don’t form a completely closed loop between the body and heart. Hemolymph (the arthropod equivalent of blood) is circulated through the body by a series of heartlike dorsal pumps and by movement of the insect’s body. The hemolymph moves through vessels that eventually lead to open sinuses called hemocoels. Here, the surrounding internal organs are literally bathed in the nutrient-bearing fluid, which eventually percolates back and reenters the hearts through tiny valves called ostia.
The key here is that unlike animals like vertebrates, insect circulatory systems aren’t involved in transporting gases like O2 and CO2 or exchanging those gases with body tissues.*58 A mosquito’s oxygen requirements are met through a series of openings (spiracles) along both sides of its thorax and abdomen. Air passes from the environment into the spiracles (which can also be closed to prevent water loss), and then through a complex of tubes called tracheae. The tracheae get smaller and smaller, finally branching into microscopic vessels called tracheoles, through which the air finally arrives to supply the tissues and cells.
This system works just fine for small creatures, but there are limitations. For example, tracheal respiration is probably a key reason why mosquitoes (and other insects like bed bugs) aren’t a whole lot bigger in size than they are. Larger animals are composed of too many cells to be efficiently supplied by this type of respiratory system.
Some of you might be saying, “Wait a minute, what about those pictures of ancient dragonflies with three-foot wing spans? How did they get enough oxygen?”
The answer is that there is evidence from the Carboniferous period (290–360 million years ago) that widespread forests and lush plant life resulted in a higher percentage of atmospheric O2 than exists in today’s atmosphere. This extra O2 was apparently enough to support larger specie
s of insects that employed the same tracheal system as their smaller modern-day cousins. Still, during the Carboniferous or even during the age of dinosaurs (when some creatures reached gigantic proportions), there is absolutely no evidence of Mothra-sized insects (or, for that matter, the twin fairies who controlled them).
To help carry out myriad circulatory functions, the average human has between 1.2 and 1.5 gallons (4.5 to 5.7 liters) of blood in their body at any given time. Blood plasma constitutes 55 percent of that volume, while cells (red and white blood cells) and platelets (classified together with cells as “formed elements”) make up the remaining 45 percent. Water makes up about 92 percent of blood plasma, with dissolved stuff making up the remaining 8 percent. The majority of these solutes are proteins produced in the liver.
To be considered a tissue, blood needs to be composed of several cell types—and it is. Blood cells (or corpuscles) come in two flavors: erythrocytes (red blood cells) and leukocytes (white blood cells). Erythrocytes (from erythrós, the Greek for “red”) are by far the most numerous, making up over 99 percent of blood cells. Function-wise, they’re very much like the old Kentucky Fried Chicken (as opposed to the new KFC). That is “they do one thing and they do it right.” The one thing they do is carry oxygen, and they do it right because each erythrocyte is literally stuffed full of an iron-containing pigment molecule called hemoglobin. Hemoglobin acts like an oxygen magnet, picking it up where it’s plentiful (like in the lungs right after you take a breath) and dumping it off in places where it’s in short supply (like tissues, whose cells require a constant supply of O2> and nutrients). Hemoglobin is so effective at carrying O2> (compared to say, water) that without it a person would need to have seventy-five gallons of fluid circulating in their body to carry the required O2>