Experiments With Blood Flow

Show that blood circulates by performing one of William Harvey's classic demonstrations.

If you didn't know anything about how your body works, how would you guess that your blood moves through your body? Do your internal organs manufacture blood from the food and the water that you put into yourself? Is the blood then sent outward from the “factory” inside your torso to feed your muscles and the rest of your body? If you've done a few dissections and observed blood vessels, especially the ones connecting the intestines to the liver, you might suppose that the nutrients are absorbed from your food in your intestines after every meal and then sent to that giant “nutrient sponge” for storage. Then your liver sends the nutrition out to the rest of your body gradually and steadily throughout the day, through the blood tubes? This is what some of the first scientists in history thought, and if you don't know any differently it makes perfect sense, doesn't it?

But if you've done enough exploring, you've discovered that there are actually two sets of tubes, both rooted in the torso, and both spreading in branches throughout the body. Are they for carrying different materials? (If you dissect a cadaver, as ancient scientists sometimes did, you might find one set full of blood, and the other empty. Maybe the second set is for carrying air?) Maybe both tubes are for carrying blood, but then why would you have two sets of tubes carrying the same material and going to the same places? Until and unless we can untangle the complicated mess of tubes in the torso, the only clues we have are the subtle differences between the two tube systems. One has slightly thicker, almost muscular walls, while the other set has only thin sheets of sealant as walls. The thin-walled set goes everywhere throughout the body, and we can often see them puffing up in the surface of the skin. The thicker-walled set runs to all parts of the body, except we never see them come near to the surface, except maybe in the joints. The body tries as hard as it can to hide, bury, and protect this thicker-walled set of tubes. And the thicker-walled set pulses. You can feel your pulse in several places, but these places are almost always in the joints, where the thick-walled tubes come near to the surface. The thin-walled set doesn't seem to do anything but trickle. Nowadays, we call the thin-walled, limp vessels that you can sometimes see puffed up in your skin your “veins.” And the deep, pulsing set of tubes are your “arteries.”

Before William Harvey, many people guessed that maybe blood moves in a more complicated way than simple one-way flow from the guts to the extremities, but they were mostly guessing. William Harvey finally convinced the world that something different is happening. Blood does not take a one-way trip from where it is manufactured to where it is consumed. Instead, blood is being constantly recycled and recirculated. It goes around and around in loops, with various ingredients being added or taken away at various points in the cycle. And William Harvey used a few simple but clear and convincing proofs to persuade the world that this is true.

One of Harvey's proofs was a simple calculation. By dissecting a heart, you can easily estimate how much blood it can hold. If you understand how the flaps inside the heart work, you know that this much blood, or a substantial fraction of it, must pass through the heart with each heartbeat. It must be drawn in from the vena cava (your primary vein) every time the heart relaxes, and expelled into the aorta (your primary artery) with each contraction. By measuring your pulse rate, you can easily estimate how many times your heart beats in a day. And by multiplying these two numbers, you can easily estimate how much blood flows through your heart every day. Even if your estimates are extremely conservative, when you compare this quantity to the quantity of food and water that you consume every day, you discover something astounding.

Another of Harvey's proofs is a simple demonstration with a ligature. People had known for a long time that if you tie a cord tightly around a limb, it becomes pale and cold. If you leave it on long enough, the limb eventually becomes painful and can suffer permanent damage. Obviously, this is because you are depriving the extremity of blood. The extremity needs a constant supply of fresh blood from the chemical factory in your torso to keep it nourished and warm. You can demonstrate this easily and safely in the modern world with a simple blood-pressure cuff. (If you want to try this yourself, make sure you have a cuff with a manual inflation bulb, and not an automatic one with an air pump, so that you can control what is happening.) Simply tighten the thumb-wheel to close the release valve, wrap the cuff around your arm, and pump up the pressure. (Obviously, you will want to keep the release valve and the velcro strap easily accessible, so that you can quickly release the pressure and/or remove the cuff if it becomes necessary. But I've done this demonstration many times and I've never had any emergencies.) If you tighten down the cuff like this, and then wait for a minute or two, you will notice the extremity becoming pale and cold.

A High-Pressure Ligature on the Left Arm

(For this to work well, the cuff needs to be well-placed on your arm and the pressure needs to be fairly high, so that no blood is getting through. And your results may vary depending on your individual anatomy. In my case, the middle sections of my fingers would show a fairly dramatic change of color, but the difference in color in my forearm and the back of my hand wasn't that strong. The change in temperature was not huge, but it was definitely noticeable. If I offered to let children feel my two hands, they would usually remark that the pale one was definitely colder than the other. You can also try pressing against the skin to form a pale spot, and you should find that the color returns very slowly if at all.)

So far, we haven't really found anything surprising. Your extremity needs fresh blood from your torso to stay nourished and warm, and you are stopping the flow of blood, so it becomes pale and cold. But here's the fun part. What happens if we tighten the ligature half way? When you apply a medium-pressure ligature, something remarkable happens. The limb does not become pale and cold. In fact, the opposite thing happens! The limb becomes flushed and red, as if it were inflamed!

A Medium-Pressure Ligature on the Left Arm

(The effect is quite vivid if you start with a strong ligature and a pale limb, and then gradually release the pressure. In my experience, the effect isn't quite as dramatic if you start with a normal limb and then gradually increase the pressure.)

Can you figure out what's going on? How can blocking the flow of blood result in more blood in the extremity? Think about the two tube systems — the limp, trickling veins, and the pulsing arteries — and maybe you can realize what's happening.