The Book of Blood: From Legends and Leeches to Vampires and Veins

by Hp Newquist

  Where does blood itself begin? Inside your bones.

  The hard outer part of bones protects a soft, spongy material called the marrow. You can see marrow if you break open a chicken bone; it’s darker than the outside of the bone and has a different texture. Your marrow is dark red and is filled with stem cells, which are your body’s master control cells. They can produce any other kind of cell, depending on what your body needs. In the marrow, stem cells’ job is to create red and white blood cells as well as platelets.

  Your bone marrow creates nearly three million red blood cells every second. The average red blood cell lives about four months before it wears out and needs to be replaced. If red blood cells weren’t replenished quickly, you would suffer from a condition called anemia, which happens when there are too few red blood cells in your bloodstream.

  Your skeletal system is the source of new blood.

  Red blood cells (also known as RBCs, erythrocytes, and corpuscles) are unusual because after they are created, they lose their nucleus. Most cells have a nucleus, which acts like each cell’s brain, but RBCs need that space so that hemoglobin can carry as much oxygen as possible. And moving oxygen throughout your body is the primary function of each red blood cell.

  Blood is created in the spongy marrow inside your bones.

  Once red blood cells are formed, they join with white blood cells (called leukocytes) and platelets—also made in the marrow—and start working their way out of the bone and into the bloodstream. This is done via minuscule blood vessels called capillaries, which reach out like small branches from your main blood vessels, the veins and arteries.

  All the components of your blood merge in the bloodstream.

  Capillaries go into all the tight places that larger vessels cannot, especially in your skin. Some are as thick as a human hair, and others are like microscopic threads—so small that they allow only one cell at a time to pass through. When you see blood from a scrape or a pinprick on your skin, you know that tiny capillaries have been pipped open. Because they are so small, they don’t typically gush a lot of blood.

  When new blood cells enter the bloodstream, they must get to the heart and lungs so they can begin their work. This means traveling through the circulatory system.

  The circulatory system is one big network, but it has two distinct parts. The first part is called the pulmonary loop, which is controlled by the right side of your heart. In this loop, your heart pumps blood to your lungs and back again so your blood can dump carbon dioxide and then pick up oxygen. (The process of the heart and lungs working together is called the cardiopulmonary system.)

  The second part of the circulatory system is called the systemic loop, which is controlled by the left side of the heart. This is the path that the blood uses to go from your heart out to your body and back again, delivering oxygen to organs and muscles and icking up carbon dioxide.

  Capillaries are so thin that they permeate your entire body. They would extend tens of thousands of miles if laid out end to end.

  In the pulmonary loop, blood enters the right side of the heart through a large vein called the vena cava. It goes into the top part of the heart and rushes into an open chamber called the right atrium. When the heart contracts (the first thump that you hear when your heart beats), the blood is forced down through an opening into another chamber called the right ventricle. Once it gets there, a fleshy valve closes to keep the blood from flowing backwards. Remember, in your body, blood always flows in one direction and never flows backwards.

  Blood flows from your heart to your lungs and back to bring oxygen into your body.

  The blood is then forced from the right ventricle out of the heart (the second part of the thump-thump) through a blood vessel called the pulmonary artery. This leads the blood into capillaries in your lungs. Here is where the blood starts its real work.

  The blood flushes into all the capillaries in the lungs, which are housed in little broccoli-shaped protrusions known as alveoli. There are anywhere from 300 to 700 million alveoli in the lungs; your lungs have that many alveoli because they need to expose as many capillaries to air as possible.

  Blood returns to your heart through the vena cava.

  * * *

  BLOOD PRESSURE AND PULSE

  Your blood pressure is a measure of how hard your blood is pressing against your blood vessels. This is important because it tells doctors and nurses if your heart is beating at the appropriate strength and if there are potential problems with your arteries' being clogged.

  Blood pressure is recorded using two numbers. The first is systolic pressure, which is the pressure that occurs when your heart beats, forcing blood through your body. The second is the diastolic pressure, the pressure that happens when the heart is between beats and is not forcing blood into your arteries. The numbers are based on the force, or pressure, that blood exerts on the walls of the blood vessels as it flows through them. The numbers are reported in millimeters. For adults, an average number is 120 systolic and 80 diastolic, usually read as “120 over 80."

  Your pulse is something different altogether. It is an indicator of how many times per minute your heart is beating. You can check your own pulse by placing a finger or two gently on “pulse points," where your arteries are close to the surface of your skin. These include the front of your wrist just under your thumb and the crook of your elbow. After exercise, you'll notice that your pulse rate goes way up due to your heart's working harder. If you're sleepy, your pulse will slow down.

  * * *

  When you take a breath, fresh air comes into your lungs. As your lungs fill up, air passes over the alveoli and the capillaries. Hemoglobin attracts oxygen molecules to itself from out of the air and binds itself to them. Once the hemoglobin is filled with oxygen, it turns the RBCs bright red.

  The next beat of the heart pushes this oxygen-fresh blood out of your lungs so that another supply of blood can come in. The fresh blood flows back into your heart, only it goes into the left side this time. It flows into the left atrium through your pulmonary vein. When the left atrium is filled, the blood flows down into the left ventricle. Again, valves keep the blood from flowing backwards.

  As your heart beats, the fresh blood is pumped out of the left ventricle through the aorta. This is the body’s largest blood vessel, almost an inch wide. The force of blood going through the aorta is incredible. It could easily squirt your blood across a room if it weren’t enclosed inside your body. This force is necessary to push the blood through the thousands of miles of vessels that are woven throughout your body. blood flow to body

  After being filled with oxygen, blood is pushed back into your body through your aorta.

  After leaving your aorta, the blood enters the circulatory system. Think of the circulatory system as a river with tributaries and streams branching off it. Instead of all the liquid just going down one big river, it flows into various branches all at the same time. Those branches of the river split off into other, smaller branches, and finally into little streams and brooks where the flow is barely a trickle. The same thing happens with blood. It pushes into all available arteries, and then it works its way, via small arteries called arterioles, into your brain and other organs. The route gets even smaller as the arterioles branch into the capillaries.

  Once blood gets moving, it has a long trip ahead of it ... through miles and miles of your body.

  This satellite photo shows a river with tributaries that allow the water to branch out into the smallest areas. In the same way, the circulatory system branches out through your body with veins, arteries, and capillaries.

  CHAPTER 6

  WHERE THE BLOOD FLOWS

  Blood carries oxygen to every part of the body, especially the brain, which gets 20 percent of all the blood coming out of the heart. Other organs also get a substantial amount, as do the muscles. While blood is moving, oxygen separates from hemoglobin and is absorbed by cells in the organs. These cells use oxygen to keep themselves alive and perform
their own chemical processes.

  RBCs eventually squeeze into the smallest capillaries, wide enough to allow only single cells to pass through. Here, the last of the oxygen is given up. At this point in the journey, the capillaries connect to similarly small vessels called venules, or little veins. This is where the deoxygenated blood begins to make its return trip to the heart.

  Because the oxygen is gone, blood loses its bright red color. It is now a weird maroon-purple color. After leaving oxygen behind, the RBCs start attracting carbon dioxide molecules. These are the waste molecules that organs and tissue cells get rid of while they’re doing their work (such as growing, helping your muscles move, and making your nervous system respond to commands from your brain). As it binds with carbon dioxide, blood moves from the capillaries to the venules and on to the veins.

  The circulatory system is the highway by which your blood reaches every part of your body.

  Just as arteries take blood away from the heart, veins deliver blood back to the heart. But unlike the thick stream of blood that rushes through the aorta at the start of its journey, the blood begins its return trip as single cells working their way through microscopic venules. And instead of flowing from the main circulatory river out into branches, it travels back from the little branches to ever bigger branches until it joins the main flow of blood through the veins. These veins then converge and send the blood on to the vena cava, where it enters the right side of the heart.

  This carbon dioxide-filled blood is forced from the heart’s right ventricle into the lungs. There it goes into the alveoli. Now think back: we started this story with you taking a breath, bringing air into your lungs, and filling up the alveoli with oxygen. The end of the journey, or what happens before you take that new breath of fresh oxygen, is that you exhale, forcing air out of your lungs. When you breathe out, the lungs remove carbon dioxide from the blood before refreshing it with oxygen. That’s why you breathe out carbon dioxide and breathe in oxygen. It’s as if your blood is a riverboat unloading die waste an instant before being loaded up with fresh and valuable cargo.

  This whole process takes about thirty seconds. That seems like a very short period of time for a gallon of blood to make an entire circuit through the body. Keep in mind that you have a lot of blood, and not all of it goes straight to the same place each time it leaves the heart. It is all flowing continuously; there are 110 gaps or spaces between the blood as it moves. It’s happening nonstop, like a hose that’s been left on.

  Even though blood’s main function is to carry oxygen, that’s not the only thing it does. What blood does when it reaches certain organs, and what certain organs do to the blood, is an entirely different side of our story.

  THE OTHER SIDE

  Everything you put in your body ends up in your blood. Food, medicine, air, water—all are plucked from your stomach, lungs, and small intestines by blood. Because each of your organs has its own function, the way it interacts with your blood is very different.

  THE BRAIN

  The brain needs more blood than any other part of your body because every part of your body depends on the brain working properly. Without blood and oxygen, the brain quickly grows weak, which can result in brain damage or death. And without your brain telling everything how to work, your body will shut down, like a car without any gas or electricity.

  More blood goes to your brain than to any other place in your body.

  Almost 20 percent of all the fresh blood coming out of your heart goes straight to your brain. The carotid arteries, which run up both sides of your neck, are two of the most important blood vessels in your body because they provide the biggest rush of blood straight to your brain.

  To make sure that blood always gets to your brain, there is a ring of arterial connections that provide additional pathways into the brain. The ring is called the Circle of Willis. It is named after Thomas Willis, a physician who wrote the first textbook on the brain, in 1664. It’s very easy to recognize the ring because it looks like a human stick figure with a large head. The many pathways in the Circle of Willis ensure that blood gets into the brain even if one artery is damaged or blocked.

  Most of the blood work in the brain occurs in a three-layered brain wrapping called the meninges. The layers of the meninges are the pia, arachnoid, and dura.

  The Circle of Willis ensures that blood gets to the brain by a number of different routes.

  The dura is basically like bubble wrap for the brain. It cushions the brain and prevents it from moving around inside your skull.

  Underneath that is the arachnoid layer, which contains crisscrossing blood vessels that look very much like a spider web. In fact, it is called the arachnoid layer because of the scientific name for spiders, which is “arachnid.” The blood vessels in the arachnoid weave through your cerebrospinal fluid, a clear liquid that floats around your brain. Cerebrospinal fluid has many purposes, one of the most important being that it flows through small spaces in the brain and keeps it clean by removing any garbage that accumulates, such as dead cells. This waste soaks back into the bloodstream via the arachnoid and is washed out of your head.

  The third meninges layer is called the pia mater. It is the layer closest to the brain and fits snugly around the whole brain like a piece of plastic shrink-wrap. The pia contains the primary blood vessels that supply fuel—glucose and oxygen—to the brain. At the same time, the pia also picks up cell waste and brings it back into the bloodstream for disposal.

  This illustration shows that the meninges cover the brain like wrapping paper so that not even blood can get through.

  The interesting thing is that blood goes into the pia, but never actually enters the main parts of the brain. These parts, the cerebrum, cerebellum, and medulla, are where thinking, dreaming, and body control take place. Blood doesn’t get to them because of what is known as the blood-brain barrier. This barrier is made up of blood vessels that are so tiny, only molecules of oxygen and glucose can squeeze through. Not even RBCs or germs can pass the barrier. It’s like a water filter: it lets only the pure stuff enter the brain.

  THE SMALL INTESTINE

  The nutrients that are so essential to your brain and the rest of your body come by way of the food you put into your mouth every day. Once food has been partially digested in the stomach, it makes its way to the small intestine, which is a coiled tube more than twenty feet long. In the small intestine, food is broken down even further, with help from digestive juices supplied by your pancreas and liver. Much of this is done with a strong acidlike chemical called bile. What remains after this breakdown is a watery, liquid full of various nutrients such as sugars and salts, along with fats, vitamins, minerals, and other substances. This liquid is the raw fuel that your body needs.

  Everything you put into your mouth ends up in your blood ... and these are the organs that play a vital role in helping blood do its job.

  The inside of the small intestine is lined with hairlike projections called villi. Like the alveoli in the lungs, they are full of capillaries. The villi soak up the nutrient liquid from the small intestine, and this liquid enters the capillaries. From there the nutrients go to the liver to be cleaned, and then flow to other organs in the body to be used with oxygen as fuel by the cells.

  Anything that the blood doesn’t need from the small intestine heads out to the large intestine and is passed out of your body.

  THE LIVER

  The liver is the largest organ in your body and looks like a slightly squashed football. It is tucked in above your stomach on the right side. It’s one of the most complex organs, involved in as many as five hundred body processes. The liver is also the only organ that can repair itself—and grow back its parts—after being damaged.

  The liver is essential to blood for a number of reasons, especially because it makes sure that blood is carrying the right substances to the rest of the body. The chemicals in the liver go after anything that doesn’t belong in your blood and breaks it down.

  W
hen blood leaves the villi and capillaries in the small intestine, it heads to the liver. Blood enters the liver through two pathways. The first is the hepatic portal vein (“hepatic” comes from the Greek word for liver, “hepar”). At the same time, the hepatic artery is also bringing oxygen-rich blood into the liver. This means that blood from the heart and blood on its way back to the heart are passing through the liver at the same time. The liver is the one place where arterial and venous blood mix together so they can be cleaned simultaneously.

  The liver is your blood system’s biggest filter.

  Chemicals in the liver break down the substances that shouldn’t be in the blood and filter them out. These include caffeine, alcohol, many drugs, bacteria, and even random particles that may have found their way into your blood. They could be anything from food molecules to dirt that might have slipped past your small intestine or entered your body through a wound. The liver also adds a protein called fibrinogen to the blood. Fibrinogen is the basis for fibrin, a threadlike substance that floats in the bloodstream and forms a kind of net when external bleeding occurs. Fibrin is one of the things that create blood clots and keep you from bleeding to death when you are injured.