Issue 105 / May - June 2015
The Impeccable Sanitation of the Blood
Think of a marvelous machine that consists of pipes, pumps, processors, and plugs. This machine grinds and grates, pumps and pours, moves and maneuvers. It constructs and consumes constantly. Despite all this action and activity, it never rusts or ruptures. I believe most of you know what I'm trying to get at. Yes, this machine is indeed the human body. The brain, the heart, the lungs, and the kidneys are in a constant state of function. A central player in all these functions is the vital fluid we call blood. It continuously monitors, cleans, nurtures, and balances without wasting anything, and does all these while keeping itself pure and pristine. How does it maintain its constitution and purity without wasting even a single molecule, while carrying out numerous tasks all over the body? This, my friend, is what I will try to explain in this article.
One of the processes that occurs in the body is called "inflammation." Inflammation occurs when a cut into the skin also punctures a blood vessel. This situation directly exposes the blood to the air. Inflammation occurs in a few steps. First, the blood vessels near the wound are expanded (which causes the swelling that we see near the cut) and special proteins called "fibrins" are brought in. These fibrins bind to each other to form a net-like structure. We are all quite familiar with this net, which we call a blood clot. This net stops the bleeding and cuts the interaction between the air and the blood within a few minutes.
Next, it is time to quickly eliminate any foreign objects that got into the tissue. Special immune cells called "macrophages" are sent to the crime scene to clean up. Macrophages are large white blood cells that "eat" microbes and other foreign objects using a process called "phagocytosis". After the scene is all cleaned up, these macrophages excrete special molecules that induce tissue repair and return the blood flow to normal. An important note here is the specific order of these events. Like every single process in the body, they occur in the most purposeful way possible. What do I mean? I mean that, first the wound is closed urgently; second, macrophages are sent in; third comes the tissue repair. Any other order would have greatly lowered the effectiveness of inflammation. Imagine that the wound is closed after the macrophages are sent in. Then, by the time the macrophages killed all the microbes, twice as many would have entered the scene. The body seems to know every single event beforehand and plans its defense accordingly.
Let's say a microbe managed to sneak into the blood before inflammation occurred, and is long gone to another region of the body. Does the microbe win? Unfortunately for the microbe, it has to pass another test. This time the tester is the lymphatic system. The lymphatic system is the sewage system of the body. When the blood transfers its nutrients to the tissue, the fluid goes through the "interstitial area" (the empty space between organs). During this process, some of the fluid stays in this area and starts to accumulate. This is where the lymphatic system kicks in. The lymphatic system consists of many tubes running parallel to the blood vessels and recollects any excess fluid and transports them to the subclavian vein near the neck. This way, excess fluids of the body and all of the molecules in them, are reintroduced into the circulation. If there is a problem with this process, an abnormality called "edema" occurs. Of course, the blood is a very sensitive fluid because it travels through the whole body and seeps into almost every single cell. If a microbe were to get into it, it would easily spread and cause disease. So, the lympatics first does a checkup on the body fluid. This checkup occurs at special nodes in the system called lymph nodes found all over the body. Two of the most famous lymph nodes are the spleen and the tonsils. Within these nodes are lymphocytes, special immune cells that "tag" bacteria and other microbes to be later destroyed by macrophages. Thus, the blood is continuously cleaned and kept safe from harmful microbes.
Last but certainly not the least, the final inspection the blood goes through takes place in the kidneys. The aforementioned two checkpoints prevent the entrance of any foreign materials into the blood, and the elimination of any microbes lucky enough to somehow make it through. So, the only task to be completed is the elimination of excess molecules formed in the metabolism. For example, the blood in the veins (the vessels that carry carbon dioxide formed by the respiration of cells) is carried to the lungs where the carbon dioxide is exhaled. But, a much more precise mechanism comes into play in the kidneys. Blood vessels that come from all around the body form a knot-like structure in the kidney called the "glomerulus." This knot-like shape increases the surface area of filtration. The blood running from the glomerulus is then filtered into the "Bowman capsule," which surrounds the glomerulus.
But wait! The sanitation system is not satisfied with this first filtration and "thinks" that the filtrate is not ready to be excreted by the urine. So, a more delicate filtration occurs right after the filtrate enters the "proximal tubule." While passing through this tubule, essential molecules are immediately reabsorbed into the body. The most valuable of all these molecules is glucose, since it is the main source of energy in the body. The proximal tubule reabsorbs around 98 % of all the glucose, while the distal tubule scouts out the rest. After the tubules are done with the filtration, not a single glucose molecule is left in the urine. As a matter of fact, the presence of even a few glucose molecules in the urine leads to a diagnosis of "renal glycosuria."
After the proximal tubule, the filtrate goes into the "loop of Henle," where it is dipped into a high-concentration environment. Water travels passively (without the need for energy) from low-concentration to high-concentration areas. In the loop of Henle, the urine is low-concentration, so water runs back into the body. Thus, any excess water in the urine is effectively and economically reabsorbed. The big machine that consists of the glomerulus, the Bowman capsule, the tubules, and the loop of Henle is called a "nephron." Everything described above occurs in a single nephron. The average number of nephrons in one kidney is around 1,000,000. The human bladder holds around 150 ml of urine on average. So, each nephron is actually responsible for 0.00015 ml of urine production. The kidneys filter over 1,000 liters of blood each day, so our blood is kept just as we want it. Millions of tiny nephrons work in unison to take in huge amounts of blood and they know exactly what to leave and what to keep, 24 hours a day, 7 days a week.
Our blood is our life source. It is the milk of the organs, and our organs would dry up without it. Believe it or not, our organs are quite picky. If they are to receive anything they don't like, they will start acting up. In order to keep the organs happy, the three mechanisms mentioned above have to work hard and not make a single mistake. These mechanisms are, of course, also made up of cells. These miniscule cells "know" exactly what their clients on the ends of the body like and don't like, and prepare the blood composition accordingly. Only one word can describe these wondrous mechanisms: Impeccable.
Brian Turk is a medical student from New Jersey. He writes on medicine, health, and biology on a freelance basis.