Blood carries the stuff of life ‹ oxygen, nutrients, water, - throughout the body; and at the same time, carries waste products, like carbon dioxide, to where it can be expelled. The blood also carries hormones, from the endocrine glands to the appropriate destinations; and carries immune system cells to where they are needed to fight foreign invaders.
Blood is comprised of: 1) plasma - the fluid component, 2) a red mass - erythrocytes, red cells that carry oxygen; and 3) a whitish substance called buffy coat, which carries leukocytes (white cells), which are part of the body's immune system; and platelets - pieces of cells whose function is to stop bleeding. [Erythrocytes, or hematocrit, make up about 45% of blood; leukocytes and platelets make up less than 1%, and plasma makes up the rest.
Blood plasma is straw-colored and sticky, and is 90% water; but it also contains ions (like sodium and chloride), nutrients (like amino acids and simple sugars), waste products (like urea, ammonia, and carbon dioxide), oxygen, hormones, and vitamins. Plasma also contains three types of proteins - albumin, globulins, and fibrinogen. Albumin helps keep the water in the bloodstream. Globulins contain antibodies and proteins that carry lipids (fat), iron, and copper through the bloodstream. And fibrinogen helps blood clotting. Blood plasma that stands, coagulates, or clots; and a clear liquid called serum is left behind - ah ha! the fluid that leaks from scabs. [There will be a test, so pay attention.]
Now lets talk about something called formed elements, or blood cells. They are erythrocytes, leukocytes, and platelets. Actually, neither platelets nor erythrocytes are really cells. They are just cell frag ments - no nuclei (RNA or DNA) or organelles (miniorgans). Only leukocytes are true cells. Because most blood cells are incomplete, they don't divide; instead they are continuously renewed by cell division in bone marrow.
These red blood cells are packed with hemoglobin, which is an oxygen-carrying protein. There are about 25 trillion erythrocytes in the body of a healthy adult. [I wonder who counted them, and how?] They are very efficient transporters of oxygen - one reason being that they don't use any oxygen for energy while doing their job ‹ they are completely anaerobic. The oxygen being transported is transferred from the bloodstream into other body cells through capillaries. Erythrocytes have the ability to contort themselves enough to squeeze through the capillaries and diffuse into other cells.
Erythrocytes live about 100-120 days - a long life for blood cells. They originate in red bone marrow, where they rid themselves of their nucleus and organelles before entering the bloodstream.
These white blood cells (see Note 1) are critical to the body's defense system. These are complete cells, having both nuclei and organelles. These white blood cells are constantly ready to repel invading organisms - bacteria, viruses, and parasites. Leukocytes, while a component of blood, are able to travel outside of the bloodstream to engage the enemy at local sites of infection. They are able to get outside of the bloodstream by squeezing through the porous walls of the capillaries.
There are actually five distinct kinds of leukocytes, which fact I was going to ignore, but the differences are interesting, so let's plunge in: 1) neurophils specifically chase after and kill bacteria - they "eat" smaller ones, and for larger ones they release a chemical that poisons them. The chemical is so strong that it actually kills some of the neurophils, which then become part of pus. [With that thought in mind, I think it's time for lunch.] 2) Eosinophils don't eat bacteria, rather, their most important job is to fight parasitic worms, which they do by releasing an enzyme that gets on the worms and digests them - take that you nasty little buggers; 3) basophils contain histamine and other "stuff" that works against inflammation - they increase permeability of nearby capillaries to let in more leukocytes; 4) lymphocytes are the most important part of the immune system, and like leukocytes, they do their work outside of the bloodstream in the connective tissues, and attack specific foreign molecules; and finally 5) monocytes also pass into connective tissue, where they are transformed into macrophages (phagocytic (see Note 2) cells).
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Note 1 - White blood cells, called leukocytes, originate in bone marrow. Some are released into the bloodstream, while the rest remains in the marrow awaiting release in large numbers to attack invading bodies.
Note 2 - A phagocyte is a generalized term for a cell that eats foreign cells, molecules and small bits of garbage.
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Okay, so where do blood cells come from, you are asking. They are manufactured in bone marrow, through a process called hematopoisis. [Now if someone wants to go somewhere and do something with you, and you don't want to go; just tell them that you are busy hematopoisising - maybe later.]
Bone marrow comes in two varieties: 1) red, which actively generates blood cells, and 2) yellow, which is dormant except in emergencies that require more than normal numbers of blood cells. Red marrow gets its color from immature erythrocytes, while yellow marrow's color comes from the profuse amount of fat cells that are in it. Red marrow occupies the axial skeleton (see Note 3), the upper portion of the humerus and femur bones, and the hip and shoulder girdles. Yellow marrow is found in all other parts of the long bones. New red blood cells move out into the bloodstream as they reach maturity, and, in that way, new blood cells are constantly moving into the bloodstream. Bone marrow, in addition to blood cells, also contains macrophages, which along with macrophages in the spleen and liver, clean the blood.
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Note 3 - The axial skeleton consists of the skull, spine, and rib cage (including the sternum).
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Now that we have discussed the blood and the pump, it is time for the circulatory system; that is, how the blood gets to and from the heart.
In general, dirty (deoxygenated, or oxygen-poor) blood returns to the heart through a network of veins, and clean (reoxygenated, or oxygen-rich) blood leaves the heart through a network of arteries. Two notable exceptions arise with the connections between the heart and lungs. Dirty blood enters the heart from where it is shipped to the lungs via pulmonary arteries Clean blood returns to the heart from the lungs via pulmonary veins. A simple way to remember the flow is that veins carry blood into the heart, and arteries carry it from the heart.
There are three unique types of blood vessels: arteries, veins, and capillaries. From the heart, arteries provide the paths for blood to get to the rest of the body. As they get farther from the heart, arteries branch off into smaller vessels called arterioles. They feed into a network of smaller vessels called capillaries. Nutrients in the blood "leak" out into body tissues. The depleted blood is then moved through the capillary beds and picked up by small vessels called venules, which, on their trip back to the heart, start coming together to form larger vessels called veins, which ultimately return blood to the heart for restoration of oxygen, and recirculation.
All except the smallest vessel walls are constructed of three layers: 1) tunica intima, the innermost, 2) tunica media, and 3) the tunica adventitia, the outermost.
The tunica intima provides a slick surface to allow blood to flow through vessels unimpeded.
The tunica media is made of a combination of rings of smooth muscle fiber and elastin, which contract and relax under control by the autonomic nervous system. This series of contractions helps propel blood throughout the body.
The tunica adventitia is a large layer of connective tissue that protects vessel walls, strengthens them, and anchors them to surrounding tissues.
Now, back to vessel types. First, the arteries. As you remember, they carry blood away from the heart.
Elastic Arteries are the large arteries nearest the heart - the aorta and its main branches.
These vessels contain a lot of elastin, which acts to smooth out the pulsing action caused by the heart. The smoothing out process saves the smaller, more fragile vessels from bursting as a result of the strong pumping from the heart.
Hardening of the arteries, or arteriosclerosis,
causes higher pressure in the larger vessels, and can eventually
result in an arterial rupture.
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Arterioles are the smallest arteries, and directly connect with the capillaries. Arteriole diameter is regulated by contraction and relaxation of the muscle in the vessel walls, to regulate the amount of blood sent to specific capillary beds (e.g., to active skeletal muscles, or the stomach during digestion).
These smallest of the blood vessels form networks, or capillary beds, in nearly all body tissues. They function to deposit nutrients in body cells, and, at the same time, to collect waste products for disposal. The thin, porous, capillary walls allow nutrients and oxygen from the blood to pass into tissue fluid; and cell wastes, like carbon dioxide, to enter the bloodstream on its return trip to the heart/lungs. In addition to reoxygenation, other specialized tasks are done: carbon dioxide enters the lungs to be exhaled, digested nutrients are picked up from the small intestines, hormones from the endocrine glands are ferried to their destinations, and nitrogenous wastes are removed from the kidneys for elimination.
There are two types of capillaries: 1) fenestrated, and 2) continuous. Both with very specific jobs. Fenestrated capillaries have pores, or windows, through which exchanges between blood and tissues can easily take place. Also, capillaries in synovial joints where water molecules are constantly required to maintain levels of synovial fluid, which lubricates them. Continuous capillaries, on the other hand, have no pores. They reside mostly in organs, like skin, skeletal muscles, and the central nervous system (the brain, essentially).
Particularly unique among capillaries are those of the brain. A feature of brain capillaries is the "bloodbrain barrier." Very low permeability ensures that, under normal circumstances, blood doesn't "leak" into brain tissues. There are special transport mechanisms that carry only specific, vitally needed molecules into the brain.
Previously mentioned capillary beds are a network of capillaries where nutrient/waste exchanges are made. Blood can pass through a bed without making exchanges, as with an inactive muscle; but it can alternatively go through a full exchange, as with an active muscle in need of oxygen. Sphincters open or close to control the flow.
As previously stated, veins carry oxygen-poor blood from the capillaries back to the heart for reoxygenation. We have seen how the blood flow is slowed down (smoothed) after leaving the heart, in order to prevent damage to the capillaries; for that reason, venous bloodflow is slow (i.e., the pressure is lower).
Venules are the smallest veins. They pick up "dirty" blood from the capillaries, and join together to form the larger veins. The largest veins actually hold about 65% of a body's blood. Vein walls are not as thick as arteries, and contain less elastin, but are not likely to burst because of the lower pressure. Veins also have a larger center opening, to accommodate the large blood volume.
Because of the lower pressure in veins, they have a series of valves to help the blood return to the heart by closing to prevent backflow. The limbs are particularly valve-dense, because of the strong effect of gravity. Varicose veins are a symptom of weak valves. As the valves weaken, blood pooling occurs, especially in the legs; and the veins swell as circulation is retarded.
Now that we have covered the general features of the blood's circulatory system, it would be time to move on to the specifics of aortic and venous flows, but we won't, except to say that the pathway of reoxygenated blood out of the heart (left ventricle) is via the aorta, and its major branches. The aorta starts upward out of the heart, through the ascending aorta, and immediately begins to curve downward. . This curve is called the aortic arch. The aortic arch sprouts some branches that deliver blood to the neck and head, arms, and the upper part of the chest cavity. The descending aorta, and its major branches carry blood to the rest of the body. Initially, the aorta carries re-oxygenated blood, but nutrients are picked up mainly at the small intestine, where they are broken down from food during the digestive process.
Now that we didn't talk about the aorta, a few words need to be added about the cardiac and cardiopulmonary circulation. Cardiac circulation was included in last month's discussion of the heart, and that won't be repeated, but cardiopulmonary circulation is a different story.
A crucial element in the circulation of blood is re-oxygenation - performed in the lungs, of course.
Oxygen-poor blood returning to the heart, enters through the Superior Vena Cava (blood from above the diaphragm), and the Inferior Vena Cava (blood from below the diaphragm) into the right atrium, from where it is pumped into the right ventricle, and then to the pulmonary trunk, which splits into the right and left pulmonary arteries, and finally into the right and left lungs for re-oxygenation. Oxygen-rich blood is returned to the left atrium, then pumped to the left ventricle, and the cycle continues.
And so ends our discussion of blood, the vessels that carry it throughout the body, and the circulatory system itself. Next month we will explore the lungs and the respiratory system.
If it moves, kick it. If it doesn't move, kick it until it does.
Oh, and, "eschew obfuscation!"
[Not a soccer rule, but maybe it should be.]
Q I keep hearing about plyometric exercises, but what are they?
C.R., Richmond, VA
A I have the eerie feeling that this subject has been addressed before in this Newsletter, but for-the-life-of-me I can't find it, so it will be briefly discussed here, and then more thoroughly discussed in a future issue. They are exercises specifically designed for power and strength. They are designed to train the neural system, which plays an extensive role in developing power and strength. The goal is to recruit a large number of type II muscle fibers (fast twitch) quickly and simultaneously. Using very heavy weights and few reps (1-3) is one way - often done for legs, as by a power lifter going for maximum squat weight. Plyometrics have a similar goal: explosive power, but for different purposes. Jumping exercises are used for the legs, and a medicine ball is used for the upper body. The problem with weights is that there is a deceleration at the end of the initial movement (eccentric contraction); however, jumping and throwing maintain acceleration through a full range of motion (i.e., you don't stop your arm at the end of a throw, you follow through - the same with a jump). Someone like a power lifter would still rely mainly on weight training, but for someone like a basketball player, where explosive jumping power is a need, plyometrics can be an important aspect of his/her training; but these explosive movements aren't for beginners. A good program of flexibility, cardiovascular endurance, and general muscular conditioning should precede a plyometric program. Without a good beginning fitness level, plyometrics can cause injuries.
Jump training can start with something like squat jumps, done with body weight or dumbbells; and can be done two-legged or as split jumps, where you start with one leg forward and switch to the other leg forward before landing.
True plyometric jumps include jumping up-and-down as fast as you can, without squatting - that would slow you down. The ultimate is box-jumping, to be covered in a later issue.
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