White blood cells (leukocytes) are fewer in number and their duty is to fight against germs. How this process works is to be expounded by the immune system under a separate title. The red blood cells are my main building blocks and they are born by the dividing of the main cells in the bone marrow. After passing through a few phases, they lose their nucleus and are filled instead with hemoglobin, a magnificent substance containing iron. Hemoglobin’s most vital function is its binding oxygen and then carbon dioxide after releasing it. Hemoglobin reaches everywhere, traveling with the blood stream. When it comes to the lungs, hemoglobin dumps the carbon dioxide and replaces is with oxygen. Then it supplies this oxygen to the cells and removes the carbon dioxide, which is produced by burning organic compounds. So its short life passes with the same ceaseless cycle to continue your life. Hemoglobin molecules’ longevity is approximately 120 days. They contain no cell elements like ribosome, mitochondria, and nucleus and therefore cannot repair themselves. They simply die when they get old. Sad? Not at all! Red blood cells fulfill the duty they were created for and leave the stage for new ones. They are broken down in the liver and bone marrow and the iron they contain is absorbed. A certain part is transformed into bilirubin, giving bile its yellow color. As you see, nothing is truly wasted.

The red blood cells in circulation number around 25 trillion, and this number does not vary greatly, as the dying ones are constantly replaced. Their measuring gives doctors an idea about possible diseases. The amount depends on various factors’ reciprocal balance. A hormone (erythropoietin) secreted by the kidneys increases the rate of production of red blood cells, in response to falling levels of oxygen in the tissues. If you lose blood due to an accident or medical operation, the stem cells in the bone marrow receive an emergency alert to produce more red blood cells. On the other hand, if you get a blood transfer, stem cells are ordered to stop producing, due to the excess of red blood cells. You see, even such basic knowledge about bodily systems fills the learner with wonder.

Deficiency of red blood cells, scientifically known as anemia, should not be ignored. It results in pallor and weariness; you feel like sleeping more. In order to avoid this condition, your body needs different things such as group B vitamins (B6, B11, B12), vitamin C, amino acids, and iron. Since it is hard to pinpoint the deficient substance, doctors generally prescribe iron-rich multivitamin supplements.

Red blood cells divide into four types, which determine the blood groups A, B, 0, and AB. In addition to the blood group, another feature known as Rh (rhesus) factor is important to know particularly before a blood transfer. Transferring the wrong type of blood may result in death.

Platelets, which are scale-shaped cells and circulate with me are not independent; they are pieces which came off bigger cells. In a cubic millimeter of blood, 250 to 350 thousand of these little scales are found and their duty is of vital importance. If it weren’t for these pieces, the slightest cut could cause death because your bleeding would not stop. Clotting is a great blessing. It usually blocks the surface of a wound within five minutes, stopping the flow of blood and saving your life. Clotting is realized through particular molecules in these minute scales as a result of a complex chain of reactions using enzymes, vitamins, and salts. Every step of this chain of reactions is another stitch to fix the wound. Other blood cells pile up and stick together behind this net and they dry up. If such clotting occurred inside the blood vessels, it would make a disastrous effect by blocking the bloodstream. I also have enzymes to break down little amounts of such clotting. As you see, everything is splendidly organized.

Peter! A blood test reveals very critical medical data. As I visit every organ, I exchange certain substances with them. Therefore, detection of an unusual substance in me can be an early warning for a disease. Nowadays, it even helps an early diagnosis of cancer.

It is not so easy for me to explain the wisdom behind all of my duties and capabilities. But to give you an idea, there are specialized departments for studying just me at medical faculties and research institutions throughout the world.

The muscle cells are responsible for the strength and tone of the vessels. Today, we know that the role of the endothelial layer goes beyond a simple physical barrier. In addition, there are twenty-five different substances secreted by the endothelial cells that play a role in blood clotting, cell proliferation, the regulation of vessel permeability, and the functioning of the immune system. The endothelial cells are 10-15 &μm wide and 20-25 μm long. They are located in the inner vessel wall in a single-cell layer. The total endothelial area in the body of an adult is around 5000-6000 m², and it weighs around 2.5 kg. Endothelial cells during inflammation Capillaries consist of an endothelial structure; they can only be seen under a microscope, but their total length is nearly 96,000 km. The blood brought by the arteries is conveyed to the vein through capillaries. At this stage, the gas, liquids, and nutrients are brought out through the vessels and the cells and tissues around are supplied with oxygen and nutrients. In return, the liquids that they discharge and other waste matter are conveyed to the vein through capillaries. This matter-exchange, which occurs both inside and outside of the capillaries, is regulated thanks to the permeability of the endothelial cell layer and the pressure balance of the capillary system. During cardiac failure and inflammation, liquid release is increased due to a pressure imbalance and the liquid retrieval is not sufficient to make up for the amount lost. This results in swelling in the area in question. Here, we need to underline that inflammation, which appears with symptoms such as edema, redness, fever, and pain, is not a harmful process. On the contrary, it is a miraculous defensive mechanism granted to our body; inflammation protects the body against serious damage. For instance, the inflammation that forms around a bee sting prevents the venom from spreading throughout the body. The endothelial cells are given an important role in the inflammation as well. The chemical molecules secreted by the endothelial cells in the inflamed spot cause the vessels to react by enlarging and thus perfusion is increased. Later, the endothelial layer becomes ready for leukocytes to settle; these are used in neutralizing the substance that caused the inflammation in the first place.

Balancing blood pressure

The layer of smooth muscle cells is stimulated with chemicals secreted by the endothelial cells and the tone of the vessels are controlled through the constriction and relaxation of the vessels. Therefore, an important duty in the regulation of blood pressure is given to these cells. During aninfection, bacteria circulate in the blood stream and the blood pressure falls extremely low. Tissue nutrition is upset (septic shock) and an excess of muscle-relaxing substance is released by the endothelial structure. Veins and arteries become too relaxed and there is a considerable drop in blood pressure (hypotension). On the other hand, with problems like atherosclerosis, the endothelial cells cannot fulfill their duty and due to a deficiency in nitrogen oxide, they become immune to the stimulus to relax the muscles. The resulting problem in this situation is hypertension.

Endothelial cells prevent hemorrhage

In order for a hemorrhage to stop the vessels that are bleeding need to narrow down. This is very important in the first stages of blood loss, particularly when there is a problem with blood clotting. When a hemorrhage begins, the endothelial cells are ordered to excrete a substance called endothelin. This starts the narrowing down of the bleeding vessels. Endothelin is not excreted in normal vessels. When the umbilical cord of a newborn is cut, it prevents the baby from losing blood.

Endothelial cells in blood clotting

The duty of endothelial cells can prevent or facilitate blood clotting, depending on the situation. First of all, they prevent the blood cells from adhering to the vessel walls and prevent clotting inside the vessels. Imagine water flowing through a pipe. The speed of the flow is greater in the center and lower at the periphery. Therefore, in the long run, some residue forms inside the pipe. In the veins and arteries, the flow of blood near the walls is also slower. To prevent the formation of any residue, both the endothelial cells and the blood cells are created with negative loaded surfaces and the blood cells are pushed towards the center. In addition, a substance called prostocyclin (PGI2) is excreted and the thrombocytes change their structure. As a result, residue formation and clotting is prevented along the vessel walls. In a case of any long term damage to the endothelium (e.g. due to smoking, diabetes, or hypertension), the relevant protection mechanism fails, and clotting inside the vessels results in thrombosis. Some serious cases can even necessitate the amputation of a limb. The endothelial cells can also facilitate clotting when necessary. In case of bleeding due to a wound, they function contrarily and help the blood to clot to prevent blood loss.

Endothelial cells in the bone marrow, the liver, and spleen

As a divine blessing, the endothelial cells form a looser layer in these organs and vessel permeability is increased. Thanks to this increase, matter Exchange with blood is easily realized; blood reaches these organs, which are responsible for the constant control of the contents of the blood, easily.

Endothelial cells in the brain and eyes

The endothelial cells in organs like the brain and eyes are very closely integrated forming a barrier between the blood and the organs. This is to such an extent that the major nutrients of the brain, like glucose and oxygen, pass without any obstacles, but several chemicals, including medication, are blocked by the selective-permeability of this protective mechanism. Research has proven that various substances injected into the bloodstream reach almost all the tissues except for the brain. Thanks to the efficient protective mechanism that has been given to these minute cells, the brain is saved from a great deal of negative effects. Even a single cell is not left to chance and nothing happens randomly. As can be seen throughoutthe universe, opposites are made to work hand in hand in the human body as well in a splendid harmony for the continuation of life.

References

• Vinay Kumar, Abul K. Abbas, Nelson Fausto, Richard Mitchell, Robbins Basic Pathology, W.B. Saunders; 8th edition, 2007. Hall, John E., Arthur C. Guyton, Textbook of Medical Physiology,