Peter, normally you only see me when you have a cut on your skin and do not care much about me. I am a living tissue such as your bones, muscles, and nerves. My basic difference from other connective tissues is that I am dispersed in the intermediary fluid, blood plasma. If I weren't riding the plasma, I would not be able to reach the remotest cells of your body and help meet their needs. My constituents are a crowded group made up of two types of basic cells and cell parts.

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.