Why is blood red? It is because of the hemoglobin contained in single cells known as erythrocytes (red blood cells). A red blood cell is shaped like a circular plate, with a 7-micrometer diameter on average. Hemoglobin is a chemical compound made of protein and pigment that contains iron, and has a tendency to bond with oxygen easily. Human life is possible thanks to hemoglobin, which distributes oxygen to the whole body after bonding with it in the lungs. In human blood, there are about 4.5 million red blood cells in one cubic millimeter, and they play a very important role. Many different symptoms are possible if the number of red blood cells fluctuates. Take anemia, for example. This is caused by a reduced number of red blood cells, and a person with anemia experiences dizziness and other symptoms.
 
There is another kind of cell in our blood, called a leukocyte (white blood cell). This cell is a little larger than a red blood cell, and there are 5,000-7,000 in one cubic millimeter of human blood. White blood cells also have a very important job, which is to kill germs or foreign substances in the body. For example, an increased number of white blood cells implies that there are inflammations or bacterial infections somewhere in the body. Therefore, counting the number of red and white blood cells is very important for medical purposes.
When you do not feel good and go to see the doctor, the first thing they do is to draw your blood. An inspection device called a blood cell counter helps a great deal. This device contributes to health by counting the number of red and white blood cells in blood using optical and electrical means.
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