Red blood cells are the most common type of cell found within the human body. They make up about 45% of blood volume. There are roughly 250 million red blood cells in one tiny drop of blood! A red blood cell will live anywhere from 3 - 4 months. Then it will die in the liver and spleen when it is attacked and destroyed by macrophages. It's interesting to note, that red blood cells are of use even after they die. This is so because when they are destroyed they release iron that will be stored and used in the liver.
Red blood cells have the main function or job to carry oxygen from the lungs to every cell in the body. This is all made possible by hemoglobin which is a compound found within red blood cells. Hemoglobin captures oxygen in the lungs and as the blood passes through body tissues, it will release the oxygen to cells throughout the body. An analogy can be made to a computer. A computer has a case, just as a red blood cell does. A computer cases only function is to store all the components of that computer. A red blood cells only function is to store oxygen. What's inside the computer case or the red blood cell is what's important. Yes the computer has many components inside its case that perform all the work. A red blood cell has its main component, hemoglobin, inside its case and it performs all the work of capturing and releasing oxygen.
As mentioned above, red blood cells are made of mostly an iron and protein compound called hemoglobin. Red blood cells are packed with this compound; hence they do not have many common components of cells such as a nucleus. The outer layer of a red blood cell is like a little bubble that is extremely flexible. Its flexibility is necessary because the cell has to travel through the smallest blood vessels and capillaries to carry oxygen to wherever it's needed. Red blood cells are always disk shaped with a centered dent on each of its sides.