Red blood cells (RBC), or erythrocytes, are the most common type of cell in your blood. Their main job is to transport oxygen to your tissues. This oxygen is transported using a protein called haemoglobin. Haemoglobin is what gives blood its red colour. Without haemoglobin, blood with be a light straw colour. Contrary to what you might think, deoxygenated blood is not blue. It is in fact a darker shade of red. Your blood only looks blue when viewed through the vessel wall and your skin.
The shape of the RBC actually allows the cell to perform its function optimally. The cytoplasm of a RBC is full of haemoglobin, so much so that sometimes the cells are referred to bags of haemoglobin. The cell is bi-concave (see below) and is the only cell in the human body that does not have a nucleus. This adaptation allows the RBC to pack in more haemoglobin and remain flexible to fit through the small capillaries of your circulation. The RBC also lacks most organelles that other cells contain.
RBCs live around 120 days before they are degraded and news ones are made. Your body makes about 2.4 million new RBCs every second to replace the lost ones. A single RBC can circulate your entire body in 20 seconds.
There are many diseases that involve RBCs including malaria, thalassemia, and sickle-cell anemia. In sickle-cell anemia, the haemoglobin molecule is abnormal resulting in deformation of the RBC to look like a sickle (see below). This can cause the cells to get caught in vessels and lead to pain, rupturing of the vessel or a stroke. An interesting side effect of the sickle-cell anemia mutation is the resistance of these people to malaria infection, which may explain why the mutation is still around.
Header Credit: Blausen Wikimedia