Red blood cells deliver oxygen to the body. Each one contains a concentrated solution of hemoglobin, a molecule that can bind to oxygen and later release it where oxygen levels are low. About a third of a red blood cell is hemoglobin, most of the rest is water.
The technical name for red blood cells is erythrocytes. The “erythro” prefix comes from the Greek for red. Red blood cells are red because oxygenated hemoglobin absorbs green and blue light.
There are around 30 trillion red blood cells in the body. Each one has about 270 million molecules of hemoglobin. Each molecule of hemoglobin can carry eight atoms of oxygen. That means that the body has a total carrying capacity of approximately 60 billion trillion oxygen atoms.
A red blood cell lacks the cell components that are normally found inside other cells, such as a nucleus or mitochondria. Consequently, it does not use up the oxygen that it carries, but it is incapable of self-repair. Red blood cells normally last around 120 days before they are removed by macrophages in the spleen that detect damage. In hereditary spherocytosis, the red blood cells survive for between 10 and 30 days depending on the severity of the condition.
To replace those that are destroyed, about 2.5 million new red blood cells are created every second. The kidneys detect lowered levels of oxygen and release erythropoietin (also known as EPO) which stimulates stem cells in the red bone marrow to differentiate into red blood cells in a process called erythropoiesis.
The new red blood cells are released into the blood stream before they are fully mature. These immature red blood cells are called reticulocytes. Unlike mature red blood cells, reticulocytes still contain some RNA, that is visible as dark granules under a microscope.
It takes a day or two for a reticulocyte to mature. During this time the red blood cell becomes stronger, loses surface area and its final shape is formed: biconcave for normal red blood cells; spherical in hereditary spherocytosis.
Normally about 1% of red blood cells are reticulocytes, but in spherocytosis this figure is substantially higher, around 4% for a mild form of spherocytosis and over 10% in severe cases. This is because red blood cells do not last as long, but also because the rate of red blood cell production is increased. This happens because the persistent anemia continually prompts the kidneys to produce erythropoietin in an attempt to compensate.
Red blood cells are no longer produced during an aplastic crisis and this can lead to abnormally low reticulocyte levels. For more information on this, see the article Spherocytosis: hemolytic crisis and aplastic crisis.