Anemia

Anemia is a group of conditions that occurs when there's a reduced number of healthy, functional red blood cells, or RBCs, in the blood.

Now, RBCs are primarily responsible for carrying and delivering oxygen to the body’s tissues. They’re produced through erythropoiesis, which occurs within the bone marrow under the direction of erythropoietin, a hormone produced in the kidneys.

Here, stem cells called proerythroblasts differentiate into erythroblasts, a type of immature RBC that synthesizes hemoglobin. Hemoglobin is a protein that binds and carries oxygen and is made up of four polypeptide chains called globins, which consist of two α and two β chains. Each chain contains a heme group, which includes an iron atom, where oxygen can bind. One oxygen molecule can bind to each protein chain, allowing one hemoglobin molecule to carry up to four oxygen molecules. And each RBC can contain hundreds of hemoglobin molecules!

Eventually, erythroblasts differentiate into reticulocytes, another type of immature RBC. The bone marrow releases reticulocytes into the bloodstream where they ultimately become mature RBCs, which are called erythrocytes. Erythrocytes are especially effective in gas exchange, due to their flexible, biconcave shape, which makes them able to fit through narrow blood vessels, while increasing surface area to conduct gas exchange.

Now, anemia can result from underproduction, increased destruction, or excessive loss of RBCs.

First, underproduction can occur if the kidneys don’t produce enough erythropoietin, which can happen in chronic kidney disease. Without enough erythropoietin stimulating the bone marrow, not enough RBCs are produced. Also, if there’s not enough building blocks to support erythropoiesis, such as iron, vitamin B12, and folate, less fully functional RBCs can be produced. So, for example, lack of iron can lead to iron deficiency anemia.

Additionally, if the bone marrow is damaged from conditions like autoimmune disorders or certain types of cancer, erythropoiesis can be inhibited. For instance, an autoimmune disorder may cause the immune system to attack bone marrow so it's unable to produce new blood cells, resulting in aplastic anemia.

Next, destruction, or hemolysis, of RBCs occurs when RBCs are broken down faster than they're produced. This is seen with inherited conditions like sickle cell anemia, where a genetic mutation alters hemoglobin, leading to RBCs that are inflexible, sickled, and short-lived. RBCs can also be destroyed by certain medications like penicillin or methyldopa; infections like malaria; autoimmune disorders like systemic lupus erythematosus, or lupus; or physical damage by medical devices like mechanical heart valves.