Hemoglobin (abbreviated to Hb or Hgb) is an iron-containing protein in your red blood cells responsible for many functions in your body. Its main role is carrying oxygen from the lungs to the tissues and transporting carbon dioxide from the tissues back to the lungs. 

Hemoglobin is a very large molecule. It consists of four iron-containing molecules called heme and four protein molecules – two alpha and two beta chains that are connected. It is specially designed to bind oxygen in the areas with high oxygen tension and release it where the tension is low. Hemoglobin also helps maintain the disc-like shape of red blood cells, allowing them to flow easily through blood vessels.  

To make sure the tissues have enough oxygen, a sufficient hemoglobin level must be maintained. The amount of hemoglobin in your body is usually measured as a part of a complete blood count test (CBC). 

Low hemoglobin levels, usually referred to as anemia, might result from iron deficiency, vitamin B12 or folic acid deficiency, internal or external blood loss such as menstrual bleeding, or bleeding resulting from surgery or injury. Low hemoglobin levels are also present in thalassemias which are groups of genetic diseases where the production of alpha and beta chains is reduced. 

High hemoglobin levels can be caused by polycythemia vera, a blood disease where the production of red blood cells is increased. It can be caused by lung disease, certain types of kidney diseases, or heart disease as well. Elevated hemoglobin levels are usually present in people living at high altitudes and smokers.   

There were cases where some athletes were abusing the drug erythropoietin for blood doping. Erythropoietin chemically increases the production of red blood cells. The increase in the number of red blood cells means that the level of hemoglobin and oxygen in the athlete's body was increased, improving their performance.