Hemoglobin is a general term that describes the molecule in erythrocytes that contains a heme group and iron. You also have what are called unstable hemoglobins which are usually genetic conditions were amino acid substitutions have taken place, one example is called Heinz bodies. Nevertheless, the primary function of hemoglobin is to transport oxygen to all body cells. All cells in the body must have a constant supply of oxygen to survive.
When hemoglobin levels are low cells, tissues, and organs are deprived of needed oxygen. Depending on exactly how low the hemoglobin level is determines the degree of anoxia. A slight drop in hemoglobin is compensated for by the body and has no major adverse effects. However, a very low hemoglobin level can be quite serious and can lead to organ damage and failure.
I noticed in your comments, "if it is not treated with iron and vitamins". Be advised that iron may or may not be needed. It depends on why the hemoglobin level is low. Iron supplements are not a cure all and furthermore they may be harmful, even toxic, if iron is not needed. Iron toxicity is a very serious situation.
I suggest you consult with your physician to determine the exact cause of the low hemoglobin. Treatment depends on the nature of the problem.
There could be many explanations regarding low hemoglobin levels. One of the main conditions that causes low hemoglobin levels is aplastic anemia. When a person has aplastic anemia, it means that their body does not produce enough new cells. Symptoms are fatigue and excessive bleeding. Other symptoms of low hemoglobin levels are irritability, and dizziness. Low hemoglobin levels can become dangerous so it is important to follow doctors orders.
Sometimes, slightly low hemoglobin levels are normal. Sometimes pregnant women have low hemoglobin levels and women who have just begun their menstrual cycles as well.
A low hemoglobin count is generally defined as less than 13.5 grams of hemoglobin per deciliter (135 grams per liter) of blood for men and less than 12 grams per deciliter (120 grams per liter) for women. In children, the definition varies with age and sex. The threshold differs slightly between medical practices.
The main purpose of hemoglobin is to bind the oxygen. When there is not a lot of some of the symptoms experience resemble that of lack of oxygen. The symptoms of low hemoglobin depend on the person, but some of the main symptoms are:
Inability to concentrate
Shortness of breath
Pale skin, nail beds, and gums
Anemia - low hemoglobin is one of the main causes of anemia.
Some less common symptoms are:
Swelling of the limbs
Blood in the stool
Hemoglobin is the respiratory pigment, the red pigment, of all vertebrates. It is present in erythrocytes, where it has a role in transport of oxygen and CO2.
When the death of red blood cells occurs, their membrane suffering a rupture, hemoglobin is hydrolyzed and Fe is recovered. Porphyrin cycle bursts and the fragments are metabolized by the liver.
After the process, for each hemoglobin molecule metabolized is resulting a molecule of CO. It is a natural process and it is a source of CO for the human body, carbon monoxide which is removed through the exhaled air.
The final metabolic product is bilirubin, a yellow pigment, whose intensity of color means the level of destroying of erythrocytes. If the rate of degradation of hemoglobin is too fast, it can block capillaries, mainly the kidney capillaries, causing various diseases.
The decreased hemoglobin, not associated with the decreasing of number of red blood cells is called anemia. Anemia has as main cause the deficiency of Fe. Because of this deficiency, decreased synthesis of hemoglobin, red blood cells will be hypochromic type (low-pigmented), and smaller than normal.
In haemolysis (destruction of red blood cells faster than synthesis), associated jaundice is caused of bilirubin metabolite and circulating hemoglobin, which can cause kidney failure.
Most people understand the importance of hemoglobin carrying oxygen, but it is also important in the transport of carbon dioxide. Hemoglobin works carrying both oxygen and carbon dioxide based on affinity. The affinity of hemoglobin for oxygen and carbon dioxide is based on the concentration of these two dissolved gases. If there is a high concentration of oxygen, the affinity for oxygen is high and hemoglobin tends to release carbon dioxide. On the other hand, if there is a high concentration of carbon dioxide, then the affinity for the gases reverses; the hemoglobin will have a high affinity for the carbon dioxide and will then release the oxygen. Therefore if someone has a low hemoglobin level, not only will their tissues not receive enough oxygen, the blood will not be able to remove the excess carbon dioxide. Double jeopardy, I'd say!