Hemoglobin (American English) or haemoglobin (British English), abbreviated Hb or Hgb, is the iron-containing oxygen-transport metalloprotein in the red blood cells (erythrocytes) of almost all vertebrates (the exception being the fish family Channichthyidae) as well as the tissues of some invertebrates. Haemoglobin in blood carries oxygen from the lungs or gills to the rest of the body (i.e. the tissues). There it releases the oxygen to permit aerobic respiration to provide energy to power the functions of the organism in the process called metabolism. A healthy individual has 12 to 20 grams of haemoglobin in every 100 ml of blood.

    In mammals, the protein makes up about 96% of the red blood cells' dry content (by weight), and around 35% of the total content (including water). Haemoglobin has an oxygen-binding capacity of 1.34 mL O2 per gram, which increases the total blood oxygen capacity seventy-fold compared to dissolved oxygen in blood. The mammalian hemoglobin molecule can bind (carry) up to four oxygen molecules.

    Hemoglobin is involved in the transport of other gases: It carries some of the body's respiratory carbon dioxide (about 20–25% of the total[9]) as carbaminohemoglobin, in which CO2 is bound to the heme protein. The molecule also carries the important regulatory molecule nitric oxide bound to a globin protein thiol group, releasing it at the same time as oxygen.

    Haemoglobin is also found outside red blood cells and their progenitor lines. Other cells that contain haemoglobin include the A9 dopaminergic neurons in the substantia nigra, macrophages, alveolar cells, lungs, retinal pigment epithelium, hepatocytes, mesangial cells in the kidney, endometrial cells, cervical cells and vaginal epithelial cells. In these tissues, haemoglobin has a non-oxygen-carrying function as an antioxidant and a regulator of iron metabolism.Excessive glucose in one's blood can attach to hemoglobin and raise the level of hemoglobin A1c.

    Haemoglobin and haemoglobin-like molecules are also found in many invertebrates, fungi, and plants. In these organisms, haemoglobins may carry oxygen, or they may act to transport and regulate other small molecules and ions such as carbon dioxide, nitric oxide, hydrogen sulfide and sulfide. A variant of the molecule, called leghaemoglobin, is used to scavenge oxygen away from anaerobic systems, such as the nitrogen-fixing nodules of leguminous plants, before the oxygen can poison (deactivate) the system.

    What is haemoglobin?

    Haemoglobin is the protein molecule in red blood cells that carries oxygen from the lungs to the body's tissues and returns carbon dioxide from the tissues back to the lungs.

    Haemoglobin also plays an important role in maintaining the shape of the red blood cells. In their natural shape, red blood cells are round with narrow centres resembling a doughnut without a hole in the middle. Abnormal haemoglobin structure can, therefore, disrupt the shape of red blood cells and impede their function and flow through blood vessels.

    What does a low haemoglobin level mean?

    A low haemoglobin level is referred to as anaemia or low red blood count. A lower than a normal number of red blood cells is referred to as anaemia and haemoglobin levels reflect this number. There are many reasons (causes) for anaemia.

    Overview of Anaemia.

    Anaemia is a condition in which the number of red blood cells is low.

    Red blood cells contain haemoglobin, a protein that enables them to carry oxygen from the lungs and deliver it to all parts of the body. When the number of red blood cells is reduced, the blood cannot carry an adequate supply of oxygen. An inadequate supply of oxygen in the tissues causes the symptoms of anaemia.

       Causes.

    The causes of anaemia are numerous, but most can be grouped within three major mechanisms that produce anaemia:

    ·         Blood loss (excessive bleeding)

    ·         Inadequate production of red blood cells

    ·         Excessive destruction of red blood cells

    Anaemia caused by excessive bleeding.

    Anaemia may be caused by excessive bleeding. Bleeding may be sudden, as may occur as a result of an injury or during surgery. Often, bleeding is gradual and repetitive (chronic bleeding), typically due to abnormalities in the digestive or urinary tract or heavy menstrual periods. Chronic bleeding typically leads to low levels of iron, which leads to worsening anaemia.

    Anaemia due to inadequate red blood cell production.

    Anaemia may also result when the body does not produce enough red blood cells (see also Formation of Blood Cells.). Many nutrients are needed for red blood cell production. The most critical are iron, vitamin B12, and folate (folic acid), but the body also needs trace amounts of copper, as well as a proper balance of hormones, especially erythropoietin (a hormone that stimulates red blood cell production).     Without these nutrients and hormones, the production of red blood cells is slow and inadequate, or the red blood cells may be deformed and unable to carry oxygen adequately.

    Chronic disease also may affect red blood cell production. In some circumstances, the bone marrow space may be invaded and replaced (for example, by leukaemia, lymphoma, or metastatic cancer), resulting in decreased production of red blood cells.

    Anaemia due to excessive red blood cell destruction.

    Anaemia may also result when too many red blood cells are destroyed. Normally, red blood cells live about 120 days. Scavenger cells in the bone marrow, spleen, and liver detect and destroy red blood cells that are near or beyond their usual life span. If red blood cells are destroyed prematurely (hemolysis), the bone marrow tries to compensate by producing new cells faster. When the destruction of red blood cells exceeds their production, hemolytic anaemia results. Hemolytic anaemia is relatively uncommon compared with the anaemia caused by excessive bleeding and decreased red blood cell production. Hemolytic anaemia may result from disorders of the red blood cells themselves, but more often it results from other disorders that cause red blood cells to be destroyed.

    Symptoms.

    Symptoms vary depending on the severity of the anaemia and how rapidly it develops. Some people with mild anaemia, particularly when it develops slowly, have no symptoms at all. Other people may experience symptoms only during physical exertion. More severe anaemia may cause symptoms even when people are resting. Symptoms are more severe when mild or severe anaemia develops rapidly, such as when bleeding that occurs when a blood vessel ruptures.

    Mild anaemia often causes fatigue, weakness, and paleness. In addition to these symptoms, more severe anaemia may cause faintness, dizziness, increased thirst, sweating, a weak and rapid pulse, and rapid breathing. Severe anaemia may cause painful lower leg cramps during exercise, shortness of breath, and chest pain, especially if people already have impaired blood circulation in the legs or certain types of lung or heart disease.

    Some symptoms may also give clues to the cause of the anaemia. For example, black tarry stools, blood in the urine or stool, or coughing up blood suggests that anaemia is caused by bleeding. Dark urine or jaundice (a yellowish tinge to the skin or the whites of the eyes) suggests that red blood cell destruction may be the cause of anaemia. A burning or prickling feeling in the hands or feet may indicate a vitamin B12 deficiency.

    Anaemia in older adults.

    Many disorders that cause anaemia, such as cancer, including blood cancers such as myelodysplasia and multiple myeloma, tend to be more common among older people. Thus, many older people develop anaemia. Anaemia of chronic disease and iron-deficiency anaemia caused by abnormal bleeding are the most common causes of anaemia among older people. Anaemia is not a normal consequence of ageing, and a cause should always be sought when anaemia is identified.

    Symptoms of anaemia are basically the same regardless of age. Also, even when anaemia is mild, older people are more likely to become confused, depressed, agitated, or listless than younger people. They may also become unsteady and have difficulty walking. These problems can interfere with being able to live independently. However, some older people with mild anaemia have no symptoms at all, particularly when anaemia develops gradually, as it often does.

    In older people, anaemia caused by vitamin B12 deficiency may be mistaken for dementia because this type of anaemia may affect mental function.

    Having anaemia may shorten the life expectancy of older people. Thus, identifying the cause and correcting it are particularly important.

    Diagnosis

    ·         Blood tests

    Sometimes anaemia is detected before people notice symptoms when routine blood tests are done.

    Low levels of haemoglobin or a low hematocrit (the percentage of red blood cells in the total blood volume) found in a blood sample confirm the anaemia. Other tests, such as examining a blood sample under a microscope and, less often, examining a sample taken from the bone marrow, help determine the cause of the anaemia.