Hemoglobin is an iron-rich protein within red blood cells that transports oxygen throughout the body. It binds to inhaled oxygen in the lungs and releases it to tissues and organs for metabolism. Measuring hemoglobin concentration is a common diagnostic step, usually performed as part of a routine blood test. The results provide direct insight into the blood’s capacity to deliver oxygen, indicating a person’s overall physical health.
The Role of Hemoglobin in the Body
The hemoglobin molecule is a tetramer, consisting of four folded protein chains called globins. Each globin chain is associated with a single heme group, a ring-like structure containing a central ferrous iron ion. This iron allows the molecule to reversibly bind to oxygen.
A single hemoglobin molecule can bind up to four oxygen molecules, one at each of its heme sites. Binding occurs efficiently in the high-oxygen environment of the lungs. The molecule carries the oxygen through the bloodstream, releasing it in peripheral tissues where oxygen concentration is lower.
This process is necessary for cellular respiration, which is how cells generate energy for bodily functions. Hemoglobin also carries carbon dioxide waste from the tissues back to the lungs for exhalation. Without adequate functioning hemoglobin, cells cannot produce sufficient energy, leading to fatigue and impaired organ function.
Understanding Measurement and Units
Hemoglobin concentration is usually determined through laboratory analysis of a blood sample, often included in a Complete Blood Count (CBC). This test quantifies the amount of hemoglobin protein present in a specific volume of blood. Results are reported using precise units reflecting this mass-per-volume relationship.
The most commonly used unit in the United States and many other countries is grams per deciliter (g/dL). A deciliter represents 100 milliliters of blood, meaning the result indicates the mass of hemoglobin, in grams, found within that specific volume. This unit provides a standardized measure of the blood’s oxygen-carrying capacity.
In some clinical settings, particularly across Europe and Canada, hemoglobin may be reported using the International System of Units (SI) as millimoles per liter (mmol/L). This unit measures the amount of substance (moles) of hemoglobin per liter of blood. While g/dL remains the prevalent unit for general understanding, values can be converted to mmol/L using a specific conversion factor for comparison.
What Your Results Mean: Normal Ranges
Hemoglobin levels are not universal and vary based on factors like age, biological sex, and environmental conditions such as altitude. Laboratories use these established reference ranges to determine if a result falls within an expected healthy window.
For adult males, the typical range is generally considered to be between 13.5 and 18 grams per deciliter (g/dL). Adult females usually have a slightly lower range, with common values between 12 and 16 g/dL. This difference is attributed to variations in body size, hormonal influences, and menstrual blood loss.
Children and infants have ranges that fluctuate significantly as they grow. Newborns often have very high initial levels, sometimes reaching 14 to 24 g/dL. These levels drop sharply in the first few months, stabilizing into a child’s range of approximately 9.5 to 14 g/dL.
These numbers serve as benchmarks, but slight deviations can occur based on the specific equipment and methodology used by the testing laboratory. Individuals living at high altitudes, where oxygen is less concentrated, may naturally have slightly higher hemoglobin levels as their bodies adapt.
Common Causes of Abnormal Levels
Results that fall outside the normal reference range can indicate underlying health issues requiring further investigation. A low hemoglobin level signals that the blood is not carrying enough oxygen, a condition commonly referred to as anemia.
The most frequent cause of low hemoglobin is iron deficiency, as iron is a necessary component for the formation of the heme group. Other nutritional deficiencies, particularly a lack of Vitamin B12 or folate, can also impair the body’s ability to produce healthy red blood cells. Chronic conditions, such as long-term blood loss from a gastrointestinal ulcer or heavy menstrual bleeding, deplete the body’s iron stores over time. Furthermore, chronic kidney disease can suppress the production of erythropoietin, a hormone that stimulates red blood cell formation, leading to lower hemoglobin counts.
Conversely, a high hemoglobin level can also be a cause for concern, often indicating the body is making too many red blood cells, a state known as erythrocytosis. This elevation may be a response to chronic low oxygen levels in the blood, such as those caused by smoking or long-standing lung or heart diseases. Living at a high altitude can also trigger the body to produce more red blood cells to compensate for the thinner air.
Dehydration can result in a falsely high reading because the blood plasma volume is reduced, concentrating the hemoglobin within the remaining fluid. In rare instances, a high count can be caused by a bone marrow disorder like polycythemia vera, where the marrow produces an excessive number of all blood cells, including those carrying hemoglobin.