Function of Red Blood Cells

Red blood cells, also called erythrocytes, are the most abundant cell type in the blood. Other major blood components include plasma, white blood cells, and platelets. The primary function of red blood cells is to transport oxygen to body cells and deliver carbon dioxide to the lungs. A red blood cell has what is known as a biconcave shape. Both sides of the cell's surface curve inward like the interior of a sphere. This shape aids in a red blood cell's ability to maneuver through tiny blood vessels to deliver oxygen to organs and tissues. Red blood cells are also important in determining human blood type. Blood type is determined by the presence or absence of certain identifiers on the surface of red blood cells. These identifiers, also called antigens, help the body's immune system to recognize its own red blood cell type.

Red Blood Cell Structure

Red Blood Cells
The main function of red blood cells (erythrocytes) is to distribute oxygen to body tissues, and to carry waste carbon dioxide back to the lungs. Red blood cells are biconcave, giving them a large surface area for gas exchange, and highly elastic, enabling them to pass through narrow capillary vessels. DAVID MCCARTHY/Getty Images

Red blood cells have a unique structure. Their flexible disc shape helps increase the surface area-to-volume ratio of these extremely small cells. This enables oxygen and carbon dioxide to diffuse across the red blood cell's plasma membrane more readily. Red blood cells contain enormous amounts of a protein called hemoglobin. This iron containing molecule binds oxygen as oxygen molecules enter blood vessels in the lungs. Hemoglobin is also responsible for the characteristic red color of blood. Unlike other cells of the body, mature red blood cells do not contain a nucleus, mitochondria, or ribosomes. The absence of these cell structures leaves room for the hundreds of millions of hemoglobin molecules found in red blood cells. A mutation in the hemoglobin gene can result in the development of sickle-shaped cells and lead to sickle cell disorder.

Red Blood Cell Production

Bone Marrow
Bone marrow, scanning electron micrograph (SEM). Bone marrow is the site of blood cell production. Differentiating white blood cells (blue), part of the body's immune system, and red blood cells, which carry oxygen around the body, are seen amongst reticular fibres (brown). Reticular fibres make up the connective tissue framework of the bone marrow. STEVE GSCHMEISSNER/Science Photo Library/Getty Images

Red blood cells are derived from stem cells in red bone marrow. New red blood cell production, also called erythropoiesis, is triggered by low levels of oxygen in the blood. Low oxygen levels can occur for various reasons including blood loss, presence in high altitude, exercise, bone marrow damage, and low hemoglobin levels. When the kidneys detect low oxygen levels, they produce and release a hormone called erythropoietin. Erythropoietin stimulates the production of red blood cells by red bone marrow. As more red blood cells enter blood circulation, oxygen levels in the blood and tissues increase. When the kidneys sense the increase in oxygen levels in the blood, they slow the release of erythropoietin. As a result, red blood cell production decreases.

Red blood cells circulate on average for about 4 months. According to the American Red Cross, adults have around 25 trillion red blood cells in circulation at any given time. Due to their lack of a nucleus and other organelles, adult red blood cells can not undergo mitosis to divide or generate new cell structures. When they become old or damaged, the vast majority of red blood cells are removed from circulation by the spleen, liver, and lymph nodes. These organs and tissues contain white blood cells called macrophages that engulf and digest damaged or dying blood cells. Red blood cell degradation and erythropoiesis typically occur at the same rate to ensure homeostasis in red blood cell circulation.

Red Blood Cells and Gas Exchange

Alveoli
Illustration of air sacs (alveoli) in the human lung. Several clusters of alveoli are shown here, two of which are shown sliced open. The ducts (top right) supplying the alveoli with air are called bronchioles. Each alveolus is wrapped in a fine network of tiny blood capillaries, as shown here at center. Red blood cells flowing over the alveoli pick up oxygen, which is then carried to other parts of the body. The blood flowing into the lungs is deoxygenated (blue). That flowing out is oxygenated (red). The lungs are composed almost entirely of structures like these. The millions of tiny alveoli together provide an enormous surface area for the absorption of oxygen. John Bavosi/Science Photo Library/Getty Images

Gas exchange is the primary function of red blood cells. The process by which organisms exchange gases between their body cells and the environment is called respiration. Oxygen and carbon dioxide are transported through the body via the cardiovascular system. As the heart circulates blood, oxygen depleted blood returning to the heart is pumped to the lungs. Oxygen is obtained as a result of respiratory system activity.

In the lungs, pulmonary arteries form smaller blood vessels called arterioles. Arterioles direct blood flow to the capillaries surrounding lung alveoli. Alveoli are the respiratory surfaces of the lungs. Oxygen diffuses across the thin endothelium of the alveoli sacs into the blood within the surrounding capillaries. Hemoglobin molecules in red blood cells release the carbon dioxide picked up from body tissues and become saturated with oxygen. Carbon dioxide diffuses from the blood to the alveoli, where it is expelled through exhalation. The now oxygen rich blood is returned to the heart and pumped to the rest of the body. As the blood reaches systemic tissues, oxygen diffuses from the blood to surrounding cells. Carbon dioxide produced as a result of cellular respiration diffuses from the interstitial fluid surrounding body cells into the blood. Once in the blood, carbon dioxide is bound by hemoglobin and returned to the heart via the cardiac cycle.

Red Blood Cell Disorders

Sickel Cell Anemia
This image shows a healthy red blood cell (left) and a sickle cell (right). SCIEPRO/Science Photo Library/Getty Images

Diseased bone marrow can produce abnormal red blood cells. These cells may be irregular in size (too large or too small) or shape (sickle shaped). Anemia is a condition characterized by the lack of production of new or healthy red blood cells. This means that there are not enough functioning red blood cells to carry oxygen to body cells. As a result, individuals with anemia may experience fatigue, dizziness, shortness of breath, or heart palpitations. Causes of anemia include sudden or chronic blood loss, not enough red blood cell production, and the destruction of red blood cells. Types of anemia include:

  • Aplastic anemia: A rare condition in which not enough new blood cells are produced by bone marrow due to stem cell damage. Development of this condition is associated with a number of different factors including pregnancy, exposure to toxic chemicals, side effect of certain medications, and certain viral infections (HIV, hepatitis, or Epstein-Barr virus).
  • Iron-deficiency anemia: A lack of iron in the body results in not enough red blood cell production. Causes include sudden blood loss, mensturation, and insufficient iron intake or absorption from food.
  • Sickle cell anemia: This inherited disorder is caused by a mutation in the hemoglobin gene that causes red blood cells to take on a sickle shape. These abnormally shaped cells get stuck in blood vessels, blocking normal blood flow.
  • Normocytic anemia: This condition results from a lack of red blood cell production. The cells that are produced however, are of normal size and shape. This condition may result from kidney disease, bone marrow dysfunction, or other chronic disease.
  • Hemolytic anemia: Red blood cells are prematurely destroyed, typically as a result of an infection, autoimmune disorder, or blood cancer.

Treatments for anemia vary based on severity and include iron or vitamin supplements, medication, blood transfusion, or bone marrow transplantation.

Reference:

  • Learn About Blood, American Red Cross. Accessed 9 August 2013 (http://www.redcrossblood.org/learn-about-blood)