Introduction:

Blood has two major components:

• Plasma is a clear yellow liquid that contains proteins, nutrients, hormones, electrolytes, and other substances. It constitutes about 55% of blood.
• White and red blood cells and platelets make up the balance of blood. The white cells are the infection fighters for the body, and platelets are necessary for blood clotting. The important factors in anemia, however, are red blood cells.

Red blood cells (RBCs), also known as erythrocytes, carry oxygen throughout the body to nourish tissues and sustain life. Red blood cells are the most abundant cells in our bodies. Men have about 5.2 million red blood cells per cubic millimeter of blood, and women have about 4.7 million red blood cells per cubic millimeter of blood. To understand red blood cells and their role in anemia, it is useful to know certain facts about them.

Hemoglobin and Iron. Each red blood cell contains about 280 million hemoglobin molecules. Hemoglobin is a complex molecule and the most important component of red blood cells. It is composed of protein (globulin) and a molecule (heme), which binds to iron.

In the lungs, the heme component binds to oxygen in exchange for carbon dioxide. The red blood cells carry the oxygen to the body's tissues, where the hemoglobin releases the oxygen in exchange for carbon dioxide, and the cycle repeats. The oxygen is used in the mitochondria, the power source within all cells.

Structure and Shape. Red blood cells are extremely small and look something like tiny, flexible inner tubes. This unique shape offers many advantages:

• It provides a large surface area to absorb oxygen and carbon dioxide.
• Its flexibility allows it to squeeze through capillaries, the tiny blood vessels that join the arteries and veins.
• Abnormally shaped or sized erythrocytes are typically destroyed and eliminated.

Blood Cell Production (Erythropoiesis). The actual process of making red blood cells is called erythropoiesis. (In Greek, erythro means "red" and poiesis means "the making of things.") The process of manufacturing, recycling, and regulating the number of red blood cells is complex and involves many parts of the body:

• The body carefully regulates its production of red blood cells so that enough are manufactured to carry oxygen but not so many that the blood becomes thick or sticky (viscous).
• Most of the work of erythropoiesis occurs in the bone marrow.
• If the body needs more oxygen (at high altitudes, for instance), the kidney triggers the release of erythropoietin (EPO), a hormone that increases production of red blood cells in the bone marrow.
• The lifespan of a red blood cell is 90 - 120 days. The liver and spleen remove old red blood cells from the blood.
• When old red blood cells are broken down for removal, iron is returned to the bone marrow to make new cells.

Sickle Cell Disease and Hemoglobin

Sickle cell disease occurs from genetic changes which causes a portion of the hemoglobin molecules to be abnormal:

• Hemoglobin A (HbA). HbA is the hemoglobin molecule found in normal red blood cells during childhood and adulthood. People without sickle cell anemia have primarily this type of hemoglobin in their blood cells.
• Hemoglobin S (HbS). HbS (S is for sickle) is the abnormal variant of hemoglobin A, which occurs in sickle-red blood cells and is the primary characteristic of the disease. The difference between hemoglobin A (HbA) and hemoglobin S (HbS) lies in only one protein out of about 300 that are common to both. This protein lies along an amino-acid chain called beta-globin, where even a tiny abnormality has disastrous results.

Hemoglobin is the most important component of red blood cells. It is composed of a protein called heme, which binds oxygen. In the lungs, oxygen is exchanged for carbon dioxide. Abnormalities of an individual's hemoglobin value can indicate defects in red blood cell balance. Both low and high values can indicate disease states.

Changes that Lead to Sickle Cell Disease

Hemoglobin F (HbF) is a form of hemoglobin that is produced during fetal development in the womb. (The F in HbF stands for fetal.) It is usually present for only a short time after birth. Normally, most HbF is later replaced by HbA, although some HbF may persist throughout life. Importantly, HbF is able to block the sickling action of red blood cells. Infants who have inherited sickle cell disease do not develop symptoms of the illness while they still have HbF present in their blood. People with the sickle cell gene who continue to carry some fetal hemoglobin are better protected, therefore, from severe forms of the disease. This knowledge is being used as the basis for therapies used in treating sickle cell disease.

The symptoms and problems of sickle cell disease are a result of the hemoglobin S (HbS) molecule:

• When the sickle hemoglobin molecule loses its oxygen, it forms rigid rods called polymers that change the red blood cells into a sickle or crescent shape.
• These abnormally sickle-shaped cells are both rigid and sticky. They stick to the walls and cannot squeeze through the capillaries. Blood flow through tiny blood vessels becomes slowed or stopped throughout the body. This deprives tissues and organs of oxygen.
• In the immediate setting, oxygen deprivation (hypoxia) can cause severe pain (the sickle cell crisis). Over time, it leads to gradual destruction in organs and tissues throughout the body.

• In a vicious cycle, oxygen deprivation in cells leads to more polymerization and increased production of sickle cells. The higher the concentration of sickle hemoglobin and the more acidic the environment, the faster the sickle cell process.
• Cell dehydration (not enough water molecules) is another major destructive factor in the sickling process of red blood cells. Dehydration increases the density of hemoglobin S within the cell, thereby speeding up the sickling process.
• Sickle cells also have a shorter life span (10 - 20 days) than that of normal red blood cells (90 - 120 days). Every day the body produces new red blood cells to replace old ones, but sickle cells become destroyed so fast that the body cannot keep up. The red blood cell count drops, which results in anemia. This gives sickle cell disease its more common name, sickle cell anemia.

The severity of sickle cell disease generally depends on a number of factors:

• The extent of oxygen loss. Prolonged oxygen deprivation contributes to the severe pain experienced as a sickle cell crisis. It also produces both short- and long-term organ damage. The lungs are specifically critical targets of the disease process. Because they supply oxygen, they can restore the sickle molecules to a normal form. Unfortunately, once the process occurs, the lungs become major sites for sickle cell damage, particularly for dangerous acute episodes of chest pain.
• The acidity of the environment. The lower the better. The organs most seriously affected are those with an acidic environment (such as the spleen and bone marrow).
• The concentration of hemoglobin S within the cell. The lower the better.
• The amount of a protective hemoglobin F (for fetal). The more the better.