Sickle cell disease is one of the most inherited blood disorders with around 100,000 people affected in the United States and millions more worldwide. Essentially, it is recognized by red blood cells (RBCs) that change from their flexible, biconcave shape to a rigid, sickle-like shape. But how do sickle cells affect blood flow and contribute to serious complications?
RBCs Function and Shape
Red blood cells contain hemoglobin, a protein that is responsible of carrying and delivering oxygen to the body tissues. However, in sickle cell disease, a genetic mutation results in the production of hemoglobin S (HbS) a protein which causes RBCs developing the abnormal shape or becoming sickled. This change profoundly alters the mechanical properties of the cell disrupting the blood flow through the vascular system and leading to some complications including: yellow eyes, fatigue, swelling, anemia, stroke, and acute chronic pain. Some of the main mechanical factors affecting the RBC flow include:
Elasticity and Deformability
Healthy RBCs exhibit elastic behavior like a rubber band; they can bounce back to their original shape after deformation. Sickle cells, however, have a high Elastic Modulus meaning they are stiffer compared to their counterpart. This is due to the polymerization of HbS protein into a rigid fiber. The high stiffness means they can’t deform effectively hence they are prone to cluster in narrow vessels leading to blockages and increasing the likelihood of strokes.
Hydrodynamic and Shear Forces
Hydrodynamic forces ensure smooth journey of the blood and the particles within through the cardiovascular system, for example, shear forces. Imagine rubbing your hands together that sliding force you feel is shear.
In blood vessels, the blood near the wall moves more slowly because of friction, while the blood in the center flows faster. This friction between the layers flowing against each other is shear force.
Shear force helps RBCs deform and travel through the smallest capillaries Fig.3. However, sickle cells respond differently to these forces. They are less deformable under stress and more inclined to premature destruction. The sickle cells have a less stable structure which weakens the cell membrane. While the lifespan of a normal RBC is around 120 days, sickle cells can only live 10-20 days. This also causes hemolytic anemia where RBCs are destroyed beyond the body’s ability to replenish them.
Adhesion and Viscous Forces
The membrane of the sickle RBCs has high adhesion due to its altered proteins. The increased adhesion to the lining of the vessels causes a buildup of cells restricting blood flow turning the vessel into a bottleneck. This can cause a painful vaso-occlusion crises, where the blood flow is blocked, depriving the organs of nutrition and oxygen which in extreme cases can lead to organ damage. The presence of the sickle cells also increases the resistance of the blood to flow, also known as increased viscosity. This will cause the blood flow to be slower and make the heart work harder to pump the thick fluid.
How RBCs Mechanics Shapes Future Research
Understanding the battle between the different forces shows why sickle cell is so challenging to manage. While there is no cure for this disease there are treatment that can help manage the symptoms. The mechanics behind sickle cell makes it a complex interplay of forces that disrupt the flow. Understanding these forces opens a fascinating window into understanding the hidden forces that work within our bodies.