The average male adult has just over 5 million red blood cells per cubic millimeter (microliter) while adult females have over 4.5 million red blood cells per microliter. This quantity of red blood cells may vary for a number of reasons – both physiological and pathological. While the body can cope with a slight increase or decrease in the red blood cell concentration within the blood, too many red blood cells or too few red blood cells can have a host of effects, some serious, on the human body.
The medical term for too few red blood cells or a low red blood cell count is anemia. This term is attributed to a low quantity of hemoglobin in the blood as well. Too many red blood cells or a high red blood cell count is known as polycythemia. There are different types of anemia and polycythemia which may be due to various causes. Depending on the type, there may also be other changes of the red blood cells in the body like its shape, size, oxygen carrying capacity of individual cells and even its color.
The primary function of red blood cells are to carry oxygen in the bloodstream. Apart from its functions, the presence of red blood cells in the human blood has a host of effects and influences the action and functioning of various other components of the cardiovascular system.
Blood is about three times the viscosity (thickness) of water and this is mainly due to the concentration of red blood cells. Too many red blood cells will make the blood more viscous (thicker) and too few red blood cells will make the blood less viscous (thinner).
Too Few Red Blood Cells
A deficiency of red blood cells will affect the oxygen-carrying capacity of the blood as well as the viscosity of blood.
Since the blood is “thinner”, blood is able to travel “faster” due to less resistance with the blood vessels of the body. This means that more blood flows through the heart in a minute than is the norm – increased cardiac output.
At the same time, the blood is carrying less oxygen which causes the blood vessels to dilate. This further decreases resistance in the peripheral circulation meaning that blood is able to travel even “faster”. This is one way that the body accommodates with the low oxygen-carrying capacity of the blood – smaller quantities of blood reaching the tissues faster is almost equivalent to large quantities of oxygen reaching the tissues at a slower rate.
However, during a period of increased demand, like exercise, the body is unable to cope due to the low red blood cell count. The heart attempts to beat faster and even the breathing rate is increased in an attempt to supply the tissues with more oxygen. However this is often insufficient in strenuous exercise or prolonged activity. Eventually the tissues become hypoxic and ischemia (tissue injury) will ensue. The heart in turn cannot cope indefinitely with the greater workload and acute heart failure will set in.
Too Many Red Blood Cells
An excess of red blood cells will affect the viscosity of blood and ultimately impact on the rate of blood flow. Since the blood is “thicker”, its flow in the peripheral circulation is sluggish. This can increase the chance of clot formation should there be an injury to the blood vessel wall or turbulent blood flow as outlined in Virchow’s triad under Thrombosis.
Although the blood flow is sluggish, the same amount of blood returns and is pushed out of the heart in a minute – cardiac output – because the blood volume is greater. However, significantly “thicker” blood may mean that the heart will have to pump harder in order to move the blood through the vessels due to increased peripheral resistance. Therefore the blood pressure may increase (hypertension).