Heart rates and lactate thresholds are two factors that govern many training regimes. Training schedules will have specific phases dedicated to building aerobic bases. However, the primary determinant of each of these factors is often overlooked - the red blood cell.

A Red Cell Biography

Name: Erythrocyte
Age range: 1-120 days
Place of birth: Longbone, marrow
Marital status: Currently single - on/off relationship with oxygen
Occupation: Energy-release operative
Key to development: Release of the hormone erythropoietin.
Plasma lactate accumulation, renal hypoxia.
Influences: Oxidative flux, osmotic shifts and foot strike stress.
Professional qualities: Highly adaptable with an iron resolve, dependable. Works well under pressure.

Aerobic means dependent upon oxygen. During respiration, oxygen seeps through the thin walls of the alveoli in the lungs and into the blood. The oxygen binds to an iron compound called haemoglobin, which is contained within the red blood cells. It is released to another iron-containing protein compound, called myoglobin. This enables available fuel sources (such as glucose, glycogen or fat) to be broken down within the muscle and energy to be released.

This is a system that works beautifully as long as there is sufficient oxygen available in the blood. If there is not enough oxygen getting to the muscles, energy is released anaerobically (without oxygen). One of the by-products of this system of energy-release is the formation of lactic acid. The accumulation of lactic acid will eventually limit performance.

For an athlete, the red blood cell count, the haemoglobin concentration and haematocrit (the percentage of red blood cells) are all-important. Many researchers have documented the high correlation between increased red cell count, with its consequent increased haemoglobin concentration and aerobic performance. Low red-cell counts and low haemoglobin concentrations have been correlated with poor performance.

Key terms

The body is constantly producing red blood cells. This is in response to a hormone called erythropoietin (EPO) being released by the kidneys. If there is an increased amount of erythropoietin within the body's circulation, then the rate of red blood cell production will be increased. Performance will improve, as more oxygen is being brought to the working muscles.

However, it is not quite as straightforward as this. In order for the reticulocytes (the new red blood cells) to work properly, they must be provided with iron. This is taken from the body's iron stores in the form of ferritin. If there is not enough iron available in the stores, then cells with low haemoglobin levels will be released. Due to the specific demands of their training, many athletes, especially women, are at serious risk of depleting their stores of iron, with the consequent lowering of haemoglobin levels. Approximately two per cent of the red-cell population is replaced every day. If the iron stores are continually depleted, then the red cells being produced by an athlete's training will not be able to work efficiently, because they will have low levels of haemoglobin. This will cause the athlete's performance to decline.

Examining new blood cells on a regular basis will give a picture of how the body is responding to training. The rates of new-cell production can be assessed, and a developing iron deficiency (when cells have a low haemoglobin content) can be detected before it begins to seriously affect performance.

The key point here is that increased red-cell production will only produce the desired effects if the cells are provided with enough iron for them to work properly. Blood tests that include the numbers of the new cells, the reticulocytes, will provide very important information for an athlete.

Red blood cell production

The body is continually producing red blood cells in a process called erythropoiesis, which as we have seen, is stimulated by the hormone erythropoietin from the kidneys. Some athletes have artificially accelerated their red blood cell counts by using an engineered form of this hormone. This manufactured hormone has achieved notoriety for its illegal use among athletes.
Red blood cell production is accelerated as a result of training. However, it is also increased with exposure to a hypoxic environment, such as low oxygen at high altitude. The production of red cells is also a compensatory mechanism to replace red cells that have been broken down (a process called haemolysis).

It is extremely important that these developing cells are provided with enough iron to work as efficiently as possible. If insufficient iron is made available to these cells then a trend towards hypochromia (being low in colour) gradually occurs as a result of low haemoglobin concentrations. Iron-deficient erythropiesis is a gradual process, which will pass through different stages before an overt condition known as anaemia prevails.

Haematological analysers

Automated haematological analysers can be used to examine blood and provide highly accurate information regarding the number of red blood cells in circulation. It works using a technique called flow cytometry. Studies of athletes at the Australian Institute of Sport have produced expected normal ranges for a range of haematological parameters as follows:

Red Blood-cell Count (RBC)

Reference interval
Males: 4.50-6.50 x 1012/l
Females: 3.80-5.80 x 1012/l
The red blood cell counts may be increased if an athlete is dehydrated (haemoconcentration). Conversely, the count can be artificially decreased due to plasma volume expansion. In iron-deficiency anaemia, the red blood cell count can fall to a low level.

Haemoglobin Concentration (Hb)

Reference interval
Males: 14.0 - 18.0g/dl
Females: 12.0 - 16.0g/dl

Haemoglobin is the red-pigmented iron-containing protein, attached to a red blood that forms a reversible bond with oxygen. Haemoglobin accounts for the largest amount of iron in the body. Low levels of haemoglobin concentration are seen in conditions of iron-deficiency anaemia. Whilst the same correlations with regard to endurance capacity have been noted with haemoglobin concentrations, care must be exercised when interpreting results as this parameter may also be influenced by plasma shifts.

Haematocrit (HCT)  

Reference interval
Males: 0. 41 - 0.53 (ratio)
Females: 0.37 - 0.47 (ratio)

Haematocrit refers to the percentage of red blood cells in the blood. It is calculated using the red-cell count and the average size of the cells. The haematocrit will also give an indication of the aerobic potential of the athlete. The volume of plasma will again influence haematocrit readings. Many athletes will have seen reference to the parameters indicated above, where cyclists are ruled ineligible to compete on health grounds if their haematocrit is too high. This is because excessively high haematocrit will adversely affect blood viscosity (its ability to flow) and will cause circulatory complications.

The parameters for the indices described above and the mathematically derived values of Mean Cell Haemoglobin (MCH) and Mean Cell Haemoglobin Concentration (MCHC), which are calculated by utitilising the red blood cell value and the haemoglobin value. Together they provide additional information about the mature red cell population. Recent advances in technology have enabled the identification of another population of red cells, the reticulocytes.

Reticulocytes  

Reference interval
0.5 -1.7%

Reticulocytes are immature red blood cells. The relative concentration of reticulocytes indicates the current rate of red-cell production. This value can be of real use to an athlete or coach who wants to quantify the efficacy of a training phase by regularly monitoring the reticulocyte count. Increased reticulocyte counts are commonly found following the inititation of interval training phases.

It is also an extremely important index when considering the training effects of altitude. Further to the information about rates of production, the reticulocyte count is also a snapshot of the current erythropoiesis and available iron supply to the newly released cells. It can offer a predictive index of the development of iron deficiency, and also can be used as a recovery index from this condition.

Are the red cells of an athlete different from those of a sedentary person?
  The red blood cells of an athlete are no different from a sedentary person but the stresses that are placed upon their cells most certainly are. The athlete's red blood cells must contend with continually fluctating oxygen saturations (oxidative flux). In addition, each cell's membrane must cope with the greater osmotic stresses of water moving into and out of the cells. Also, cells are placed under mechanical stress, particularly when passing through the exposed capillaries in the soles of the feet, where they can be haemolysed (broken down).

All of these factors have the effect of accelerating red-cell senescence (ageing), leading to red-cell destruction. The older cells will be removed from circulation and the reticulocytes, the new cells, will be produced to compensate. This is why athletes tend to have a higher reticulocyte count then the average person. Care must also be exercised when interpreting the results of an athlete's blood test because plasma shifts may falsely affect the readings. This will be discussed in part two of this article.

To conclude

A blood test provides much important information. It is often performed when an athlete is performing poorly, but it is equally important when an athlete is performing well.