By Nenad Rodic,
posted 04/05


Circulatory system transports blood throughout the body. It is made up of the heart and the blood vessels (veins, arteries, capillaries). Blood carries oxygen and nutrients to all organs and cells, and removes CO2 (carbon dioxide), lactic acid, and other waste by-products of metabolism away from them. The heart is the pump that ensures the movement of blood throughout the body.
All arteries carry oxygen-enriched blood (except for the pulmonary artery which connects the heart to the lungs). Blood that is delivered to the lungs by the pulmonary artery is low in oxygen and high in CO2 content. In the lungs, the exchange of CO2 for O2 is completed, so that CO2 is exhaled, and O2 from inhaled air is diffused into the blood. Oxygenated blood than travels via pulmonary veins to the heart, and from there, it is pumped to all parts of the body via the arteries. Once it reaches an organ, and its building blocks, the cells, blood delivers nutrients and oxygen while receiving CO2 and toxic waste products from the cells, subsequently travelling via the veins back towards the heart.
The circulatory system plays a vital role in sport physiology since the ability to deliver oxygen to muscle cells is one of the main limiting performance factors (VO2max).
Some of the limiting oxygen consumption factors related to circulatory system are:

Cardiac output is the amount of blood pumped out of the heart each minute. For left heart ventricle, the values are around 5 L/min while resting, and up to 30L/min while exercising for a trained athlete.

Blood volume: Blood plasma, which is primarily water, constitutes about 55% of the total blood volume. The rest are blood elements of which red blood cells (RBC), or erythrocytes, are the most relevant, as they represent oxygen transport vessels. RBC’s contain are filled with an iron-containing protein called hemoglobin. Hemoglobin has the ability to bind both O2 and CO2 and transport them from the lungs to all the cells, and back to the lungs again. The volume ratio of solid parts in the blood and blood plasma is called hematocrit. Normal values range between 42-46%, but people who live at high elevations can have hematocrit values up to 60%. High hematocrit means that there are more blood elements to transport oxygen to the cells, hence the potential for increased aerobic performance. It is not clear if training has any effect on RBC production) High intensity and long-distance training have damaging effect on RBC’s, especially if combined with weight loss diets. One of the ways to increase RBC count is to spend some time at high elevations. It is not advisable to train at very high altitude since this will likely cause overtraining and possibly the reverse effect on RBC count. Altitude tents are a good tool since you can train where you live and sleep at any elevation you want, by setting the tent’s controls. During the early nineties a drug called EPO (erythropoetin-alpha) appeared on the market. It is an artificially created, but naturally occurring hormone initially intended to treat chemotherapy patients from damaging effects that chemo has on the blood. But as soon as it was out, cyclists and other athletes got their hands on it to increase their hematocrit values and subsequently, their performance. Since EPO it is a natural substance found in the body, the UCI (cycling governing body) introduced a test that would suspended every rider with hematocrit over 50%. This rule excluded riders from high elevation regions, like Columbia for example. Later, tests were developed to find the artificial EPO, which sent ripples of concern within the athletic community. EPO treatment can be very effective and in my opinion is the most potent of all banned substances used today by endurance athletes. EPO can be hazardous as well, since high hematocrit values mean more and harder work for the heart muscle. This drug caused many deaths in professional sport. Many athletes use some sort of blood doping and while this situation is very unfortunate before we judge any of them we should give up coffee, pain relievers, food supplements and other things we take daily. Although not banned most of these substances are very dangerous and harmful. Some of them are taken to prevent some sort of degradation process or help recuperate. In that case they are helpful. It is the same with EPO. Some of the athletes are under such stress that the only way for them to keep their health is to take “strong stuff”. I do not condone this behavior. I think everyone should take it little bit easier, but the world today does not operate that way. So, next time you hear of an athlete being banned or arrested for the use of EPO feel sorry for them and this society that makes things like that necessary.

Capillaries are tiny blood vessels that surround every cell. The rate of oxygen diffusion form blood into a cell’s interior depends on the degree of development of the capillary net. It is of utmost importance to stimulate capillary development throughout the muscles, as this seems to be the limiting factor not only for oxygen transport, but the transport of free fatty acids from the blood into the cells as well. Number of capillaries in the muscle tissue can be increased through low-intensity, long-distance training that is well bellow anaerobic threshold or even below maximum aerobic function (180-age=heart rate, approximately).
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This system consists of nasal passages, the bronchi, and the lungs. Inhaled air travels from the mouth or nose through large tubes called bronchi; they branch further into smaller tubes called bronchioles ending in small sacks called alveoli. Capillaries also surround alveoli and it is in the alveoli where O2-CO2 exchange occurs. I find respiratory statistics to be not very important since it is not likely that amount of air you can inhale at once, or per minute will be a limiting factor of your performance. As long as your pulmonary function is normal (no cold, bronchitis or pneumonia) there should be no interference with your performance.
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By Nenad Rodic, founder of