Oxygen (O), or more specifically dioxide (O₂), needs to be transported from the atmosphere to your muscles so that it can be utilised in creating energy in the aerobic (with oxygen) energy system. Without it you would only be able to create energy anaerobically (without oxygen). This would be unsustainable and you would run out of energy within a couple of minutes.
To facilitate this transportation you have the cardio-respiratory system. This consists of two parts. The first is the respiratory system, which is comprised of an airway and the lungs. The second is the cardiovascular system, which is composed of a pump, a high pressure delivery system, a diffusion network and a collection and return network.
So lets start at the beginning. Your intake of breath. To initiate your intake the respiratory centre in your brain signals to the body to increase the volume inside the lungs. This is achieved by rising the ribs up and out and bringing the diaphragm (a muscle situated underneath the lungs) down. The higher volume means that there is now a lower pressure within the lungs then there is in the atmosphere. This causes simple diffusion to take place as air rushes into the lungs to balance the pressure difference.
The air enters through the mouth and nose into the nasal cavity, where it is cleaned, moistened and warmed. It then travels down through the pharynx, larynx and trachea, at which point it enters the chest and the lungs.
In the lungs it travels through the bronchi, into the bronchioles and finally into the alveoli. You can imagine these three parts as an inverted tree within each lung, as they start of as thick pipes and then branch down, smaller and smaller, until they become the tiny alveoli sacks. The reason for this huge amount of branching is to make sure every part of the lungs is used, providing a large surface for diffusion of the O₂ into the blood stream. In fact the diffusion surface is so large that if you could flatten both lungs out it would cover half of a tennis court!
Surrounding the alveoli are capillaries, which are extremely thin blood vessels. Because the wall of the alveoli and capillaries is only 1 cell thick O₂ can pass from the lungs to the blood stream very easily. It is encouraged to do so by the fact that the pressure of O₂, also known as the partial pressure of O₂ (PO₂), within the blood stream is lower then the PO₂ within the lung. This is because the O₂ that was in the blood has been taken up by cells and muscles on its way around the body, resulting in the amount within the blood, and so the related pressure, being low.
Because the PO₂ in the blood is low and the PO₂ within the lungs is high a pressure gradient is created, from low in the blood to high in the lungs. This facilitates a process known as simple diffusion. Simple diffusion is a random process that causes particles to go from an area of high pressure, or concentration, to an area of lower pressure, or concentration. It is the same thing that happens when you spray a can of deodorant into one corner of the room and then it spreads so that it can be smelt on the other side of the room.
To facilitate O₂ transportation in the blood you have red blood cells. These contain a protein molecule called haemoglobin. Haemoglobins can each hold 4 particles of O₂, and each red blood cell contains around 250 million haemoglobins. This means that each red blood cell can carry 1,000,000,000 molecules of O₂ if it is fully saturated!
Once the blood has entered the blood stream it is sent back to the heart, where it is subjected to the strongest pump in the body, the left ventricle. This is the section of the heart that sends the blood out and to the body. As the heart contracts it squirts the blood from the left ventricle, through the aorta (the bodies largest artery) and then into subsequent arteries around the body.
This is a very high pressure system and is only possible if the system is sealed. If the system suffers a major compromise and is no longer sealed then the pressure required cannot be maintained and this, along with the blood loss, can cause death if it is not corrected quickly.
As the blood makes its initial journey it provides no oxygen to the cells of the body as the arteries have very thick walls. This is required due to the fact they have to deal with such a high pressure environment. The arteries, much like the bronchi in the lungs, then begins to branch of into smaller arteries which eventually branch down into capillaries again. These capillaries are found everywhere in the body and surround every single cell. They have thin walls, as do the cells, to allow diffusion to take place again.
An incredibly complex and intricate system is used to regulate where the O₂ is released, using PO₂, temperature, PH balance, chemoreceptors and the partial pressure of carbon dioxide (PCO₂). To keep it simple though just understand that the body always knows where the O₂ is required. As it travels around the body the affinity of the O₂ to the haemoglobin is increased and decreased, to encourage or discourage its release to the adjacent cells.
Once the blood has been through the capillaries it enters the venous system, which comprises of veins and returns the blood back to the heart, where it can be sent through the lungs to be oxygenated again.
It is worth noting that this whole system can be improved at basically every level with cardiovascular training. This allows your body to extract and deliver O₂ more efficiently and quicker, which is why as your cardio fitness improves you are able to train harder and for longer.
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