If cancer affects your bone marrow, the marrow will produce fewer and fewer of the blood cells essential to your well being.
Bone marrow produces red blood cells, which carry oxygen. With too few red blood cells, your body does not have enough oxygen and you become tired and breathless. Giving you oxygen may not be a solution. It is not lack of oxygen intake that is causing the problem; it is lack of cells to carry the oxygen.
Low white-cell counts increase your risk of infection. Too few platelets reduces the ability of blood to clot.
Low blood counts may be caused by treatment, such as radiation. In that case, the effects on your bone marrow may wear off over time and the cell counts may go up again. Low blood counts may also result when cancer destroys the cell-making capacity of the bone marrow or when the cancer kills blood cells faster than the marrow can make new ones.
If the cancer has destroyed the bone marrow's ability to produce blood cells, palliative measures are available to help maintain your quality of life but these measures will not cure the problem.
If blood counts are low due to treatments, such as radiation, medication may "jump start" the bone marrow. Usually the medicine is given in a series of injections. You may be able to give the injections yourself. If this is successful, the blood counts will rise and the symptoms - breathlessness and fatigue - may lessen.
Eventually, the toxic effect of the cancer on all body systems will probably increase, making fatigue and breathlessness unavoidable. You may be less able to perform daily tasks simply because you don't have the energy.
Blood transfusions can increase the number of red blood cells available.