In humans,heart muscle cells begin contracting very early in development, before any sign of development of the arms and legs in the embryo.They continue contracting once every second or so, without rest, until death,perhaps 70 or 80 years later. Throughout life, there is little margin for error for these cells. If they do not function adequately,even for a very short period,heart failure may result, with often fatal consequences. For heart cells to carry out their relentless workload, they require very careful regulation of their extracellular environment.
Angina is felt as oppressive, constrictive pain under the breastbone, often spreading down the inner left arm and sometimes across to the right shoulder. It occurs in relation to exertion and is relieved by rest. The pain of a heart attack is similar, but of greater intensity and longer duration. The pain appears to originate in the heart muscle itself rather than the coronary arteries, which supply blood to the heart muscle.
Both angina and heart attacks are result of hypoxia, which occurs when the oxygen supply to cardiac muscle cells is insufficient to meet their needs. The most common cause of hypoxia is arteriosclerosis, which is a blockage of the coronary arteries that prevents normal blood supply to cardiac muscle cells. Because there is insufficient oxygen, cells change from aerobic to anaerobic respiration and start to produce lactic acid.Less energy is available to cardiac muscle cells due to this change.
When the heart is functioning normally, much of the energy used by cardiac muscle cells is used to pump sodium ions of cell and to maintain the inside of the cell negative with respect to the outside (i.e., to polarise the membrane). With each contraction, ions move rapidly across the cell membrane and polarization is lost. Energy is required to re-establish the balance of ions and the polarity of the membrane before a cell can contract again.
During hypoxia, lactic acid accumulates, causing angina and, if the amount of energy available to cells is insufficient to maintain normal ionic balance and membrane polarization, the cells will be unable to contract normally. If the hypoxia is extreme or maintained for some time, heart cells will begin to die, causing a heart attack (a myocardial infarction).
In addition to adequate oxygen levels, the normal function of cardiac muscle cells requires that levels of ions (particularly calcium, sodium and potassium) in the surrounding extracellular fluid be maintained within narrow limits. The concentration of calcium ions is important, but in ordinary circumstances it is unlikely to change sufficiently to alter heart function. The concentration of sodium ions is unlikely to increase dangerously, but sodium ions can be diluted by drinking huge quantities of water or by drowning in fresh water. This causes cardiac muscle cells to contract very quickly without coordination and the heart fails to pump efficiently.
If there is excess potassium in extracellular fluid, the heart will become extremely relaxed and dilated, and heart rate will slow. Two to three times the normal level of potassium will cause such weakness of the heart that death will result.
© 2017 Barake Charles