Trained, Moderately Trained, and Untrained Individuals

The heart is an organ of specially designed contractile muscles that are designed to function as pumps. This organ system comprises of actually two separate pumps, the right heart that pumps blood through the lungs and the left heart that pumps blood through peripheral organs. In turn each of these hearts is a pulsatile 2-chamber pump composed of an atrium and a ventricle.Each atrium is a weaker primer pump for the ventricular cavity, the pumping force generated by atrial muscular contractions helping to move blood into the ventricle. The ventricles then supply the main pumping force that propels blood either through the pulmonary circulation by the right ventricle or through the peripheral circulation by the left ventricle.

Special mechanisms within the heart cause a continuing succession of contractions of these chambers throughout the life, and this is equivalent to contractions of specially designed muscle fibers that enclose these cavities (Tortora, G.J. and Grabowski, S.

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R., 2003).These contractions of the cardiac muscles happen in a regular pattern in health, and this is regulated by a system called cardiac rhythmicity. The regulations that govern cardiac rhythmicity is in turn affected by many physiologic factors within the human body, and to be able to express different levels of cardiac functions in different states in different individuals and under different environmental stimuli, the human body has been provided with an in-built mechanism known as cardiac rhythmicity.Cardiac rhythmicity essentially is transmission of action potentials through the cardiac musculature in different rates causing the heart to beat rhythmically, the time scale of rhythm varying according to the given physiologic state that the body is in (Tortora, G.J. and Grabowski, S.

R., 2003).The stimulus for cardiac contraction arises entirely within the heart. Innervation of the cardiac muscles comes from both branches of the autonomic nervous system, allowing for external regulation of the heart rate, strength of contraction, and provision of some degree of sensory feedback.As in other types of muscle and in nerve, the muscle cells of the heart have an excitable and selectively permeable cell membrane that is responsible for both resting potentials and action potentials.

Cardiac muscles actually form a functional syncytium with cells acting in concert both mechanically and electrically.They essentially develop a communication system among themselves despite their very small size for muscle cells. This is biologically necessary for organized function (Tortora, G.J.

and Grabowski, S.R., 2003).

These small muscle cells also make each cell more critically dependent on external environment, and cardiac function, therefore, may be greatly responsive to the electrolyte and metabolic alterations arising elsewhere in the body. Hormonal messengers, such as, norepinephrine also have quick access to cardiac muscle cells leading to alterations in the patterns of their functions. The length-tension property of the cardiac myocytes is the basis of the remarkable capacity of the heart to adjust to a wide range of physiologic conditions and requirements (Froelicher, V.

F. and Myers, J., 2006).The term excitation-contraction coupling refers to the mechanism by which the action potential causes the myofibrils of the cardiac myocytes to contract. These electrical phenomena are the results of ionic concentration differences and several ion-selective membrane channels, many of which are voltage and time dependent.

The closer association of electrical and mechanical events is one key to the inherent properties of cardiac muscle that suit it to its role in an organ that is largely self-regulating. Furthermore, cardiac muscles in its natural location do not exist as separate strips of tissue, rather present as interwoven bundles of fibers in the heart walls, arranged so that contractions leading to shortening results in reduction of volume of the heart chamber, and the force generated results in an increase in pressure in the chamber (Froelicher, V.F. and Myers, J., 2006).The mechanisms controlling the circulation can be divided into neural control mechanisms, hormonal control mechanisms, and local control mechanisms.Cardiac performance at any given point is the result of the integration of all these three control mechanisms.

It has been well acclaimed that neural control of heart involves sympathetic and parasympathetic branches of the autonomic nervous system (Tortora, G.J. and Grabowski, S.

R., 2003). However, in some situations, factors other than blood volume and arterial pressure regulation strongly influence cardiovascular control mechanisms.These situations include the flight-or-fight response, diving, thermoregulation, standing, and exercise. Stimulation of the parasympathetic nerves to the heart causes the hormone acetylcholine to be released at the vagal endings. This hormone has two major effects on the heart. First, it decreases the rate of rhythm of the sinus node and second, it decreases the excitability of the A-V junctional fibers between the atrial musculature and the A-V node (Froelicher, V.F.

and Myers, J., 2006).