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The heart is a pear-shaped organ located in the thoracic cavity.

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The Heart (unit 1) The heart is a pear-shaped organ located in the thoracic cavity. It lies underneath the sternum between the lungs. The heart consists of three layers. 1. The pericardium; which is an outer, double layered bag containing a thin film of fluid. Its function is to reduce friction between the heart and other parts of the thoracic cavity and to maintain the heart shape. 2. The myocardium, or cardiac muscle; contracts in the same way as skeletal muscle. The heart has a pacemaker which sends impulses throughout the myocardium. Because of its united structure, all cells forming the entire myocardium muscle sheet contract together, producing a heartbeat. This is the all or none law. ...read more.


sends out impulses. The S.A. node is located in the right atrial wall. It is the rate at which the pacemaker emits impulses that determines the heart rate. Cardiac output also changes during exercise. Cardiac output is defined as the stroke volume (amount of blood ejected from a ventricle at each heart beat) multiplied by the heart rate. During the initial stages of exercise, increased cardiac output is due to an increase in both heart rate and stroke volume. When the level of exercise exceeds 40% to 60% of the individual's capacity, stroke volume has either plateaued or begun to increase at a much slower rate. Thus further increases in cardiac output are largely the result of increases in heart rate. The more blood pumped, the more oxygen is available to the exercising muscles. ...read more.


This decrease in heart rate is called bradycardia. The heart adapts to aerobic exercise over time so it can pump more blood per stroke. If I play a match at the beginning of the season when I am untrained, cardiac output can increase to four times resting capacity. In the untrained female, it goes from pumping 4 to 5 litres a minute at rest to 16 to 20 litres a minute during exercise, primarily through an increase in heart rate. In trained athletes such as Serena who trains throughout the year, the cardiac output can increase up to eight times the resting output. This is brought about not only by an increase in heart rate, but also by a training-induced increase in the stroke volume - the amount of blood ejected with each heartbeat. Rachel Jones ...read more.

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