The heart has four chambers; two atria and two ventricles. During Serena’s match, there are many short-term responses taking place concerning the heart. The heart pumps blood containing oxygen, fluids and nutrients to the active muscles. Blood flow then drains the metabolic waste products away. There is an increased blood flow to the working muscles due to increased metabolic activity. This increased metabolic activity causes an increased concentration of carbon dioxide and lactic acid content in the blood, which increases acidity of the blood. A decrease in pH of the blood can decrease glycolytic enzyme activity. The heart is regulated by the autonomic nervous system (ANS), and determines the rate at which the pacemaker, the Sino-atrial node (S.A. node) 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. And as muscles train, they're better able to extract and use the oxygen to produce more work.
Serena heart is more efficient than mine. This is because she does year round training, whereas my training is only seasonal i.e. in the summer. Her heart would have adapted to this training by becoming bigger and stronger. This is called cardiac hypertrophy. Hypertrophy increases the force of the contraction of the heart muscle, which allows the heart to pump blood with increased efficiency. Therefore her heart is able to pump more blood out of her left ventricle per systole. I.e. she has a larger stroke volume at rest and during exercise. This is advantageous because the heart can pump more blood per beat since the walls can contract more forcefully. The heart does not need to beat as often per minute to achieve the same amount of blood flowing to body tissues. 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.
