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HEART AND EXERCSE

Posted on Tuesday, 30 October 2012



The heart is an organ which pumps blood, which in turns carries oxygen and nutrients to the cells of the body and transports away the waste products such as carbon dioxide, lactic acid etc.

Cardiac Output

CO is defined as the volume of blood pumped by the heart in one minute, and is expressed in liters per minute or milliliters per minute. CO is the product of heart rate times stroke volume (the amount of blood pumped with each beat of the heart). For example if HR equals 72 beats per minute and stroke volume equals 70 ml of blood, then the CO is equal to 5,040 ml/min or 5.04 lts./min (72x70). CO can also be calculated from the amount of oxygen consumed per minute and the amount of oxygen taken up by the blood as it flows through the lungs. These relationships can be expressed by the Fick principle as follows:

For example if the oxygen content of the venous blood entering the lings is 16 volumes percent, that of the arterial blood leaving the lungs is 20 volumes percent and the oxygen consumption of the body 200 ml per minute, the amount of oxygen used per minute equals the amount of oxygen taken up by the lungs per minute. From the above data it can be seen that each 100 ml of blood flowing through the lungs picks up 4 ml of oxygen. Since the total amount of oxygen absorbed into the blood from the lungs each minute is 200 ml, then a total of fifty 100 ml portions of blood must flow through the lungs each minute to absorb this amount of oxygen. Thus the cardiac output is

The blood samples to measure the oxygen content by the Fick's procedure is taken by Cardiac catheterization and the oxygen consumption rate is measure by a respirometer apparatus.

Another method used to measure CO is the dye dilution method. Other methods include the carbon dioxide rebreathing test and radioisotope test.

It should be pointed out that Fick technique is considered most accurate to measure the CO under resting or steady state conditions of exercise, while CO in rapid changing conditions the dye dilution technique is more accurate.

Cardiac Output during rest

At rest in the supine position, the normal cardiac output in adults is approximately 5 liters per minute. This is generally achieved with a heart rate of 70beats per min for the untrained and 45 beats per min for endurance trained person. Since the trained persons cardiac output at rest is also about 5 lits, then the decrease in heart rate must be offset by an increased in stroke volume if the cardiac output formula, the calculated stroke volume for the untrained person would be around 71.4 ml of blood per beat, whereas the stroke volume for the untrained person would be about 111.1 ml per beat.

Cardiac Output during exercise

During exercise upto 40 to 60 percent of maximal capacity, cardiac output in trained athletes may be increased to 40 lts per min. At this level of work, it is known that this 5-7 fold increase in cardiac output is due to increases in both heart rate and stroke volume. At levels beyond 40 to 60 percent of maximum, increases in cardiac output are mainly a function of heart rate increases. At the same time, it should be emphasized that since heart rate in strenuous exercise increases approximately the same in both athletes and non athletes, the greater changes in cardiac output attained by the trained athletes is due to their greater ability for increasing the stroke volume of the heart. This is more than double the size of the stroke volume for untrained subjects. Again substituting the heart rate values in the cardiac output formula, the calculated stroke values for the untrained person would be around 100 ml of blood per beat, whereas the stroke volume for the trained person would be approximately 200 ml per beat.

The regulation of cardiac output involves the regulation of heart rate and stroke volume.

Heart rate

The impulse that causes the heart to contract rhythmically originates within the heart muscle itself, in the right atrium known as the pacemaker or S-A node. Unlike skeletal muscles, the heart muscle possesses autorhythmicity. However both the nervous and chemical factors are involved in the regulation of the heart rate during rest and exercise.

The autonomic nervous system which supplies the parasympathetic or vagus nerves and the sympathetic nervous system or the accelerator nerves to the S-A node plays an important role in regulating heart rate.

Stimulation of the parasympathetic fibers cause the release of acetylcholine (Ach) from their ends, which slows the rate of impulse formation in the S-A node and also slows the rate of conduction through the A-V bundle which slows the impulse into the ventricles. Such impulses are cardio-inhibitory and the final result is a slower heart rate.

Stimulation of the sympathetic fibers causes the release of norepinephrine from their ends. The norepinephrine speeds up both the S-A node rates and the conduction rates. Such impulses are called cardio-acceleratory which results in a faster and stronger heart rate.

This different effect of the two nerves is referred to as reciprocal innervation of the heart muscle.

The nerves controlling the heart arise from specific areas of the medulla of the brain called cardio-inhibitory and cardio-acceleratory centers, the control of the heart rate is predominantly through reflexes.

The cardio-acceleratory centers are affected by several afferent stimulation sources which are referred to as pressor. They are

(i) proprioceptive reflexes originating in the working muscles and joints to contribute to increases in heart rate.

(ii) impulses arising in the chemoreceptors of the carotid body and the aortic body as a result of decreased pH or increased carbon dioxide results in an increased heart rate.

(iii) impulses arising in the adrenal medulla cause a discharge of norepinephrine and epinephrine hormones into the blood stream and an increase in heart rate.

The cardio inhibitory or depressor afferent sources are from the activity of the stretch receptors in the carotid sinus and the aortic arch. The activities from these receptors tend to slow the heart rate down.

Two other factors that are important are an increase in body temperature and a fall in the blood oxygen content, also play a large role in the increased heart rate.

Heart Rate Response to Exercise

Heart rate increases linearly with increasing oxygen consumption in both the trained and untrained individuals. During exercise the heart rate of a well trained person is consistently lower at any given workload or Vo2 than that of the untrained person

Endur
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