The body's response to exercise- Regular aerobic activity results in a type of cardiac hypertrophy (growth). In this case the heart increases in efficiency and size. The wall of the left ventricle thickens, increasing the strength of its contractions. Thi

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Physiology task 2

Regular aerobic activity results in a type of cardiac hypertrophy (growth). In this case the heart increases in efficiency and size. The wall of the left ventricle thickens, increasing the strength of its contractions. This has an important effect on heart rate, stroke volume and cardiac output.

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Steady state

Once you undertake exercise, there is a raise in energy usage. This is reflected in the increased oxygen consumption.

Under some conditions where the work rate is constant, the pattern of this increased oxygen consumption shows an initial rise for a few minutes and then it levels off. Once this plateau has been reached, oxygen consumption remains relatively steady over the period of exercise. This is known as steady state exercise. For example, if you exercise for 20 minutes continuously jogging at the same speed, 20 minutes of an aerobic class or 20 minutes of continuous same speed swimming, a number of responses will be different. The athlete’s heart and respiratory states increase to accommodate the demands placed on the body. A grater amount of ATP is synthesised and a number of neuromuscular changes occur. After 3 or 4 minutes of exercise your body adapts to the increase of exercise intensity, so your increase physiological demands levels out. For the remaining 15minutes or so, the athlete will undergo what is known as steady state exercise.

Cardiovascular responses

Heart rate

Before exercise the average heart rate for is between 60-80 beats per minute in untrained men and women, but this is usually much lower in trained athletes (between 40-60 beats per minute). The heart rate increases during exercise and it does so in relation to the intensity of the exercise undertaken.

Stroke volume

Stroke volume increases to its highest levels during sub maximal exercise and does not increase further during maximal exercise.

Stroke volume achieves its maximum amounts at between 40 and 50% of VO2 max. In trained athletes, this usually means 120-140 beats per minute. The biggest increase in stroke volume happens in the transition from rest to moderate exercise. During maximal exercise, stroke volume does not increase from its peak at 120-140 BPM as the left ventricle is at this point already full to capacity. The body tolerate maximal exercise for as long as it can by increasing heart rate and maintaining stroke volume, but the body will tire and fatigue if the demands of the maximal exercise are too great.

Cardiac output

Each litre of blood carries about 200ml of oxygen. The oxygen carrying capacity of blood normally varies only a bit because haemoglobin content varies little regardless of the exercise intensity. Approximately 5 litres of blood are circulated around the body each minute at rest for trained or untrained athletes, so this works out to being approximately 1 litre of oxygen being available the body.  

An increase in cardiac output has huge benefits for trained athletes as they can transport more blood to the working muscles and, therefore more oxygen is available.

The formula for cardiac output is stroke volume x heart rate, for example if the stroke volume was 70 ml and the heart rate is 70 BPM the cardiac out put would be:

70 x 70 =  4,900 litres of blood per minute.

However, a key point of steady state exercise is that the resting heart rate decreases while the stroke volume increases. A trained athlete can have a stroke volume of 100ml and a resting heart rate of 50 BPM. Therefore the average cardiac output of a trained athlete would be:

100x 50= 5,500 litres of blood per minute

This means that there is more oxygen available to the body’s working muscle therefore giving higher performance levels than the average non trained athlete.

Blood pressure

During steady state exercise, enlargement of the blood vessels in the working muscles increases the area for blood flow. The contractions and relaxation of the skeletal muscles force blood through the vessels and returns to the heart.

Blood flow

Increased energy expenditure from exercise requires adjustment to the blood flow that affects the whole cardiovascular system.

Vasodilation

During exercise the vascular part of active muscles increases through dilation of arterioles, which is a blood vessel that braches off from an artery. This process is known as vasodiliation and involves an increase in the diameter of the blood vessels resulting in an increased blood flow to the muscle area supplied by the vessels (arterioles).

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Vasoconstriction

Vessels can also shut down blood flow to tissues, which can temporarily lessen blood supply. This process is known as vasoconstriction and involves a decrease in the diameter of blood vessels by contraction of the involuntary muscle fibres in the vessels walls, resulting in the reduction of blood flow. For example, kidney function shows the capacity to control renal blood flow. At rest renal blood flow accounts for about 20% of cardiac output during maximal exercise, renal blood flow decreases to about 1% of cardiac output.

Thermoregulation

Steady state exercise increases sweating. This beneficial initial response is ...

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