TRANSPORT IN ANIMALS. THE HEART ACTION. CHANELLE MCKEN. ECG(ELECTROCARDIOGRAM). This is a graphic record of the electrical activities of the heart , as monitored at specific locations on the body surface.
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THE HEART ACTION
To demonstrate thickening of a heart muscle for example due to long-standing high blood pressure.
Bradycardia- Indicate a heart rate that is slower than normal. Underactivity of the thyroid gland and changes in the electrical activity of the sino-atrial node (SAN) can also give rise to bradycardia.
At a certain point an action potential is set in the cells. A wave of excitation similar to nerve impulse passes across the muscle fibres of the heart as the action potential spreads from the SAN. It causes the muscle fibres to contract.
In the body, however, the demands on the blood system are constantly changing and the heart rate has to be adjusted accordingly. This is achieved by control systems, one nervous and the other chemical.
The amount of blood flowing from the heart over a given period of time is known as the cardiac output and depends upon the volume of blood pumped out of the heart at each beat, the stroke volume and the heart rate ( number of beats per minute).
Cardiac output = stroke volume * heart rate
Nervous control of the heart rate - the nervous control of the cardiovascular system is located in the medulla. Part of its function is to control heart rate. Certain nerve link the medulla with the heart.
Impulses passing along the vagus nerves reduce the heart rate. Other nerves, which are part of the sympathetic nervous system, have their origin in the cardiac accelerator region of the medulla. These run parallel to the spinal cord and enter the SAN. Stimulation by the nerves results in an increase in the heart rate .
Sensory nerve fibres from stretch receptors within the walls of the aortic arch, the carotid sinus and the vena cava run to the cardiac inhibitory center in the medulla.
As the volume of blood passing to any these vessels increases so does the stretching walls of these vessels.
Consequently the walls of the vena cava are stretched by large quantities of blood and the heart rate is increased. At the same time the increased blood flow to the heart places the cardiac muscle of the heart under increased pressure.
Increased stroke volume stretches the aorta and carotids which in turn , via stretch reflexes, signal the cardiac inhibitory center to slow the heart rate. Therefore there is an automatic fail safe mechanism which serves to prevent the heart from working too fast, and to enable it to adjust its activity in order to cope effectively with the volume of blood passing through it at any given time.
Adrenaline is secreted by the medulla of the adrenal glands. The adrenal medulla also secretes smaller amounts of the hormone noradrenaline which has similar effects to adrenaline. Both stimulate the heart.
Short term effects that affects the heart rate is exercise. During period of heavy exercise the output of blood from the left ventricle of the heart may increase from the resting condition.
The heart, like all muscles, gets stronger with exercise. Long- term training results in a stronger heart and a higher cardiac output.