TRANSPORT IN ANIMALS

1. TRANSPORT IN ANIMALS THE HEART ACTION CHANELLE MCKEN

2. ECG(ELECTROCARDIOGRAM) • This is a graphic record of the electrical activities of the heart , as monitored at specific locations on the body surface. • In an ECG test, the electrical impulses made while the heart is beating are recorded and usually shown on a piece of paper. • This is known as an electrocardiogram, and records any problems with the heart's rhythm, and the conduction of the heart beat through the heart which may be affected by underlying heart disease.

3. Functions of an ECG • The ECG can be used to discover different types of heart disease. • To assess if the patient has had a heart attack or evidence of a previous heart attack. • Used to monitor the effect of medicines used for coronary artery disease. • Reveals rhythm problems such as the cause of a slow or fast heart beat.

4. To demonstrate thickening of a heart muscle for example due to long-standing high blood pressure. • To see if there are too few minerals in the blood.

6. Definition of terms • Arrhythmia- Abnormal patterns of cardiac electrical activity. ECG analysis is useful in detecting and diagnosing cardiac arrhythmias. • Tachycardia- Indicates a faster-than-normal heart rate. It may be caused by anxiety, anger and laughter, and overactivity of the thyroid gland. Severe tachycardia is often the result of changes in the electrical activity of the heart.

7. 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.

8. Internal factors that control heart action • The basic rate of the heartbeat is controlled by the activity of the SAN. The SAN can stimulate the heartbeat on its own, but the rate at which it beats can be varied by stimulation from the autonomic nervous system. • The cells in the SAN slowly become depolarised during atrial diastole. This means that the charge across the membrane is gradually reduced.

9. 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. • The SAN is known as the pacemaker because each wave of excitation begins here and acts as the stimulus for the next wave of excitation. • Even when the heart is removed from the body and placed into an artificial medium it will continue to beat rhythmically, although more slowly.

10. 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. • This is a homeostatic response whose overall function is to maintain constant conditions within the bloodstream even though conditions around it are constantly changing.

11. 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 • It is the cardiac output which is the important variable in supplying blood to the body. One way of controlling cardiac output is to control the heart rate.

12. 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. • The medulla has two regions affecting the heart rate, the cardiac inhibitory center which reduces the heart rate, and the cardiac accelerator center which stimulates the heart rate. • Two parasympathetic nerves, called the vagus nerves, leave the inhibitory center and run , one on either side of the trachea, to the heart. Here nerve fibres lead to the SAN and AVN.

13. 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 .

14. 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. • Impulses received from the aorta and the carotids decrease the heart rate, while those from the vena cava stimulate the accelerator center which increases the heart rate. • It is the coordinated activity of the inhibitory and accelerator centers in the medulla that controls the heart rate.

15. As the volume of blood passing to any these vessels increases so does the stretching walls of these vessels. • This stimulates the stretch receptors increasing the number of nerve impulses transmitted to the centers in the medulla. For example, under conditions of intense activity body muscles contract strongly and this increases the rate at which venous blood returns to the heart.

16. 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. • Cardiac muscle responds automatically to this pressure by contracting more strongly during systole and pumping out an increased volume of blood. Stroke volume is increased.

17. 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. • Hormonal control of heart rate- a number of hormones affect heart rate, either directly or indirect.

18. 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. • Cardiac output and blood pressure are increased by increasing heart rate. • Thyroxine, produced by the thyroid gland, raises basal metabolic activity, with greater demand for oxygen and production of more heat. As a result, vasodilation followed by increased blood flow occurs and this leads in turn to increased cardiac output. Heart rate is directly affected by thyroxine.

19. 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. • This brought about by an increased heart rate and a more complete emptying of the ventricles (stroke volume). • In anticipation of exercise, during its early phase, the sympathetic nervous system and adrenaline stimulate an increased heart rate, however during a period of prolonged exercise, the rate is maintained by further nervous and hormonal factors.

20. The heart, like all muscles, gets stronger with exercise. Long- term training results in a stronger heart and a higher cardiac output.