Describe the circulation of blood through the heart and explain how the cardiac cycle is controlled.
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Access Biology Describe the circulation of blood through the heart and explain how the cardiac cycle is controlled. The blood circulatory system is responsible for providing a continuous flow of blood throughout the body to provide oxygen and nutrients to the tissues and remove the waste products and carbon dioxide. The circulatory system also acts as a means of transport for hormones and antibodies, and serves to distribute heat. The heart is the pump which tirelessly carries out the task of moving the blood around the body, beating at about 60 times a minute for an entire lifetime. It takes approximately one minute for blood to circulate from the heart, around the body and back again. The heart consists of four chambers, two atria (labelled auricles on diagram) and two ventricles, and is mainly composed of muscle. It is lined with a thin layer of cells called the Endocardium, and is enclosed within a double layered membrane sac called the Pericardium, which attaches it to the breastbone, diaphragm and thorax membranes. The deoxygenated blood is carried into the heart directly by the Vena Cava vein; the Superior Vena Cava drains blood from the head, neck and arms, and the Inferior Vena Cava drains the blood from the rest of the body, and firstly enters the Right Atrium.
The electrical stimulation that the muscle needs in order to contract is generated in the Sinus Node, or Pacemaker, which is an area of specialised cells located in the wall of the Right Atrium. This sets the heart beat rhythm, and is under the control of the autonomic nervous system which determines the heart rate, depending on the amount of carbon dioxide in the blood, which varies due to action, exercise or emotional state. The atria is caused to contract first, and the electrical impulse then reaches the ventricles shortly after through the Atrioventricular node, across a band of neuromuscular fibres called the Bundle of His, and spreads across the ventricles through the Purkinje fibres. These cause the ventricle to contract shortly after the atria, in a sequential rhythm of contraction and relaxation. The contraction stage is known as systole, and the relaxation stage is called diastole. The area between the atria and the ventricles is insulated by another fibrous band which prevents the signal from prematurely entering the ventricle, and throwing this rhythm into chaos. How does coronary heart disease effect the normal functions of the heart? Coronary heart disease is the name given to the disorder arising from the narrowing of the coronary arteries, which prevents the heart muscle from receiving a sufficient amount of blood; this is called Ischaemia.
This sudden decrease in oxygen supply to the heart is almost always fatal. Persons who survive a heart attack must under go careful rehabilitation, and are at risk from recurrence. The immediate cause of death in a heart attack is called ventricular fibrillation or cardiac arrest. This is where the heart loses its rhythm and beats weakly and rapidly. The normal heart beat can be re established by a massive electrical shock across the chest. People suffering from severe angina due to atherosclerotic disease may be given nitrate which is sprayed of dissolved under the tongue, and help the heart to work more efficiently. Some people take aspirin to thin the blood and reduce clotting. If these measures cause no improvement, the next step is an operation called a coronary bypass, where a section of vein from the leg is removed and grafted across the blocked coronary artery to form a bridge through which the blood can travel. Another treatment is called catheterisation, or percutaneous transluminal coronary angioplasty. This operation involves inserting a wire with a tiny balloon into an artery in the leg, and feeding it through the aorta into the blocked coronary artery. When the balloon reaches the area blocked by atherosclorosis, the balloon is inflated so as to compress the plaque, and re-establish normal blood flow.
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