McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

volume in the atria is greater than the pressure in the ventricles, blood flows through the atrioventricular (AV) valves into the ventricles. The valve on the right side of the heart is called the tricuspid valve because it is composed of three leaflets or cusps. The valve on the left side of the heart, called the mitral or bicuspid valve, is composed of two leaflets or cusps (see Figure 42.1). Just before the ventricles are stimulated to contract, the atria contract, pushing about one more tablespoon of blood into each ventricle. The much more powerful ventricles then contract, pumping blood out to the lungs through the pulmonary valve or out to the aorta through the aortic valve and into the systemic circulation. The con­ traction of the ventricles is referred to as systole . Each period of systole followed by a period of diastole is called a cardiac cycle . The heart’s series of one-way valves keeps the blood flowing in the correct direction: • Deoxygenated blood enters the right atrium, flows through the tricuspid valve to the right ventricle, and flows through the pulmonary valve to pulmonary arteries and the lungs. • Oxygenated blood from the lungs returns through the pulmonary veins to the left atrium, flows through the mitral valve into the left ventricle, and then flows through the aortic valve to the aorta and the rest of the body. The AV valves close very tightly when the ventri­ cles contract, preventing blood from flowing backwards into the atria, thereby keeping blood moving forwards through the system. The pulmonary and aortic valves open with the pressure of ventricular contraction and close tightly during diastole, keeping blood from flowing backwards into the ventricles. These valves operate much like one-way automatic doors: you can go through in the intended direction, but if you try to go the wrong way, the doors close and stop your movement. The proper functioning of the cardiac valves is important in main­ taining the functioning of the cardiovascular system. Cardiac conduction Each cycle of cardiac contraction and relaxation is con­ trolled by impulses that arise spontaneously in certain pacemaker cells of the sinoatrial (SA) node of the heart. These impulses are conducted from the pacemaker cells by a specialised conducting system that activates all of the parts of the heart muscle almost simultaneously. These continuous, rhythmic contractions are controlled by the heart itself; the brain does not stimulate the heart to beat. This safety feature allows the heart to beat as long as it has enough nutrients and oxygen to survive, regardless of the status of the rest of the body. This property protects the vital cardiovascular function in many disease states; it is the same property that allows the heart to continue functioning in a person who is “brain dead”.

SA node

Left atrium

AV node

Bundle of His

Right atrium

Left ventricle

Right ventricle

Left bundle branch Purkinje fibres

Right bundle branch

The conduction system of the heart consists of the SA node, atrial bundles, AV node, bundle of His, bundle branches and Purkinje fibres (Figure 42.2). The SA node, which is located near the top of the right atrium, acts as the pacemaker of the heart. Atrial bundles conduct the impulse through the atrial muscle. The AV node, which is located near the bottom of the right atrium, slows the impulse and allows the delay needed for ven­ tricular filling. The AV node then sends the impulse from the atria into the ventricles by way of the bundle of His, which enters the septum and then divides into three bundle branches. These bundle branches, which conduct the impulses through the ventricles, break into a fine network of conducting fibres called the Purkinje fibres, which deliver the impulse to the ventricular cells. Automaticity The cells of the impulse-forming and conducting system are rather primitive, uncomplicated cells called pale or P cells. Because of their simple cell membrane, these cells possess a special property that differentiates them from other cells: they can generate action potentials or electri­ cal impulses without being excited to do so by external stimuli. This property is called automaticity . All cardiac cells possess some degree of automaticity. During diastole or rest, these cells undergo a spontane­ ous depolarisation because they decrease the flow of potassium ions out of the cell and probably leak sodium into the cell, causing an action potential. This action potential is basically the same as the action potential of the neuron (see Chapter 19). The action potential of the cardiac muscle cell consists of five phases: • Phase 0 occurs when the cell reaches a point of stimulation. The sodium gates open along the cell membrane and sodium rushes into the cell, resulting in a positive flow of electrons into the cell—an electrical potential. This is called depolarisation. FIGURE 42.2  The conducting system of the heart. Impulses originating in the sinoatrial (SA) node are transmitted through the atrial bundles to the atrioventricular (AV) node and down the bundle of His and the bundle branches by way of the Purkinje fibres through the ventricles.