McKenna's Pharmacology for Nursing, 2e

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P A R T 8  Drugs acting on the cardiovascular system

CHAPTER SUMMARY ■■ The heart is a hollow muscle that is divided into a right and a left side by a thick septum and into four chambers—the two upper atria and the two lower ventricles. The right side of the heart receives all of the deoxygenated blood from the body through the veins and directs it into the lungs. The left side of the heart receives oxygenated blood from the lungs and pumps it out to every cell in the body through the arteries. ■■ The heart is responsible for pumping oxygenated blood to every cell in the body and for picking up waste products from the tissues. ■■ The cardiac cycle consists of a period of rest, or diastole, when blood is returned to the heart by veins, and a period of contraction, or systole, when the blood is pumped out of the heart. ■■ The heart muscle possesses the properties of automaticity (the ability to generate an action potential in the absence of stimulation) and conductivity (the ability to rapidly transmit an action potential). ■■ The heart muscle is stimulated to contract by impulses generated in the heart, not by stimuli from the brain. The autonomic nervous system can affect the heart to increase (sympathetic) or decrease (parasympathetic) activity. ■■ In normal sinus rhythm, cells in the SA node generate an impulse that is transmitted through the atrial bundles and delayed slightly at the AV node before being sent down the bundle of His into the ventricles. When cardiac muscle cells are stimulated, they contract. (dysrhythmias), which can upset the normal balance in the cardiovascular system and lead to a decrease in cardiac output, affecting all of the cells of the body. ■■ Heart muscle contracts by the sliding of actin and myosin filaments in a functioning unit called a sarcomere. Contraction requires energy and calcium to allow the filaments to react with each other and slide together. ■■ The heart muscle needs a constant supply of blood, which is furnished by the coronary arteries. Increase in demand for oxygen can occur with changes in heart rate, preload, afterload or stretch on the muscle. ■■ The cardiovascular system is a closed pressure system that uses arteries (muscular, pressure or resistance vessels) to carry blood from the heart, veins (flexible, distensible capacitance vessels) to return blood to the heart and capillaries (which connect arteries to veins) to keep blood flowing from areas of high pressure to areas of low pressure. ■■ Alterations in the generation of conduction of impulses in the heart cause arrhythmias

■■ Blood pressure is maintained by stimulus from the sympathetic system and reflex control of blood volume and pressure by the renin–angiotensin system and the aldosterone–ADH system. Alterations in blood pressure (hypotension or hypertension) can upset the balance of the cardiovascular system and lead to problems in blood delivery. ■■ Fluid shifts out of the blood at the arterial ends of capillaries to deliver oxygen and nutrients to the tissues. It moves out due to the hydrostatic or fluid pressure of the arterial side of the system. Fluid returns to the system at the venous end of the capillaries because of the oncotic pull of proteins in the vessels. Disruptions in these pressures can lead to oedema or loss of fluid in the tissues.

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ONLINE RESOURCES

An extensive range of additional resources to enhance teaching and learning and to facilitate understanding of this chapter may be found online at the text’s accompanying website, located on thePoint at http://thepoint.lww.com. These include Watch and Learn videos, Concepts in Action animations, journal articles, review questions, case studies, discussion topics and quizzes.

WEB LINK

To explore the virtual cardiovascular system, consult the following Internet source: www.InnerBody.com

BIBLIOGRAPHY

Barrett, K. E. & Ganong, W. F. (2010). Ganong’s Review of Medical Physiology (23rd edn). New York: McGraw-Hill. Braunwald, E., Bonow, R. O., Mann, D. L., Zipes, D. P. & Libby, P. (2012). Braunwald’s Heart Disease: A Textbook of Cardiovascular Medicine (9th edn). Philadelphia: Elsevier Saunders. Goodman, L. S., Brunton, L. L., Chabner, B. & Knollmann, B. C. (2011). Goodman and Gilman’s Pharmacological Basis of Therapeutics (12th edn). New York: McGraw-Hill. Guyton, A. & Hall, J. (2011). Textbook of Medical Physiology (12th edn) . Philadelphia: Saunders Elsevier. Hurst, J. W., Fuster, V., Walsh, R. A. & Harrington, R. A. (Eds.). (2011). Hurst’s the Heart (13th edn). New York: McGraw-Hill. Porth, C. M. (2011). Essentials of Pathophysiology: Concepts of Altered Health States (3rd edn). Philadelphia: Lippincott Williams & Wilkins. Porth, C. M. (2009). Pathophysiology: Concepts of Altered Health States (8th edn). Philadelphia: Lippincott Williams & Wilkins.