EXAM 3.pdf - NORMAL CARDIOVASCULAR SYSTEM ANATOMY AND...

Unformatted text preview: NORMAL CARDIOVASCULAR SYSTEM ANATOMY AND PHYSIOLOGY The cardiovascular system consists of the heart, blood, and vessels (including arteries, capillaries, and veins). Its function is to perfuse the organs and tissues with blood. Heart Cardiac Structure and Function LOCATION OF THE HEART. The heart is located in the mediastinum within the thoracic cavity. It is enclosed by three membranes. The outermost is the fibrous pericardium, which forms a loose-fitting pericardial sac around the heart. The second, or middle, layer is the parietal pericardium, a serous membrane that lines the fibrous layer. The third and innermost layer, the visceral pericardium or epicardium, is a serous membrane on the surface of the heart muscle. Between the parietal and visceral layers is serous fluid, which prevents friction as the heart beats. STRUCTURE OF THE HEART AND CORONARY BLOOD VESSELS. The walls of the four chambers of the heart are made of cardiac muscle (myocardium) and are lined with endocardium. Endocardium is smooth epithelial tissue that prevents abnormal clotting. The epithelium also covers the valves of the heart and continues into blood vessels, at which point it is called the ​endothelium. Coronary circulation provides oxygenated blood throughout the myocardium and returns deoxygenated blood to the right atrium via the coronary sinus. The two main coronary arteries are the first branches of the ascending aorta, just outside the left ventricle (​Fig. 21.1​). The superior chambers of the heart are the thin-walled right and left atria, which are separated by the interatrial septum. The lower chambers are the thicker walled right and left ventricles, which are separated by the interventricular septum. Each septum is made of myocardium that forms a common wall between the two chambers. CORONARY BLOOD FLOW. The right atrium receives deoxygenated blood from the coronary sinus, from the upper body by way of the superior vena cava, and from the lower body by way of the inferior vena cava (see ​Fig. 21.1​). This blood flows from the right atrium through the tricuspid valve into the right ventricle. Backflow during ventricular systole (contraction and emptying) is prevented by the tricuspid, or right atrioventricular (AV) valve (​Fig. 21.2​). The right ventricle pumps blood through the pulmonary semilunar valve to the lungs by way of the pulmonary trunk and arteries. The pulmonary semilunar valve prevents backflow of blood into the right ventricle during ventricular diastole (relaxation and filling). The left atrium receives oxygenated blood from the lungs by way of the four pulmonary veins. This blood flows through the mitral, or left AV valve (also called the bicuspid valve) into the left ventricle. The mitral valve prevents backflow of blood into the left atrium during ventricular systole. The left ventricle pumps blood through the aortic semilunar valve to the body by way of the aorta. The aortic valve prevents backflow of blood into the left ventricle during ventricular diastole. The tricuspid and mitral valves consist of three and two cusps, respectively. These cusps, or flaps, are connective tissue covered by endocardium. They are anchored to the floor of the ventricle by the chordae tendineae and papillary muscles. The papillary muscles are columns of myocardium that contract along with the rest of the ventricular myocardium. This contraction pulls on the chordae tendineae and prevents hyperextension of the AV valves during ventricular systole (see ​Fig. 21.2​). Although each ventricle pumps the same amount of blood, the much thicker walls of the left ventricle pump with approximately five times the force of the right ventricle to distribute the blood throughout the body. This difference in force is reflected in the large difference between systemic and pulmonary blood pressure. Cardiac Conduction Pathway and Cardiac Cycle The cardiac conduction pathway is the pathway of electrical impulses that generates a heartbeat. The sinoatrial (SA) node in the wall of the right atrium is autorhythmic and depolarizes about 100 times per minute, initiating each heartbeat. (While at rest, parasympathetic fibers dominate and slow the SA node to about 75 beats per minute.) For this reason, the SA node is called the pacemaker, and a normal heartbeat is called a normal sinus rhythm. From the SA node, impulses travel on a specific path (​Fig. 21.3​). If the SA node becomes nonfunctional, the AV node can initiate each heartbeat, but at a slower rate of 40 to 60 beats per minute. The ​bundle of His is capable of generating the beat of the ventricles, but at the much slower rate of about 20 to 35 beats per minute. FIGURE 21.1​ Anterior view of the heart and major blood vessels. FIGURE 21.2​ Frontal section of the heart showing internal structures and cardiac blood flow. A cardiac cycle is the sequence of mechanical events that occurs during each heartbeat. Simply stated, the two atria contract simultaneously, followed by the simultaneous contraction of the two ventricles (a fraction of a second later). The contraction (emptying), or systole, of each set of chambers is followed by relaxation (filling), or diastole, of the same set of chambers. The events of the cardiac cycle create the normal heart sounds. The first of the two major sounds (the “lub” of “lub-dub”) is caused by the closure of the AV valves during ventricular systole. The second sound is created by the closure of the aortic and pulmonary semilunar valves. Cardiac Output Cardiac output is the amount of blood ejected from the left ventricle in 1 minute (the right ventricle pumps a similar amount). It is determined by multiplying stroke volume by heart rate. Stroke volume is the amount of blood ejected by a ventricle in one contraction. It averages 60 to 80 mL/beat. With an average resting heart rate of 75 beats per minute, average resting cardiac output is 5 to 6 L (approximately the total blood volume of an individual that is pumped within 1 minute). Ejection fraction is a measure of ventricular efficiency. It is normally 55% to 70% of the total amount of blood within the left ventricle that is ejected with every heartbeat. During exercise, venous return increases and stretches the ventricular myocardium, which in response contracts more forcefully. This is known as ​Starling’s law of the heart, and the result is an increase in stroke volume. More blood is pumped with each beat. At the same time, the heart rate increases, causing cardiac output to increase by as much as four times the resting level (or more for fit athletes).heart contracts forefully= Regulation of Heart Rate high stroke volume The heart generates its own electrical impulse, which begins at the SA node. The nervous system, however, can change the heart rate in response to environmental circumstances. In the brain, the medulla oblongata receives sensory input and alters heart function (​Fig. 21.4​). Hormones and the Heart The hormone epinephrine is secreted by the adrenal medulla in stressful situations. It is sympathomimetic in that it increases the heart rate and force of contraction and dilates the coronary vessels. This in turn increases cardiac output and systolic blood pressure. Aldosterone is a hormone produced by the adrenal cortex. It is important for cardiac function because it helps regulate blood levels of sodium and potassium, both of which are needed for normal electrical activity of the myocardium. FIGURE 21.3​ Conduction pathway. The atria of the heart secrete a hormone of their own, called atrial natriuretic peptide or atrial natriuretic hormone. As its name suggests, atrial natriuretic peptide increases the excretion of sodium by the kidneys by inhibiting secretion of aldosterone by the adrenal cortex. Atrial natriuretic peptide is secreted when a higher blood pressure or greater blood volume stretches the walls of the atria. The loss of sodium is accompanied by the increased loss of water in urine. This decreases blood volume and, therefore, blood pressure as well. Blood Vessels Arteries and Veins Arteries and arterioles carry blood from the heart to capillaries. Their walls are relatively thick and consist of three layers. Arteries carry blood under high pressure. The outer layer of fibrous connective tissue prevents rupture of the artery. The middle layer of smooth muscle and elastic connective tissue contributes to the maintenance of normal blood pressure, especially diastolic blood pressure, by changing the diameter of the artery. The diameter of arteries is regulated primarily by the sympathetic division of the autonomic nervous system. By use of the smooth muscle, the arteries can also alter where the greatest volume of blood is directed. The inner layer, or lining, of the artery is simple squamous epithelium, called endothelium, which is very smooth to prevent abnormal clotting. Veins and venules carry blood from capillaries to the heart. Their walls are relatively thin because they have less smooth muscle than arteries. However, sympathetic impulses can bring about extensive constriction of veins. This becomes important in situations such as severe hemorrhage. The lining of veins is, like arteries, endothelium that prevents abnormal clotting; at intervals, it is folded into valves to prevent backflow of blood. Valves are most numerous in the veins of the extremities, especially the legs, where blood must return to the heart against the force of gravity. Capillaries Capillaries carry blood from arterioles to venules and form extensive networks in most tissues. The exceptions are cartilage, covering/lining epithelia, and the lens and cornea of the eye. Capillary walls are a continuation of the lining of arteries and veins. They are one-cell thick to permit the exchange of gases, nutrients, and waste products between the blood and tissues (​Fig. 21.5​). Blood flow through a capillary network is regulated by a precapillary sphincter, a smooth muscle fiber ring that contracts or relaxes in response to tissue needs. In an active tissue such as exercising skeletal muscle, for example, the rapid oxygen uptake and carbon dioxide production cause dilation of the precapillary sphincters to increase blood flow. At the same time, precapillary sphincters in less active tissues constrict to reduce blood flow. This is important because the body does not have enough blood to fill all of the capillaries at once; the fixed volume must constantly be shunted or redirected to where it is needed most. FIGURE 21.4​ Factors affecting heart rate. Exchange between blood and tissue fluids occurs primarily due to diffusion and/or filtration at the capillaries. Diffusion is important to gas exchange. Filtration is a vital mechanism for homeostasis of extracellular fluids. Some of this tissue fluid returns to the capillaries, and some is collected in lymph capillaries. Lymph is returned to the blood by lymph vessels. Should blood pressure within the capillaries increase, more tissue fluid than usual is formed, too much for the lymph vessels to collect. This may result in tissue swelling, called ​edema. Blood Pressure Blood pressure is the force of the blood against the walls of the blood vessels. It is measured in millimeters of mercury (mm Hg), systolic over diastolic. The normal average of systemic arterial pressure is 120/80 mm Hg. Blood pressure decreases in the arterioles and capillaries, and the systolic and diastolic pressures merge into one pressure. As blood enters the veins, blood pressure decreases further and approaches zero as it flows into the right ventricle. As mentioned previously, the blood pressure in the capillaries is of great importance. Normal blood pressure is high enough to permit filtration for nourishment of tissues but low enough to prevent rupture. FIGURE 21.5​ Structure of an artery, arteriole, capillary network, venule, and vein. The arterioles (and veins during increased sympathetic stimulation) are usually in a state of slight constriction that helps to maintain normal blood pressure, especially diastolic pressure. This contributes to peripheral resistance; it is regulated by the vasomotor center in the medulla, which receives input via the glossopharyngeal and vagus nerves. Blood pressure is also affected by many other factors. If heart rate and force increase, blood pressure increases within limits. If the heart is beating very fast, the ventricles are not filled before they contract, cardiac output decreases, and blood pressure drops. The strength of the heart’s contractions depends on adequate venous return, which is the amount of blood that flows into the atria. Decreased venous return results in weaker contractions. Venous return depends on several factors: constriction of the veins to reduce pooling, the skeletal muscle pumping to squeeze the deep veins of the legs, and the diaphragm’s downward pressure during inhalation to compress the abdominal veins as the thoracic veins are decompressed. The valves in the veins prevent backflow of blood and thus contribute to the return of blood to the heart. The elasticity of the large arteries also contributes to normal blood pressure. When the left ventricle contracts, the blood stretches the elastic walls of the large arteries, which absorb some of the force. When the left ventricle relaxes, the arterial walls recoil, exerting pressure on the blood. Normal elasticity, therefore, lowers systolic pressure, raises diastolic pressure, and maintains normal pulse pressure. Pulse pressure is the difference between the systolic and diastolic pressures. The usual ratio of systolic to diastolic to pulse pressure is 3:2:1. Renin-Angiotensin-Aldosterone Mechanism renin horm ones The ​kidneys are of great importance in the regulation of blood pressure​. If blood flow through the kidneys decreases, renal filtration decreases and urinary output decreases to preserve blood volume. Decreased blood pressure stimulates the kidneys to secrete renin, which initiates the renin-angiotensin-aldosterone mechanism, raising blood pressure (​Fig. 21.6​). Other hormones that affect blood pressure include those of the adrenal medulla, norepinephrine and epinephrine, which increase cardiac output and cause vasoconstriction in skin and viscera. Antidiuretic hormone is released from the posterior pituitary. It directly increases water reabsorption by the kidneys, thus increasing blood volume and blood pressure. Atrial natriuretic peptide is secreted by the atria of the heart. It inhibits aldosterone secretion and thereby increases renal excretion of sodium ions and water, which decreases blood volume and subsequently blood pressure. Circuits of Circulation The two circuits of circulation are pulmonary and systemic (see ​Fig. 21.2​). Pulmonary circulation begins at the right ventricle, which pumps deoxygenated blood toward the lungs for gas exchange at the alveoli. Oxygenated blood returns to the left atrium by way of the pulmonary veins. Low pressure in the pulmonary capillaries prevents filtration in pulmonary capillaries. This keeps tissue fluid from accumulating in the alveoli of the lungs, which can otherwise result in pulmonary edema. Systemic circulation begins in the left ventricle, pumping oxygenated blood into the aorta, the many branches of which eventually give rise to capillaries within the tissues. Deoxygenated blood returns to the right atrium by way of the superior and inferior vena cava and the coronary sinus. The hepatic portal circulation is a special part of the systemic circulation in which blood from the capillaries of the digestive organs and pulmonary ciculation @right ventricle =pulmonary edema spleen flows through the portal vein and into the sinusoids in the liver before returning to the heart. This pathway permits the liver to regulate the blood levels of nutrients such as glucose, amino acids, and iron and to remove potential toxins such as alcohol or medications from circulation. Aging and the Cardiovascular System clots The aging of blood vessels, especially arteries, is believed to begin in childhood, although the effects are not apparent until later in life (​Fig. 21.7​). ​Atherosclerosis is the deposition of lipids in the walls of arteries over a period of years. The deposited lipids can narrow the arteries’ lumens and form rough surfaces that may stimulate intravascular clot formation. Atherosclerosis decreases blood flow to the affected organ. With age, the heart muscle becomes less efficient, and maximum cardiac output and heart rate both decrease, although resting levels may be more than sufficient (“Gerontological Issues: Orthostatic Hypotension”). Valves may become thickened by fibrosis, leading to heart murmur. Gerontological Issues reasons asses Orthostatic Hypotension.​ The older adult is at increased risk for developing orthostatic hypotension, which could precipitate a fall. This is often due to a combination of age-related changes, immobility, chronic illnesses, and medications. It is important to assess blood pressure and pulse while the patient is lying, sitting, and standing and to teach the patient to sit up and stand slowly before walking. teach CARDIOVASCULAR DISEASE risk An estimated 92.1 million American adults have one or more types of cardiovascular disease (Benjamin et al., 2017) (“Cultural Considerations”). Lifestyle plays a leading role in risk factors for cardiovascular disease. Americans continue to be sedentary and eat excess calories. Ways to improve cardiovascular health include not smoking, exercising, eating healthy, and maintaining normal blood pressure, blood glucose, total cholesterol levels, and weight. In women, the greatest cause of death is cardiovascular disease. The movement Go Red for Women (​ ​) gives women encouragement and tools to prevent cardiovascular disease and live healthy. • WORD • BUILDING • atherosclerosis:​ athere—porridge + sklerosis—hardness Cultural Considerations how to improve Adults in the United States are more likely to die of heart disease than any other cause, reasons regardless of their racial or ethnic heritage. However, certain minority groups face a for high greater risk than others. Many intertwined factors likely contribute to these high heart disease rates, including lower average income, which affects where people live, and heart disease access to healthy food, safe places to exercise, and quality health care. Certain racial and ethnic groups have higher rates of hypertension, tend to develop hypertension at an earlier age, and are less likely to undergo treatment to control their high blood pressure. Following are some examples: rate racial & ethnic ● Mexican American men and women tend to have higher rates of elevated blood pressure. ● Obesity continues to be higher for African American and Mexican American women. ● Only 50% of Native Americans, 44% of Asian Americans, and 38% of Mexican Americans have had their cholesterol checked within the past 2 years. ● Coronary heart disease mortality is higher for African Americans. ● Stroke is the only leading cause of death for which mortality is higher for Asian American males. Strategies to improve disparities should focus on culturally competent engagement in health promotion and disease prevention education. NURSING ASSESSMENT OF THE CARDIOVASCULAR SYSTEM Nursing assessment of the cardiovascular system includes a patient health history and physical examination (“Gerontological Issues: Atypical Symptoms”). If the patient is experiencing an acute problem, focus on the most serious signs and symptoms and physical data until the patient is stabilized (​Table 21.1​). asses Gerontological Issues Atypical Symptoms.​ Older adults commonly have signs and symptoms that are not typical of a myocardial infarction. For example, the only symptom of myocardial infarction in an older patient may be dyspnea. Chest pain, a typical symptom, may not be present. It is important for the older adult to have a complete assessment for this reas...
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