Chapter 11 The Cardiovascular System When most people hear the term cardiovascular system

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Chapter 11 The Cardiovascular System

When most people hear the term cardiovascular system, they immediately think of the heart. We have all felt our own heart "pound" from time to time, and we tend to get a bit nervous when this happens. The crucial importance of the heart has been recognized for a long time. However, the cardiovascular system is much more than just the heart, and from a scientific and medical standpoint, it is important to understand why this system is so vital to life.

Most simply stated, the major function of the cardiovascular system is transportation. Using blood as the transport vehicle, the system carries oxygen, nutrients, cell wastes, hormones, and many other substances vital for body homeostasis to and from the cells. The force to move the blood around the body is provided by the beating heart. The cardiovascular system can be compared to a muscular pump equipped with one-way valves and a system of large and small plumbing tubes within which the blood travels. Blood (the substance transported) is discussed in Chapter 10. Here we will consider the heart (the pump) and the blood vessels (the network of tubes).

Cardiovascular System: The Heart

  • Describe the location of the heart in the body and identify its major anatomical areas on an appropriate model or diagram.

    • See figure 11.1 on page 329

      • Located within the bony thorax and is flanked on each side by the lungs with the apex (more pointed region) directed toward the left hip and rests on the diaphragm, ~at the level of the fifth intercostal space while the broader posterosuperior aspect, the base, where the great vessels emerge points toward the right shoulder and lies beneath the second rib

    • See figures 11.2 a and b on pages 330 and 331

      • The superior atria are receiving chambers and are not important in the pumping activity of the heart while the inferior ventricles with thicker walls are the discharging chambers, or pumps, of the heart

      • The septum that divides the heart longitudinally is referred to as the interventricular or interatrial septum depending on which chamber it divides

      • The superior and inferior venae cavae bring deoxygenated blood to the right side of the heart where it is pumped out through the pulmonary trunk that splits into the right and left pulmonary arteries headed for the lungs to pick up oxygen while the pulmonary veins bring the oxygenated blood back to the left side of the heart to be pumped out the aorta which divides into the systemic arteries that feed all body tissues

  • Trace the pathway of blood through the heart.

    • Body  superior and inferior venae cavae  right atrium  tricuspid valve  right ventricle  pulmonary semilunar valve  left and right pulmonary arteries  lungs  left and right pulmonary veins  left atrium  bicuspid valve  left ventricle aortic semilunar valve  aorta  body

  • Compare the pulmonary and systemic circuits.

    • See figure 11.3 on page 332

    • The left side of the heart is the systemic circuit pump that supplies oxygenated blood to the body and return to the heart via the venae cavae while the right side is the pulmonary circuit pump that sends deoxygenated blood to the lungs to pick up oxygen and return to the heart via the pulmonary arteries and veins

  • Explain the operation of the heart valves.

    • See figure 11.4 on page 333

      • The atrioventricular (AV) valves located between the atrial and ventricular chambers on each side of the heart prevent backflow into the atria when the ventricles contract

        • The left AV valve, the bicuspid/mitral valve has two cusps, or flaps, of endocardium

        • The right AV valve, the tricuspid valve, has three cusps

        • Tiny white cords called the chordate tendineae (heart strings) anchor the cusps to the walls of the ventricles so that when the heart is relaxed and blood is passively filling its chambers, the AV valve flaps hang limply into the ventricles but as the ventricles contract they press on the blood in their chambers and in intraventricular pressure begins to rise causing the AV valve flaps to be forced upward and closed so that the flaps prevent backflow when the ventricles are contracting

      • The semilunar valves guard the bases of the two large arteries leaving the ventricular chambers

        • Pulmonary semilunar valve and aortic semilunar valves both have three cusps that tightly fit together when the valves are closed – when the ventricles contract and force blood out, the cusps are forces open and flatten against the walls of the arteries and when the ventricles relax letting blood flow backwards, the cusps fill with blood and close preventing further backflow

    • Each set of valves operates at a different time – the AV valves are open during heart relaxation and closed when the ventricles are contracting while the semilunar valves are closed during heart relaxation and are forced open when the ventricles contract

  • Name the functional blood supply of the heart.

    • The blood supply that oxygenates and nourishes the heart is provided by the right and left coronary arteries that branch from the base of the aorta and encircle the heart in the atrioventricular groove at the junction of the atria and ventricles

      • The coronary arteries and their major branches are compressed when the ventricles are contracting and fill when the heart is relaxed

    • The myocardium is drained by several cardiac veins which empty into a large vessel on the backside of the heart called the coronary sinus, which empties into the right atrium

  • Name the elements of the intrinsic conduction system of the heart and describe the pathway of impulses through this system.

    • The intrinsic conduction system, nodal system, contains special tissue found nowhere else in the body that causes heart muscle depolarization in only one direction – from the atria to the ventricles and it enforces a contraction rate of ~75 beats/minute on the heart so the heart beats as a coordinated unit

    • The sinoatrial (SA) node is a crescent-shaped node of tissue located in the right atrium along with the atrioventricular (AV) node at the junction of the atria and ventricles, the atrioventricular (AV) bundle (bundle of His) and the right and left bundle branches located in the interventricular septum and the Purkinje fibers, which spread within the muscle of the ventricle walls comprise the intrinsic conduction system

      • The SA node has the highest rate of depolarization in the whole system and starts each heartbeat and sets the pace for the whole heart giving it the name pacemaker

      • From the SA node, the impulse spreads through the atria to the AV node, and then the atria contract – at the AV node, the impulse is delayed briefly to give the atria time to finish contracting then is passes rapidly through the AV bundle, the bundle branches, and the Purkinje fibers, resulting in a wringing contraction of the ventricles that begins at the heart apex and moves toward the atria ejecting blood superiorly into the large arteries leaving the heart

  • Define systole, diastole, stroke volume, and cardiac cycle.

    • Systole and diastole refer to contraction and relaxation of the ventricles while the cardiac cycle refers to the events of one complete heartbeat, during which both atria and ventricles contract and then relax

      • Mid-to-late diastole – heart completely relaxed, pressure in the heart is low, and blood is flowing passively into and through the atria into the ventricles, semilunar valves are closed and AV valves are open, then the atria contract forcing blood into the ventricles

      • Ventricular systole – ventricular contraction (systole) begins and pressure within the ventricles increases closing the AV valves and when the intraventricular pressure is higher than the pressure in the large arteries leaving the heart, the semilunar valves are forced open, and blood rushes through them out of the ventricles – during this time, the atria are relaxed so their chambers are again filling with blood

      • Early diastole – ventricles relax, the semilunar valves snap shut and for a short time, the ventricles are completely closed chambers – the intraventricular pressure drops and when it drops below the pressure in the atria, the AV valves are forced open, and the ventricles again begin to refill rapidly with blood

    • Cardiac output (CO) is the amount of blood pumped out by each side of the heart in one minute – it is the product of the heart rate (HR) and the stroke volume (SV), which is the volume of blood pumped out by a ventricle with each heartbeat – increases as the force of ventricular contraction increases

      • CO = HR (75 beats/min) X SV (70 ml/beat)

      • CO = 5250 ml/min

  • Define heart sounds and murmur.

    • Using a stethoscope two distinct sounds during each cardiac cycle can be heard – lub and dup in a sequence – lub-dup, pause, lub-dup, pause…

      • Lub is caused by the closing of the AV valves and is longer and louder while dup occurs when the semilunar valves close at the end of systole and is short and sharp

    • Murmurs are abnormal or unusual heart sounds and most often indicate valve problems

      • Blood flows silently as long as the flow is smooth and uninterrupted but when it strikes obstructions, its flow becomes turbulent and generates sounds that can be heard with a stethoscope

  • Explain what information can be gained from an electrocardiogram.

    • An ECG traces the flow of current through the heart and has three recognizable waves

      • P wave – small and signals the depolarization of the atria immediately before they contract

      • QRS complex – results from depolarization of the ventricles and proceeds the contraction of the ventricles

      • T wave – results from currents flowing during the repolarization of the ventricles

    • Abnormalities in the shape of the waves and changes in their timing send signals that something may be wrong with the intrinsic conduction system or may indicate a myocardial infarct (an area of heart tissue in which the cardiac cells have died

  • Describe the effect of the following on heart rate: stimulation by the vagus nerve, exercise, epinephrine, and various ions.

    • During times of physical or emotional stress, the nerves of the sympathetic division of the autonomic nervous system stimulate the SA and AV nodes and the cardiac muscle itself

      • When demand declines, the parasympathetic nerves, primarily the vagus nerves, slow and steady the heart

    • Epinephrine mimics the effect of the sympathetic nerves and increases the heart rate - and various ions can create an electrolyte imbalance that can slow or speed up the heart rate

      • Reduced levels of ionic calcium in the blood slows the heart while increased levels cause prolonged contractions and may cause the heart to stop

      • Reduced levels of potassium ions in the blood causes the heart to beat feebly with abnormal heart rhythms

    • Physical factors such as age, gender, exercise, body temperature can also alter the heart rate

      • Heart rate is faster in the fetus and gradually decreases throughout life

      • Average heart rate is faster in females than men

      • Heat increases heart rate by boosting the metabolic rate while cold has the opposite reaction

      • Exercise acts through the nervous system controls to increase heart rate – exercise also adds heat with the activity of the muscles

Cardiovascular System: Blood Vessels

  • Compare and contrast the structure and function of arteries, veins, and capillaries.

    • Arteries carry blood away from the heart while veins carry blood toward the heart – arteries change into successively smaller arteries and then into arterioles, which drain into capillaries, which in turn empty into venules which successively enlarge into the great veins that enter the heart

    • Except the capillaries, the walls of blood vessels have three coats, or tunics

      • Tunica interna – lines the lumen or interior of the vessels, is a thin layer of endothelium resting on a scanty layer of loose connective tissue – the cells fit closely together and form a slick surface that decreases friction as blood flows through

      • Tunica media – bulky middle coat made mostly of smooth muscle and elastic tissue controlled by the sympathetic nervous system and is active in changing the diameter of the vessels

        • Some of the larger arteries have elastic laminae, complete sheets of elastic tissue in addition to the scattered elastic fibers

      • Tunica externa – is the outer-most tunic composed mostly of fibrous connective tissue and functions basically to support and protect the vessels

    • The walls of arteries are usually much thicker than veins with the tunica media being much heavier because they are closer to the heart and must be able to expand as blood is forces into them and then recoil passively as blood flows away and their walls must be strong and stretchy enough to take these continuous changes in pressure

    • Veins are far from the heart and the pressure in them tends to be low all the time so they have thinner walls with larger lumens than corresponding arteries and valves to prevent backflow

      • Skeletal muscle activity also enhances venous return – as the muscles surrounding the veins contract and relax, the blood is pushed through the veins towards the heart

      • The respiratory pump also helps return blood to the heart – during inhalation, the drop of pressure that occurs in the thorax causes the large veins near the heart to expand and fill

    • The transparent walls of the capillaries are only one cell layer thick with only the tunica interna allowing exchanges to be easily made between the blood and the tissue cells

      • The tiny capillaries tend to form interweaving networks called capillary beds and the flow from arterioles to venules through the capillary bed is called microcirculation

        • There are two types of vessels (1) vascular shunt – vessels that directly connects the arteriole and venule at opposite ends of the bed and (2) true capillaries – the actual exchange vessels

          • A cuff of smooth muscle fibers called a precapillary sphincter surrounds the root of each true capillary and acts as a valve to regulate the flow of blood into the capillary

  • Identify the body's major arteries and veins and name the body region supplied by each.

    • See figure 11.11 and 11.12 on pages 345 and 347

    • Read pages 344 – 348

  • Discuss the unique features of special circulations of the body: arterial circulation of the brain, hepatic portal circulation, and fetal circulation.

    • Arterial supply of the brain and Circle of Willis – see page 348 and figure 11.13

      • Internal carotid arteries are branches of the common carotid arteries and run through the neck and enter the skull through the temporal bone and once inside the cranium, each divides into the anterior and middle cerebral arteries, which supply most of the cerebrum

      • Vertebral arteries pass upward from the subclavian arteries at the base of the neck and within the skull, the vertebral arteries join to form single basilar artery, which serves the brain stem and cerebellum as it travels upward – at the base of the cerebrum, the basilar artery divides to form the posterior cerebral arteries, which supply the posterior part of the cerebrum

      • The anterior and posterior blood supplies of the brain are united by small communicating arterial branches and the result is a complete circle of connecting blood vessels called the circle of Willis, which surrounds the base of the brain and protects the brain because it provides more then one route for blood to reach brain tissue in case of a clot or impaired blood flow

    • Hepatic portal circulation – see page 349 and figure 11.14

      • The veins of the hepatic portal circulation drain the digestive organs, spleen, and pancreas and deliver this blood to the liver through the hepatic portal vein where the liver can process the nutrients just delivered to the blood flow from the digestive system

        • The liver is drained by the hepatic veins that enter the inferior vena cava

      • The inferior mesenteric vein drains the terminal part of the large intestine drains into the splenic vein, which itself drains the spleen, pancreas, and the left side of the stomach

      • The superior mesenteric vein drains the small intestine and the first part of the colon to join to form the hepatic portal vein – the left gastric vein, which drains the right side of the stomach, drains directly into the hepatic portal vein

    • Fetal circulation – see page 350 and figure 11.15 – all nutrient, excretory, and gas exchanges occur through the placenta with nutrients and oxygen move from the mother’s blood into the fetal blood and fetal wastes move in the opposite direction

      • The umbilical cord contains three blood vessels – one large umbilical vein and two smaller umbilical arteries – the umbilical vein carries blood rich in nutrients and oxygen to the fetus while the umbilical arteries carry carbon dioxide and debris-laden blood from the fetus to the placenta

      • As blood flows superiorly toward the heart of the fetus, most of it bypasses the immature liver through the ductus venosus and enters the inferior vena cava, which carries the blood to the right atrium of the heart

      • Since fetal lungs are nonfunctional, two shunts bypass the blood so that some of the blood entering the right atrium is shunted directly into the left atrium through the foramen ovale, a flap-like opening in the interatrial septum and the blood that does manage to enter the right ventricle is pumped out the pulmonary trunk where it meets a second shunt, the ductus arteriosus, which is a short vessel that connects the aorta and the pulmonary trunk

      • At birth, the foramen ovale closes and the ductus arteriosus collapses and is converted to fibrous ligamentum arteriosum and as blood stops flowing through the umbilical vessels, they become obliterated, and the circulatory patter becomes that of an adult

  • Define blood pressure and pulse and name several pulse points.

    • Blood pressure – is the pressure the blood exerts against the inner walls of the blood vessels, and it is the force that keeps blood circulating continuously even between heartbeats

    • Pulse – the alternating expansion and recoil of an artery that occurs with each beat of the left ventricle creating a pressure wave that travels through the entire arterial system

      • Pulse points – you can feel a pulse in any artery lying close to the body surface by compressing the artery against firm tissue – also called pressure points because these same points are compressed to stop blood flow into distal tissues during hemorrhage

        • Temporal, facial, carotid, brachial, radial, femoral, Popliteal, posterior tibial, dorsalis arteries are all pulse/pressure points

  • List factors affecting and/or determining blood pressure.

    • Measuring blood pressure – systolic pressure is the pressure in the arteries at the peak of ventricular contraction and diastolic pressure is the pressure when the ventricles are relaxing

      • Blood pressures are reported in millimeters of mercury with the systolic pressure written first = 120/80

    • Effect of various factors on blood pressure – arterial blood pressure is directly to cardiac output and peripheral resistance where peripheral resistance is the amount of friction encountered by the blood as it flows through the blood vessels

      • Neural factors – the autonomic nervous system – sympathetic division is responsible for causing vasoconstriction, or narrowing of the blood vessels, which increases the blood pressure

      • Renal factors – the kidneys – alter blood volume by adding or retaining water to/from the blood stream to increase or decrease blood volume and thus blood pressure – certain kidney cells release the enzyme renin into the blood, which triggers a series of chemical reactions that results in the formation of angiotensin II, a potent vasoconstrictor chemical that stimulates the adrenal cortex to release aldosterone, a hormone that enhances sodium ion reabsorption by the kidneys, which causes water to follow increasing blood volume and blood pressure

      • Temperature – cold temperatures has a vasoconstricting effect while heat has a vasodilating effect

      • Chemicals – aka drugs – epinephrine and nicotine increase blood pressure while alcohol and histamine decrease blood pressure

      • Diet – diets low in salt, saturated fats, and cholesterol help reduce blood pressure

  • Define hypertension and atherosclerosis and describe possible health consequences of these conditions.

    • Hypertension – aka high blood pressure – a condition of sustained elevated arterial pressure of 140/90 or higher – the silent killer as the heart is forced to pump against increased resistance (work harder) causing the myocardium to enlarge and when finally strained beyond its capacity to respond, the heart weakens and its wall become flabby – also ravages blood vessels causing small tears in the endothelium that accelerate the progress of atherosclerosis

    • Atherosclerosis – when arteries are narrowed by the damming-up process that occurs from the inside out – the walls of the vessels thicken and then protrude into the vessel lumen until the vessel closes completely possibly by a roaming blood clot or arterial spasm – eventually results in arteriosclerosis as enlarging plaques hinder diffusion of nutrients from the blood to deeper tissues of the artery wall, smooth muscle cells in the tunica media die and the elastic fibers deteriorate and are gradually replaced by nonelastic scar tissue leading eventually to myocardial infarcts, strokes, and kidney failure

  • Describe the exchanges that occur across capillary walls.

    • Substances exchanged first diffuse through an intervening space filled with interstitial fluid – substances also tend to move to and from body cells according to their concentration gradients taking one of four routes across the plasma membranes of the single layer of endothelial cells forming the capillary wall

      • Diffuse directly through (cross) if they are lipid-soluble such as respiratory gases

      • Lipid-insoluble substances may cross within vesicles – endo- or exocytosis

      • Limited passage of fluid and small solutes is allowed by intercellular clefts (gaps or area of plasma membrane not joined by tight junctions)

      • Very free passage of small solutes and fluids is allowed by fenestrated capillaries – found where absorption is a priority (intestinal capillaries or those serving endocrine glands) or where filtration occurs (kidney)

        • Fenestra – an oval pore or opening that is usually covered by a delicate membrane that is much more permeable than other regions of the plasma membrane

Developmental Aspects of the Cardiovascular System

  • Describe briefly the development of the cardiovascular system.

    • The heart begins as a simple tube in the embryo and begins beating and pumping blood by the fourth week – becomes a four-chambered structure acting as a double pump by week seven with special vascular shunts to bypass the nonfunctioning, collapsed lungs and liver until birth

    • The heart will hypertrophy and its cardiac output will increase substantially with exercise regularly and aerobically – becomes a more powerful and efficient pump

  • Name the fetal vascular modifications, or "fetal shunts," and describe their function before birth.

    • See fetal circulation above

  • Explain how regular exercise and a diet low in fats and cholesterol may help maintain cardiovascular health.

    • The benefit of aerobic exercise is that it clears fatty deposits from the blood vessel walls, helping to slow the progress of atherosclerosis

    • A diet low in animal fat, cholesterol, and salt lowers the risk of cardiovascular diseases as well as avoiding stress and eliminating cigarette smoking

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