The cardiovascular system • The cardiovascular system provides the transport system that keeps blood continuously circulating. -The heart is no more than the transport system pump. - Blood vessels are the delivery routes. • Using blood as the transport medium, the heart continually propels oxygen, nutrients, wastes, and many other substances into the blood vessels that service body cells.
Heart Anatomy -About the size of a fist. -Cone-shaped , has a mass of between 250 and 350 grams. -Enclosed within the mediastinum ,the medial cavity of the thorax. -Extends obliquely for 12 to 14 cm from the second rib to the fifth intercostal space . -Rests on the superior surface of the diaphragm. -Anterior to the vertebral column and posterior to the sternum.
-If you press your fingers between the fifth and sixth ribs just below the left nipple, you can easily feel your heart beating where the apex contacts the chest wall. Hence, this site is referred to as the point of maximal intensity(PMI) - The lungs flank the heart laterally and partially obscure it. - Approximately two-thirds of its mass lies to the left of the midsternal line.
Coverings of the Heart The heart is enclosed in a sac called the pericardium: 1-The loosely fitting superficial part of this sac is the fibrous pericardium. This tough, dense connective tissue layer: (1) protects the heart, (2) anchors it to surrounding structures, and (3) prevents overfilling of the heart with blood.2-Deep to the fibrous pericardium is the serous pericardium, a thin, slippery, two-layer serous membrane: -Its parietal layer lines the internal surface of the fibrous pericardium. -the visceral layer, also called the epicardium over the external heart surface and is an integral part of its wall. • Between the parietal and visceral layers is the slitlike pericardial cavity, which contains a film of serous fluid. • The serous membranes, lubricated by the fluid, glide smoothly past one another during heart activity, allowing the mobile heart to work in a relatively friction-free environment.
HOMEOSTATIC IMBALANCE Pericarditis, inflammation of the pericardium, is characterized by pain deep to the sternum. -Over time, it may lead to adhesions in which the visceral and parietal pericardia stick together and impede heart activity. -In severe cases, excess fluid compresses the heart, limiting its ability to pump blood. This condition in which the heart is compressed by fluid is called cardiac tamponade(tam″pŏ-nād′). -Physicians treat it by inserting a syringe into the pericardial cavity and draining off the excess fluid.
Layers of the Heart WallThe heart wall is composed of three layers : 1-The superficial epicardium is the visceral layer of the serous pericardium. It is often infiltrated with fat, especially in older people. 2-The middle layer, the myocardium (“muscle heart”), is composed mainly of cardiac muscle and forms the bulk of the heart. It is the layer that contracts.
3-The third layer, the endocardium is a glistening white sheet of endothelium.it lines the heart chambers. The endocardium is continuous with the endothelial linings of the blood vessels leaving and entering the heart.
Chambers and Associated Great Vessels -The heart has four chambers ,two superior atria and two inferior ventricles . -The internal partition that divides the heart longitudinally is called the interatrial septum where it separates the atria, and the interventricularseptum where it separates the ventricles. -The right ventricle forms most of the anterior surface of the heart. -The coronary sulcus or atrioventricular groove, encircles the junction of the atria and ventricles like a crown .
Atria: The Receiving Chambers -The right and left atria are remarkably free of distinguishing surface features. -The interatrial septum bears a shallow depression, the fossaovalis that marks the spot where an opening, the foramen ovale, existed in the fetal heart . -Because they need contract only minimally to push blood “downstairs” into the ventricles, the atria are relatively small, thin-walled chambers. -As a rule, they contribute little to the propulsive pumping activity of the heart.
- Blood enters the right atrium via three veins : • The superior vena cava returns blood from body regions superior to the diaphragm; (2) the inferior vena cava returns blood from body areas below the diaphragm; and (3) the coronary sinus collects blood draining from the myocardium. - Four pulmonary veins enter the left atrium, coming from the lungs.
Ventricles: The Discharging Chambers-Together the ventricles make up most of the volume of the heart. - The conelike papillary muscles, which play a role in valve function, project into the ventricular cavity. The ventricles are the actual pumps of the heart . -The right ventricle pumps blood into the pulmonary trunk, which routes the blood to the lungs where gas exchange occurs. -The left ventricle ejects blood into the aorta (a-or′tah), the largest artery in the body.
Pathway of Blood Through the HeartThe heart is actually two side-by-side pumps, each serving a separate blood circuit. • The blood vessels that carry blood to and from the lungs form the pulmonary circuit . • The blood vessels that carry blood supply to and from all body tissues constitute the systemic circuit. • The right side of the heart is the pulmonary circuit pump. Blood returning from the body is relatively oxygen-poor and carbon dioxide–rich. It enters the right atrium and passes into the right ventricle, which pumps it to the lungs via the pulmonary trunk .
-The left side of the heart is the systemic circuit pump. Freshly oxygenated blood leaving the lungs is returned to the left atrium and passes into the left ventricle, which pumps it into the aorta to the body tissues. Then the blood, once again loaded with carbon dioxide and depleted of oxygen, returns through the systemic veins to the right side of the heart, where it enters the right atrium through the superior and inferior venaecavae.
Although equal volumes of blood are pumped to the pulmonary and systemic circuits at any moment, the two ventricles have very unequal workloads. -The pulmonary circuit, served by the right ventricle, is a short, low-pressure circulation, whereas the systemic circuit, associated with the left ventricle, takes a long pathway through the entire body . -The walls of the left ventricle are three times as thick as those of the right ventricle, and its cavity is nearly circular. -Consequently, the left ventricle can generate much more pressure than the right and is a far more powerful pump.
Coronary Circulation-The coronary circulation is the shortest circulation in the body. -The arterial supply of the coronary circulation is provided by the right and left coronary arteries, both arising from the base of the aorta and encircling the heart in the coronary sulcus. HOMEOSTATIC IMBALANCE Blockage of the coronary arterial circulation can be serious and sometimes fatal: -Angina pectoris is thoracic pain caused by the temporary lack of oxygen but do not die. -Myocardial infarction (MI), is cell death due to prolonged coronary blockage. -Because adult cardiac muscle is essentially amitotic, most areas of are repaired with noncontractile scar tissue. - Damage to the left ventricle, which is the systemic pump, is most serious.
Heart ValvesBlood flows through the heart in one direction. This one-way traffic is enforced by four valves that open and close in response to differences in blood pressure on their two sides.1-Atrioventricular ValvesThe two atrioventricular (AV) valves, one located at each atrial-ventricular junction, prevent backflow into the atria when the ventricles are contracting. • The right AV valve, the tricuspid valve has three flexible cusps • The left AV valve, with two flaps, is called the mitral valve (mi′tral). It is sometimes called the bicuspid valve. • Attached to each AV valve flap are tiny white collagen cords called chordaetendineae“heart strings” which anchor the cusps to the papillary muscles protruding from the ventricular walls.
When the heart is completely relaxed, blood flows into the atria and then through the open AV valves into the ventricles. • When the ventricles contract, the valve edges meet, closing the valve . • The chordaetendineae and the papillary muscles serve to anchor the valve flaps in their closed position. • If the cusps were not anchored in this manner, they would be blown upward into the atria, in the same way an umbrella is blown inside out by a gusty wind.
2-Semilunar ValvesThe aortic and pulmonary (semilunar, SL) valves guard the bases of the aorta and pulmonary trunk, respectively and prevent backflow into the associated ventricles. • Each SL valve is fashioned from three pocketlikecusps, each shaped roughly like a crescent moon . • When intraventricular pressure rises above the pressure in the aorta and pulmonary trunk, the SL valves are forced open . • When the ventricles relax, and the blood flows backward toward the heart, it fills the cusps and closes the valves.
HOMEOSTATIC IMBALANCE -An incompetent valve forces the heart to repump the same blood over and over because the valve does not close properly and blood backflows. - valvularstenosis(“narrowing”), the valve flaps become stiff and constrict the opening. This stiffness compels the heart to contract more forcibly than normal. In both instances, the heart’s workload increases and, ultimately, the heart may be severely weakened. Under such conditions, the faulty valve (most often the mitral valve) is replaced .
Heart Physiology-The ability of cardiac muscle to depolarize and contract is intrinsic; that is, it is a property of heart muscle and does not depend on the nervous system. Even if all nerve connections to the heart are severed, the heart continues to beat rhythmically (as demonstrated by transplanted hearts). -Nevertheless , the healthy heart is amply supplied with autonomic nerve fibers that can alter the basic rhythm of heart activity set by intrinsic factors. Setting the Basic Rhythm: The Intrinsic Conduction SystemThe independent activity consists of noncontractile cardiac cells specialized to initiate and distribute impulses throughout the heart in an orderly, sequential manner. Thus, the heart beats as a coordinated unit.
Sequence of Excitation 1-Sinoatrial node. The crescent-shaped sinoatrial (SA) node is located in the right atrial wall, just inferior to the entrance of the superior vena cava. A minute cell mass with a mammoth job, the SA node typically generates impulses about 75 times every minute. (However, its inherent rate in the absence of extrinsic neural and hormonal factors is closer to 100 times per minute.) Because no other region of the conduction system or the myocardium has a faster depolarization rate, the SA is the heart’s pacemaker, and its characteristic rhythm, called sinus rhythm, determines heart rate.
2-Atrioventricular node. From the SA node, the depolarization wave spreads to the atrioventricular (AV) node, located in the inferior portion of the interatrial septum immediately above the tricuspid valve. At the AV node, the impulse is delayed allowing the atria to respond and complete their contraction before the ventricles contract. Once through the AV node, the signaling impulse passes rapidly through the rest of the system.
3-Atrioventricular bundle. From the AV node, the impulse sweeps to the atrioventricular (AV) bundle (also called the bundle of His) in the superior part of the interventricular septum. 4-Right and left bundle branches. The AV bundle persists only briefly before splitting into two pathways—the right and left bundle branches, which course along the interventricular septum toward the heart apex. 5-Purkinje fibers complete the pathway through the interventricular septum, penetrate into the heart apex, and then turn superiorly into the ventricular walls.
HOMEOSTATIC IMBALANCE Defects in the intrinsic conduction system can cause irregular heart rhythms, or arrhythmias (ah-rith′me-ahz): fibrillation, a condition caused by ischemia . It is a rapid and irregular contractions in which control of heart rhythm is taken away from the SA node by rapid activity in other heart regions. The heart in fibrillation has been compared with a squirming bag of worms. Fibrillating ventricles are useless as pumps; and unless the heart is defibrillated quickly, circulation stops and brain death occurs. Defibrillation is accomplished by electrically shocking the heart . The hope is that the SA node will begin to function normally and sinus rhythm will be reestablished.
Because the only route for impulse transmission from atria to ventricles is through the AV node, any damage to the AV node, referred to as a heart block. -In total heart block no impulses get through and the ventricles beat at their intrinsic rate, which is too slow(20-40/min) to maintain adequate circulation. -In partial heart block, only some of the atrial impulses reach the ventricles. In both cases, pacemakers are used to recouple the atria to the ventricles as necessary.
Modifying the Basic Rhythm: Extrinsic Innervation of the Heart • The sympathetic nervous system (the “accelerator”) increases both the rate and the force of heartbeat. • The Parasympathetic activation (the “brakes”) slows the heart. It sends inhibitory impulses to the heart via branches of the vagus nerves. The cardiac centers are located in the medulla oblongata (cardioacceleratory center and cardioinhibitory center ) • They sends impulses to the SA and AV nodes.
ElectrocardiographyThe electrical currents generated in and transmitted through the heart spread throughout the body and can be detected with an electrocardiograph. A graphic record of heart activity is called an electrocardiogram .An ECG is a composite of all the action potentials generated by nodal and contractile cells at a given time and not, as sometimes assumed, a tracing of a single action potential. A typical ECG has three waves : 1-The first, the small P wave, lasts about 0.08 s and results from movement of the depolarization wave from the SA node through the atria.
2-The large QRS complex results from ventricular depolarization. It has a complicated shape because the paths of the depolarization waves through the ventricular walls change continuously, producing corresponding changes in current direction. 3- The T wave is caused by ventricular repolarization. Because atrialrepolarizationtakes place during the period of ventricular excitation, the wave representing atrialrepolarization is normally obscured by the large QRS complex being recorded at the same time.
Heart SoundsDuring each heartbeat, two sounds can be distinguished when the thorax is auscultated (listened to) with a stethoscope. These heart sounds, often described as lub-dup, are associated with closing of heart valves. The basic rhythm of the heart sounds is lub-dup, pause, lub-dup, pause, and so on. • The first sound, which occurs as the AV valves close, signifies the point when ventricular pressure rises above atrial pressure .The first sound tends to be louder, longer, and more resonant than • the second sound, which is a short and sharp sound heard as the SL valves close at the beginning of ventricular relaxation (diastole).
HOMEOSTATIC IMBALANCE Blood flows silently as long as the flow is smooth and uninterrupted. If it strikes obstructions, however, its flow becomes turbulent and generates heart murmurs that can be heard with a stethoscope. • Heart murmurs are fairly common in young children (and some elderly people) with perfectly healthy hearts, probably because their heart walls are relatively thin and vibrate with rushing blood. • Most often, however, murmurs indicate valve problems. If a valve is incompetent, a murmur is heard as the blood backflows or regurgitates through the valve. • A stenotic valve, in which the valvular opening is narrowed, restricts blood flow through the valve.
Mechanical Events: The Cardiac CycleThe heart alternately contracts, forcing blood out of its chambers, and then relaxes, allowing its chambers to refill with blood. The terms systole (sis′to-le) and diastole (di-as′to-le) refer respectively to these contraction and relaxation periods. The cardiac cycle includes atrial systole and diastole followed by ventricular systole and diastole`(one complete heart beat). These mechanical events always follow the electrical events seen in the ECG. The average heart beats approximately 75 times per minute , so the length of the cardiac cycle is normally about 0.8 seconds.