Cardiovascular system

Cardiovascular system

1. The cardiovascular system provides the transport system “hardware” that keeps blood continuously circulating to fulfill this critical homeostatic need.

2. The Pulmonary and Systemic Circuits

   • 1. The right side of the heart is the pulmonary circuit pump. It pumps blood through the lungs, where the blood picks up oxygen and dumps carbon dioxide. The left side of the heart is the systemic circuit pump. It pumps blood through the body’s tissues, supplying them with oxygen and nutrients and removing carbon dioxide.

3. Heart Anatomy

   • Size, Location, and Orientation
     • 1. The human heart, about the size of a clenched fist, is located obliquely within the mediastinum of the thorax.
   • Coverings of the Heart
     • 2. The heart is enclosed within a double sac made up of the outer fibrous pericardium and the inner serous pericardium (parietal and visceral layers). The pericardial cavity between the serous layers contains lubricating serous fluid.
   • Layers of the Heart Wall
     • 3. Layers of the heart wall, from the interior out, are the endocardium, myocardium (reinforced by a fibrous cardiac skeleton), and epicardium (visceral layer of the serous pericardium).
   • Chambers and Associated Great Vessels
     • 4. The heart has two superior atria and two inferior ventricles. Functionally, the heart is a double pump. 5. Entering the right atrium are the superior vena cava, inferior vena cava, and coronary sinus. Four pulmonary veins enter the left atrium. 6. The right ventricle discharges blood into the pulmonary trunk; the left ventricle pumps blood into the aorta.
   • Heart Valves
     • 7. The atrioventricular (AV) valves (tricuspid and mitral) prevent backflow into the atria when the ventricles are contracting; the semilunar (SL) valves (pulmonary and aortic) prevent backflow into the ventricles when the ventricles are relaxing.
   • Pathway of Blood Through the Heart
     • 8. Oxygen-poor systemic blood enters the right atrium, passes into the right ventricle, through the pulmonary trunk to the lungs, and back to the left atrium via the pulmonary veins. Oxygenladen blood entering the left atrium from the lungs flows into the left ventricle and then into the aorta, which provides the functional supply of all body organs. Systemic veins return the oxygen-depleted blood to the right atrium.
   • Coronary Circulation
     • 9. The right and left coronary arteries branch from the aorta and via their main branches (anterior and posterior interventricular, right marginal, and circumflex arteries) supply the heart itself. Venous blood, collected by the cardiac veins (great, middle, and small), empties into the coronary sinus. 10. Blood delivery to the myocardium occurs during heart relaxation.

4. Cardiac Muscle Fibers

   • Microscopic Anatomy
     • 1. Cardiac muscle cells are branching, striated, generally uninucleate cells. They contain myofibrils consisting of typical sarcomeres. 2. Intercalated discs containing desmosomes and gap junctions connect adjacent cardiac cells. The myocardium behaves as a functional syncytium
   • Mechanism and Events of Contraction
     • 3. As in skeletal muscle, the membrane depolarization of contractile myocytes causes opening of sodium channels and allows sodium to enter, which is responsible for the rising phase of the action potential curve. Depolarization also opens slow Ca21 channels; Ca21 entry prolongs the period of depolarization (creates the plateau). Ca21 released by the SR and entering from the extracellular space couples the action potential to sliding of the myofilaments. Compared to skeletal muscle, cardiac muscle has a prolonged refractory period that prevents tetany.
   • Energy Requirements
     • 4. Cardiac muscle has abundant mitochondria and depends almost entirely on aerobic respiration to form ATP.

5. Heart Physiology

   • Electrical Events
     • 1. Certain noncontractile cardiac muscle cells exhibit automaticity and rhythmicity and can independently initiate action potentials. Such cells have an unstable resting potential called a pacemaker potential that gradually depolarizes, drifting toward threshold for firing. These cells compose the intrinsic conduction system of the heart.
       • 2. The conduction system of the heart consists of the SA and AV nodes, the AV bundle and bundle branches, and the subendocardial conducting network. This system coordinates the depolarization of the heart and ensures that the heart beats as a unit. The SA node has the fastest rate of spontaneous depolarization and acts as the heart’s pacemaker; it sets the sinus rhythm.
         • 3. Defects in the intrinsic conduction system can cause arrhythmias, fibrillation, and heart block.
           • 4. The autonomic nervous system innervates the heart. Cardiac centers in the medulla include the cardioacceleratory center, which projects to the T1–T5 region of the spinal cord, which in turn projects to the cervical and upper thoracic sympathetic trunk. Postganglionic fibers innervate the SA and AV nodes and the cardiac muscle fibers. The cardioinhibitory center exerts its influence via the parasympathetic vagus nerves (X), which project to the heart wall. Most parasympathetic fibers serve the SA and AV nodes.
             • 5. An electrocardiogram (ECG) is a graphic representation of the cardiac conduction cycle. The P wave reflects atrial depolarization. The QRS complex indicates ventricular depolarization; the T wave represents ventricular repolarization.
   • Heart Sounds
     • 6. Normal heart sounds arise chiefly from turbulent blood flow during the closing of heart valves. Abnormal heart sounds, called murmurs, usually reflect valve problems.
   • Mechanical Events: The Cardiac Cycle
     • 7. A cardiac cycle consists of the events occurring during one heartbeat. During mid-to-late diastole, the ventricles fill and the atria contract. Ventricular systole consists of the isovolumetric contraction phase and the ventricular ejection phase. During early diastole, the ventricles are relaxed and are closed chambers until the atrial pressure exceeds the ventricular pressure, forcing the AV valves open. Then the cycle begins again. At a normal heart rate of 75 beats/min, a cardiac cycle lasts 0.8 s. 8. Pressure changes promote blood flow and valve opening and closing.
   • Cardiac Output
     • 9. Cardiac output, typically 5 L/min, is the amount of blood pumped out by each ventricle in 1 minute. Stroke volume is the amount of blood pumped out by a ventricle with each contraction. Cardiac output 5 heart rate 3 stroke volume.
       • 10. Stroke volume depends to a large extent on the degree to which venous return stretches cardiac muscle. Approximately 70 ml, it is the difference between end diastolic volume (EDV) and end systolic volume (ESV). Anything that influences heart rate or blood volume influences venous return, hence stroke volume.
         • 11. Activation of the sympathetic nervous system increases heart rate and contractility; parasympathetic activation decreases heart rate but has little effect on contractility. Ordinarily, the heart exhibits vagal tone.
           • 12. Chemical regulation of the heart is effected by hormones (epinephrine and thyroxine) and ions (particularly potassium and calcium). Ion imbalances severely impair heart activity.
             • 13. Other factors influencing heart rate are age, sex, exercise, and body temperature.
               • 14. Congestive heart failure occurs when the pumping ability of the heart cannot provide adequate circulation to meet body needs. Right heart failure leads to systemic edema; left heart failure results in pulmonary edema.

6. Developmental Aspects of the Heart

   • Before Birth
     • 1. The heart begins as a simple (mesodermal) tube that is pumping blood by the fourth week of gestation. The fetal heart has two lung bypasses: the foramen ovale and the ductus arteriosus. 2. Congenital heart defects are the most common of all birth defects. The most common of these disorders lead to inadequate oxygenation of blood or increase the workload of the heart.
   • Heart Function Throughout Life
     • 3. Age-related changes include sclerosis and thickening of the valve flaps, declines in cardiac reserve, fibrosis of cardiac muscle, and atherosclerosis. 4. Risk factors for cardiac disease include dietary factors, excessive stress, cigarette smoking, and lack of exercise.

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