Heart Physiology
Heart Physiology
Electrical Events
Certain Noncontractile cardiac muscle cells exhibit automatically and rhythimicity and can independently imitate href="http://www.chempapy.blogspot.com/">action potentials
Such cells have an unstable resting potential called a pacemaker potential that gradually depolarizes drifting toward threshold for firing
These cells comprise the intrinsic conduction system of the heart
The conduction or nodal system of the heart consists of the sinuatrial Node (SA) and the Atrioventicular (AV) nodes, The Av Bundle and bundle branches and the Purkinje fibers
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 hearts pacemaker; It sets the sinus rhythm
Defects in the intrinsic conduction system can cause arhtmias,fibrillation and heart block
The heart is innervated by autonomic nevous system
Autonomic cardiac centers in the medulla include the sympathetic cardioaccelatory center, which projects to the T1-T5 region of the spinal cord which in turn projects to the cervical and upper thoracic chain ganglia
Postganglionic fibers innervate the SA and AV nodes and the cardiac muscle fibers. The parasympathetic cardio inhibitory center exerts its influence via the vagus nerves (X), which project to the heart wall. Most parasympathetic fibers. Serve the SA and AV nodes
An electrocardiogram is a graphic representation of the cardiac conduction cycle. The P wave reflects atrial depolarization. The QRS complex indicates ventricular depolarization
Mechanical Events: The cardiac cycle
Cardiac cycle refers to events occuring during one heartbeat
During mid to late diastole the ventricles fill and the atria contracts
Ventricular systole consists of isovolumetric contraction phase and ventricular ejection phase
During early diastole, the ventricles are relaxed and are closed chambers until increasing atrial pressure forces the AV valves to open and the cycle begins again
At a normal heart reate of 75beats/min a cardiac cycle lasts 0.8s
Pressure changes promote blood flow and valve opening and closing
CARDIAC CYCLE
AND HEART SOUNDS
The cardiac cycle is the sequence of events in one heartbeat. In its simplest form, the cardiac cycle is the simultaneous contraction of the two atria, followed a fraction of a second later by the simultaneous contraction of the two ventricles. Systole is another term for contraction. The term for relaxation is diastole. You are probably familiar with these terms as they apply to
blood pressure readings. If we apply them to the cardiac cycle, we can say that atrial systole is followed by ventricular systole. There is, however, a significant difference between the movement of blood from the atria to the ventricles and the movement of blood from the ventricles to the arteries. The events of thecardiac cycle are shown in. In this traditional representation, the cardiac cycle is depicted in a circle, because one heartbeat follows another, and the beginning
of atrial systole is at the top (12 o’clock). The size of the segment or arc of the circle indicates how long it takes. Find the segment for atrial systole and the one for ventricular systole, and notice how much larger (meaning “longer”) ventricular systole is. Do you think this might mean that ventricular contraction is more important than atrial contraction? It does, as you will see. as you read the following. We will begin at the bottom (6 o’clock) where the atria are in the midst of diastole and the ventricles have just completed their systole. The entire heart is relaxed and the atria are filling with blood.Blood is constantly flowing from the veins into both atria. As more blood accumulates, its pressure forces open the right and left AV valves. Two-thirds of the atrial blood flows passively into the ventricles (which brings us to 12 o’clock); the atria then contract to pump the remaining blood into the ventricles.Following their contraction, the atria relax and the ventricles begin to contract. Ventricular contraction forces blood against the flaps of the right and left AV valves and closes them; the force of blood also opens the aortic and pulmonary semilunar valves. As the ventricles continue to contract, they pump blood into the arteries. Notice that blood that enters the arteries must all be pumped. The ventricles then relax, and at the same time blood continues to flow into the atria,
and the cycle begins again.
Normal heart sounds arise chiefly from the closing of heart valves. Abnormal heart sounds called murmurs usually reflect valve problems
Cardiac Output
Cardiac output typically 5L/min is the amount of blood pumped out by each ventricle in 1minute
Stroke volume is the amount of blood pumped out by a ventricle with each contraction. Cardiac output = Heat rate x Stroke volume
Stroke volume depends to alarge extent on the degree of stretch of cardiac muscle by venous return. Approximately 70ml, It is the difference between end diastolic volume 9 EDV) and the end systolic volume influences venous return, Hence stroke volume
Activation of the sympathetic nervous system increases heart rate and contractility: parasympathetic activation decreases heart rate and contractility.Ordinally the heart exhibits vagal tone
Chemical regulation of the heart is affected by hormones (epinephrine and thyroxine) and ions (sodium, potassium and calcium). Imbalances in ions severely impair heart activity
Other factors influencing heart rate are age, sex, exercise and body temperature.
Congestive heart failure occurs when the pumping ability of the heart is inadequate to provide normal circulation to meet body needs.
Right hear failure lead to systemic edema; Left heart failure leads to pulmonary edema
Developmental Aspects of the Heart
The heart begins as a simple (Mesodermal) Tube that is pumping blood bt the fourth week of gestation. The fetal heart has two lung bypasses: The foramen ovale and the ductus arteriosus
Congenital heart defects Account for more than half of infant deaths. The most common of these disorder lead to inadequate oxygenation of blood or increase the work load on the heart
Age related changes include sclerosis and thickening of the valve flaps, declines in cardiac reserve, fibrosis of cardiac muscle and atherosclerosis
Risk factors for cardiac disease include dietary factor,excessive stress, cigarette smoking and lack of excercise
Related clincial terms
Asystole (a-sis’to-le) situation in which the heart fails to contract
Cardiac Catherization: Diagnostic procedure involving passage of a fine catheter( tubing) through a blood vessel into the heart; Blood samples are withdrawn and pressures within the heart can be measured; findings help to detect valve problems, heart deformities and other heart malfunctions
Cor pulmonale (kor pul-mun-na’le; cor=heart, pulmo=lung) A life threatening condition of right sided heart failure resulting from elevated blood pressure in the pulmonary circuit (pulmonary hypertension); Acute cases may develop suddenly due to a pulmonary embolism; Chronic cases are usually associated with chronic lung disorders such as emphysema
Endocarditis ( en”do-kar-di’tis) Inflammation of the endocardium,usually confined to the endocardium of the heart valves
Endocarditis often results from infection by bacteria that have entered the blood steram but may result from fungal infection or an autoimmune response
Drug addicts may develop endocarditis by injecting themselves with contaminated needles
Heart palpitation A heartbeat that is unusually strong, fast or irregular so that the person becomes aware of it; may be caused by certain drugs, emotional pressures (“nevervous heart) or heart disorders
Mitral valve prolapse Valve disorder affecting up to 5 % of the population; Most often seen in young women and appears to have a genetic basis; One or more of the flaps of the mitral valve become incompetent and billow into the left atrium during ventricular systole, allowing blood regurgitation; Mitral valve prolapse is a common indication for valve replacement surgery
Myocarditis ( mi”o-kar-di/tis; myo=muscle, card=heart, it is= inflammation) Inflammation of the cardiac muscle layer (myocardium) of the heart; sometimes follows an untreated streptococcal infection in children; may weaken the heart and impair is ability to act as an effective pump
Endocarditis (en”do-kar-di’ tis) Inflammation of the endocardium,usually confined to the endocardium of the heart valves. Endocarditis often results from infection by bacteria that have entered the bloodstream but may result from fungal infection or autoimmune response. Drug addicts may develop endocarditis by injecting themselves with contaminated needles
REFERENCES
•Campbell, N.A., J.B. Reece, L.G. Mitchell, and M.R. Taylor (2003) Biology: Concepts and Connections, Benjamin/Cummings
•Hoar, W.S. (1983) General and Comparative Physiology, Prentice-Hall
•Snell, R.S.(2003) Clinical Neuroanatomy for Medical Students. Lippincott Williams &Wilkins
•Rosenzweig, M.R., S.M. Breedlove, and A.L. Leiman (2002) Biological Psychology: An Introduction to Behavioural, Cognitive and Clinical Neuroscience. Sinauer Associates
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