Heart
The adult heart has the shape of a blunt cone and is approximately the size of a
closed fist. The blunt, rounded point of the cone is the apex; the larger, flat
part at the opposite end of the cone is the base.The heart is located in the
thoracic cavity between the lungs. The heart, trachea, esophagus, and associated
structures form a midline partition called the mediastinum
The heart is enclosed in a double-walled sac called the pericardium. The
parietal pericardium, the outer layer of the pericardium, consists of a
tough layer of fibrous connective tissue and a thin moist serous layer.
The inner layer of the pericardium is called visceral pericardium or the
epicardium. It covers the surface of the heart. Between the parietal
and viscerial pericardia is a space called the pericardial cavity.
The heart wall consists of three layers: the epicardium, the
myocardium, and the endocardium. The epicardium, previously
mentioned, is the outermost layer of the heart wall. It consists of
serous membrane. The myocardium, the thickest layer, is cardiac
muscle tissue and the endocardium consists of simple squamous
endothelium overlying a thin areolar tissue layer
Heart Blood Flow
The pathway of a red blood cell from the right atrium and
back. (1) right atrium -> (2) -> right AV valve -> (3) right
ventricle -> (4) pulmonary valve -> (5) pulmonary trunk ->
(6) pulmonary arteries -> (7) lungs, for exchange of gases
(not shown) -> (8) pulmonary veins -> (9) left atrium -> (10)
left AV valve -> (11) left ventricle -> (12) aortic valve ->
(13) aorta -> (14) other systemic vessels -> (15) inferior
and superior venae cavae -> (16) back to the right atrium.
The pathway from 5 to 8 is the pulmonary circuit, and
thepathway from 13 to 15 is the systemic circuit.
Blood Flow through the Heart
The heart operates as two parallel pumps. The right side of the heart circulates blood through the lungs; the left circulates blood
through the other parts of the body.
Histology
Heart muscle demonstrating the structure
and arrangement of the individual muscle
fibers
Conduction System
Gap Junctions
Gap junctions (electrical synapses) exist between the cardiac muscle cells. They are ion channels between cells that allow cells
to stimulate each other. This promotes a unified action.
Conduction System
The conduction system transmits electrical impulses via specialized cardiac fibers, ensuring that the heart muscle contracts in a coordinated
fashion.
Sinoatrial Node
Atrioventricular Node
Atrioventricular Bundle
Right and left bundle branches
Purkinje fibers
The timing of the heart’s beating is coordinated by special conducting fibers. The two atria contract as a unit slightly before the two ventricles
do so. The SA node is the pacemaker, which initiates each heartbeat.
Electrocardiogram
The normal ECG consists of a P wave, a QRS complex, and a T wave. The P
wave, which is the result of action potentials that cause depolarization of the
atrial myocardium, signals the onset of atrial contraction. The QRS complex is
composed of three individual waves: the Q, R, and S waves. The QRS complex
results from ventricular depolarization and signals the onset of ventricular
contraction. The T wave represents the repolarization of the ventricles and
thus precedes ventricular relaxation. A wave representing repolarization of
the atria cannot be seen because it occurs during the QRS complex.
Electrocardiogram
The time between the beginning of the P wave and the beginning of the QRS
complex is the PQ interval, commonly called the PR interval because the Q
wave is often very small. During the PR interval, which lasts approximately 0.16
second, the atria contract and begin to relax. The ventricles begin to
depolarize at the end of the PR interval. The QT interval extends from the
beginning of the QRS complex to the end of the T wave, lasts approximately
0.3 second, and represents the approximate length of time required for the
ventricles to contract and begin to relax.
Electrocardiogram
Electrical currents generated in the heart can be detected by electrodes placed on the surface of the body. An electrocardiogram is a graphic
display of the heart's electrical activity.
Cardiac Cycle
The heart's contractile activity results in pressure changes that propel blood through the circulatory system. These pressure changes occur in
a cyclic nature and are referred to as the cardiac cycle.
Systole and Diastole
The term systole means "to contract," and diastole means "to dilate." Atrial systole is contraction of the atrial myocardium, and atrial
diastole is relaxation of the atrial myocardium. Similarly, ventricular systole is contraction of the ventricular myocardium, and ventricular
diastole is relaxation of the ventricular myocardium.
When the words "systole" and "diastole" are used without reference to specific chambers, however, they mean ventricular systole or
diastole. The
atria (a) empty during atrial systole and (b) fill with blood during atrial diastole.
Quiescent period: Atria and ventricles in diastole. AV valves open,blood flows into atrium to ventricles. 70% of blood passively enters ventricles
during this period. Atrial pressure is greater than ventricular pressure.
Atrial systole: SA node fires, the atria depolarize and stimulates atrial systole, pumping the remaining 30% of blood into ventricles. The volume
of blood in ventricles following end-diastolic volume (EDV) is about 130ml.
Isovolumetric contraction: Atria repolarize and relax. Ventricles depolarize and begin to contract. Ventricle pressure rises above atrial
pressure and AV valves close. Semilunar valves are also closed. Ventricles contract; but there is no change in volume because all valves are
closed.
Ventricular ejection: As ventricles contract, ventricular pressure builds, finally exceeding arterial pressure. The semilunar valves open and
blood is ejected. The volume ejected per contraction (approximately 70ml) is called the stroke volume. The lbood remaining in the ventricle
(about 60ml) is called the end-systolic volume.
Isovolumetric relaxation: Ventricles relax; blood from the aorta and pulmonary trunk briefly flows backwards but then fills the cusps of the
semilunar valves, promting them to close. This creates a small pressure rebound to the dicrotic notch of the aortic pressure curve.
Ventricular filling: Ventricular diastole occurs. Ventricular pressure drops below atrial pressure. AV valvesopen and blood flows rapidly into
ventricles (rapid ventricular filling.) Pressure gradient decreases;blood flows into ventricles slower (diastasis.) Remaining blood enters ventricle
following atrial systole. Ventricular filling includes phases 1 and 2.
Heart Sounds
The closing of valves in the heart produces sounds that are often described as "lub dub". The "lub" is produced by the closing of the AV valves.
The "dub" is produced by the closing of semilunar valves.
Heart Murmurs - Normal Aortic Valve
A normal aortic valve only allows blood to flow from the left ventricle into the aorta when the ventricular pressure exceeds that in the aorta.
Closing of the aortic valve produces the "dub" heart sound.
Heart Murmurs - Valvular Insufficiency
Valvular insufficiency is a condition in which the cusps of the valve do not form a tight seal when the valve is closed. Aortic valvular
insufficiency results in an abnormal sound during ventricular diastole.
Heart Murmurs - Valvular Stenosis
Valvular stenosis is a condition that results from abnormal constriction of the valve's ability to open fully. Aortic stenosis can be heard during
ventricular systole.
Cardiac Output
Heart rate and stroke volume and thus cardiac output can be altered by a baroreceptor reflex. Baroreceptors located in the aortic
arch and the carotid arteries detect changes in blood pressure. If blood pressure decreases, the baroreceptors inform the
cardioregulatory center in the medulla oblongata. The medulla oblongata increases sympathetic stimulation to the SA node thus
increasing heart rate to the myocardium, which increases calcium availability and thus increases stroke volume.