Functions of the Heart

The heart functions include

• The heart generates blood pressure.

• The heart routes blood through the systemic and pulmonary circulation.

• The pumping action of the heart and the valves of the heart ensure a one-way flow of blood through the heart and blood vessels.

• The heart helps regulate blood supply to tissues.

Size, Form, and Location of the Heart

• The heart is approximately the size of a fist and is located in the pericardial cavity.

Anatomy of the Heart

Pericardium

• The pericardial sac consists of a fibrous and serous pericardium. The fibrous pericardium is lined by the parietal pericardium.

• The outer surface of the heart is lined by the visceral pericardium (epicardium).

• Between the visceral and parietal pericardium is the pericardial cavity, which is filled with pericardial fluid.

External Anatomy

• Atria are separated externally from the ventricles by the coronary sulcus. The right and left ventricles are separated externally by the interventricular sulci.

• The inferior and superior venae cavae enter the right atrium. The four pulmonary veins enter the left atrium.

• The pulmonary trunk exits the right ventricle, and the aorta exits the left ventricle.

Blood Supply to the Heart

• The left and right coronary arteries originate from the base of the aorta and supply the heart. Blood returns from heart tissue to the right atrium through the coronary veins and coronary sinus.

Heart Chambers

• There are four chambers in the heart. The left and right atria receive blood from veins, function mainly as reservoirs. Contraction of the atria completes ventricular filling.

• The atria are separated internally from each other by the interatrial septum.

• The ventricles are the main pumping chambers of the heart. The right ventricle pumps blood into the pulmonary trunk and the left ventricle, which has a thicker wall, pumps blood into the aorta.

• The ventricles are separated internally by the interventricular septum.

Heart Valves

• The heart valves ensure one-way flow of blood.

• The tricuspid valve (three cusps) separates the right atrium and right ventricle, and the bicuspid valve (two cusps) separates the left atrium and left ventricle.

• The papillary muscles attach by the chordae tendineae to the cusps of the tricuspid and bicuspid valves and adjust tension on the valves.

• The aorta and pulmonary trunk are separated from the ventricles by the semilunar valves.

• The skeleton of the heart is a plate of fibrous connective tissue that separates the atria from the ventricles, acts as an electrical barrier between the atria and ventricles, and supports the valves of the heart.

Route of Blood Flow Through the Heart

• The left and right sides of the heart can be considered as separate pumps.

• Blood flows from the systemic vessels to the right atrium and from the right atrium to the right ventricle. From the right ventricle blood flows to the pulmonary trunk and from the pulmonary trunk to the lungs. From the lungs blood flows through the pulmonary veins to the left atrium, and from the left atrium blood flows to the left ventricle. From the left ventricle blood flows into the aorta and then through the systemic vessels.

Histology of the Heart

Heart Wall

• The heart wall consists of the outer epicardium, the middle myocardium, and the inner endocardium.

Cardiac Muscle

• Cardiac muscle is striated and depends on ATP for energy. It depends on aerobic metabolism.

• Cardiac muscle cells are joined by intercalated disks that allow action potentials to be propagated throughout the heart.

Electrical Activity of the Heart

Action Potentials in Cardiac Muscle

• Action potentials in cardiac muscle are prolonged compared with those in skeletal muscle and have a depolarization phase, a plateau phase, and a repolarization phase.

• The depolarization is due mainly to opening of the voltage-gated sodium ion channels, and the plateau phase is due to opened voltage-gated calcium ion channels. Repolarization at the end of the plateau phase is due to the opening of potassium ion channels for a brief period.

• The prolonged action potential in cardiac muscle ensures that contraction and relaxation occurs and prevents tetany in cardiac muscle.

• The SA node located in the upper wall of the right atrium is the normal pacemaker of the heart and cells of the SA node have more voltage-gated calcium ion channels than other areas of the heart.

Conduction System of the Heart

• The conduction system of the heart is made up of specialized cardiac muscle cells.

• The SA node produces action potentials that are propagated over the atria to the AV node.

• The AV node and atrioventricular bundle conduct action potentials to the ventricles.

• The right and left bundle branches conduct action potentials from the atrioventricular bundle through Purkinje fibers to the ventricular muscle.

• An ectopic beat results from an action potential that originates in an area of the heart other than the SA node.

Electrocardiogram

• The ECG is a record of electrical events within the heart.

• The ECG can be used to detect abnormal heart rates or rhythms, conduction pathways, hypertrophy or atrophy of the heart, and the approximate location of damaged cardiac muscle.

• The normal ECG consists of a P wave (atrial depolarization), a QRS complex (ventricular depolarization), and a T wave (ventricular repolarization).

• Atrial contraction occurs during the P-Q interval, and the ventricles contract and relax during the Q-T interval.

Cardiac Cycle

• Atrial systole is contraction of the atria, and ventricular systole is contraction of the ventricles. Atrial diastole is relaxation of the atria, and ventricular diastole is relaxation of the ventricles.

• During atrial systole, filling of the right ventricle is completed.

• During ventricular systole, the tricuspid valve closes, and blood forces open the pulmonary semilunar valve; blood flows into the pulmonary trunk. Also, the bicuspid valve closes, and blood forces open the aortic semilunar valve; blood flows into the aorta.

Heart Sounds

• The first heart sound results from closure of the tricuspid and bicuspid valves. The second heart sound results from closure of the aortic and pulmonary semilunar valves.

• Abnormal heart sounds are called murmurs. They can result from incompetent (leaky) valves or stenosed (narrowed) valves.

Regulation of Heart Function

• Cardiac output (volume of blood pumped per ventricle per minute) is equal to the stroke volume (volume of blood ejected per beat) times the heart rate (beats per minute).

Intrinsic Regulation of the Heart

• Intrinsic regulation refers to regulation that is contained in the heart.

• As venous return to the heart increases, the heart wall is stretched, and the increased stretch of the ventricular walls is called preload.

• An increase in preload causes the stroke volume to increase (Starling's law of the heart) and heart rate to increase.

• Afterload is the pressure against which the ventricles must pump blood.

Extrinsic Regulation of the Heart

• Extrinsic regulation refers to nervous and hormonal mechanisms.

• Sympathetic stimulation increases stroke volume and heart rate; parasympathetic stimulation decreases heart rate.

• The baroreceptor reflex detects changes in blood pressure and causes a decrease in heart rate and stroke volume in response to a sudden increase in blood pressure or an increase in heart rate and stroke volume in response to a sudden decrease in blood pressure.

• Emotions influence heart function by increasing sympathetic stimulation of the heart in response to exercise, excitement, anxiety, or anger and by increasing parasympathetic stimulation in response to depression.

• Alterations in body fluid levels of carbon dioxide, pH, and ion concentrations, as well as changes in body temperature, influence heart function.