The Mammalian Heart

The Heart is the organ that controls the circulatory system in mammals (and other animals).

It pumps blood around the body. 

Mammals have a double circulatory system, so the heart must pump blood to the lung and to the rest of body simultaneously.

Location and shape

The human heart is in the middle of the thorax, with its apex pointing to the left.

The human heart is situated in the middle mediastinum, at the level of thoracic vertebrae T5-T8.

A double-membraned sac called the pericardium surrounds the heart and attaches to the mediastinum.

The back surface of the heart lies near the vertebral column, and the front surface sits behind the sternum and rib cartilages.

The upper part of the heart is the attachment point for several large blood vessels – the venae cavae, aorta and pulmonary trunk.

The largest part of the heart is usually slightly offset to the left side of the chest and is felt to be on the left because the left heart is stronger and larger, since it pumps to all body parts.

Because the heart is between the lungs, the left lung is smaller than the right lung and has a cardiac notch in its border to accommodate the heart.

An adult heart has a mass of 250–350 grams (9–12 oz).

The heart is typically the size of a fist: 12 cm (5 in) in length, 8 cm (3.5 in) wide, and 6 cm (2.5 in) in thickness.

Chambers

The heart has four chambers, two upper atria, the receiving chambers, and two lower ventricles, the discharging chambers.

The right atrium and the right ventricle together are sometimes referred to as the right heart. Similarly, the left atrium and the left ventricle together are sometimes referred to as the left heart.

The ventricles are separated from each other by the interventricular septum, visible on the surface of the heart as the anterior longitudinal sulcus and the posterior interventricular sulcus.

Valves

The heart has four valves, which separate its chambers. One valve lies between each atrium and ventricle, and one valve rests at the exit of each ventricle.

The valves between the atria and ventricles are called the atrioventricular valves.

Between the right atrium and the right ventricle is the tricuspid valve. The tricuspid valve has three cusps, which connect to chordae tendinae and three papillary muscles named the anterior, posterior, and septal muscles, after their relative positions.

The mitral valve lies between the left atrium and left ventricle. It is also known as the bicuspid valve due to its having two cusps, an anterior and a posterior cusp. These cusps are also attached via chordae tendinae to two papillary muscles projecting from the ventricular wall.

The papillary muscles extend from the walls of the heart to valves by cartilaginous connections called chordae tendinae.

These muscles prevent the valves from falling too far back when they close.

During the relaxation phase of the cardiac cycle, the papillary muscles are also relaxed and the tension on the chordae tendineae is slight. As the heart chambers contract, so do the papillary muscles. This creates tension on the chordae tendineae, helping to hold the cusps of the atrioventricular valves in place and preventing them from being blown back into the atria.

Two additional semilunar valves sit at the exit of each of the ventricles.

The pulmonary valve is located at the base of the pulmonary artery. This has three cusps which are not attached to any papillary muscles. When the ventricle relaxes blood flows back into the ventricle from the artery and this flow of blood fills the pocket-like valve, pressing against the cusps which close to seal the valve.

The semilunar aortic valve is at the base of the aorta and also is not attached to papillary muscles. This too has three cusps which close with the pressure of the blood flowing back from the aorta.

Right heart

The right heart consists of two chambers, the right atrium and the right ventricle, separated by a valve, the tricuspid valve.

The right atrium receives blood almost continuously from the body’s two major veins, the superior and inferior venae cavae.

A small amount of blood from the coronary circulation also drains into the right atrium via the coronary sinus, which is immediately above and to the middle of the opening of the inferior vena cava.

In the wall of the right atrium is an oval-shaped depression known as the fossa ovalis, which is a remnant of an opening in the fetal heart known as the foramen ovale. 

The right atrium is connected to the right ventricle by the tricuspid valve.

The walls of the right ventricle are lined with trabeculae carneae, ridges of cardiac muscle covered by endocardium.

The right ventricle tapers into the pulmonary trunk, into which it ejects blood when contracting.

The pulmonary trunk branches into the left and right pulmonary arteries that carry the blood to each lung. The pulmonary valve lies between the right heart and the pulmonary trunk.

Left heart

The left heart has two chambers: the left atrium, and the left ventricle, separated by the mitral valve.

The left atrium receives oxygenated blood back from the lungs via one of the four pulmonary veins.

The left atrium is connected to the left ventricle by the mitral valve.

The left ventricle is much thicker as compared with the right, due to the greater force needed to pump blood to the entire body.

The left ventricle pumps blood to the body through the aortic valve and into the aorta.

Two small openings above the aortic valve carry blood to the heart itself, the left main coronary artery and the right coronary artery.

 The Heart Wall

The heart wall consists of three layers:

Epicardium – This is the outermost layer of the heart and a thin layer of membrane that protects and lubricates the outer its section.

Myocardium – This is a muscular layer and consists of muscle tissue. It contributes to the thickness and responsible for the pumping action.

Endocardium – It is the innermost layer that lines within the heart and keeps blood from sticking and prevents the formation of harmful blood clots.

Cardiac Impulse

The electrical conduction system of the heart transmits signals generated usually by the sinoatrial node to cause contraction of the heart muscle.

The pacemaking signal generated in the sinoatrial node travels through the right atrium to the atrioventricular node, along the Bundle of His and through bundle branches to cause contraction of the heart muscle.

This signal stimulates contraction first of the right and left atrium, and then the right and left ventricles. This process allows blood to be pumped throughout the body.

Cardiac Conduction System

This pathway is made up of 5 elements:

The sino-atrial (SA) node

The atrio-ventricular (AV) node

The bundle of His

The left and right bundle branches

¡The Purkinje fibres

STEP 1: PACEMAKER IMPULSE GENERATION

The first step of cardiac conduction is impulse generation.

The sinoatrial (SA) node(also referred to as the pacemaker of the heart) contracts, generating nerve impulses that travel throughout the heart wall.

This causes both atria to contract.

The SA node is located in the upper wall of the right atrium. It is composed of nodal tissue that has characteristics of both muscle and nervous tissue.

STEP 2: AV NODE IMPULSE CONDUCTION

The atrioventricular (AV) node lies on the right side of the partition that divides the atria, near the bottom of the right atrium.

When the impulses from the SA node reach the AV node, they are delayed for about a tenth of a second.

This delay allows atria to contract and empty their contents into the ventricles prior to ventricle contraction.

STEP 3: AV BUNDLE IMPULSE CONDUCTION

The impulses are then sent down the atrioventricular bundle.

This bundle of fibers branches off into two bundles and the impulses are carried down the center of the heart to the left and right ventricles.

The bundle of His is an important part of the electrical conduction system of the heart, as it transmits impulses from the atrioventricular node to the ventricles of the heart.

The intrinsic rate of the bundle of His is 20 or less beats per minute. 

The bundle of His branches into the left and the right bundle branches, which run along the interventricular septum.

The left bundle branch further divides into the left anterior and the left posterior fascicles.

These bundles and fascicles give rise to thin filaments known as Purkinje fibers.

These fibers distribute the impulse to the ventricular muscle.

The ventricular conduction system comprises the bundle branches and the Purkinje networks. It takes about 0.03–0.04 seconds for the impulse to travel from the bundle of His to the ventricular muscle.

—STEP 4: PURKINJE FIBERS IMPULSE CONDUCTION

At the base of the heart the atrioventricular bundles start to divide further into Purkinje fibers.

When the impulses reach these fibers they trigger the muscle fibers in the ventricles to contract. The right ventricle sends blood to the lungs via the pulmonary artery. The left ventricle pumps blood to the aorta.