Anatomy of the Heart

The heart is located in a central tissue mass that divides the ribcage into two cavities between the two lungs. This central tissue consists of the heart, its associated blood vessels, the esophagus, and the trachea.

The axis of the heart is tilted so that the apex (tip) of the heart points toward the lower left. The large arteries and veins running to and from the heart are connected to the upper end or base of the heart.

Below is a cross section through the body at the level indicated in the diagram above.

The heart functions as a dual pump with each half of the heart beating together in unison during a single cardiac cycle. That is, the atria contract together and then the ventricles contract together.

Following a single erythrocyte through the basic circuits of the cardiovascular system:
From the head, trunk, and extremities blood enters the right atrium through the superior and inferior vena cavae. The right atrium pumps blood through the right atrioventricular (tricuspid) valve to the right ventricle. From the right ventricle, blood flows through the pulmonary valve to the lungs via the pulmonary arteries (deoxygenated blood). In the lungs the blood becomes more red in color as it is oxygenated. Blood returns to the heart via the pulmonary veins (oxygenated blood) to the left atrium. The left atrium pumps blood through the left atrioventricular (bicuspid or mitral) valve to the left ventricle. From the left ventricle, blood flows through the aortic valve to the arch of the aorta and out to the systemic arteries of the body. These arteries pass the blood to capillaries where the tissues of the body use the oxygen from the blood for aerobic cell respiration. As blood passes out of the capillary beds of the body it is collected by veins that return the deoxygenated blood to the heart. The superior and inferior vena cavae return deoxygenated venous blood to the right atrium of the heart.



Heart Valves

Heart valves function to ensure a one-way flow of blood through the heart. The valves are not made of muscle, but rather are composed of sheets of tough connective tissue (leaflets) that act like flaps. The heart valves open and close passively because of pressure differences on either side of the valve. When pressure is greater behind the valve, the leaflets are blown open and the blood flows through the valve. However, when pressure is greater in front of the valve, the leaflets snap shut and blood flow is stopped. The motion of a heart valve is analogous to the motion of the front door of your house. The door, which only opens in one direction, opens and closes due to pressure on the door.

Atrioventricular (AV) and Semilunar Valves

The atrioventricular valves (AV valves), which separate the atria from the ventricles, allow blood to flow from the atria to the ventricles, but prevent flow in the opposite direction. The right AV valves is called the tricuspid valve. The left AV valve is called the mitral valve. The opening and closing of the AV valves is dependent on pressure differences between the atria and ventricles. When the ventricles relax, atrial pressure exceeds ventricular pressure, the AV valves are pushed open and blood flows into the ventricles. However, when the ventricles contract, ventricular pressure exceeds atrial pressure causing the AV valves to snap shut.

To ensure that the AV valves do not evert (turn inside-out), they are attached to small papillary muscles by tough tendons called the cordae tendineae. Papillary muscles contract in synchrony with the ventricles, thus maintaining constant tension on the valve leaflets.

The semilunar valves (pulmonary valve and aortic valve) are one-way valves that separate the ventricles from major arteries. The aortic valve separates the left ventricle from the aorta, while the pulmonary valve separates the right ventricle from the pulmonary artery. As the ventricles contract, ventricular pressure exceeds arterial pressure, the semilunar valves open and blood is pumped into the major arteries. However, when the ventricles relax, arterial pressure exceeds ventricular pressure and the semilunar valves snap shut.

Heart Sounds are associated with Valve Closure

Normal heart sounds are caused by the closing of heart valves. As valves snap shut, the walls of the chambers and major arteries vibrate. We hear these vibrations as two distinct sounds; lub-dup. The first sound, "lub", is associated with the closing of the AV valves. The second sound, "dup", is associated with the closing of the semilunar valves

CHAMBERS OF THE HEART AND
CIRCUITS OF THE CARDIOVASCULAR SYSTEM

RIGHT ATRIUM
This chamber receives blood from the body via the:
superior vena cava = returns blood from the head and upper extremities
inferior vena cava = returns blood from the trunk and legs

RIGHT ATRIOVENTRICULAR VALVE (TRICUSPID VALVE)
This valve is found between the right atrium and the right ventricle
Both the right and the left atrioventricular valves are stabilized by papillary muscles and the chordae tendineae.

RIGHT VENTRICLE
This chamber receives blood from the right atrium
The wall of this chamber is more muscular than the right atrium, but less muscular than the left ventricle




PULMONARY SEMILUNAR VALVE
As compared to the atrioventricular valves, this smaller valve has self-supporting cusps with no chordae tendinae or papillary muscles. When closed, the 3 symmetrical cusps support one another.

PULMONARY TRUNK & PULMONARY ARTERIES
These blood vessels carry blood from the heart to the lungs.
The pulmonary trunk branches to form the right and left pulmonary arteries.

PULMONARY VEINS
These four veins carry blood from the lungs into the left atrium.

LEFT ATRIUM
This chamber receives blood from the 4 pulmonary veins and pumps blood through the left atrioventricular valve.

LEFT ATRIOVENTRICULAR VALVE (BICUSPID OR MITRAL VALVE)
This valve has two flaps and stronger papillary muscles and chordae tendinae due to strength of the left ventricle.

LEFT VENTRICLE
This chamber has a much thicker muscular wall than the right ventricle (blood has to travel farther through body) and it forms the apex (tip) of the heart. This chamber pumps blood through the aortic semilunar valve



AORTIC SEMILUNAR VALVE
This valve keeps blood from flowing backward into the left ventricle as blood is pumped from the left ventricle into the arch of the aorta.

ARCH OF AORTA
This is the largest artery in the body
It carries blood from the left ventricle to the upper and lower body.

OTHER IMPORTANT PARTS OF THE HEART

papillary muscles = muscles in the ventricles connected to the chordae tendinae. These muscles help stabilize the AV valves during ventricular contraction.

chordae tendinae = fibrous cords that keep the AV valve cusps from swinging into the atria & prevent regurgitation of blood.


CARDIAC MUSCLE CELLS
Cardiac muscle cells are uninucleate, somewhat striated cells, that contain lots of mitochondria and are interconnected branching fibers that are connected at specialized cell-to-cell junctions.

An electrical impulse generated in one part of the heart spreads to the entire heart via the conduction system of the heart. Cardiac cells can generate electrical impulses (action potentials) without any input from the nervous system.

Electrical Conduction System of the Heart

Action potentials that originate in the SA node spread to the myocardial cells of the atria through gap junctions between cells. Depolarization of the atria stimulates contraction of the atrial myocardium. Action potentials cannot directly spread from the atrial myocardium to the ventricular myocardium due to the presence of the non-conducting fibrous skeleton that separates them. Rather, the impulse travels to the ventricles through a system of specialized cells called the conduction system.

The conduction system is composed of the:

1. atrioventricular node (AV node)
2. Bundle of His
3. bundle branches
4. Purkinje fibers.

Cardiac Cycle

The heart undergoes a constant cycle of contractions and relaxations. The period of ventricular contraction is called systole. The period of ventricular relaxation is called diastole.

Events in Diastole

Diastole begins as the ventricles start to relax. Soon the pressures within the aorta and pumonary artery exceed ventricular pressures, causing the semilunar valves to close. As the ventricular pressure falls below the atrial pressure the AV valves open and the ventricles fill with blood. The ventricles fill to about 80% of capacity prior to contraction of the atria, the last event in diastole. Atrial contraction forces the final 20% of the end-diastolic volume (the volume of blood that exists in the ventricles at the end of diastole) into the ventricles.

Summary of Diastole

1. Ventricles relax
2. pulmonary and aortic valves close
3. AV valves open
4. ventricles fill (about 80% of capacity)
5. atria contract (ventricles fill another 20%)

Events in Systole

As the ventricles start to contract, the ventricular pressure soon exceeds the atrial pressure, causing the AV valves to close. As the ventricles continue to contract, the ventricular pressure exceeds the arterial pressures causing the semilunar valves open. Blood is forcefully ejected out of the ventricles and into the aorta and pulmonary artery.

Summary of Systole

1. ventricles contract
2. AV valves close
3. aortic and pulmonary valves open
4. blood is ejected
5. atria relax and fill with blood

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