Anatomy and Physiology Notes: The Heart

Heart and heart wall layers
1. The heart is located in the left side of the mediastinum.
2. The heart consists of three layers.
a. The epicardium is the outermost layer of the heart.
b. The myocardium is the middle layer and is the
actual contracting muscle of the heart.
c. The endocardium is the innermost layer and lines
the inner chambers and heart valves.


Pericardial sac
1. Encases and protects the heart from trauma and infection
2. Has two layers
a. The parietal pericardium is the tough, fibrous outer membrane that attaches anteriorly to the lower half
of the sternum, posteriorly to the thoracic vertebrae, and inferiorly to the diaphragm.
b. The visceral pericardium is the thin, inner layer that closely adheres to the heart.
3. The pericardial space is between the parietal and visceral layers; it holds 5 to 20 mL of pericardial fluid, lubricates the pericardial surfaces, and cushions the heart.


There are four heart chambers
1. The right atrium receives deoxygenated blood from the body via the superior and inferior vena cava.
2. The right ventricle receives blood from the right atrium and pumps it to the lungs via the pulmonary artery.
3. The left atrium receives oxygenated blood from the lungs via four pulmonary veins.
4. The left ventricle is the largest and most muscular chamber; it receives oxygenated blood from the lungs via the left atrium and pumps blood into the systemic circulation via the aorta.

There are four valves in the heart.
1. There are two atrioventricular valves, the tricuspid and the mitral, which lie between the atria and ventricles.
a. The tricuspid valve is located on the right side of the heart.
b. The bicuspid (mitral) valve is located on the left side of the heart.
c. The atrioventricular valves close at the beginning of ventricular contraction and prevent blood from flowing
back into the atria from the ventricles; these valves open when the ventricle relaxes.

2. There are two semilunar valves, the pulmonic and the aortic.
a. The pulmonic semilunar valve lies between the right ventricle and the pulmonary artery.
b. The aortic semilunar valve lies between the left ventricle and the aorta.
c. The semilunar valves prevent blood from flowing back into the ventricles during relaxation; they open during
ventricular contraction and close when the ventricles begin to relax.

Sinoatrial (SA) node

1. The main pacemaker that initiates each heartbeat
2. It is located at the junction of the superior vena cava and the right atrium.
3. The sinoatrial node generates electrical impulses at 60 to 100 times per minute and is controlled by the sympathetic and parasympathetic nervous systems.

Atrioventricular (AV) node
1. Located in the lower aspect of the atrial septum
2. Receives electrical impulses from the sinoatrial node
3. If the sinoatrial node fails, the atrioventricular node can initiate
and sustain a heart rate of 40 to 60 beats/min.

The bundle of His
1. A continuation of the AV node; located at the interventricular septum
2. It branches into the right bundle branch, which extends down the right side of the interventricular septum, and the left bundle branch, which extends into the left ventricle.
3. The right and left bundle branches terminate into Purkinje fibers.

Purkinje fibers
1. Purkinje fibers are a diffuse network of conducting strands located beneath the ventricular endocardium.
2. These fibers spread the wave of depolarization through the ventricles.
3. Purkinje fibers can act as the pacemaker with a rate between 20 and 40 beats/min when higher pacemakers fail


Coronary arteries 
1. The coronary arteries supply the capillaries of the myocardium with blood.
2. The right coronary artery supplies the right atrium and ventricle, the inferior portion of the left ventricle, the posterior septal wall, and the sinoatrial and atrioventricular nodes.
3. The left main coronary artery consists of two major branches, the left anterior descending and the circumflex arteries.
4. The left anterior descending artery supplies blood to the anterior wall of the left ventricle, the anterior ventricular septum, and the apex of the left ventricle.
5. The circumflex artery supplies blood to the left atrium and the
lateral and posterior surfaces of the left ventricle.