Anatomy of the Heart

The pulmonary circuit involves blood flow from the heart to the lungs for oxygenation and back to the heart

The systemic circuit involves blood flow from the heart to the rest of the body and back to the heart

The heart is located in the mediastinum, between the lungs

It is about the size of a fist

The heart has a conical shape

The pericardium is a double-layered sac that encloses the heart

It protects the heart

It anchors the heart

It prevents overfilling of the heart with blood

The heart wall has three layers: the epicardium, myocardium, and endocardium

The epicardium is the outer layer

The myocardium is the middle layer composed of cardiac muscle

The endocardium is the inner layer

The heart has four chambers: two atria and two ventricles

Surface features of the heart include the atria, ventricles, coronary sulcus, and interventricular sulci

The heart has four valves: the tricuspid, mitral (bicuspid), pulmonary, and aortic valves

These valves ensure unidirectional blood flow

Blood flow through the heart:

• Blood enters the right atrium through the superior and inferior vena cavae
• Blood passes through the tricuspid valve to the right ventricle
• Blood passes through the pulmonary valve to the pulmonary trunk and arteries
• In the lungs, blood releases carbon dioxide and picks up oxygen
• Oxygenated blood returns to the heart via the pulmonary veins into the left atrium
• Blood passes through the mitral valve to the left ventricle
• Blood passes through the aortic valve to the aorta, which distributes blood to the body

The myocardium is nourished by the right and left coronary arteries, which branch off of the aorta

It is drained by cardiac veins that empty into the coronary sinus, which then empties into the right atrium

Cardiac muscle cells are short, branched, and interconnected

They contain one or two nuclei

They are striated

They contain many mitochondria

Intercalated discs join cardiac muscle cells

They contain gap junctions that allow electrical signals to pass quickly from cell to cell

The sinoatrial (SA) node is the heart's pacemaker

It initiates the electrical impulses that cause the heart to beat

The atrioventricular (AV) node delays the impulse briefly before it spreads to the ventricles

The atrioventricular (AV) bundle conducts the impulses to the bundle branches

The Purkinje fibers distribute the impulses throughout the ventricles

The SA node fires spontaneously because its cells have unstable resting membrane potentials

This is due to a slow influx of sodium ions

The SA node excites the myocardium by generating an action potential that spreads through the atria

This causes the atria to contract

An electrocardiogram (ECG) is a recording of the electrical activity of the heart

A normal ECG has a P wave (atrial depolarization), a QRS complex (ventricular depolarization), and a T wave (ventricular repolarization)

Blood pressure changes control the opening and closing of the heart valves

Valves open when pressure is greater behind the valve and close when pressure is greater in front of the valve

Heart sounds are caused by the closing of the heart valves

The first sound (S1 or "lub") is caused by the closing of the AV valves

The second sound (S2 or "dub") is caused by the closing of the semilunar valves

Cardiac cycle:

• Ventricular filling: AV valves are open, blood flows into ventricles
• Atrial contraction: Atria contract, pushing more blood into ventricles
• Isovolumetric contraction: Ventricles contract, AV valves close ("lub")
• Ventricular ejection: Semilunar valves open, blood is ejected into aorta and pulmonary trunk
• Isovolumetric relaxation: Ventricles relax, semilunar valves close ("dub")

During the cardiac cycle, blood volume increases in the ventricles during filling

Blood volume decreases during ejection

Sympathetic nerves increase heart rate and contractility

Parasympathetic nerves decrease heart rate

Cardiac output is the amount of blood pumped by each ventricle in one minute

It is calculated by multiplying heart rate by stroke volume

Cardiac output is important because it determines how much oxygen and nutrients are delivered to the body

Cardiac output is governed by heart rate and stroke volume

Nervous and chemical factors that alter heart rate:

• Sympathetic stimulation increases heart rate
• Parasympathetic stimulation decreases heart rate
• Epinephrine increases heart rate

Nervous and chemical factors that alter stroke volume:

• Increased preload increases stroke volume
• Increased contractility increases stroke volume
• Increased afterload decreases stroke volume

The right and left ventricles pump equal volumes of blood

This is achieved through adjustments in contractility and stroke volume

Exercise increases cardiac output by increasing heart rate and stroke volume

During exercise, cardiac reserve is utilized