Heart Location and Structure

Heart Location

• Located in the thoracic cavity as a hollow, muscular, cone-shaped organ
• The general location is in the mediastinum
• Found within the pericardial cavity and between the pleural cavities
• Enclosed by the pericardium, which anchors and protects it
• The esophagus and trachea are posterior to it

Heart's Internal Structure

• The heart has four chambers or compartments
• Two superior chambers, termed atria
• Two inferior chambers, termed ventricles
• The interatrial and interventricular septum prevent blood mixing between chambers
• Ventricular walls are thicker than atrial walls because they pump blood into the systemic and pulmonary circulations

Right Atrium

• Receives deoxygenated blood from the body through three vessels
• The three vessels are the superior vena cava, inferior vena cava, and coronary sinus
• From this chamber, blood flows to the right ventricle

Right Ventricle

• The right ventricle pumps deoxygenated blood into the pulmonary trunk

Left Atrium

• The left atrium receives oxygenated blood via the pulmonary veins

Left Ventricle

• The left ventricle pumps oxygenated blood into the aorta
• The left ventricle walls are thicker than the right requiring more force to send blood throughout the systemic circulation

Systemic Circulation

• Supplies the body's tissues and organs with oxygenated blood
• Newly deoxygenated blood returns to the right atrium via the superior vena cava, inferior vena cava, and coronary sinus

Heart at Microscopic Level

• There are two functional units (syncytia)
• These are the atrial syncytium and ventricular syncytium
• The atria then ventricles contract, and then the heart relaxes

Heart Cells

• Two different types of cells exist in the heart wall
• These are contracting cells and cells that generate an electrical signal
• Cells generating electrical signals form the cardiac conduction system in each heartbeat

Cardiac Conduction System

• Starts at the sinoatrial node (SA node)
• The SA node is part of the cardiac conduction system that generates an electrical signal most rapidly
• The SA node spreads signals over the entire atrial syncytium, causing atrial contraction
• Signals then spread to the atrioventricular node (AV node)
• From the AV node signals pass through the atrioventricular bundle (AV bundle)
• Signals arrive in the interventricular septum then pass through two bundle branches (right and left)
• At the apex, fibers branch extensively, forming Purkinje fibers

Electrocardiograms (ECGs)

• ECGs assess the cardiac conduction system
• Determines whether the electrical activity of the heart is working properly
• Electrodes are placed on the body in 2 upper limb leads, 2 lower limb leads, and 6 precordial leads
• Enables assessment of the heart from 12 different angles to pinpoint abnormality locations

Electrical Current & Waves

• The current arising from the SA node, called depolarization, is a positive current
• Positive current travels through the atrial walls
• A negative current then restores the electrical potential of the atrium to normal after the positive current passes, called repolarization
• The first wave is the P wave
• The second group of waves is the QRS complex consisting of the Q, R, and S waves
• The third wave is the T wave
• Isoelectric lines occur when there is no change occurring in the electrical state of the heart
• This happens between the P wave and QRS complex and between the QRS complex and T wave

Heart Valves

• Heart valves ensure one-way blood flow
• They are composed of dense, fibrous connective tissue which is covered in endocardium
• There are 4 valves organized as two pairs: atrioventricular (AV) and semilunar (SL)

Atrioventricular Valves (AV)

• The tricuspid and mitral valve are AV valves

Valve Function

• AV valves close when ventricles contract and the pressure in the ventricles exceeds pressure in the atria
• Chordae tendinae and papillary muscles contract along with ventricles, creating tension that prevents the free edges of the valves from swinging upward into the atria
• AV valves open after ventricular relaxation, when atrial pressure exceeds ventricular pressure

Semilunar Valves (SL)

• Blood passes from the right ventricle to the pulmonary trunk and from the left ventricle into the aorta through these valves
• The pulmonic and aortic valves are semilunar valves valves
• When closed, the cusps fall into the center of the pulmonary trunk and aorta to prevent backflow of blood from the vessel into the ventricle
• During ventricular contraction pressure increases, and when ventricular pressure exceeds pressure in the aorta and pulmonary trunk, the semilunar valves open
• During ventricular relaxation, pressure drops, and when ventricular pressure falls below the pressure in the aorta and pulmonary trunk, the semilunar valves close

Cardiac Cycle

• When the heart is relaxed, the semilunar valves are closed and the AV valves are open
• Blood returns to the right atrium through the superior and inferior vena cava and coronary sinus
• On the left side, blood returns to the heart from the pulmonary veins from the lungs
• During atrial contraction, the pressure in atria increases
• Atria relax during ventricular contraction when The pressure exceeds the pressure in the atria
• As ventricles contract ventricular pressure climbs, exceeding pressure in the aorta and pulmonary trunk
• Finally ventricles stop contracting and begin to relax
• When valves close, vibrations occur in the blood passing through the heart, producing heart sounds (sound one) that are carried to the body’s surface and can be heard with a stethoscope

Heart Sounds

• S1 (sound one) and S2 (sound two) are the heart sounds
• S2 (sound two) indicates start of ventricular diastole

Heart Position

• The heart is positioned deep to the sternum, slightly to the left of the midline in the chest cavity
• The point of maximal impulse is the most accurate location to check heart rate
• Palpating assesses the number of beats per minute
• Auscultation allows assessment of heart rate as well as heart sounds

Auscultation & Heart Rate

• Physicians often auscultate in multiple locations to assess heart sounds related to the aortic, pulmonic, tricuspid, and mitral valves
• A valve that does not close all the way makes a swishing sound
• A valve that does not open all the way makes a clicking sound

Electrical Changes during the Cardiac Cycle

• The electrical changes start at the SA node
• Excitation of the atria creates the P-wave on an ECG
• A signal is sent to the AV node, delayed for 1/10th of a second
• This then passes to the AV bundle which is the only electrical connection between the atrial and ventricular syncytium
• Excitation of the ventricles creates the QRS complex on an ECG
• The heart relaxes and ventricles repolarize, creating the T wave on the ECG

Pressure Changes during the Cardiac Cycle

• Pressure changes in the heart prevent backflow of blood, causing the valves to open and close during the cardiac cycle
• The P wave on an ECG is followed closely by increased atrial pressure during atrial contraction (atrial systole)
• After atrial systole, the atria contract, and pressure remains low
• The QRS complex on an ECG is followed almost immediately by increased ventricular pressure
• The period of ventricular contraction is called ventricular systole
• When ventricular pressure exceeds aortic pressure, the semilunar valves open
• Once this pressure peaks, the ventricles stop contracting, and pressure in ventricles falls below pressure in the aorta and pulmonary trunk until ventricular pressure continues to fall

Ventricular Volume

• Ventricular volume is fairly high during relaxation, or diastole
• During atrial systole a little more blood is pushed into the ventricles, slightly increasing volume
• Ventricles contract, pressure increases and volume decreases
• Blood is ejected to the aorta and pulmonary trunk
• AV valves open and ventricular volume begins to increase again

Assessing Pulse Demo & Heart Rate

• Pulse and heart rate are typically the same in a person with healthy cardiovascular function
• Pulse and heart rate may differ if someone has poor peripheral circulation or arterial disease
• Pulse is assessed in elastic arteries, which can distend and retract

Locations to Assess Pulse

• Carotid artery
• Brachial artery (at the antecubital fossa)
• Radial artery
• Femoral artery
• Dorsalis pedis
• Posterior tibial

Cardiac Output

• Cardiac output is the volume of blood ejected per minute
• CO is vital to maintain blood flow circulating through the body to supply O2 and nutrients to cells and carry away metabolic waste
• Cardiac output is influenced by heart rate (HR) and stroke volume (SV)
• Cardiac output can be calculated as HR x SV
• Stroke volume is the volume of blood ejected during a single heartbeat
• Stroke volume is calculated as End diastolic volume (EDV) - End systolic volume (ESV)
• Cardiac output can be increased or decreased to meet the needs of the body

Autonomic Nervous System

• Plays a role in regulating cardiac output through two branches: parasympathetic and sympathetic

Sympathetic Division

• In the heart, the sympathetic nervous system innervates the SA node, AV node, and contractile cells of the myocardium
• When activated, causes the SA node to depolarize more quickly and can shorten the delay at the AV node
• Tachycardia is defined as HR > 100 bpm
• Sympathetic activation also causes contractile cells of myocardium to contract more forcefully, leading to increased stroke volume

Parasympathetic Division

• In the heart, the parasympathetic nervous system innervates the SA node and the AV node
• Parasympathetic nervous system slows the SA node’s rate of self-excitation
• Parasympathetic signals are carried to the SA node by the vagus nerve
• Bradycardia is defined as HR < 60 bpm