The myocarduim consists of 2 cell types

contractile cells

conducting cells

Individual cells branch and join neighboring cells end-to end at junctions called intercalated disk.

Intercalated Disks

Consist of

Desmosomes are protein complexes that bind adjacent cells together allowing force generated in one cell to be transferred to the adjacent cell

Gap junctions are protein complexes that form pores between adjacent cells which electrically connect adjacent cells to one another as ions are able to pass freely between cells (electrical synapse )

THE CONDUCTING MYOCARDIUM

1% of the myocardium consists of autorhythmic or conducting myocytes which spontaneously generate action potentials, allowing the heart to beat without any outside signal (myogenic)

4 distinct groups of autorhythmic cells are in the heart

Sinoatrial (SA) node (the primary pacemaker)

Atrioventricular (AV) node

Bundle of His (and bundle branches)

Purkinjie fibers

In a resting adult the SA node initiates an AP approximately every 0.8 seconds (75 per minute) determining the frequency (sinus rhythm) of systole and diastole of the atria and ventricles resulting in a heart rate of 75 beats per minute (bpm)

The frequency of the APs can be altered by the antagonistic branches of the ANS to raise or lower the heart rate (HR) when appropriate

The Sympathetic NS increases HR from rest

When HR > 100 bpm = tachycardia

The Parasympathetic NS decreases HR from rest

When HR < 60 bpm = bradycardia

The cardiac cycle has 5 phases which are associated with the blood pressure and blood volume changes that occur within the ventricles during ventricular diastole and systole

Passive ventricular filling

Atrial systole

Isovolumetric contraction

Ventricular ejection

Isovolumetric relaxation

At the beginning of ventricular filling, the semilunar valves are closed, BOTH atria and ventricles are in diastole whereby blood from the great veins pass through the atria through opened AV valves passively filling the ventricles (accounts for 85% of ventricular filling)

During ventricular diastole, blood pushes against the top side of the semilunar valves forcing them downward into a closed position producing the second heart sound (dup) and against the top side of the AV valves forcing them downward into an opened position

The P wave of the ECG causes atrial systole whereby blood is ejected from the atria to finish the filling of the diastolic ventricles (accounts for 15% of ventricular filling)

The volume of blood in each ventricle at the end of the filling phase is called End Diastolic Volume (EDV) and is approximately 135 mL

As atrial systole comes to an end the QRS complex of the ECG causes ventricular systole, which begins with a short isovolumetric phase

The semilunar valves remain closed while ventricular pressure rises above atrial pressure causing the AV valves to close (lub)

The ventricle becomes a closed chamber with no blood entering or leaving the ventricle as contraction continues to further increase the pressure in the ventricles

During ventricular systole, blood pushes against the bottom side of the AV valves forcing them upward into a closed position producing the first heart sound (lub) and against the bottom side of the semilunar valves forcing them upward into an opened position

Flaps of the AV valves connect on the ventricular side to collagenous tendons called chordae tendineae

The opposite ends of the chordae tendineae are tethered to finger-like extensions of the ventricular myocardium called papillary muscles

These muscles provide stability for the chordae tendineae but cannot actively open or close the AV valves

During ventricular systole, the chordae tendineae prevent the valve from being pushed back into the atrium

If the chordae tendineae fail the valve is pushed back into the atrium during ventricular systole and is referred to a prolapse

Ventricular pressure continues to rise until it overcomes the pressure in the arteries opening the semilunar valves and ejecting blood into the arteries

Approximately 70 mL of the blood in the ventricle is ejected (stroke volume) which leaves 65 mL of blood remaining in the ventricles ( End Systolic Volume (ESV))

The left and the right ventricles pump the same volume of blood into the systemic and pulmonary circuits but at very different pressures (120 mmHg vs. 25 mmHg )

Because the blood that is ejected from the left ventricle has a further distance to travel (head to toes), the outer wall of the left ventricle is notably thicker (more myocardium) than the right which, when contracted, produces a higher blood pressure capable of moving blood a greater distance.

Ventricular contraction comes to an end, whereby ventricular pressure becomes less than the pressure in the great arteries causing a backflow of blood into the ventricles closing the semilunar valves (dup) –

semilunar valve closure causes a brief rise in the arterial pressure called the dicrotic notch as blood rebounds off the valve

Following the closure of the semilunar valves, the ventricles once again become closed chambers with no blood entering or leaving, as the AV valves remain closed.

As the ventricles continue to relax, the pressure continues to fall in until it becomes less than the pressure in the atria causing the AV valves to open which ends isovolumetric relaxation and begins passive ventricular filling

CO is the volume of blood pumped by a single ventricle in one minute and is a measure of the cardiac performance

Directly related to both the heart rate (HR) and stroke volume (SV)

HR is the number of heart beats per minute

normal resting HR = 75 beats/min

SV is the volume of blood ejected out by a ventricle each systole (beat) = EDV ─ ESV

normal resting SV = 70 ml/beat

HR x SV = CO

(75 beats/min) x (70 ml/beat) = 5250 ml/min = 5.25 L/min

The entire blood volume is completely circulated around the body every minute

During exercise CO can increase to 30 L/min

The CO can be altered to meet the needs of your body

Deliver O2, nutrients, hormone to the cells of the body as quickly as they are used

Remove CO2, urea, lactic acid from the cells of the body as quickly as they are produced

At certain times, the needs of your body change

Skeletal muscles during exercise use O2 and produce CO2 faster requiring an increase in the delivery rate of O2 and removal rate of CO2

During sleep, O2 is used and CO2 is produced more slowly requiring a decrease in the delivery rate of O2 and removal rate of CO2

CO can be changed by either changing HR or SV

If HR or SV increases, the CO increases, sending blood through the cardiovascular system faster

If HR or SV decreases, the CO decreases, sending blood through the cardiovascular system slower

Both HR and SV are controlled by the 2 antagonistic branches of the Autonomic Nervous System

Cardioacceleratory (sympathetic) center in the medulla oblongata can increase both the HR and SV

Cardioinhibitory (parasympathetic) center in the medulla oblongata can decrease the HR only

APs from the cardioacceleratory center propagate along the sympathetic cardiac nerve which synapse with the SA node

Sympathetic neurons exocytose norepinepherine (an adrenergic agent) onto the SA node

Norepinephrine binds to β-(beta) adrenergic receptors of SA nodal cells resulting in an increase in the frequency of APs in the SA node

APs from the cardioinhibitory center propagate along the Vagus nerve which synapses with the SA node

Releases the neurotransmitter acetylcholine (a cholinergic agent) onto the SA node

Acetylcholine binds muscarinic cholinergic receptors of SA nodal cells resulting in a decrease in the frequency of APs in the SA node

Ventricular contractility

The force produced by the working ventricular myocytes during systole

Controlled by hormones, neurotransmitters and other chemical substances (drugs)

The force applied to working ventricular myocytes before they contract

The amount of pressure in the ventricles at the end of ventricular filling

Aids ejection of blood out of the ventricles

The afterload on the ventricles

The force applied to working ventricular myocytes after they begin to contract

The amount of pressure in the arteries pushing on the closed semilunar valves

Opposes ejection of blood out of the ventricles