Heart Physiology

Heart Physiology

• Heart pumps 6000 quarts of blood per day• Nearly the entire blood volume is pumped

through once per minute

• Regulated by– Autonomic nervous system• Increases and decreases heart rate

– Intrinsic conduction system (nodal)• Built into heart, sets rhythm

Intrinsic Conduction System

• Sinoatrial node (SA node)– Right atrium– Starts heartbeat– Sets pace “pacemaker”

• Atrioventricular node (AV node)– Junction of atria and ventricles– Impulse from SA node causes atria to contract

Intrinsic Conduction System

• Pause in contraction• Then rapid sequential contraction– Bundle of His– Bundle branches• At septum

– Purkinje fibers• In ventricle walls• Begins at apex, travels to atria

Heart Beat

• Regular heartbeat 75 beats per minute– Tachycardia- increase (>100 beats/min)– Bradycardia- decrease (<60 beats/min)

• Cardiac cycle– ONE heartbeat– Takes about 0.8 seconds– In a healthy heart, atria contract at same time

Cardiac Cycle

• Atria contract @ same time• Atria relax as ventricle contracts

• Systole– Contraction of ventricle

• Diastole– Relaxation of ventricle

The ventricle DOES the PUMPING!

Cardiac Cycle Stages

1. Mid to Late Diastole– Heart relaxes– Low pressure– Blood flowing from atria to ventricles– Semilunar Valves CLOSED– AV valves OPEN

Atria contract, forces blood into ventricles

Cardiac Cycle Stages

2. Ventricular Systole– Ventricular contraction– Pressure increases to close AV valves– Semilunar valves open– Atria relax

Blood enters arteries

Cardiac Cycle Stages

3. Early Diastole– Ventricles relax– Semilunar valves close– Ventricles closed until pressure drops in atria– Forces AV valves to open

Cycle begins again

Heart Sounds

• 2 Distinct sounds during contraction– “lub, dup”– Lub – • AV valves closing• Deeper sound

– Dup – • Semilunar valves closing• Faster sound

Cardiac Output

• Volume of blood pumped by each side of the heart per minute

• Product of – Heart rate (beats per minute)– Stroke volume (blood pumped out of ventricle

with each beat (mL per minute)

CO = HR X SV

Stroke Volume

• Starling’s Law

– Critical factor in how much blood is pumped through per minute is due to how much the cardiac muscle is stretched before contraction

– Stretching results from venous return

Heart Rate

• Relatively constant• Increases with stroke volume increases• Neural control– Sympathetic nervous system• Controls SA, AV valves• Increases heart rate

– Parasympathetic nervous system• Slows heart rate during periods of rest

• Hormonal control– Epinephrine, thyroxine both increase heart rate

Heart Rate

• Affected by physical factors– Age, sex, exercise, temperature

– Fetal heart rate 140-160 beats per minute• Decrease throughout life• Female 72-80 • Male 64-72

– Heat increases rate, cold decreases– Exercise (nervous control)