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 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 beats per minute Decrease throughout life Female Male – Heat increases rate, cold decreases – Exercise (nervous control)