Heart Actions Heart chambers function together in a

Heart Actions Heart chambers function together in a coordinated fashion

Cardiac Cycle • Atrial walls contract – While ventricles relax • Ventricular walls contract – While atria relax • This series of events: CARDIAC CYCLE • When a chamber is relaxed blood flows INTO it • Systole: Systole contraction • Diastole: Diastole relaxation

Cardiac cycle= Heart beat

Heart sounds and muscle fibers • Heartbeat: lub-dup • The opening and closing of the atrioventricular valves (A-V valves) • Functional syncytium: syncytium mass of merging cardiac cells that function as a unit – Atrial syncytium – Ventricular syncytium

Cardiac Conduction System • Parts of cardiac muscle do not contract but instead conduct impulses • These make up the cardiac conduction system which coordinates the cardiac cycle • Sinoatrial (S-A) node: node specialized muscle tissue in right atrium that reach threshold on its own—pacemaker

Cardiac Conduction System • S-A Node self excites and sends impulse • Atrial syncytium sends impulses on fibers to: • Atrioventricular (A-V) node which delay the impulse and prevent atria and ventricles from contracting at the same time • From A-V node impulse travels on a thick mass of nerve fibers (A-V bundle) that branch into the ventricles as Purkinje fibers • Purkinje fibers are attached to papillary muscles and signal the ventricular syncytium to contract

Electrocardiogram • A recording of the electrical changes in the myocardium during a cardiac cycle – P wave—depolarization of atrial fibers – QRS complex—depolarization of ventricular fibers – T wave—repolarization of ventricular fibers

Regulation of Cardiac Cycle • During certain times (e. g. exercise) parts of the body need more blood • S-A node controls heart rate • Parasympathetic nerve fibers from medulla oblongata (cardiac center) • Sympathetic nerve fibers secrete norepinephrine in response to nerve impulses • Aorta and carotid arteries have baroreceptors which sense “stretching” • Cerebrum and hypthalamus also send impulses to cardiac center • Important ions influencing heart action: K+, Ca 2+