Chapter 18- Cardiac Physiology
Cardiac muscle contraction
- The sinoatrial node or SA node is the physiological pacemaker of the heart.
o It depolarizes faster than any other part of the autorhythmic system.
o 90-95 action potential (= heart beats) per minute
- The atrioventricular or AV node depolarizes slower than the SA node (40-60
potentials per minute)
- The atrioventricular bundle or bundle of His is the only electrical connection
between atria and ventricles.
- It splits into right ad left bundle branches that run inside the interventricular
septum toward the apex of the heart.
- Purkinje fibers spread all over the myocardium of both ventricles (30-40
- Defects in the cardiac conduction system may be cause of an irregular
heartbeat (arrhythmia) and even lead to fibrillations
o Abnormal pacemaker called ectopic focus takes over.
▪ AV nodes take over and develop 40-60 bpm.
o Partial or total heart block may develop and few or no impulses from
the SA node reach the ventricles.
- There are two cardiac center located in the medulla oblongata.
o Cardioacceleratory centers sends signals via sympathetic fibers to SA
and AV node, cardiac muscle cells, and coronary arteries.
▪ Increase heart rate, increase contractility, and increase the
blood flow to the heart.
o Cardioinhibitory center only inhibits SA and AV nodes through
parasympathetic fibers in the vagus nerve
Electrocardiography - Electrocardiogram (ECG, EKG) can measure the flow of ions across the
sarcolemma during repolarization and depolarization and displaying in form
of static and dynamic recordings.
o P Wave is caused by depolarization of SA node
o QRS complex is caused by ventricular depolarization
o T wave is caused by ventricular repolarization
- AV valves up and the valves close
o First audible sound of the cardiac cycle is called the first heart sound
- Course of the semilunar valves
o Second heart sound occurs later in the heart cycle.
- Both heart sounds are not closing sounds of the valves, but are caused by
turbulent blood flow.
- Abnormal sounds are called heart murmurs.
o Cause: malfunction of a heart valve
o If a valve doesn’t close completely the condition is called the
insufficiency or regurgitation
- Eight basic valve defects with impairment of blood flow.
o Tricuspid stenosis: obstructs the flow from right atrium and right
o Tricuspid regurgitation: blood flows back from right ventricle to right
o Pulmonary stenosis: obstructs flow into the pulmonary trunk
o Pulmonary regurgitation: blood flows back into the right ventricle
o Mitral stenosis: obstructs the flow from the left atrium and left
ventricle o Mitral regurgitation: blood flow back from the left ventricle and left
o Aortic stenosis: blood flow back into the aorta
o Aortic regur