Physiology of the Heart

Heart Works Hard
The heart pushes the body’s supply of approx. 6000 quarts of blood through blood vessels daily.

Setting the Basic Rhythm
Cardiac muscle cells contract spontaneously and independently, and occur in a regular and continuous way. Cardiac muscle can beat independently. Muscle cells in different areas of heart have different rhythms. Atrial cells contract 60 times per minute, while ventricular cells contract 20-40 times per minute. The heart needs a unifying control system or else the heart would be uncoordinated and inefficient.


Regulating heart activity
The intrinsic conduction system (nodal system) is built into the heart tissue and sets basic rhythm. It is composed of tissue that is a cross between muscle and nervous tissue. It causes heart muscle depolarization from the atria to the ventricles. The intrinsic conduction system enforces contraction of 75 beats per minute.


The intrinsic conduction system contains the sinoatrial node (SA), which is located in the right atrium. It starts each heartbeat and pace for the whole heart, which is why it is often called the ‘pacemaker.’ It also contains the atrioventricular (AV) node, located in the junction of the atria and ventricles. The atrioventricular (AV) bundles and bundle branches are located in the interventricular septum. Purkinje fibers spread within the muscle of the ventricular walls.


Cardiac Output
Cardiac output (CO) is the amount of blood pumped out by each side of the heart in one minute. It is the product of heart rate (HR) and stroke volume (SV)
The average adult cardiac output is HR (75 bpm) x SV (70 ml/beat), which amounts to 5250 ml/min. The entire blood supply passes through the body once each minute. CO varies with demands of the body, and increases when SV increases, the hearts beat faster, or both. It decreases when either or both decreases.


Regulation of Stroke Volume
The more the heart muscle is stretched, the stronger the contraction. Venous return is the amount of blood entering the heart and defending its ventricles. It prevents the back up of blood circulation by forcing the ventricle that pumps less blood to pump out an equal amount. When the volume or speed of the blood flow increases, the stroke volume and face increases as well.

Neural (ANS) Controls
The sympathetic nervous system stimulates the SA and AV nodes, as well as the cardiac muscle itself during physical or emotional stress. The heart beats more rapidly. The parasympathetic nervous system slows and steadies the heart during non crisis times.


Effects of Hormones and Ions on Heart Activity
Epinephrine mimics the effect of sympathetic nerves, and epinephrine and thyroxine both increase heart rate. Reduced levels of ionic calcium
depress the heart if found in the blood. It causes prolonged contractions that the heart may stop entirely. The excess or lack of potassium and/or sodium modifies heart activity, which may include deficit of potassium ions in the blood. When this occurs, the heart beats feebly and abnormal rhythms appear.