CARDIOVASCULAR SYSTEM PHYSIOLOGY

HEART ACTIONS A cardiac cycle is a complete heartbeat During a cardiac cycle, the pressure in the heart chambers rises and falls These pressure changes open and close the valves The first part of a heart sound (lub) occurs when the AV valves are closing The second part of a heart sound (dub) occurs when the semilunar valves are closing

CARDIAC CONDUCTION SYSTEM The heart is autorhythmic, able to contract itself without nervous or hormonal stimulation Components include: SA node, AV node, and specialized cardiac muscle tissue and cells These components coordinate the events of the cardiac cycle (heartbeat) SA node – located in the right atrium near the opening of the superior vena cava; initiates one impulse after another (pacemaker) The impulse spreads to other areas of the myocardium due to cardiac muscle cells and gap junctions AV node – located in the inferior part of the right atrium The impulse passes from the AV node to the AV bundle (group of fibers) and the Purkinje fibers carry the impulse to distant regions of the myocardium

ELECTROCARDIOGRAM Recording of the electrical changes in the myocardium during a cardiac cycle Electrodes are placed on the skin and connected to wires that respond to weak electrical changes When the SA node triggers a cardiac impulse, the atrial fibers depolarize, producing an electrical change = P wave When the cardiac impulse reaches the ventricular fibers, they rapidly depolarize producing an electrical change = QRS complex The electrical changes that accompany ventricular muscle fiber repolarization slowly produce a T wave, ending the ECG pattern Physicians use ECG patterns to assess the hearts ability to conduct impulses

BLOOD PRESSURE The force the blood exerts against the inner walls of the blood vessels The maximum pressure achieved during ventricular contraction is called the systolic pressure When the ventricles relax, the arterial pressure drops, and the lowest pressure that remains in the arteries before the next ventricular contraction is called the diastolic pressure A sphygmomanometer measures blood pressure The cuff is wrapped around the arm so that is surrounds the brachial artery; air is pumped into the cuff until the cuff pressure exceeds the pressure in that artery squeezing the vessel closed and stopping its blood flow; as air is slowly released from the cuff, the air pressure inside it decreases; when the cuff pressure is just slightly lower than the systolic blood pressure in the brachial artery, the artery opens for a small volume of blood to spurt through, producing a sharp sound heard through the stethoscope (systolic pressure); as the cuff pressure continues to drop, louder sounds are heard; when the cuff pressure is just slightly lower than that within the fully opened artery, the sounds disappear (diastolic pressure) Normal blood pressure is 120/80

CARDIAC OUTPUT For the body to function properly, the heart needs to pump blood at a sufficient rate to maintain an adequate and continuous supply of oxygen and other nutrients to the brain and other vital organs Cardiac output = describes the amount of blood your heart pumps each minute A healthy heart pumps about 5-6 liters of blood every minute when a person is resting

CARDIAC OUTPUT Cardiac Output: It is determined by: 1. The amount of blood pumped by the left ventricle during each beat (stroke volume) 2. The number of heart beats per minute The amount of blood ejected by a ventricle during each contraction (single beat) is called stroke volume Cardiac output is an indication of the blood flow through peripheral tissues; without adequate blood flow, homeostasis cannot be maintained; helps keep blood pressure at the levels needed to supply oxygen-rich blood to your organs

CARDIAC OUTPUT For an adult at rest… Stroke volume = 80 mL (average) Heart rate = 75 beats per minute (average) Therefore, Cardiac Output (CO) = Stroke Volume x Beats per Minute CO = 80 mL/beat x 75bpm CO = 6000mL/min (6L/min)