Stroke Volume/Heart Rate/Cardiac Output
3. Afterload
Afterload is a measure of the pressure that must be generated by the ventricles to force the semilunar valves open. The greater the afterload, the smaller the stroke volume. Arteriosclerosis (narrowing of the arteries) and high blood pressure increase afterload and reduce stroke volume.
Heart rate is regulated by the following three factors:
Autonomic Nervous System Chemicals Age, gender, physical fitness
Heart rate (HR)
number of heartbeats per minute.
Perfusion
the constant flow of oxygenated blood to tissues. Good perfusion requires: • A well-functioning heart • An adequate volume of blood • Appropriately constricted blood vessels If perfusion fails, cellular and eventually patient death occur.
Parasympathetic nervous system —
vagus nerve stimulation reduces heart rate and AV conduction through release of acetylcholine.
Stroke volume (SV)
volume of blood ejected by each ventricle during a single contraction. Standard Stroke Volume = 70 mL
Cardiac output (CO)
volume of blood pumped out of the right or left ventricle per minute. CO = SV × HR.
2. Contractility
Contractility is the degree to which cardiac muscle cells contract as a result of extrinsic influences. Positive inotropic factors, such as certain hormones (epinephrine or thyroxin), drugs (digitalis), or elevated levels of Ca 2+, increase contractility, while negative inotropic factors, such as certain drugs (calcium channel blockers) or elevated levels of K +, decrease contractility.
Stroke volume is regulated by the following three factors:
Preload Contractility Afterload
1 Preload
Preload is the degree to which cardiac muscle cells are stretched by the blood entering the heart chambers. According to the Frank‐Starling law of the heart, the more the chamber is stretched, the greater the force of its contraction. Because the end‐diastolic volume (EDV) is a measure of how much blood enters the ventricles, the EDV is an indicator of ventricle preload.
Cardiac Output
https://www.youtube.com/watch?v=4SYOMCSI2ZE
Sympathetic nervous system —
increases heart rate, automaticity, AV conduction, and contractility through release of norepinephrine and epinephrine
