CH18 Cardiac Output

Exam 1 Review: Chapter 18: Cardiac Output

cardiac output - The amount of blood pumped out by the ventricles per minute, or simply the product of heart rate (HR) and stroke volume (SV); the stroke volume at rest in the standing position averages between 60 and 80 ml of blood in most adults, thus at a resting heart rate of 80 beats per minute the resting cardiac output will vary between 4.8 and 6.4 L per min.

heart rate (HR) - The number of cardiac cycles (heartbeats) per unit of time, usually expressed as beats per minute (bpm); a normal resting HR is 72-75 beats per minute (bpm).

cardiac reserve - The difference between resting and maximal cardiac output; non-athletes have a cardiac reserve about 4 times their normal cardiac output while trained athletes and our astronaut corps have cardiac reserves up to 7 times their normal cardiac output.

preload - The tension in the myocardium, usually considered for the ventricles when they are filled, at the end of diastole, before contraction begins; the amount of preload is influenced by venous return.

contractility - A muscle's ability to move by shortening in response to a stimulus by means of the ATP powered sliding of myosin and actin.

afterload - The tension in the myocardium, usually considered for the ventricles when they have emptied, at the end of systole, after contraction; the amount of afterload is influenced by the degree of arterial (pulmonary trunk or aorta) pressure.

Frank-Starling law of the heart - The fundamental principle of cardiac performance which states that the force of contraction of the cardiac muscle is proportional to its initial length. (The energy set free at each contraction is a simple function of cardiac filling (preload). When the diastolic filling of the heart is increased or decreased with a given volume, the displacement of the heart increases or decreases with this volume.)

Atrial (Bainbridge) Reflex - A sympathetic reflex of the heart; when venous return is increased, the atrial walls are stretched, that stretch is reported to the CNS which initiates sympathetic impulses which stimulate the sino-atrial node with norepinephrine, causing an increase in both heart rate and contractile force.

positive inotropic effect - The result of any stimulus to the heart which increases heart rate and contractile force, and, therefore, cardiac output, such effects are independent of the length-tension relationship influencing the myocardium.

positive inotropic agent - Any drug whose action on the heart increases heart rate and contractile force, and, therefore, cardiac output.

negative inotropic effect - The result of any stimulus to the heart which decreases heart rate and contractile force, and, therefore, cardiac output, such effects are independent of the length-tension relationship influencing the myocardium.

negative inotropic agent - Any drug whose action on the heart decreases heart rate and contractile force, and, therefore, cardiac output.

Explain:

1. the Frank-Starling Law of the Heart.

The Frank-Starling Law of the Heart is the fundamental principle of cardiac performance which states that the force of contraction of the cardiac muscle is proportional to its initial length. (The energy set free at each contraction is a simple function of cardiac filling (preload). When the diastolic filling of the heart is increased or decreased with a given volume, the displacement of the heart increases or decreases with this volume.)

The Frank-Starling Law of the Heart refers to the length-tension relationship of cardiac muscle where length (stretch) is determined by EDV (end diastolic volume) and tension = contractile force at a given length will determine SV (stroke volume). As the ventricles become overfilled (to the right on the curve beyond EDV = ~250 mL), the heart becomes inefficient and stroke volume levels off and eventually declines.

2. the mathematical formula describing cardiac output as a function of stroke volume and heart rate.

CO = HR x SV Cardiac Output = Heart Rate x Stroke Volume

Cardiac Output is the amount of blood pumped by each ventricle during one minute. It is a function of heart rate and stroke volume and is directly proporitonal to the product of those two variables. An increase in either heart rate or stroke volume or both will increase cardiac output; a decrease in either heart rate or stroke volume or both will decrease cardiac output.

3. the direct neural and chemical regulation of cardiac output.

Neural The Autonomic Nervous System regulates cardiac output (CO). Sympathetic stimulation with norepinephrine increases CO while Parasympathetic stimulation with acetyl choline decreases CO.

Chemical Epinephrine and norepinephrine from the adrenal medulla increase CO; the thyroid hormones, T₃& T₄ increase CO. Extracellular fluid levels of potassium (K⁺) and calcium (Ca⁺⁺) also influence cardiac output (CO).

Neural	The Autonomic Nervous System regulates cardiac output (CO). Sympathetic stimulation with norepinephrine increases CO while Parasympathetic stimulation with acetyl choline decreases CO.
Chemical	Epinephrine and norepinephrine from the adrenal medulla increase CO; the thyroid hormones, T₃& T₄ increase CO. Extracellular fluid levels of potassium (K⁺) and calcium (Ca⁺⁺) also influence cardiac output (CO).