17.5 Cardiac Output and Regulation

What 2 factors influence the EDV?

1. length of time the ventricle spends in diastole 2. amt. of blood returning to the right atrium from the systemic circuit (venous return)

E. Regulation of cardiac output

Although the heart is auto rhythmic, it still requires regulation to ensure that cardiac output meets the body's needs. Cardiac output is regulated primarily by the nervous and endocrine systems, which influence both heart rate and stroke volume.

C. Factors that influence stroke volume:

Although the stroke volume averages about 70ml per beat, it may range from 50 to 120ml. The exact stroke volume may be difficult to measure directly, and often a measurement called the ejection fraction is used in its place.

1. Calculate SV by subtracting the amount of blood in the ventricle at the end of a contraction (ESV) from the amount of blood in the ventricle after it has filled during diastole(EDV)

In an average heart, the resting stroke volume is equal to about 70 mL 120ml (EDV)-50ml(ESV)=70mL(SV)

2. Cardiac innervation and regulation by the parasympathetic nervous system:

PNS exerts essentially the opposite effects on the heart. It innervates the heart by the left and right vagus nerves CNX. The nerves of the parasympathetic division of the ANS release the neurotransmitter acetylcholine which affects the SA node, decreasing its rate of action potential generation.

2. To find the cardiac output, multiply the heart rate by the stroke volume:

a. 72 beats/min (HR) x 70ml/beat(SV) = 5040 ml/min, or 5 liters/min (CO)

4. Other factors that influence cardiac output:

electrolyte concentration in the extracellular fluid, body temp, age, physical fitness, exercise

3. Cardiac regulation by the endocrine system

hormonal regulation of cardiac output occurs in various forms, such as thyroid hormone, glucagon, aldosterone, antidiuretic hormone, and atrial natriuretic peptide

ejection fraction

percentage of blood out of the total amount that is ejected with each ventricular systole, and is equal to the stroke volume divided by the EDV.

3 factors influence the stroke volume

preload, contractility, afterload

Heart failure

reduces the hearts ability to function effectively as a pump and may be left ventricular or right ventricular failure. Treatment is aimed at increasing cardiac output via lifestyle modifications, medications, and surgery if needed.

1. Preload: Degree of cardiac muscle cell stretch

refers to the length or degree of stretch of the sarcomeres in the ventricular cells before they contract. The degree of preload is largely determined by the EDV, or the amount of blood that has drained into the ventricle by the end of the filling phase.

B. Determination of Cardiac Output: What must be known to calculate the cardiac output for a ventricle?

stroke volume and heart rate

1. Cardiac innervation and regulation by the autonomic nervous system:

the 2 branches of the autonomic nervous system regulate our autonomic functions. The sympathetic division of the ANS innervates the heart via a set of sympathetic nerves that release the neurotransmitter norepinephrine which increases cardiac output with both positive chronotropic and inotropic effects.

3. Afterload: amount of back pressure

the force that the right and left ventricles must overcome in order to eject blood into their respective arteries. largely determined by the blood pressure in the arteries of the pulmonary and systemic circuits.

2. Contractility

the heart's intrinsic pumping ability, or ability to generate tension. Increasing contractility will increase the stroke volume and therefore decrease the ESV.

D. Factors that influence heart rate

the other determinant of cardiac output is the heart rate. Under normal conditions, the rate at which the SA node generates action potentials determines the heart rate. Factors that influence the rate at which the SA node depolarizes are known as chronotropic agents.