Adequate cardiac output is necessary in order to supply oxygen and other nutrients to all body tissues. Cardiac output is influenced by stroke volume and heart rate. Factors affecting the stroke volume include PRELOAD, CONTRACTILITY, and AFTERLOAD.
In the following conditions, indicate what factor/s influence the cardiac output: PRELOAD, CONTRACTILITY, AFTERLOAD, HEART RATE. Explain why the factor/s affects cardiac output in 1 to 2 sentences.
a. High blood pressure
Hypertension or high blood pressure, the most common disorder affecting the heart and blood vessels, is when the force of blood pushing against the walls of the blood vessels is consistently high. This condition increases the afterload, which forces the ventricles to work harder to eject blood and causes the cardiac output to decrease.
b. Massive bleeding
Massive bleeding depresses cardiac output due to decreased blood volume, decreased venous return and filling of the heart, and decreased stroke volume.
c. The drug dopamine
Several synthetic drugs, like dopamine, have been developed that mimic the effects of epinephrine and norepinephrine by stimulating the influx of calcium ions from the extracellular fluid. The high concentration of calcium increases the strength of contraction or contractility and heart rate, thus causing the cardiac output to increase.
d. Running a sprint
Exercises such as running a sprint elevate cardiac output and accelerate metabolic rate due to the increased oxygen demand. The contraction of skeletal muscles helps return blood to the heart more rapidly, which increases cardiac output by increasing preload.
e. Hyperthyroidism with increased secretion of thyroid hormones
Hyperthyroidism with increased secretion of thyroid hormones stimulates the sinoatrial (SA) node and causes premature electrical activity in the heart, resulting in enhanced cardiac contractility and increased heart rate thus causing increased stroke volume and cardiac output.
f. Massive myocardial infarction (cardiac tissue death)
Myocardial infarction or heart attack is where the cardiac tissue begins to die due to complete obstruction to blood flow. It causes reduced cardiac output due to impaired contractility and excessive preload or afterload.
g. Hypothermia
Hypothermia or lowered body temperature (35℃ or below) slows down the cellular metabolism, so the need for oxygen is reduced. Because of this, there is a decreased cardiac output due to the decreased heart rate and contractility.
h. Emotional distress
In response to emotional distress, the adrenal medullae release epinephrine and norepinephrine, which enhances the heart’s pumping effectiveness. These hormones, similar to how the norepinephrine released by cardiac accelerator nerves affect cardiac muscle fibers, increase both the heart rate and contractility, resulting in an increased stroke volume and cardiac output.
Reference
G. J. & Derrickson, B. (2014). Principles of Anatomy and Physiology (14th Edition). New York: John Wiley & Sons, Inc.