ACTIVITY 1
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 includes PRELOAD, CONTRACTILITY and AFTERLOAD.
In the following conditions, indicate what factor/s influence the cardiac output: PRELOAD, CONTRACTILITY, AFTERLOAD, HEART RATE. Explain the why the factor/s affects cardiac output in 1 to 2 sentences.
a) High blood pressure
High blood pressure, often known as hypertension, thickens the tunica media in blood arteries, speeds up the onset of atherosclerosis and coronary artery disease, and raises systemic vascular resistance. Increased afterload in the heart from hypertension makes the ventricles work harder to expel blood, lowering the stroke volume and ultimately, the cardiac output.
b) Massive bleeding
Massive bleeding produces a drop in central venous pressure, cardiac filling, and arterial pressure. This lowers preload since there is less blood, which in turn lowers stroke volume and cardiac output; but in compensation, heart rate and contractility is also increased.
c) The drug dopamine
Dopamine has both chronotropic and inotropic effects on the myocardium, causing an increase in heart rate and contractility, eventually leading to an increase in cardiac output.
d) Running a sprint
The heart can stretch (preload), pump more forcefully (contractility), or increase the amount of blood that fills the left ventricle before it pumps to increase stroke volume when running a sprint or while exercising. Consequently, cardiac output also rises.
e) Hyperthyroidism with increased secretion of thyroid hormones
Heart rate and contractility both increase as blood levels of thyroid hormones rise. Due to the increased contractility, the stroke volume is also elevated, and along with increased heart rate, raises cardiac output.
f) Massive myocardial infarction (cardiac tissue death)
The interruption of blood supply causes cardiac tissues and muscles to die during a major myocardial infarction or heart attack. Due to the muscles' inability to pump efficiently, there will be less contractility and more afterload, which also results in a decrease in stroke volume and cardiac output.
g) Hypothermia
There is a reflex adrenergic vasoconstriction (noradrenalin) of the skin during a decrease in core body temperature, often known as hypothermia. Due to increasing bradycardia, heart rate decreases and cardiac output drops below a core temperature of 34 degrees Celsius.
h) Emotional distress
The adrenal medullae release more hormones in response to emotional distress, enhancing sympathetic stimulation. Increased contractility and heart rate brought on by this condition also results in an increase in stroke volume and cardiac output.
References:
Deussen, A. (2007). Hyperthermie und Hypothermie. Der Anaesthesist, 56(9), 907–911. https://doi.org/10.1007/s00101-007-1219-4
DRUGBANK Online. (2018). Dopamine: Uses, Interactions, Mechanism of Action | DrugBank Online. Drugbank.com; DrugBank. https://go.drugbank.com/drugs/DB00988
Healthwise. (2022, January 10). Cardiac Output. Alberta.ca. https://myhealth.alberta.ca/Health/Pages/conditions.aspx?hwid=tx4080abc
Klabunde, R. E. (2021, February 2). Cardiovascular Physiology | Hemorrhagic Shock. Cvphysiology.com. https://www.cvphysiology.com/Blood%20Pressure/BP031
Tortora, G. J., & Derrickson, B. (2014). Principles of Anatomy and Physiology (14th ed.). Wiley.