THE CARDIOVASCULAR SYSTEM
Week 10
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 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
- Because contraction of the ventricles generates blood pressure (BP), and blood exerts hydrostatic pressure on the walls of a blood vessel, the contractility of high blood pressure influences cardiac output. Blood pressure is determined by the total volume of blood in the cardiovascular system, which is why narrowed, less elastic arteries caused by high blood pressure make it more difficult for blood to travel efficiently throughout your body, causing the heart to work harder and affecting cardiac output.
b. Massive bleeding
- Massive bleeding impacts cardiac output by lowering blood volume during acute blood loss, which causes a drop in central venous pressure and cardiac filling. This results in decrease in the afterload of cardiac output and arterial pressure, and the body activates many compensatory mechanisms to restore arterial pressure and blood volume to normal.
c. The drug dopamine
- Dopamine-releasing neurons help regulate skeletal muscle tone and some aspects of movement caused by skeletal muscle contraction. Dopamine has positive chronotropic and inotropic effects on the myocardium and influences cardiac output factors such as increased heart rate and cardiac contractility.
d. Running a sprint
- Increased body temperature, such as that experienced while sprinting, causes the SA node to discharge impulses more quickly, affecting cardiac output and increasing heart rate. Running, for example, works large body muscles for at least 20 minutes, increases cardiac output, and speeds up metabolic rate.
e. Hyperthyroidism with increased secretion of thyroid hormones
- Epinephrine and norepinephrine (from the adrenal medullae) increase the effectiveness of the heart's pumping and affect cardiac muscle fibers released by cardiac accelerator nerves. Thyroid hormone excess in hyperthyroidism affects cardiac contractility and increases heart rate.
f. Massive myocardial infarction (cardiac tissue death)
- Most myocardial infarctions are caused by underlying coronary artery disease wherein the blood and oxygen supply to the heart is cut off, and the muscle cells begin to suffer damage and die. Massive myocardial infarction reduces cardiac output by causing a loss of contractile mass, resulting in an acute decrease in cardiac pump function.
g. Hypothermia
- Hypothermia is a drop in core body temperature to 35°C or lower. Hypothermia reduces cerebral oxygen consumption, induces bradycardia, and increases the heart rate, which causes cellular metabolism to slow, the need for O2 decreases, and more O2 remains bound to hemoglobin.
h. Emotional distress
- Responses to stressors can be pleasant or unpleasant, and they differ between people; however, this affects the cardiac output. Emotional distress causes an increase in heart rate, an increase in blood pressure, and the release of stress hormones, all of which result in an increase in heart workload.
References:
Cunha, J. (2022, July 18). Dopamine: LBP, uses, side effects, dosages, interactions, warnings. RxList, from https://www.rxlist.com/dopamine-drug.htm
Klabunde, R. (2022). Hemorrhagic shock. Image for Cardiovascular Physiology Concepts, Richard E Klabunde PhD, from https://www.cvphysiology.com/Blood%20Pressure/BP031#:~:text=The%20reduction%20in%20blood%20volume,cardiac%20output%20and%20arterial%20pressure
Ojha, N., & Dhamoon, A. (2022). Myocardial infarction - statpearls - NCBI bookshelf. National Library of Medicine, from https://www.ncbi.nlm.nih.gov/books/NBK537076/
Torpy, J. (2007). Acute Emotional Stress and the Heart. Jama Network, from https://jamanetwork.com/journals/jama/fullarticle/208031#:~:text=Experiencing%20emotional%20or%20physical,heart%2C%20which%20can%20be%20dangerous
Tortora, G. J., & Derrickson, B. H. (2018). Principles of anatomy and physiology. John Wiley & Sons.