Week 10 POST-LECTURE ACTIVITY: THE CARDIOVASCULAR SYSTEM
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 the why the factor/s affects cardiac output in 1 to 2 sentences.
a. High blood pressure
FACTOR: Afterload
High blood pressure is caused by blocked arteries, which then cause the heart to work harder to supply the body with adequate amounts of oxygen and nutrients. Due to this, it increases the heart’s afterload, causing a decrease in stroke volume.
b. Massive bleeding
FACTORS: Preload, Contractility, Heart rate
The preload will decline as a result of excessive blood volume loss. The sympathetic division of the autonomic nervous system will therefore be stimulated, demonstrating increased contractility and heart rate, to maintain the cardiac output.
c. The drug dopamine
FACTORS: Contractility, Heart Rate
Dopamine produces positive inotropic effects on the myocardium, which enhances the cardiac output by increasing the heart rate and contractility of the cardiac muscle.
d. Running a sprint
FACTORS: Preload, Contractility, Heart rate
As we run, the muscles involved require more oxygen. In order to fulfill the oxygen demand, our cardiac output will increase 4-5 times more than our resting cardiac output – causing the cardiac muscles to contract faster, leading to an increased stroke volume, preload, and heart rate.
e. Hyperthyroidism with increased secretion of thyroid hormones
FACTORS: Cardiac Contractility, Heart rate
The excessive secretion of thyroid hormones causes enhanced cardiac contractility and an increased heart rate – which then leads to increased cardiac output.
f. Massive myocardial infarction (cardiac tissue death)
FACTOR: Contractility
Since myocardial infarction is caused by a blocked artery that impedes blood flow, the cardiac muscle doesn’t get enough oxygen. This causes impaired functioning and a reduction in the contractility of the cardiac muscles, leading to decreased cardiac output.
g. Hypothermia
FACTOR: Heart rate
During hypothermia, a person’s body is deliberately cooled to a low temperature. This results in a slow metabolism and decreased oxygen demand, causing the patient’s heart rate to slow down.
h. Emotional distress
FACTORS: heart rate, contractility, afterload
Emotional distress prompts the adrenal medulla to release the hormones – epinephrine and norepinephrine, which both have positive inotropic effects, causing an increased heart rate and cardiac contractility. Moreover, stress also increases blood pressure, causing the ventricles to pump harder and leading to increased afterload.