Question

question 84 (0.5 points)
joelle sees a saber tooth tiger running at her. her sympathetic nervous sytem get activated. her heart increases from 80 beats per minute (bpm) to 140 bpm. which explains why there is no change in her stroke volume?
her end distolic volume increased because of the increased heart rate while her end systolic volume decreased because of increased force of contraction.
her end distolic volume decreased because of the increased heart rate while her end systolic volume increased because of increased force of contraction.
her end distolic volume decreased because of the increased heart rate while her end systolic volume also decreased because of increased force of contraction.
this is impossible because everyone knows that a persons stroke volume always decreases when their heart rate increases.

Explanation:

To determine the correct answer, we analyze the relationship between heart rate, stroke volume, end - diastolic volume (EDV), and end - systolic volume (ESV). Stroke volume (SV) is calculated as \( SV=EDV - ESV \). When the sympathetic nervous system is activated (as in a fight - or - flight response like seeing a saber - tooth tiger), the heart rate increases. An increased heart rate reduces the time for the heart to fill with blood, so EDV decreases. At the same time, the increased force of contraction (due to sympathetic activation) makes the heart pump more blood out, so ESV also decreases. For the stroke volume to remain unchanged, the decrease in EDV and the decrease in ESV must be such that their difference (SV) stays the same.

• The first option is incorrect because it says EDV increases, but an increased heart rate reduces filling time and thus EDV should decrease.
• The second option is incorrect as it states ESV increases, but the increased contractility should decrease ESV.
• The fourth option is incorrect because stroke volume does not always decrease with an increase in heart rate; it can remain stable or even increase in certain situations (like the fight - or - flight response here due to the changes in EDV and ESV).
• The third option is correct as it correctly describes EDV decreasing (due to less filling time from increased heart rate) and ESV decreasing (due to increased contractility), which can result in no change in stroke volume.

Answer:

The third option (the one with the blue - circled radio button): Her end distolic volume decreased because of the increased heart rate while her end systolic volume also decreased because of increased force of contraction.