This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Physiology
Which of the following statements regarding stroke volume is TRUE? 
Select and explain. 
A. Decreases by increase in hea rate, B. Determined by pre-diastolic volume, C. Determined by afterload, D. Is equal to cardiac output
A: Decreases by increase in hea rate
Answer: A: Decreases by increase in hea rate 
Explanation: Stroke volume = cardiac output/hea rate, i.e it is inversely propoional to hea rate. As a result when hea rate increases the stroke volume decreases. This reduction in stroke volume at high hea rates is due to a decrease in the length of time the hea spends in diastole and thus a reduction in time available to the hea for filling. Stroke volume refers to quantity of blood ejected with each hea stroke. According to Starling's law of hea, stroke volume is determined primarily by the preload. As the preload increases, stroke volume also increases until it reaches a plateau. Stroke volume can also be increased by sympathetic stimulation of the hea. Ref: Physiology Secrets By Hershel Raff, 2nd edn, page 74