Question

For each description of an aspect of running the Marathon A–E, select the most appropriate option from the following list of physiological mechanisms. 

1. Brainstem respiratory control. 

2. Higher centre respiratory control. 

3. Local metabolic control of vascular 

4 Baroreceptor reflex control of vascular tone. tone. 

5. Low level sympathetic activity. 

6. High level sympathetic activity. 

7. Low vagal tone. 

8. High vagal tone. 

A. As the run proceeds in medium temperature conditions, the runner’s skin becomes markedly flushed. 

B. Despite differing levels of exertion, the runners maintain their arterial oxygen pressures close to their normal resting values. 

C. As the race proceeds the runners show marked sweating and take in fluid to maintain fluid balance. 

D. The runners have very high levels of blood flow through their leg muscles. 

E. A positive correlation has been found between a runner’s resting heart rate and time taken to complete the Marathon.

Answer 1

A. Option 5 Low level sympathetic activity. As the runners generate increased heat from their exercising muscles there is temperature reflex release of sympathetic tone to the skin blood vessels; removal of the resting constrictor tone causes vasodilation. 

B. Option 2 Higher centre respiratory control. The precise matching of ventilation (and cardiac output) to muscular activity is evidence of sophisticated cerebral activity above the level of the automatic, reflex centres in the medulla; this concept has led to the term, ‘exercise centre’. 

C. Option 6 High level sympathetic activity. Sweating is induced by activity in cholinergic sympathetic nerves to sweat glands. 

D. Option 3 Local metabolic control of vascular tone. The huge increase in muscle blood flow is due to metabolic changes generated locally by the active muscles. 

E. Option 8 High vagal tone. Endurance athletes, such as marathon runners, demonstrate their fitness by having very low (e.g. 30–40 beats per minute) heart rates due to very high resting cardiac vagal tone; this is related to their having large powerful hearts with a high stroke volume so resting cardiac output requires only a low heart rate; during exercise they can reach the usual high rates, so multiplying their resting cardiac output during maximal exercise much more (say 6 times) than the average person (say 3–4 times).