BGYC34; First Assignment (2012)
PhysioEx Lab 6 (Cardiovascular Physiology)
Activity 1: Investigating the Refractory Period of Cardiac Muscle
Results: Briefly describe what happened when you applied the electrical stimuli to the
heart. (2 marks)
There was a second beat that occurred when the stimulus was applied in the in the
second half of the beat, just after the peak ended. This would then be followed by a
compensatory pause (a longer period between beats). Give two marks for this brief
description; adjust accordingly if it isn’t all there.
Question 1: Explain why the larger waves seen on the oscilloscope represent ventricular
contraction. (1 mark)
The ventricle has more muscle mass and therefore has the largest strength contractions.
There are only two things that can contract; the atria and the ventricle and the ventricle
is bigger and more muscular than the atria. This is worth 1 mark.
Activity 2: Examining the Effect of Vagus Nerve Stimulation
Results: Briefly describe what happened when you electrically stimulated the vagus
nerve. (2 marks)
Electrically stimulating the vagus nerve should stop the heart temporarily. However,
heart rate will increase again (to the normal level) after a few moments. Give two marks
for making this statement or something along these lines.
Question 2: Explain how possible mechanisms of vagal escape would differ in an
isolated heart preparation such as the one used in this experiment versus a heart in a live
animal/human. (3 marks)
Vagal escape is the restoration of the heart beat following a slowing (or stopping) of the
heart caused by vagus nerve stimulation. In a heart in a live animal/human there are two
possible mechanisms of vagal escape: First, the Purkinjie fibres may act as pacemaker
cells. They are not innervated by the vagus nerve like the SA and AV nodes are. Second,
there may be an increase in sympathetic tone to the heart which causes the heart rate to
increase. Give two marks for the preceding statement. Give 1 mark for the following
statement. In an isolated heart such as the one used in this experiment, there is no
sympathetic innervation so the only mechanism of vagal escape would be the Purkinjie
fibres acting as pacemaker cells.
Activity 3: Examining the Effect of Temperature on Heart Rate 2
Results: Provide the heart rate values recorded when the heart was bathed with Ringer’s
solution of different temperatures. (2 marks)
Give two marks if the data of heart rate as a function of the different temperatures is
Question 3: Homeothermic animals such as humans have a thermoneutral zone (i.e., a
range of external environmental temperatures) in which internal body temperature
remains at a constant level while simultaneously maintaining metabolic rate (i.e., energy
expenditure) constant. If the environmental temperature drops below the thermoneutral
zone, humans can still keep internal body temperature constant (as long as it isn’t too
cold). However, in this case, metabolic rate does not simultaneously remain constant but,
rather, it increases.
a) In this case (below the thermoneutral zone), what would happen to heart rate (in an
intact living human)? Explain your answer. (2 marks)
In an intact living human, if environmental temperature drops below the thermoneutral