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The Cardiac Cycle in the Frog Heart
In this lab, we will observe the mechanical events of the cardiac cycle in an active frog heart. Students will
investigate the regulatory effects of temperature and chemical compounds on heart rate and contractile
strength. In addition students will induce and record the events of atrioventricular heart block and pre-mature
ventricular contractions.
Lab Objectives
• Describe the function of the myograph transducer in recording the cardiac cycle.
• Identify the phases of the cardiac cycle on a myogram recording.
• Describe the effects of temperature and various chemicals on heart rate and force of contraction.
• Compare and contrast incomplete and complete heart block. Differentiate between first-degree,
second-degree, and third-degree heart block.
• Describe how heart block was induced in lab.
• Describe how a pre-mature ventricular contraction (extra systole) was induced in lab. Define the term
“compensatory pause” and explain why it occurs following an extra systole.
• Explain the significance of a long refractory period in the heart and how it differs from other types of
• Calculate the time intervals of atrial systole / diastole and ventricular systole / diastole, as well as the
heart rate using a myogram recording.
• Calculate force of atrial and ventricular systole using a myogram recording.
• Calculate percent change in heart rate.
Introductory Notes
Frog Heart
The frog heart is a good model for our cardiac cycle lab because frogs are cold-blooded vertebrates
(ectotherms) with a low metabolic rate at room temperature. The frog heart beats slowly, allowing us to easily
observe the mechanical activity of the cardiac cycle. Amphibians can respire through their skin as long as it is
kept moist and cool thus supplying the heart with oxygen. The heart will continue to beat when excised from
the body due to its intrinsic contractile properties. Human hearts have the same ability but must be supplied
with a constant flow of warm, oxygenated blood.
The frog heart differs from the human heart anatomically in that they have three chambers instead of four. The
frog heart has a right and left atria and a single ventricle. Blood returning from the lungs and body mixes in
the single ventricle chamber however, due to the structure of the ventricle and the low pressure of the system
the blood mixing isn’t extensive. The pacemaker in the amphibian heart is the sinus venosus, which sits on the
posterior side of the heart.
Force (g)
Cardiac Cycle
The cardiac cycle consists of atrial systole and diastole followed by ventricular systole and diastole. These are
the events, which take place in one complete heartbeat. These phases are controlled by the contraction of
myocardial cells, which are stimulated by the autorhythmic cells.
Time (seconds)
Effects of Temperature on Cardiac Muscle
In humans and other endotherms, the rate and strength of the heartbeat will increase when temperature
increases, to a point, due to the affect increased temperature has on enzymatic reactions. When environmental
temperatures decrease heart rate will initially speed up to support an increase in metabolism to increase heat
production to maintain homeostasis. However, once core body temperature decreases cardiac activity will
decrease due to the decreasing rate of chemical reactions.
Frogs are ectotherms therefore, their environment dictates their core body temperature. Warm temperatures
will cause an increase in heart rate and cold temperatures will cause a decrease in heart rate.
Effects of Chemicals on Cardiac Muscle
Chemicals affecting heart rate are called chronotropic agents and chemicals affecting contractile force are
called inotropic agents.
• Epinephrine is released by the adrenal medulla when in “fight or flight”. Epinephrine along with
norepinephrine released by sympathetic fibers would increase heart rate and contractile force. It would
be considered a sympathomimetic drug.
• Acetylcholine is a neurotransmitter released by parasympathetic fibers. Acetylcholine released by the
vagus nerve binds to muscarinic receptors on the SA node thus decreasing heart rate. There are very
few parasympathetic fibers innervating the ventricles so ACh has little effect on contractile force.
Acetylcholine is considered a parasympathomimetic drug.
• Atropine is a drug derived from the Belladonna plant. It is a parasympatholytic drug that blocks
muscarinic receptors and inhibits the effects of parasympathetic activity on the heart. It will increase
heart rate if heart rate is decreased by parasympathetic stimulation.
The Absolute Refractory Period in Cardiac Muscle Tissue
Cardiac muscle tissue is slow to repolarize resulting in a relatively long absolute refractory period. This
prevents summation and tetany from occurring in cardiac muscle and insures that the ventricles have time to
fill before the next ventricular contraction.
Heart Block
Heart block occurs when the ventricles fail to respond to every depolarization of the SA node or the response
is delayed.
• First-degree block: The interval between atrial contraction and ventricular contraction is prolonged.
Results in a prolonged PR interval > .20 seconds. Also known as partial or incomplete heart block.
• Second-degree block: The atrial to ventricular contraction ratio is altered. The action potentials are not
strong enough to pass through to the ventricles every time. Can be a 2:1; 3:1; or 3:2 ratio of atrial to
ventricular contractions. Also considered partial heart block.
• Third-degree block: The conduction between the atria and ventricles is completely blocked. There will
be no correlation between atrial and ventricular contractions. The ventricles have escaped atrial control
and will resume contraction at their own slower intrinsic rate. Also known as complete heart block.
Experimental Procedures
Determination of the Phases of a Cardiac Cycle
1. Set the physiograph paper speed to 1cm/sec to determine the horizontal calibration.
2. Determine the vertical calibration for force by hanging a 1g weight from the myograph
transducer. Turn record on and note the vertical deflection. Remove the weight.
3. Record one full page of the normal cardiac cycle rhythm.
- Distinguish between the atrial and ventricular contractions.
- Label both atrial systole and diastole and ventricular systole and diastole.
- Calculate the length of time of one complete cardiac cycle.
- Calculate the heart rate.
- Calculate the grams of force of atrial systole and ventricular systole.
- Using the practice myogram handout, complete these measurements and record results on your data
The Effects of Temperature on the Frog Cardiac Cycle
1. Prepare the following solutions:
• 8°C Ringer’s solution (icy)
• 25°C Ringer’s solution (room temperature)
• 40°C Ringer’s solution (warmed on hot plate)
2. Drop warm Ringer’s solution onto the heart and record the heart contractions.
3. Rinse the heart with room temperature Ringer’s solution and record the heart contractions.
4. Drop cold Ringer’s solution onto the heart and record the heart contractions.
5. Using the practice handout, calculate the heart rate induced by each of these conditions and determine the
"percent change in temperature" of the warm and cold solutions verses room temperature. Record results
on the data sheet.
6. Rinse the heart with room temperature Ringer’s solution.
The Effects of Chemicals on the Frog Cardiac Cycle
1. Record the effects of the following chemicals on the cardiac cycle:
• .1% epinephrine
• .1% acetylcholine
• .1% atropine
2. Place 2 drops of the epinephrine solution on the heart (on the sinus venosus if you can), wait
approximately 30 seconds, and then begin recording. Record until the effects on heart rate and/or force of
contraction are observed.
3. Wash heart thoroughly with Ringer’s to rinse off epinephrine and to establish a new normal baseline.
4. Place 1-2 drops of acetylcholine on the heart, wait approximately 30 seconds, and then begin recording.
Record until the effects on heart rate and/or force of contraction are observed.
5. Do not wash off the acetylcholine solution.
6. Place 2 drops of atropine on the heart, begin recording immediately. Record until the effects on heart rate
and/or force of contraction are observed.
7. Wash heart completely with Ringer’s to rinse off chemicals and to establish a new normal baseline.
8. Using the practice handout, calculate the heart rate and contractile force induced by each of these
chemicals and determine whether each agent was chronotropic and/or inotropic in its effects. Record
results on the data sheet.
The Absolute Refractory Period
1. Make sure the stimulator power is in the off position. Set the stimulator voltage to 100 volts and the
frequency to 100 stimulations/sec. Attach a hand electrode to the stimulator.
2. Turn the stimulator power on. Safety: the electrode will be a "hot wire"–– Do Not Touch.
3. Record a few normal cardiac cycles and then touch the electrode to the heart to deliver a single stimulation
to the ventricle during various times in the cardiac cycle. (Be sure that the electrode position does not
mechanically interfere with the contraction).
4. Label the extra systole and compensatory pause on the myogram. Record your observations on the data
sheet and explain the significance of a long refractory period in heart muscle.
Heart Block
1. Loop a piece of thread around the groove between the atria and ventricle and tie in a loose slipknot. Slip
two shorter pieces of thread through the loop on opposite sides of the heart.
2. After recording a few normal cycles, gently tighten the knot around the heart and observe the changes in
the cardiac rhythm. Continue tightening until both partial and complete heart block are observed.
3. Loosen the knot and observe the recovery.
4. Record your observations on the data sheet.
5. Optional: remove the entire heart from the frog and place it in a petri dish with Ringer’s solution.
Sever the upper atria from the lower ventricle and determine each structure's intrinsic rate over one minute.
Note that the ventricle may not respond immediately after the removal from the atria. It may take up to 2
minutes for them to respond.
Lab Section:________________
Normal Cardiac Cycle:
• Vertical calibration (magnitude):
_____g / _____cm
• Horizontal calibration (paper speed): _____cm / second
• Force of atrial contraction
__________ g
• Force of ventricular contraction
__________ g
• Duration of atrial systole
__________ sec
• Duration of atrial diastole
__________ sec
• Duration of ventricular systole
__________ sec
• Duration of ventricular diastole
__________ sec
Effects of Temperature
Record the effects of each temperature on the rate of the cardiac cycle:
8° C:_________ beats/min
25° C:__________ beats/min
Calculate percent change for normal to warm beats per minute:
Calculate percent change for normal to cold beats per minute:
35° C:_________ beats/min
Effects of Chemicals
Record the effects of each chemical on the rate and contractile force of each cardiac cycle.
Chronotropic and/or
Heart Rate (BPM)
Force of Contraction (g)
Inotropic? (+ or -)
Absolute Refractory Period
• Record your observations after electrically stimulating the heart during the cycle. Label the extra systole
and compensatory pause you observed.
During which part of the cardiac cycle can an extra systole be obtained?
What causes the compensatory pause following the extra systole?
Explain the significance of a long refractory period in heart muscle.
Heart Block
• Compare and contrast the myograms attained for partial and complete heart block.