Circulatory System in Animals
Senior 6 · Biology
Circulatory System in Animals
Lesson 3 of 5
25 min
When a nurse at a health centre listens to your chest with a stethoscope, she hears two sounds with every beat: a longer "lub" and a sharper "dub". Neither is the sound of muscle contracting. Both are the sounds of valves snapping shut: first the atrioventricular valves, then the semilunar valves. The order of the sounds is the order of events in the heart, and this lesson explains it: the cardiac cycle, how each beat starts and spreads, how it is recorded in an electrocardiogram, and how the body speeds the heart up or slows it down.
Definition
Cardiac cycle: the sequence of events in one heartbeat, from the start of one beat to the start of the next. At 75 beats per minute it lasts .
Systole: contraction of a chamber of the heart. Diastole: relaxation, during which the chambers fill.
Stroke volume (SV): the volume of blood pumped by one ventricle in one beat. Cardiac output (CO): the volume pumped by one ventricle per minute, .
Myogenic: describes muscle that starts its own contractions, without a signal from a nerve. Heart muscle is myogenic.
Figure 1 plots the pressure in the left atrium, the left ventricle and the aorta through one cycle. The right side of the heart goes through the same stages at the same time, at lower pressures.
Figure 1: Pressure in the left atrium, left ventricle and aorta during one cardiac cycle at 75 beats per minute (a model drawn from typical values). Dashed lines: the moments when the valves open or close.
Every event follows the rule from Lesson 2: a valve opens when the pressure behind it is higher than the pressure in front of it, and closes when that is reversed.
| Stage (time in Figure 1) | What happens | Valves |
|---|---|---|
| Atrial systole (0 to 0.13 s) | The atria contract, topping up the ventricles, which are already about 70 % full | AV open; semilunar closed |
| Ventricular systole, first part (0.13 to 0.19 s) | The ventricles contract; their pressure rises above atrial pressure, so the AV valves close ("lub"). With all valves shut, pressure rises fast but no blood moves | AV closed; semilunar closed |
| Ventricular systole, ejection (0.19 to 0.41 s) | Ventricular pressure exceeds aortic pressure: the semilunar valves open and blood is forced into the arteries | AV closed; semilunar open |
| Diastole, first part (0.41 to 0.49 s) | The ventricles relax; their pressure falls below aortic pressure, so the semilunar valves close ("dub"), making the notch on the aortic curve | AV closed; semilunar closed |
| Diastole, filling (0.49 to 0.80 s) | Ventricular pressure falls below atrial pressure: the AV valves open and blood flows from the veins through the atria into the ventricles | AV open; semilunar closed |
Notice that the aortic pressure never falls to zero. The elastic wall of the aorta stretches during ejection and recoils during diastole, keeping blood flowing between beats.
Worked example 1: reading the pressure graph
Use Figure 1. (a) For how long is the aortic valve open? (b) Explain why the left ventricular pressure rises very steeply between 0.13 s and 0.19 s. (c) Calculate the heart rate.
Figure 2: The conducting system of the heart. Orange: specialised conducting tissue. Grey band: non-conducting tissue between atria and ventricles.
The waves of excitation produce small electrical currents that reach the skin. Electrodes on the chest, arms and legs record them as an electrocardiogram.
Figure 3: An ECG trace of three beats on standard ECG paper (drawn by Ganzaa from a model of a normal trace).
| Part of the trace | What it shows |
|---|---|
| P wave | Excitation (depolarisation) spreading over the atria, just before atrial systole |
| P–R interval | The time from the start of atrial excitation to the start of ventricular excitation, mostly the delay at the AVN; normally about 0.12 to 0.20 s |
| QRS complex | Excitation spreading through the ventricles, just before ventricular systole; large because the ventricles have much more muscle |
| T wave | Recovery (repolarisation) of the ventricles, during early diastole |
Doctors use the ECG to find problems: a heart rate above 100 beats per minute at rest (tachycardia) or below 60 (bradycardia, normal in some athletes); an irregular trace with no clear P waves (atrial fibrillation); or changes in the ST segment and T wave when heart muscle is short of oxygen during a heart attack.
Worked example 2: heart rate from an ECG
In Figure 3, the R peaks are 4 large squares apart. Each large square is 0.2 s. (a) Calculate the heart rate. (b) A second patient's trace has R peaks 2.5 large squares apart. Calculate the heart rate and comment on it.
Shortcut: heart rate , because .
The SAN sets the basic rate, but the brain adjusts it. The cardiovascular centre in the medulla oblongata sends impulses along two nerves to the SAN:
The centre responds to sensory receptors. Chemoreceptors in the carotid arteries, the aorta and the medulla detect the rise in carbon dioxide (and fall in blood pH) during exercise, so the heart speeds up and removes CO₂ and delivers O₂ faster. Baroreceptors (stretch receptors) in the aorta and carotid sinuses detect blood pressure: high pressure slows the heart, low pressure speeds it up. The hormone adrenaline, released in fear or excitement, also acts directly on the SAN.
Factors that change heart rate: exercise, emotion (adrenaline), body temperature (a fever raises it), age (it is highest in babies), fitness (trained athletes have a larger stroke volume, so a lower resting rate), caffeine and nicotine (raise it), and blood loss (the heart speeds up to maintain pressure).
Worked example 3: cardiac output at rest and in exercise
At rest, a student's stroke volume is and her heart rate 72 beats per minute. During a hard run they rise to and 170 beats per minute. Calculate the cardiac output in each case, in , and the factor by which it rises.
Hypothesis: pulse rate increases with the intensity of exercise. Independent variable: the number of step-ups per minute on a 30 cm step (0, 15, 30). Dependent variable: pulse rate, counted at the wrist for 30 s immediately after 2 minutes of exercise, then doubled. Control variables: duration of exercise, step height, the same person, rest until the pulse returns to its resting value between trials, same position when measuring.
Repeat each intensity three times, calculate a mean and standard deviation, and plot the means with error bars. Overlapping error bars suggest that a difference may not be real.
Warning
Safety
CrashCourse: "The Heart, Part 2 - Heart Throbs" (Crash Course Anatomy & Physiology #26). Watch for the SA node, the AV node and the Purkinje fibres, and how the ECG waves match the cardiac cycle.
Tip
Exam technique for the cardiac cycle. For every valve, give the pressure comparison: "The AV valve closes because the pressure in the ventricle becomes higher than the pressure in the atrium." Never write that a valve closes "because the ventricle contracts"; the examiner wants the pressure difference.
Practice: the cardiac cycle and its control
Key points
Sources and credits
Figures 1 to 3, the tables, the investigation, the worked examples and the practice questions are written by Ganzaa. Figure 1 is a model built from typical pressures, and Figure 3 a model of a normal trace.
