Body Fluids and Circulation · Lesson 6 of 10
The Cardiac Cycle: Systole, Diastole, Output and Heart Sounds
“Follow a complete heartbeat through pressure changes, valve opening and closure, stroke volume, cardiac output and heart sounds.”
• Define systole, diastole and a cardiac cycle. • Explain each major phase of one heartbeat in causal order. • Identify the valve events responsible for first and second heart sounds. • Calculate cardiac output from stroke volume and heart rate. • Relate SA-node excitation and chamber pressure to one-way blood flow.
A heartbeat is an ordered sequence, not one simultaneous squeeze
The heart alternates between relaxation, when chambers fill, and contraction, when blood is propelled forwards. Relaxation is called diastole; contraction is called systole. A complete cardiac cycle includes the coordinated events in both atria and ventricles from the beginning of one heartbeat to the next. The electrical impulse starts the sequence, but pressure differences and valve positions determine the movement of blood.
The repeating sequence of atrial and ventricular relaxation, contraction, filling and ejection that constitutes one heartbeat.
Phase one: joint diastole and ventricular filling
At the beginning of the simplified cycle, atria and ventricles are relaxed. Blood enters the atria from venae cavae on the right and pulmonary veins on the left. Because atrial pressure is greater than ventricular pressure, the tricuspid and mitral valves are open and blood passes into the ventricles. The aortic and pulmonary semilunar valves remain closed because pressure in the large arteries is greater than in the relaxed ventricles.
A significant fraction of ventricular filling occurs passively before atrial contraction. The ventricles are not filled only by the atria actively squeezing. This is especially important when interpreting how the heart works at rest.
Phase two: atrial systole completes filling
The SA node generates an impulse that spreads through both atria, leading to atrial contraction. Atrial systole pushes additional blood into the ventricles and helps complete filling. The chapter describes an approximate 30% increase in ventricular filling from atrial contraction as a useful model, although the actual contribution varies with heart rate, age and physiology. The AV node then passes the impulse to the ventricles through the specialised conduction network.
Phase three: ventricular systole and ejection
As the ventricles contract, ventricular pressure rises and forces the atrioventricular valves shut. At first the ventricles are briefly contracting while all major valves are closed; volume is essentially unchanged even though pressure is increasing. When ventricular pressure exceeds pressure in the pulmonary artery on the right and aorta on the left, the semilunar valves open. Blood is ejected into both arterial circuits. During most of this phase, the atria relax and receive returning venous blood.
The order matters: atrioventricular valves close before semilunar valves open. If the tricuspid and mitral valves remained open during vigorous ventricular contraction, pressure would drive blood backwards towards the atria. Normal one-way flow therefore depends on valve timing produced by pressure gradients.
Phase four: ventricular relaxation and the next filling phase
When ventricular muscle relaxes, its pressure falls. As arterial pressure exceeds ventricular pressure, semilunar valves snap shut and prevent retrograde flow from the aorta and pulmonary artery. The ventricles again briefly change pressure with all valves closed. Once their pressure drops below atrial pressure, the tricuspid and mitral valves open and filling resumes. The heart has returned to joint relaxation and is ready for the next SA-node impulse.
| Phase | Atrial state | Ventricular state | Valve and flow consequence |
|---|---|---|---|
| Joint diastole | Relaxed | Relaxed and filling | AV open, semilunar closed |
| Atrial systole | Contracting | Final filling | AV open; additional blood enters ventricles |
| Early ventricular systole | Relaxing | Pressure rises | AV shut; briefly all closed |
| Ventricular ejection | Relaxing | Contracting | Semilunar open; blood enters arteries |
| Early ventricular diastole | Filling | Pressure falls | Semilunar shut; then AV reopen |
What produces lub and dub?
A stethoscope reveals prominent sounds near valve-closure events. The first heart sound, commonly written 'lub', accompanies closure of the mitral and tricuspid valves at the start of ventricular systole. The second, 'dub', accompanies closure of the aortic and pulmonary semilunar valves as ventricular ejection ends. The sounds arise from vibrations linked to valve closure and surrounding blood/tissue dynamics, not simply two valves striking like hard doors.
Stroke volume, heart rate and cardiac output
The volume ejected by one ventricle in a single beat; approximately 70 mL is a useful illustrative resting value.
The volume ejected by each ventricle in one minute, equal to stroke volume multiplied by heart rate.
Using textbook resting estimates, 72 beats per minute and a stroke volume of 70 mL give 72 × 70 = 5,040 mL per minute, or about 5 L per minute. A cycle at 72 beats/min lasts 60 ÷ 72 ≈ 0.83 seconds, customarily rounded to 0.8 seconds. Under normal steady conditions, the volumes pumped by left and right ventricles per unit time are approximately equal, even though the pressures they generate are different.
During physical activity, both heart rate and stroke volume can change, increasing cardiac output to support active muscles. Cardiac output is not the combined sum of left and right ventricular output; counting both as separate flows would double-count blood that travels sequentially through pulmonary and systemic circuits. When solving a numerical question, confirm that heart rate is in beats per minute and stroke volume is in millilitres per beat.
At 80 beats/min and 75 mL/beat, cardiac output = 80 × 75 = 6,000 mL/min = 6 L/min. The units cancel as (beats/min) × (mL/beat) = mL/min.
Quick Quiz
Quiz
What occurs first as ventricular pressure rises at the start of systole?
Which event is responsible for the second heart sound?
Which valves are normally open during most passive ventricular filling?
Stroke volume is best defined as...
With HR=60 beats/min and SV=80 mL, cardiac output equals...
Why are there two major pumping phases?
Practice Problems
- Explain one complete cardiac cycle beginning with joint diastole.
- Why do semilunar valves close during ventricular relaxation?
- Identify events producing the first and second heart sounds.
- Calculate output at 75 beats/min with stroke volume 65 mL.
- Explain why cardiac output may rise during exercise.
Joint diastole allows passive filling through AV valves; SA impulse causes atrial systole; AV conduction triggers ventricular systole, AV closure then semilunar opening/ejection; ventricular relaxation closes semilunar valves and eventually reopens AV valves.
Key Takeaways
• One cardiac cycle consists of coordinated atrial and ventricular diastole and systole. • Passive ventricular filling occurs before atrial systole; the atria supplement this filling. • Rising ventricular pressure closes AV valves, then opens semilunar valves for ejection. • The first heart sound follows AV-valve closure; the second follows semilunar-valve closure. • Cardiac output = stroke volume × heart rate; typical textbook rest values give approximately 5 L/min.