The Cardiac Cycle, the ECG, Regulation and Disorders: Quick Revision

A brief note on The Cardiac Cycle, the ECG, Regulation and Disorders. Read it before a test. Try each question at the end before you open its answer.

The cardiac cycle

• About 0.8 s, repeated 72 times in a minute.
• Joint diastole: all chambers relax. Valves between the atria and the ventricles are open. Semilunar valves are shut.
• Atrial systole: the SAN fires, the atria contract, and they add about 20 to 30 per cent to the filling of the ventricles.
• Ventricular systole: the valves between the atria and the ventricles shut, the semilunar valves open, and the blood goes into the pulmonary artery and the aorta.
• Ventricular diastole: the semilunar valves shut, and the valves between the atria and the ventricles open again.
• Heart sounds: lub is the closing of the tricuspid and bicuspid valves. Dub is the closing of the semilunar valves.
• Cardiac output: stroke volume (about 70 mL) times heart rate (about 72), which is about 5 L in a minute. It is much more in an athlete.

The ECG and the control of the heart

• Three leads: one on each wrist and one on the left ankle.
• P wave: depolarisation of the atria. QRS complex: depolarisation of the ventricles, with the contraction soon after Q. T wave: repolarisation of the ventricles, and its end marks the end of the systole.
• Count the QRS complexes in a known time to find the heart rate.
• The heart is myogenic. The cardiac centre in the medulla acts through the autonomic nervous system.
• Sympathetic: increases rate, strength and output. Parasympathetic: decreases rate, speed of conduction and output. The adrenal medulla hormones increase the output.

Disorders

• Blood pressure: normal 120/80 mm Hg, systolic over diastolic. 140/90 or higher is hypertension.
• CAD (atherosclerosis): deposits of calcium, fat, cholesterol and fibrous tissue narrow the coronary arteries.
• Angina pectoris: chest pain when the heart muscle gets too little oxygen.
• Heart failure: the heart does not pump enough, and the lungs are congested. It is not cardiac arrest and not a heart attack.

A worked example

The stroke volume is 70 mL and the heart rate is 72 beats a minute. What is the cardiac output?

1. Output is the stroke volume times the heart rate: $70 \times 72 = 5040$ mL.
2. In litres, this is about 5 L in a minute.

Practice with new numbers

A program to try

The program works out the cardiac output from the stroke volume and the heart rate.

Mistakes to avoid

• Mixing up the two heart sounds. Lub is the valves between the atria and the ventricles. Dub is the semilunar valves.
• Placing the contraction of the ventricles at the T wave. It starts soon after Q.
• Mixing up the nerves. Sympathetic raises, parasympathetic lowers.
• Saying that heart failure is a stopped heart. That is cardiac arrest.

Check yourself

1. What does the P wave show?

2. What is the normal blood pressure?

3. What is the cardiac output?

4. What is angina pectoris?

Sources

• Betts J. G. et al. (2022). Anatomy and Physiology 2e, OpenStax, chapter 19, section 19.3.
• Mancia G. et al. (2023). 2023 ESH guidelines for the management of arterial hypertension. Journal of Hypertension, 41, 1874 to 2071.
• NCERT (2023). Biology, Textbook for Class XI, chapter 15.