What the Lub-Dub of Your Heartbeat Really Means

Your heart beats roughly seventy times a minute, day and night, without ever resting.

This part covers the heart and its nodal tissue, the cardiac cycle, the ECG, and double circulation, regulation and heart disorders.

The human heart has two atria and two ventricles, valves that keep blood flowing one way, and nodal tissue — SA node, AV node, bundle of His and Purkinje fibres — in which the SA node generates the most impulses and so sets the heart's rhythm.

Chambers. Two thin-walled atria sit above two thick-walled ventricles, separated by septa.

Valves:

- Tricuspid — between right atrium and right ventricle
- Bicuspid (mitral) — between left atrium and left ventricle
- Semilunar valves — at the openings of the pulmonary artery and aorta

Nodal tissue:

- SA node — in the right upper corner of the right atrium
- AV node — in the lower left corner of the right atrium, near the atrio-ventricular septum
- Bundle of His — runs from the AV node through the inter-ventricular septum
- Purkinje fibres — spread through the ventricle walls

Why the SA node is the pacemaker. All nodal tissue can generate action potentials without outside stimulation, but the SA node generates the most — about 70 to 75 per minute — so it starts and keeps the rhythm.

An everyday example. An artificial pacemaker placed under the skin takes over the SA node's job when the natural rhythm fails.

The substance. The left ventricle has the thickest wall, because it pumps blood to the whole body.

In each cardiac cycle, all chambers first relax and fill, the atria contract to top up the ventricles, the ventricles contract to push blood into the aorta and pulmonary artery, and then they relax again; stroke volume is the blood pumped per beat, and cardiac output is stroke volume multiplied by heart rate.

The stages:

- Joint diastole — all chambers relaxed; AV valves open; blood flows into the ventricles
- Atrial systole — the SA node fires; atria contract, adding about 30 per cent more blood to the ventricles
- Ventricular systole — the impulse travels through the AV node, bundle of His and Purkinje fibres; ventricles contract; AV valves close; semilunar valves open
- Ventricular diastole — ventricles relax; semilunar valves close; AV valves reopen and the cycle repeats

Heart sounds:

- Lub — closure of the tricuspid and bicuspid valves
- Dub — closure of the semilunar valves

Worked example — the numbers. A heart beats 72 times per minute, and each ventricle pumps 70 mL per beat.

$$\text{One cycle}=\frac{60\text{ s}}{72}\approx 0.8\text{ s}$$

$$\text{Cardiac output}=70\text{ mL}\times 72=5040\text{ mL per minute}\approx 5\text{ L per minute}$$

An everyday example. A doctor's stethoscope picks up lub and dub as valves snap shut.

The substance. Athletes can raise their cardiac output far above normal because both stroke volume and heart rate increase.

An ECG is a graph of the heart's electrical activity: the P wave shows depolarisation of the atria, the QRS complex shows depolarisation of the ventricles, and the T wave shows the ventricles returning to their resting state.

The waves:

- P wave — electrical excitation (depolarisation) of the atria, which leads to atrial contraction
- QRS complex — depolarisation of the ventricles
- T wave — repolarisation of the ventricles

Worked example — heart rate from a strip. An ECG shows 18 QRS complexes in 15 seconds.

$$\text{Heart rate}=\frac{18}{15}\times 60=72\text{ beats per minute}$$

An everyday example. The moving zigzag line on a hospital bedside monitor is a continuous ECG, with each tall spike a QRS complex.

The substance. Atrial repolarisation shows no separate wave — it happens during the much larger QRS complex, which hides it.

In double circulation, blood passes through the heart twice per round — via the pulmonary circuit to the lungs and the systemic circuit to the body; nerves and hormones adjust a beat that starts in nodal tissue, and disorders include hypertension, coronary artery disease, angina and heart failure.

Double circulation:

- Pulmonary — right ventricle $\to$ pulmonary artery $\to$ lungs $\to$ pulmonary veins $\to$ left atrium
- Systemic — left ventricle $\to$ aorta $\to$ body tissues $\to$ veins $\to$ right atrium

Regulation:

- The heart is myogenic — normal activity is auto-regulated by nodal tissue
- Sympathetic nerves and adrenal medullary hormones increase heart rate and cardiac output
- Parasympathetic nerves decrease heart rate and cardiac output

Disorders:

- Hypertension — blood pressure above the normal 120/80 mm Hg; repeated readings of 140/90 or higher indicate hypertension
- Coronary artery disease — deposits of calcium, fat, cholesterol and fibrous tissue narrow the coronary arteries (atherosclerosis)
- Angina pectoris — acute chest pain when too little oxygen reaches the heart muscle
- Heart failure — the heart cannot pump enough blood for the body's needs; it is not the same as cardiac arrest or a heart attack

An everyday example. A home blood pressure monitor repeatedly showing 140/90 or more is a signal to see a doctor.

The substance. A four-chambered heart keeps oxygenated and deoxygenated blood fully separate, which is what makes double circulation efficient.