Cardiac cycle

What controls the heartbeat?

The cardiac conduction system (electrical system of the heart).

What does the ECG show?

Electrical signals as waves representing heart muscle contraction/relaxation.

What are the two phases of the cardiac cycle?

Systole (contraction) and Diastole (relaxation).

What happens first in the cycle?

Atria contract → then ventricles contract.

What happens during atrial systole?

Atria contract, topping off ventricles (AV valves open, semilunar valves closed).

What is isovolumetric ventricular contraction?

Ventricles start to contract with all valves closed; pressure builds (1st heart sound).

What happens during ejection phase?

Semilunar valves open, blood exits; marked by T wave (ventricular repolarization).

What is isovolumetric relaxation?

Ventricles relax, all valves closed again (2nd heart sound).

What is passive ventricular filling?

Atria passively fill ventricles as AV valves reopen (no contraction yet).

What causes the 'lubb' sound?

AV valves closing at the start of ventricular systole.

What causes the 'dupp' sound?

Semilunar valves closing at the start of diastole.

What is a murmur?

An abnormal heart sound due to valve issues (e.g., regurgitation, stenosis).

How is cardiac output calculated?

CO = Stroke Volume (SV) × Heart Rate (HR).

What is a normal CO?

4–6 L/min.

What happens if HR or SV increases?

Cardiac output (CO) increases.

What are the 3 controls of heart activity?

Chronotropic = heart rate; Inotropic = contraction strength; Dromotropic = signal conduction speed.

What part of the brain regulates HR?

Medulla’s cardiac center.

What does the sympathetic nervous system do to HR?

Increases it (via norepinephrine).

What does the parasympathetic nervous system do?

Decreases it (via acetylcholine and vagus nerve).

What do baroreceptors do?

Detect blood pressure changes to adjust HR (e.g., drop = HR increases).

What do chemoreceptors do?

Detect low O2, high CO2, or acidosis → increase HR.

What factors affect HR?

Emotions (stress = ↑ HR), Exercise (↑ HR via epi & nerves), Temperature (heat ↑, cold ↓), Pain (can ↓ HR → fainting).

What is stroke volume?

Amount of blood pumped by one ventricle per beat (~70 mL).

What affects SV?

Nervous & hormonal signals, Blood return/stretch (preload), Electrolyte balance, Inotropic drugs.

What is Starling’s Law?

The more the heart fills (stretches), the stronger it contracts (to a point).

What is preload?

Volume of blood in ventricles before contraction.

What is afterload?

The pressure the heart must pump against (e.g., high BP = ↑ afterload).