B3.2.16 Stages in the cardiac cycle (HL) Notes

Stages In The Cardiac Cycle

1. The cardiac cycle is the sequence of events in one heartbeat, ensuring oxygen-rich blood is delivered to tissues while deoxygenated blood is sent to the lungs.
2. It consists of three key stages:
   1. Atrial Systole – Atria contract, pushing blood into the ventricles.
   2. Ventricular Systole – Ventricles contract, ejecting blood into the arteries.
   3. Diastole – The heart relaxes, refilling with blood.

Stage 1: Atrial Systole (Atria Contract)

1. What Happens?
   1. The SA node fires an electrical impulse, triggering contraction of the left atrium.
   2. This increases pressure, forcing blood through the mitral valve into the left ventricle.
   3. The ventricles remain relaxed, allowing efficient filling.
2. Key Features
   1. Duration: ~0.1 seconds.
   2. Valves: Mitral valve open, semilunar valve closed.
   3. Blood Flow: Left atrium → Left ventricle.

Stage 2: Ventricular Systole (Ventricles Contract)

1. What Happens?
   1. The AV node transmits the signal via the Bundle of His & Purkinje fibers.
   2. The left ventricle contracts, rapidly increasing pressure.
   3. The mitral valve closes (preventing backflow).
   4. When ventricular pressure > aortic pressure, the semilunar valve opens, ejecting blood into the aorta.
2. Key Features
   1. Duration: ~0.3 seconds.
   2. Valves: Mitral valve closed, semilunar valve open.
   3. Blood Flow: Left ventricle → Aorta → Body.

Stage 3: Diastole (Heart Relaxes & Refills)

1. What Happens?
   1. The ventricles relax, reducing pressure.
   2. The semilunar valve closes (prevents backflow from the aorta).
   3. The mitral valve reopens, allowing blood to flow from the left atrium into the ventricle.
   4. Most ventricular filling is passive, driven by pressure differences.
2. Key Features
   1. Duration: ~0.4 seconds.
   2. Valves: Mitral valve open, semilunar valve closed.
   3. Blood Flow: Left atrium → Left ventricle (passively).

Systolic vs. Diastolic Blood Pressure

Blood pressure is measured in mmHg (millimeters of mercury) and recorded as systolic pressure/diastolic pressure (e.g., 120/80 mmHg).

Systolic Blood Pressure (SBP)

1. The maximum pressure exerted on arterial walls during ventricular systole.
2. Normal SBP is around 120 mmHg.
3. A high SBP may indicate hypertension (high blood pressure), increasing cardiovascular risk.

Diastolic Blood Pressure (DBP)

1. The minimum pressure in the arteries during ventricular diastole, when the heart is relaxed.
2. Normal DBP is around 80 mmHg.
3. A low DBP may indicate reduced arterial elasticity or excessive relaxation of blood vessels.

Interpreting Blood Pressure Data

1. Blood pressure is typically represented as a ratio of systolic/diastolic (e.g., 120/80 mmHg).
2. This data can be represented in graphs to analyze variations in pressure throughout the cardiac cycle.

1. Systolic Pressure Peak: The peak point on the graph represents when the ventricles contract, forcing blood into the arteries.
2. Diastolic Pressure Bottom: The lowest point on the graph represents when the heart is relaxed and the pressure in the arteries is at its lowest point.

Why Is the Cardiac Cycle Important?

1. Ensures continuous circulation → Oxygen & nutrients reach tissues.
2. Prevents backflow → Heart valves direct blood efficiently.
3. Disruptions = Serious conditions → Arrhythmias, valve defects, heart failure.