Stages In The Cardiac Cycle
- 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.
- It consists of three key stages:
- Atrial Systole – Atria contract, pushing blood into the ventricles.
- Ventricular Systole – Ventricles contract, ejecting blood into the arteries.
- Diastole – The heart relaxes, refilling with blood.
This cycle is controlled by the sinoatrial node (SA node), the heart’s pacemaker, which generates electrical impulses to keep the heart beating in sync.
Stage 1: Atrial Systole (Atria Contract)
- What Happens?
- The SA node fires an electrical impulse, triggering contraction of the left atrium.
- This increases pressure, forcing blood through the mitral valve into the left ventricle.
- The ventricles remain relaxed, allowing efficient filling.
- Key Features
- Duration: ~0.1 seconds.
- Valves: Mitral valve open, semilunar valve closed.
- Blood Flow: Left atrium → Left ventricle.
- The cardiac cycle occurs simultaneously in both sides of the heart.
- However, for clarity, we will focus on the left side, which pumps oxygenated blood to the body.
- Think of this stage as filling a water balloon.
- The atrium contracts like a hand squeezing a pump, pushing blood into the ventricle (the balloon).
Stage 2: Ventricular Systole (Ventricles Contract)
- What Happens?
- The AV node transmits the signal via the Bundle of His & Purkinje fibers.
- The left ventricle contracts, rapidly increasing pressure.
- The mitral valve closes (preventing backflow).
- When ventricular pressure > aortic pressure, the semilunar valve opens, ejecting blood into the aorta.
- Key Features
- Duration: ~0.3 seconds.
- Valves: Mitral valve closed, semilunar valve open.
- Blood Flow: Left ventricle → Aorta → Body.
The closure of the mitral valve and the opening of the semilunar valve ensure that blood flows in only one direction during ventricular systole.
Stage 3: Diastole (Heart Relaxes & Refills)
- What Happens?
- The ventricles relax, reducing pressure.
- The semilunar valve closes (prevents backflow from the aorta).
