Anatomy and Physiology 12 · Biology · Lesson 7.1
The heart & cardiac cycle
- Level
- Grade 12
- Reading time
- 15 min reading
- Review
- Last reviewed 2026-09-03
- Standard
- Accuracy standard
- Errata
Abstract
Structure and the heartbeat.
What is the cardiac cycle?
Short answer
The cardiac cycle is one complete heartbeat, the sequence of contraction and relaxation that pumps blood. During systole the heart muscle contracts and forces blood out of the chambers; during diastole it relaxes and refills. The atria contract first to fill the ventricles, then the ventricles contract to eject blood to the lungs and body, with valves preventing backflow.
Concept preview
What you will be able to do
- 01Identify the four chambers and four valves of the heart and trace the path of blood through it.
- 02Distinguish pulmonary from systemic circulation and explain why a double circulation is efficient.
- 03Describe the events of the cardiac cycle (systole and diastole) and relate them to the heart sounds.
- 04Explain how the SA node and the conduction system set and coordinate the heartbeat.
Clench your fist and you are holding a fair model of your own heart — a hollow, muscular pump a little larger than a fist that sits behind the breastbone, tilted slightly to the left. It is built almost entirely of a special tissue called cardiac muscle, or myocardium, which contracts rhythmically without ever tiring and without any conscious command. Beating on average about seventy times a minute, the heart contracts more than a hundred thousand times a day and roughly two and a half billion times across a human lifetime, driving blood to every one of the body's trillions of cells. No pump built by engineers comes anywhere close to this record of tireless, self-repairing reliability, and understanding how it achieves it is the goal of this topic.
School Plan
The heart & cardiac cycle
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Related topics
Frequently asked questions
- What is the difference between systole and diastole?
- Systole is the contraction phase that pumps blood out; diastole is the relaxation phase that lets the chambers refill.
- What do the heart valves do?
- They ensure one-way flow, opening to let blood through and closing to prevent it flowing backward between beats.