# Cardiac Cycle

> CIE A-Level Biology · Transport in Mammals
> Source: https://www.owlsprep.com/study/cie-9700-u8-cardiac-cycle/

This module explains the sequential contraction and relaxation of heart chambers that drives unidirectional blood flow around the mammalian circulatory system. You will learn how pressure and volume changes control valve movement and how to interpret exam-style cardiac cycle graphs.

**Prerequisites:** [Mammalian heart structure](https://www.owlsprep.com/study/cie-9700-u8-mammalian-heart-structure/)

## Learning objectives

- Describe the full sequence of events in one mammalian cardiac cycle
- Relate pressure and volume changes to valve opening and closing
- Interpret standard pressure graphs of the cardiac cycle
- Explain how unidirectional blood flow is maintained

## Overview and Key Definitions

The cardiac cycle is one complete sequence of heart contraction and relaxation, repeating approximately 75 times per minute at rest in healthy adult humans. It is divided into two core phases: systole (contraction of a chamber) and diastole (relaxation of a chamber).

**Cardiac cycle** — A repeating sequence of contraction and relaxation of all heart chambers that completes one full heartbeat, producing one pulse of blood flow around the circulatory system.

*Example:* A resting heart rate of 75 bpm means 75 full cardiac cycles occur per minute.

**Worked example:** A person has a resting heart rate of 60 beats per minute. Calculate the duration of one full cardiac cycle.

1. Recall that one heartbeat equals one complete cardiac cycle. Convert heart rate to duration using the formula:
2. $$\text{Cycle duration (seconds)} = \frac{60}{\text{Heart rate (beats per minute)}}$$
3. Substitute the given heart rate:
4. $$\text{Duration} = \frac{60}{60} = 1\ \text{second}$$

> **Exam tip:** Always confirm heart rate is in beats per minute before calculating cardiac cycle duration in exams.

## Sequence of Events

The cardiac cycle follows a fixed ordered sequence that ensures unidirectional blood flow, from veins to atria to ventricles to arteries. Atria always contract before ventricles to allow complete filling of the ventricles before ejection.

1. **Mid-to-late diastole**: All chambers are relaxed. ~70% of blood flows passively from veins into atria, then through open atrioventricular (AV) valves into ventricles.
2. **Atrial systole**: Atria contract, pushing the remaining ~30% of blood into ventricles to complete ventricular filling.
3. **Ventricular systole**: Ventricles contract, increasing internal pressure. AV valves close first, then semilunar valves open to eject blood into the aorta and pulmonary artery.
4. **Early diastole**: Ventricles relax, pressure drops. Semilunar valves close, and the cycle begins again as ventricles start passive filling.

**Worked example:** Describe what happens immediately after ventricular systole begins.

1. When ventricles start contracting, ventricular muscle shortens, decreasing ventricular volume and rapidly increasing ventricular pressure.
2. Ventricular pressure quickly rises above the pressure in the atria.
3. This pressure difference pushes the cusps of the AV valves closed, preventing backflow of blood into the atria.
4. Ventricular pressure continues to rise until it exceeds the pressure in the aorta and pulmonary artery, pushing semilunar valves open to eject blood.

> **tip**
>
> Valve movement is always driven by pressure differences, not active muscle contraction. This is a common exam point.

## Pressure, Volume and Valve Regulation

All events in the cardiac cycle follow pressure gradients: blood always flows from a region of higher pressure to a region of lower pressure. Valves open or close to maintain this unidirectional flow, preventing backflow.

**Pressure gradient** — A difference in pressure between two adjacent regions, which drives bulk flow of fluid from higher to lower pressure.

**Worked example:** Explain why semilunar valves close at the end of ventricular systole.

1. At the end of ventricular systole, ventricles start to relax, so ventricular volume increases and ventricular pressure drops rapidly.
2. Pressure in the aorta and pulmonary artery is now higher than pressure in the relaxed ventricles.
3. This pressure difference pushes the semilunar valve cusps closed, preventing backflow of blood from the arteries back into the ventricles.

## Interpreting Cardiac Cycle Graphs

CIE exams very frequently ask candidates to interpret graphs plotting pressure changes in the left atrium, left ventricle and aorta over one cardiac cycle. Key events are always marked by the intersection of pressure lines.

- AV valves close when ventricular pressure > atrial pressure
- Semilunar valves open when ventricular pressure > aortic pressure
- Semilunar valves close when ventricular pressure < aortic pressure
- AV valves open when ventricular pressure < atrial pressure

**Worked example:** On a cardiac cycle pressure graph, the left ventricular pressure line crosses above the left atrial pressure line. What event does this correspond to?

1. Before the crossing, left ventricular pressure is lower than left atrial pressure, so the AV valve is open.
2. After the crossing, left ventricular pressure is higher than left atrial pressure.
3. Higher pressure on the ventricular side of the AV valve pushes the valve closed.
4. The crossing therefore corresponds to closure of the left AV (bicuspid) valve, at the start of ventricular systole.

**Exam command terms**

Common command terms for this topic in CIE A-Level Biology:

- **Describe** — State the sequence of events, linking pressure changes to valve movement *(Describe the stages of the cardiac cycle)*

- **Explain** — Link pressure differences to valve movement and blood flow direction *(Explain why AV valves close during ventricular systole)*

> **Exam tip:** Always label which pressure line corresponds to which chamber/artery when drawing or interpreting graphs in exams.

## Common pitfalls

- **Wrong:** Claiming that valves open and close via active muscle contraction
  - Why it fails: Valves are passive connective tissue structures with no muscle tissue
  - Correct: Explain all valve movement as a result of pressure differences across the valve
- **Wrong:** Stating that all blood entering ventricles is pumped by atrial contraction
  - Why it fails: Over 70% of ventricular filling occurs passively during diastole before atrial contraction
  - Correct: State that atrial contraction only completes ventricular filling, adding the final ~30% of blood
- **Wrong:** Claiming atria and ventricles contract at the same time
  - Why it fails: Sequential contraction is required for full ventricular filling before ejection
  - Correct: Always describe atrial systole occurring before ventricular systole
- **Wrong:** Confusing which pressure crossing corresponds to which valve event
  - Why it fails: Many students memorize events without linking them to pressure gradients
  - Correct: Remember: a valve opens when pressure is higher on the upstream side, and closes when pressure is higher on the downstream side

## Cheatsheet

| Stage | Atria State | Ventricles State | AV Valves | Semilunar Valves |
| --- | --- | --- | --- | --- |
| Mid-diastole | Relaxed | Relaxed | Open | Closed |
| Atrial systole | Contracted | Relaxed | Open | Closed |
| Early ventricular systole | Relaxed | Contracted | Closed | Closed |
| Ventricular ejection | Relaxed | Contracted | Closed | Open |
| Early diastole | Relaxed | Relaxed | Closed | Closed |

## What's next

Understanding the cardiac cycle is foundational for learning how heart function is regulated and how common cardiovascular conditions develop. Next, you can explore how electrical activity in the heart initiates and coordinates the contractions that make up the cardiac cycle, which explains how pacemaker cells set resting heart rate. This topic also links closely to how the body adjusts cardiac output during exercise, a frequently tested exam topic that builds directly on the basic sequence of the cardiac cycle you learned here. Mastery of pressure and volume changes is also required to understand heart murmurs and how structural heart defects affect blood flow.

- [Blood vessels structure and function](https://www.owlsprep.com/study/cie-9700-u8-blood-vessels-structure-and-function/)
- [Gas transport in blood](https://www.owlsprep.com/study/cie-9700-u8-gas-transport-in-blood/)

---

From [OwlsPrep](https://www.owlsprep.com) — free study guides for A-Level, IB, AP and IGCSE, written against the official syllabus. Canonical page: https://www.owlsprep.com/study/cie-9700-u8-cardiac-cycle/