--- title: "The Cardiac Cycle and Pressure in the Heart" description: "Valves open and shut because of pressure differences, not because of nerve signals, and a pressure trace shows it. Reading the trace explains the order of atrial and ventricular contraction." canonical: https://lightmysky.com/learn/science/the-cardiac-cycle-and-pressure-in-the-heart-mt_iOSVyoV3Pj source: https://lightmysky.com/learn/science/the-cardiac-cycle-and-pressure-in-the-heart-mt_iOSVyoV3Pj.md retrieved: 2026-09-12 --- > **Agent view.** This is the Markdown twin of the page, for tools and assistants. > When to use this site, and the call that answers each job: https://lightmysky.com/agent-instructions.md > API description (OpenAPI 3.1): https://lightmysky.com/openapi.json · Authentication: https://lightmysky.com/auth.md > Pricing: https://lightmysky.com/pricing.md · Catalog: https://lightmysky.com/llms.txt · Full catalog: https://lightmysky.com/llms-full.txt > Every machine-readable file on this domain: https://lightmysky.com/.well-known/ai-catalog.json > Ask for Markdown with `Accept: text/markdown`, a `.md` address, or `?mode=agent`. # The Cardiac Cycle and Pressure in the Heart Valves open and shut because of pressure differences, not because of nerve signals, and a pressure trace shows it. Reading the trace explains the order of atrial and ventricular contraction. Subject: Science · Area: The Human Body · Ages 17 to 18 Page: https://lightmysky.com/learn/science/the-cardiac-cycle-and-pressure-in-the-heart-mt_iOSVyoV3Pj ## Ready when they can - Identifies systole and diastole on a pressure against time trace. - Explains each valve opening or closing from the pressure either side of it. - Relates the trace to the volume of blood leaving the ventricle in one beat. ## Lesson: Reading the heartbeat in pressure traces One cardiac cycle is a repeating sequence with a fixed order. The atria contract first in atrial systole, pushing the last blood into the ventricles. Then the ventricles contract powerfully in ventricular systole, ejecting blood into the arteries. Finally the whole heart relaxes in general diastole while the chambers refill. Systole means contraction and diastole means relaxation and filling. Valves never decide anything themselves. Each one simply obeys the pressure difference across it. When pressure behind a valve exceeds pressure ahead, blood pushes it open. When the gradient reverses, blood tries to flow back and the cusps slam shut. As the ventricle contracts, its pressure passes atrial pressure, so the atrioventricular valves shut with the first heart sound. Pressure keeps climbing until it passes artery pressure, so the semilunar valves open and ejection begins. **Example.** A pressure against time trace shows these stages as peaks and dips. Ventricular pressure shoots up during systole and falls during diastole. When ventricular pressure falls back below artery pressure, blood tries to re-enter and the semilunar valves close. Learn to point at each wave and name the stage, because trace questions always test that skill. The trace also tells you how much blood each beat delivers. Stroke volume is the volume leaving a ventricle in one contraction: the volume before systole minus the volume after. Say a ventricle holds 130 ml before systole and 60 ml after, giving 70 ml per beat. Multiply stroke volume by heart rate to get cardiac output per minute. **Recap.** Pressure differences open and shut each valve, and the trace lets you read every stage. ## Practice 8 questions on this page, each with its working shown. ## Needs first - [Heart Rate, Stroke Volume and Cardiac Output](https://lightmysky.com/learn/science/heart-rate-stroke-volume-and-cardiac-output-mt_mjrPFHgvfx) ## Opens up - [Tissue Fluid and Its Return to the Circulation](https://lightmysky.com/learn/science/tissue-fluid-and-its-return-to-the-circulation-mt_0jw7uXpK3C) - [Pacemaker Potentials, Conduction and Reading an ECG](https://lightmysky.com/learn/science/pacemaker-potentials-conduction-and-reading-an-ecg-mt_D-WKFc7BZm)