Pressure crossings, not muscle, open and close the valves in SC131's Unit 3 cardiac cycle explanation, which follows the left heart through one beat of about 0.8 seconds. Searches like "sc 131 unit 3 assignment example", "sc131 unit 3 sample" and "sc131 unit 3 example" land here.
What a finished SC131 Unit 3 cardiac cycle explanation looks like
Three or four pages built on a pressure-volume-time figure in the Wiggers style, with left atrial, left ventricular and aortic pressures on one axis, ventricular volume beneath, and the ECG and heart sounds aligned below that. The text follows the numbered phases. Late diastole: the mitral valve open, the ventricle filling passively, atrial contraction adding the final share. Isovolumetric contraction: ventricular pressure rises past atrial pressure, the mitral valve closes with the first heart sound, and volume holds at about [120] mL while pressure climbs. Ejection: ventricular pressure passes aortic pressure near [80] mmHg, the aortic valve opens, and pressure peaks near [120] mmHg. Isovolumetric relaxation: ventricular pressure drops below aortic, the aortic valve closes with the second sound, and volume holds near [50] mL until the mitral valve reopens.
How a SC131 Unit 3 example is structured
An opening paragraph states the governing rule, that blood moves from higher to lower pressure and valves open or close when the pressure across them reverses, and names the left side as the model for both. The phases follow in order, each paragraph naming the pressures on either side of each valve, which valve changes state, and what happens to volume. Electrical events are placed ahead of the mechanical ones they cause, the P wave before atrial contraction and the QRS complex before ventricular pressure rises. A calculation section derives stroke volume from the end-diastolic and end-systolic volumes and an ejection fraction from those, around [58] percent. A short section on the right heart notes the same sequence at lower pressures. The conclusion multiplies stroke volume by heart rate to reach cardiac output, anticipating the regulation covered later.
One rule governs every valve
Stating at the start that valves follow pressure differences lets every later event be predicted from the curves rather than memorized as a separate fact.
The isovolumetric phases explained
Both periods when all valves are closed are given their own paragraphs, since pressure changing while volume holds is the part of the cycle most answers compress.
Sounds matched to closures
The first and second heart sounds are tied to atrioventricular and semilunar valve closure, placed exactly on the figure where the pressure curves cross.
Electrical before mechanical
ECG waves are aligned slightly ahead of the contractions they trigger, showing the delay between depolarization and pressure change that the figure makes visible.
Volumes turned into output
End-diastolic and end-systolic volumes produce stroke volume and ejection fraction, connecting one beat's mechanics to the quantity the body regulates.
Where marks go in SC131 Unit 3
The single most damaging claim is that a valve opens because its chamber contracts, rather than because pressure reversed, when the explanation exists to show that valves are passive. Close behind is a cycle with the isovolumetric phases missing, contraction running straight into ejection, which makes the pressure curve impossible to explain. Heart sounds attributed to valves opening, or the second sound given to the mitral valve, cost accuracy marks. An aortic valve drawn opening before ventricular pressure exceeds aortic pressure is marked as a conceptual failure by most graders. ECG waves placed after the contractions they cause read as confusion about cause and effect. Stroke volume arithmetic without units, or an ejection fraction above one hundred percent, draws a smaller deduction. Unlabeled axes and uncited reference pressures cover the rest.
Get a SC131 Unit 3 example written to your instructions
Send the SC131 Unit 3 prompt and rubric together with any Wiggers diagram or data table your section distributes. The first custom explanation, free, takes the left heart through one full beat valve by valve with figure positions marked, delivered inside 24-48h. Let us know if the right heart is required as well.
SC131 Unit 3 questions, answered
Why are there two phases when no blood moves?
Because the ventricle has to change pressure before a valve can open. After the mitral valve closes, pressure must rise above aortic pressure before the aortic valve opens, and during that interval every valve is shut. The same happens in reverse after ejection. Explaining these phases as pressure changing at constant volume is what separates a strong answer from a summary.
Does the ECG belong in the explanation?
Most prompts expect it, at least briefly, because the electrical events trigger the mechanical ones. Placing the P wave before atrial contraction, the QRS before ventricular pressure rises and the T wave before relaxation shows the causal order. A detailed ECG interpretation is not required; the point is timing, and one aligned trace on the figure usually covers it.
What numbers should the explanation use?
Typical resting values for a healthy adult are standard: aortic pressure between about 80 and 120 mmHg, end-diastolic volume near 120 mL, end-systolic volume near 50 mL, and a cycle of about 0.8 seconds at 75 beats per minute. Stating them as approximate and using them to calculate stroke volume and ejection fraction shows the numbers are understood, not decorative.