Lesson 6 of 7 / Transport in humans
How heart contraction and valves move blood
What makes each valve open or close during a heartbeat?
In this lesson: Explain how contraction, relaxation and valve action produce one-way flow through the heart.
About 5 min
The key ideaContraction raises chamber pressure; relaxation lowers it. Pressure differences determine flow and valve position.
Both ventricles work at the same time. Brief transitions with both valve sets closed are not shown; pressure determines when each valve changes position.
Explanation
A cardiac cycle is one sequence of contraction and relaxation. Systole refers to contraction and diastole to relaxation, so specify whether you mean the atria or ventricles.
During filling, relaxed ventricles have lower pressure than the atria. The atrioventricular valves open and blood flows into the ventricles. Atrial systole then pushes in the remaining blood while the ventricles are still relaxed.
The ventricles contract and their pressure rises above atrial pressure, closing the atrioventricular valves. When ventricular pressure exceeds pressure in the outgoing arteries, the semilunar valves open and blood is ejected. Both ventricles contract together.
As the ventricles relax, their pressure falls below arterial pressure and the semilunar valves close. Once ventricular pressure falls below atrial pressure, the atrioventricular valves reopen and filling resumes.
There are brief transitions when both valve sets are closed while pressure changes. The diagram focuses on filling, atrial contraction and ejection, rather than implying that semilunar valves remain open throughout every moment of systole.
Step by step
- 1
Name the contracting chamber
Atrial and ventricular systole are different phases.
- 2
Compare pressures
Blood tends to move from higher pressure to lower pressure.
- 3
Set the valves
Explain which opening permits forward flow and which closure prevents backward flow.
Worked example
The start of ventricular contraction
Why does ventricular contraction not normally force blood back into the atria?
One way to explain it
Contraction raises ventricular pressure above atrial pressure. This pressure difference closes the atrioventricular valves, preventing backflow into the atria. Blood can be ejected into the arteries once ventricular pressure exceeds arterial pressure.
Why this answer works
- Start with muscle contraction.
- Use the pressure comparison to explain valve closure.
- Do not say both valve sets open at the same time.
Is this true? "All four chambers contract at the same time."
The atria contract together first; the ventricles then contract together. This sequence supports filling followed by ejection.