A valve opens when the pressure behind it is higher than the pressure in front of it, and shuts when the pressure in front is higher. That rule gives one-way flow, and it is how most valve questions are marked.
It follows the route in tracing the double circulation and supports the next lesson on vessels.
What do “behind” and “in front” mean?
“Behind” is the side the blood is coming from. “In front” is the side it is going to. A valve is like a door that only swings one way: a push from behind opens it and a push from the front slams it shut.
Between an atrium and its ventricle there is an atrioventricular valve. At the start of each large artery there is a semilunar valve. Many veins also have small pocket valves.
How to explain a valve, step by step
- Say which chamber or vessel is contracting or relaxing.
- Say where the pressure is higher.
- Say what the valve does, as a result of that difference.
- Say which way the blood flows, or what backflow is prevented.
Use “pressure” and “higher” in the answer. A sentence such as “the valve stops blood going back” gains less credit than one giving the pressure reason.
Worked example
Explain how the valves keep blood moving one way as the left ventricle contracts and then relaxes.
Step 1, ventricle contracts: the pressure in the left ventricle rises above the pressure in the left atrium. The atrioventricular valve shuts. This stops blood flowing back into the atrium.
Step 2, blood leaves: the pressure in the left ventricle rises above the pressure in the aorta. The semilunar valve opens and blood flows into the aorta.
Step 3, ventricle relaxes: the pressure in the left ventricle falls below the pressure in the aorta. The semilunar valve shuts, so blood in the aorta cannot return to the ventricle.
Step 4, refilling: when the pressure in the left atrium is higher than in the ventricle, the atrioventricular valve opens and the ventricle fills.
Notice that each step gives two pressures and one valve action. The pressure comparison is the reason part of the answer.
The mistake to watch for
A common slip is to write that the valve “pumps” or “pushes” blood, or that it “opens when the heart beats”.
Mistaken answer: “The valve pushes the blood into the aorta when the ventricle beats.”
A valve has no muscle and moves passively. The ventricle muscle makes the pressure. The valve only opens because of that pressure.
A better answer is: “The ventricle contracts, so the pressure in it becomes higher than in the aorta. This opens the semilunar valve and blood flows out.”
Check yourself
Try these, then open each answer.
1. The pressure in an atrium is 5 units and the pressure in the ventricle below it is 12 units. Is the valve between them open or shut? Explain.
Show answer
It is shut. The pressure in front of the valve (the ventricle, 12) is higher than behind it (the atrium, 5), so blood is pushed against the valve flaps and closes them.
2. Why do veins in the legs have valves, although the blood in them is at low pressure?
Show answer
The blood has to return upwards to the heart, and low pressure could let it flow backwards under gravity. The valves close when blood tries to flow back, so flow is one way towards the heart. Squeezing by leg muscles moves the blood forward.
3. Complete: “When the ventricle relaxes, the pressure in it becomes lower than in the artery, so the ______ valve ______.”
Show answer
“…so the semilunar valve shuts.” This stops blood flowing back from the artery into the ventricle.
Where this leads next
With one-way flow explained, move on to comparing vessels by structure and function, then try the circulation and blood practice set.
Some students can state the rule but lose marks when a question gives pressure values. A teacher in online one-to-one Biology tuition can work through several of these with you until the comparison becomes automatic.