Question

The most important factor in hemostasis preventing PPH after placental separation is due to

• A. Uterine contraction and relaxation
• B. Retraction of the uterus
• C. Thrombosis of blood vessels in the myometrium
• D. Retroplacental clot

Hint:

Remember that retraction, unlike contraction, leaves the uterine muscle permanently short.

Answer 1

The Correct Option is B

Step 1: Understanding the Question.
After the placenta separates from the uterine wall, open blood vessels (spiral arteries) are left behind at the placental site. The question asks what mainly stops these vessels from bleeding.

Step 2: Key Concept.
The uterine muscle fibers run in a criss-cross, interlacing pattern around the blood vessels. When these fibers shorten and stay shortened, a process called retraction, they act like small ligatures (ties) that squeeze the vessels shut. This is often called the living ligature mechanism.

Step 3: Detailed Explanation.
Contraction is a temporary shortening of muscle that relaxes again afterward, so contraction and relaxation alone cannot keep a vessel permanently closed; bleeding would restart every time the muscle relaxes.
Retraction is different: the muscle fibers stay short even after the contraction wave passes, because the fibers rearrange and do not fully lengthen back. This permanent shortening keeps the vessels compressed continuously and is the true reason bleeding stops after separation.
Thrombosis of the blood vessels helps seal them off, but it happens after the vessels are already compressed by retraction; it is a secondary, supporting step, not the main mechanism.
The retroplacental clot is the clot that collects behind the placenta once it separates; it is a result of the bleeding process, not the cause of hemostasis.

Step 4: Final Answer.
The living ligature action of retracted uterine muscle fibers is the main reason bleeding is controlled, so the answer is retraction of the uterus.