Step 1: Understanding the Concept:
A heart murmur is an extra sound heard between or during the normal heartbeat sounds, and it points to turbulent, rather than smooth, blood flow inside the heart.
Step 2: Key Formula or Approach:
Blood flow becomes turbulent mainly when it is forced through an abnormally narrow opening or flows backward through a gap that should be sealed. The structures that open and close to control flow direction between heart chambers, and between the heart and major vessels, are the heart valves.
Step 3: Detailed Explanation:
When a valve narrows, as in mitral or aortic stenosis, blood is squeezed through a smaller opening and speeds up, creating turbulence and a murmur. When a valve fails to close fully, as in mitral or aortic regurgitation, blood leaks backward against the normal direction of flow, again creating turbulence. The SA node and AV node instead control the heart's electrical timing, so their disorders show up as irregular rhythms, not murmurs, and the pulmonary vein is a low-pressure return vessel not directly linked to murmur formation.
Step 4: Final Answer:
Murmurs are generally heard in disorders affecting the heart valves.