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The reason why you feel lightheaded after a BM is due to the vasovagal effect.of the vagus nerve (The vagus nerve is located in the brainstem and snakes down from the brain to the heart and on through to the abdomen)

This is caused by straining which create a parasympathetic effect on the vagus nerve. which then release acetycholine and acetycholine tend to lower heart rate and blood pressure The vasovagal response is caused by excessive venous pooling that paradoxically results in vasodilatation and bradycardia rather than the appropriate physiologic responses of vasoconstriction and tachycardia.

The resulting bradycardia reduces cerebral blood flow to a level inadequate to maintain consciousness.

See Simple equation of sequence of events:

Straining>vagus nerve mediated> acetycholine>parasympathetic effects(vasodilation and bradycardia and hypotension or low blood pressure)>reduced blood flow to the brain > lightheadedness, fainting spell ( syncope)

One measure to combat this : avoid straining too hard and avoid constipation.