During an erection vs menstruation:
An adult person has an average of about 5 Liters [≈ 5'000 mL] of circulating blood volume.
During a full erection, the penis contains about 100-150 mL of blood. So an erection sequesters ~2–3% of total blood volume, which is physiologically trivial. In any case, an erection amounts to a temporary sequestration of blood {redistribution; not consumption} because during a penile erection, blood is reassigned due to localised swelling, not totally lost. Thus after detumescence, the blood returns to normal circulation anyway and the overall blood loss = 0% net.
The human body routinely tolerates much larger "missing" volumes of blood for example:
- During menstruation, a female typically loses ~30-80 mL of blood per cycle (average ≈ 35 mL) {<2% of total blood volume}. This is why menstruation could be viewed as energetically expensive & evolutionarily unusual ('coz most mammals reabsorb the lining). Nonetheless, this loss is not dangerous because:
- it happens gradually (over days).
- the bone marrow rapidly replenishes red cells.
- the liver and gut tightly regulate any resulting iron imbalance.
- Blood donation removes ~450-500 mL [10 × more than menstruation] without harm. This loss consists of a fraction of approximately ~9-10% of total blood volume. The human body still takes the necessary time to recover from this:
- plasma volume is restored within 24-48 hours.
- red blood cells are replaced over 4-8 weeks.
- iron depletion may take weeks to months to fully replenish.
In medical terms, surgeons would consider a blood depletion of ~750-1500 mL (approximately 15-30% of total blood volume) to be clinically significant.
