Regulatory mechanisms are effective enough that a bleeding patient may remain asymptomatic until roughly what fraction of blood volume is lost?

1. Falling blood volume lowers both pressure and flow, and that state is called hypovolemia. 2. Compensating mechanisms defend blood pressure so well that early losses produce no symptoms. 3. A patient may stay asymptomatic until 10 to 20 percent of blood volume has gone. 4. This is exactly why an apparently comfortable trauma patient still needs intravenous fluid replacement. 5. Losses approaching half of blood volume would be far past the silent window and well into shock. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 20 "The Cardiovascular System: Blood Vessels and Circulation", section 20.2 Blood Flow, Blood Pressure, and Resistance_