Head-out immersion causes a blood shift from the limbs and into the thorax. The fluid shift is largely from the extravascular tissues and the increased atrial volume results in a compensatory diuresis. Plasma volume, stroke volume, and cardiac output remain higher than normal during immersion. The increased respiratory and cardiac workload causes increased blood flow to the cardiac and respiratory muscles. Stroke volume is not greatly affected by immersion or variation in ambient pressure, but bradycardia reduces the overall cardiac output, particularly due to the diving reflex in breath-hold diving.