This NY Times article about the body's reaction to risks explains it well:
To get an inkling of how this physiology works, consider the following scenario, in which a trader grapples with a rumor that the Fed may raise rates later that afternoon:
As 2:15 — the time of the announcement — approaches, trading on the screens dwindles. The floor goes quiet. The trader feels intellectually prepared. But the challenge he faces requires more than cognitive skill. He needs fast reactions, and energy for the hours ahead.
Consequently, his metabolism speeds up, ready to break down energy stores in liver, muscle and fat cells. Breathing accelerates, drawing in more oxygen, and his heart rate speeds up. Cells of the immune system take up position at vulnerable points of the body, ready to deal with injury and infection. And his nervous system, extending from the brain down into the abdomen, redistributes blood — constricting flow to the gut, giving him butterflies, and to the reproductive organs, since this is no time for sex — shunting it to major muscle groups in the arms and thighs as well as to the lungs, heart and brain.
The announcement will bring volatility, and a chance to make money. The trader feels a surge of energy as steroid hormones are synthesized by their respective glands and injected into his bloodstream. Steroids are powerful, dangerous chemicals — they change almost every detail of body and brain: his growth rate, lean-muscle mass, mood, even the memories he recalls — and for that reason their use is tightly regulated by the International Olympic Committee and the hypothalamus, the brain’s drug enforcement agency.
These past hours, the trader’s testosterone levels have been climbing. This steroid hormone, produced by men (and, in lesser quantities, by women) primes the trader for the challenge ahead, just as it does athletes preparing to compete and male animals to fight. Rising levels increase confidence and, crucially, appetite for risk. For the trader this is a moment of transformation, what the French since the Middle Ages have called “the hour between dog and wolf.”
The stress hormones adrenaline and cortisol surge out of the adrenal glands, and the cortisol travels to the brain, where it stimulates the release of dopamine, a chemical operating along neural circuits known as the pleasure pathways. At high levels, cortisol provides a nasty, stressful experience. But in small amounts, in combination with dopamine — one of the most addictive drugs known to the human brain — it delivers a narcotic hit, a rush that convinces traders that there is no other job in the world.
Finally, at 2:14, the trader leans into his screen, pupils dilated, breathing rhythmic, muscles coiled, body and brain fused for impending action. An expectant hush descends on global markets.
This scenario illustrates just how sensitive the body is to information. We do not regard prices on a screen as a computer would, dispassionately; we react physically. Our body and brain rev up and down together, and this natural fusion makes us better risk-takers.
So yes, my body does respond to risk. And it can't just keep taking a beating. So even though my mind can handle things, sometimes my body can't, and vice versa.