What is the physical response to a high level chess game? Chess is used as an example of how the body can be influenced purely by brain activity:

In the middle of a chess Grand Master tournament these people are maintaining blood pressure for 6 hours' running that you only see in a marathon runner


These people are going through thousands of calories per day doing nothing more than thinking

and even that:

one of these chess Grand Masters who's just taken down an opponent ... has the exact same physiology as some ... baboon on the Savanna who's just ripped the stomach open of his rival!

Also note, it's mentioned here that Fischer was

built like an olympic athlete

In terms of heart rate, breathing, energy consumption ('calories'), and other biometric indicators, what are the typical physiological responses of the human body to an important, high-level classical chess game?
European Journal of Applied Physiology studied the physiological consequences of the tension caused by playing chess in 20 male chess players, by following heart rate, heart rate variability, and respiratory variables. We observed significant increase in the heart rate (75-86 beats/min), in the ratio low frequency (LF)/high frequency (HF) of heart rate variability (1.3-3.0) and also a decrease in mean heart rate variability with no changes in HF throughout the game. These results suggest a stimulation of the sympathetic nervous system with no changes in the parasympathetic system. The respiratory exchange ratio was rather elevated (over 0.89) at the start and significantly decreased during the game (0.75 at the end), indicating that energy expenditure progressively switched from carbohydrate to lipid oxidation. The changes in substrate oxidation and the sympathetic system seem to be due to high cognitive demands and bring new insight into adaptations to mental strain.