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"This relationship could be a compensatory mechanism where lower lung function is compensated by higher cardiac output and higher brain perfusion," he said.

Many of the signs of aging are invisible, slow, and subtle — changes in cell division capacity, cardiac output and kidney function don’t exactly show up in the mirror.

"A pulse rate of 240bpm, the cardiac output, no radial pulse - I thought that would attract cardiologists from all over the country," said Mr Christie.

Pregnant people’s hearts also work more — a 50 percent increase in cardiac output.

“Therefore, any factors that hamper evaporation, such as high humidity, reduced cardiac output, reduced air flow — or some drugs — can cause body temperature to rise, which can culminate in life-threatening heatstroke.”