Physiologic Time & Real Time
As defined by Mordenti:
"...physiologic time is measured by biologic clocks which use internal, physiologic parameters such as heartbeats, breath cycles or blood circulation velocities as units of measurement. When physiologic events in different mammals are measured by biological clocks, they occur in equivalent physiologic time.... Thus, the lifespan of an elephant and a mouse is the same when measured with a biological clock (i.e., heartbeats), although their life spans vary significantly when measured in years."
A neat explanation of the term comes from Stephen Jay Gould. He asks us to muse upon the allotted lifetimes of mammals. Gerbils live only a few years, most dogs get beyond twelve, elephants and the big whales live about as long as we do. If, says Gould, our reaction is pity for the shorter-lived species, we should consider just how we should measure the length of a life.
The calendar and clock we use for measuring 'real' time are misleading. Think about heartbeats and respiration, says Gould. Your heart or mine might beat seventy-two times per minute, while we breathe eighteen times per minute. The next time you hold your cat, take note of its pulse and breath. Both are much more rapid than yours.
It holds that any mammal's heart beats about four times per breath. However, pulse and respiration each decrease roughly as the fourth root of body mass. That means a 25lb (11kg) whippet breathes twice as fast as a 400lb (180kg) gorilla.
So we should instead look at measuring time by the number of heartbeats of an animal - it then turns out that all animals live for roughly a billion heartbeats. With a heartbeat clock, our lifetimes would all be roughly the same. Humans are something of an anomoly, but for a number of well-established. Even so, even the human lifespan of 2.5 billion beats is within the same order-of-magnitude as the rest of the animal kingdom.
Gauging life in heartbeats or breaths reveals, once more, how deeply subjective time is. The second law of thermodynamics tells us that we experience time moving only from past to future. That's because the world we perceive is statistically irreversible. And the rate at which we see time moving in only one direction, is dictated by our hearts and lungs.
The content on this site has been drawn from a wide range of Web & printed resources and edited/summarised to form a single point of reference and overview of the Billion Heart Beats.