Why Snakes Don't Faint!
Practical Reptile Keeping|March 2017

Paul Donovan considers the remarkable ways in which snakes can function, thanks to their circulatory systems, which are highly tuned to reflect their individual lifestyles.

Paul Donovan

When I was writing an article on giraffes for another magazine, it raised the question - when a giraffe bends down, why does blood not rush to its brain? My mind then switched to snakes. When a snake climbs a tree, what prevents blood rushing to its tail? Equally, when one of these reptiles then descends back to the ground, why does blood not immediately rush to its head?

High and low blood pressure

The answer, of course, is that a system must be in place to prevent this from happening, because otherwise it would be extremely dangerous for the animal. And of course, there is! When lying flat on the ground, a terrestrial snake’s circulatory system works with relative ease, as gravitational pressure is not placing any undue stress on it.

But it becomes a different situation, however, once a snake climbs up or down a tree. This is when gravity comes into play, by increasing the demands on the cardiovascular system as, all of a sudden, the body has changed position significantly. When a snake begins to climb, the blood pressure in the centre of the body rises, that in the head area decreases, and blood begins to pool in the tail.

As the snake climbs up a tree, its blood vessels could suddenly deform, causing the walls to become distended as a result of gravitational pressure being placed on the system. This would then lead to plasma escaping from the capillaries. As blood pressure drops, vital organs would not receive sufficient blood, and would be at risk of failing, threatening the snake’s survival. These reptiles therefore have a circulatory system which is adapted to work against the forces of gravity.

The importance of posture

You might assume that it is only those snakes which climb up, over or down things which have to cope with gravitational influences on their bodies, but this is not the case. Take, for example, a rattlesnake which is threatened and adopts its defensive S-shaped posture. Its body has now taken on a series of more oblique angles which means its circulatory system must reflect, albeit on a temporary basis, the gravitational pressure gradient being imposed on it. The same is also true of course in the case of a cobra which suddenly rears up.

In these two instances, as the snake assumes a vertical position, both its blood flow and blood pressure will be affected. This is because the blood above the heart creates gravitational pressure against which the heart must work. As gravitational pressure is then trying to pull the blood down, the heart is working with the aim of pumping it up to this part of the body, with this task suddenly having become more difficult.

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