Location of the heart and blood supply in tree snakes

An employee of my laboratory, J. Donald recently showed that in skids and other tree snakes, the blood vessels located behind the heart are richly innervated, whereas there are significantly fewer nerve endings in the vessels in front of the heart. Nerves of various types can be represented in both vessels. Their purpose is not exactly known, but the very nature of the innervation of blood vessels is such that nerves probably play an important role in regulating muscle tone in the walls of blood vessels, especially in the back of the body, where blood can accumulate.


Nerves controlling vasoconstriction in most animals are part of reflex arcs starting from special sensory organs called baroreceptors. The localization of baroreceptors in snakes has not yet been established, however, it is believed that they are located in and around the heart, as in many other vertebrates in which they are found. Baroreceptors are located in the walls of blood vessels and are excited when the pressure in these vessels increases and the wall stretches. The arrival of nerve impulses from baroreceptors leads to a change in the rhythm of the heart and a contraction of the smooth muscles of the vessels; this reaction is a reflex, the central part of which is located in the brain. Simultaneous changes in the activity of the heart and vascular musculature leads to the fact that blood pressure returns to the previous level. This type of regulation is called negative feedback.


If the baroreceptors are really localized in or near the heart, then the location of the heart in the front of the body in tree snakes should give another advantage. The effect of gravitational pressure is most pronounced in the area of the ends of the body, and therefore the closer the baroreceptors are to the head, the more effectively they can be used to control changes caused by gravitational phenomena and maintain normal conditions for the brain.


However, there are two small drawbacks in the location of the heart very close to the head. Firstly, when a tree snake hangs from a branch head down, it may be difficult to maintain blood supply to the tail (although this is not as important as for the brain). Secondly, when the head is up, the blood from the tail should return to the heart, overcoming the heaviness of almost the entire column of blood in the veins. How did nature get around these difficulties?


Snakes, unlike mammals, have no valves in their veins that prevent the reverse flow of blood. Instead, they have three mechanisms that promote the upward movement of blood: contraction of vascular smooth muscles by signals from baroreceptors; contraction of skeletal muscles, squeezing veins; and finally, dense skin. When a tree snake crawls up for a long time, it periodically stops and wriggles in place, so that waves of muscle contractions run through the body from the back to the head. These contractions squeeze the veins, propel the blood forward and increase the central venous pressure (i.e. pressure in large veins) in the heart area. As a result, the flow of venous blood to the heart increases, and it can effectively maintain blood pressure.


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