Reptilian Hearts: The Ultimate Survival Machines That Can Keep
Beating Without Oxygen, Brain, or Body
Turtles are fascinating
creatures that can live for a long time, sometimes even longer than humans. But
what happens when they die? Do they stop moving immediately, or do they keep
twitching for a while? And what about their hearts? Do they stop beating as
soon as the turtle is declared dead, or do they keep pumping blood for hours,
days, or even weeks?
In this article, we will
explore the amazing phenomenon of reptilian hearts that can keep beating after
death, even when separated from the body. We will also look at the reasons
behind this phenomenon, such as the slow metabolism, high concentration of
ions, and special cardiac tissue of reptiles. We will focus on turtles as an
example, but also mention other reptiles that share this trait.
Part 1: How to Tell If a
Turtle Is Dead or Alive
Before we dive into the
topic of post-mortem heartbeats, we need to address a more basic question: how
can we tell if a turtle is dead or alive? This may seem like an easy question,
but it is actually quite tricky. Turtles are notoriously difficult to diagnose
for death, because they have several adaptations that allow them to survive in
harsh conditions and appear lifeless.
One of these adaptations is
hibernation. Turtles can lower their body temperature and metabolic rate to
conserve energy and survive cold winters. During hibernation, they may bury
themselves in mud or sand, or hide under rocks or logs. Their breathing and
heartbeat become very slow and faint, and they may not respond to external
stimuli. To an inexperienced observer, they may look dead, but they are
actually alive and waiting for warmer weather.
Another adaptation is anoxia
tolerance. Turtles can survive without oxygen for a long time, thanks to their
ability to store glycogen in their liver and muscles, and use anaerobic
metabolism to produce energy. They can also reduce their oxygen demand by
shutting down non-essential organs and functions. This allows them to stay
underwater for hours or even days without coming up for air. Some turtles can
even breathe through their cloaca (the opening for excretion and reproduction)
or their skin, by absorbing dissolved oxygen from the water. When turtles are
exposed to anoxia (lack of oxygen), they may enter a state of torpor (reduced
activity and responsiveness), which can also make them look dead.
A third adaptation is
decapitation survival. Turtles can survive having their heads cut off for a
short period of time, because their brains are not essential for controlling
their basic bodily functions. Their spinal cord and peripheral nerves can still
send signals to their muscles and organs, allowing them to move and breathe.
Their hearts can also keep beating without any input from the brain, thanks to
their special cardiac tissue that we will discuss later.
These adaptations make it
hard to determine if a turtle is dead or alive by just looking at it. Some
signs that may indicate death are:
- Rigor
mortis (stiffening of the muscles)
- Putrefaction
(decomposition of the body)
- Livor
mortis (pooling of blood in the lower parts of the body)
- Algor
mortis (cooling of the body)
- Clouding
of the eyes
- Absence
of reflexes
- Absence
of heartbeat
However, these signs are not
always reliable or conclusive. Rigor mortis may not occur in cold-blooded
animals like turtles, or may be delayed by low temperatures. Putrefaction may
also be slowed down by low temperatures or dry environments. Livor mortis may
not be visible in turtles with dark shells or skin. Algor mortis may not be
significant in ectothermic animals like turtles, whose body temperature depends
on the environment. Clouding of the eyes may be caused by dehydration or
injury, not necessarily death. Absence of reflexes may be due to hibernation or
torpor, not necessarily death. And absence of heartbeat may be due to faintness
or irregularity, not necessarily death.
Therefore, to confirm if a
turtle is dead or alive, it is necessary to perform a thorough physical
examination and use diagnostic tools such as stethoscope, electrocardiogram
(ECG), ultrasound, or necropsy (post-mortem examination). However, even these
methods may not be definitive, as we will see in the next part.
Part 2: How the Reptilian Heart Keeps Beating After Death
One
of the most remarkable features of reptilian hearts is their ability to keep
beating after death, even when separated from the body. This phenomenon has
been observed in various reptile species, such as turtles, snakes, lizards, and
crocodiles. But how is this possible? What makes the reptilian heart so
resilient and independent?
The
answer lies in the structure and function of the reptilian cardiac tissue.
Unlike mammalian hearts, which have a specialized pacemaker region called the
sinoatrial node (SA node) that initiates and regulates the heartbeat, reptilian
hearts have multiple pacemaker regions distributed throughout the atria and
ventricles. These regions are composed of nodal cells, which are modified
cardiac muscle cells that can generate spontaneous electrical impulses without
any external stimulation. These impulses travel through specialized conducting
fibers called nodal tissue, which connect the different pacemaker regions and
coordinate their activity.
The
main pacemaker region in reptiles is located in the sinus venosus (SV), a
thin-walled chamber that receives blood from the body and empties into the
right atrium. The SV generates impulses at a regular rate and sends them to the
atria and ventricles through the nodal tissue. However, if the SV is damaged or
removed, other pacemaker regions can take over and maintain the heartbeat. The
most important of these backup pacemakers are located in the atrioventricular
node (AV node), which is situated at the junction of the atria and ventricles,
and in the ventricular apex (VA), which is located at the tip of the ventricle.
The
AV node and VA can generate impulses independently of the SV, but at a slower
rate. The AV node also acts as a gatekeeper that regulates the transmission of
impulses from the atria to the ventricles, preventing them from contracting too
fast or too slow. The VA can also influence the contraction of the ventricle by
sending retrograde impulses (backward impulses) to the rest of the cardiac
tissue.
The
presence of multiple pacemaker regions and conducting fibers in reptilian
hearts gives them a high degree of autonomy and adaptability. They can adjust
their rate and rhythm according to various factors, such as temperature, oxygen
level, hormonal status, and neural input. They can also survive injury or ischemia
(lack of blood supply) better than mammalian hearts, because they have more
backup systems and alternative pathways for electrical conduction.
This
also explains why reptilian hearts can keep beating after death, even when
separated from the body. As long as they have enough oxygen and nutrients, they
can generate their own impulses and contract without any input from the brain
or nervous system. They can also resist decay longer than other organs, because
they have a low metabolic rate and a high concentration of ions that prevent
bacterial growth.
However,
this does not mean that reptilian hearts are immortal or invincible. They still
depend on external factors for optimal functioning, such as temperature
regulation, hormonal balance, and neural modulation. They also have limits to
their endurance and resilience, especially when exposed to extreme conditions
or prolonged stress. Therefore, it is important to monitor and evaluate their
health and performance using appropriate diagnostic tools and methods.
Conclusion:
The
reptilian heart exhibits remarkable post-mortem contractility, even when
isolated from the body. This phenomenon is attributed to the distinctive
structure and function of the reptilian cardiac tissue, which has multiple
pacemaker regions and conducting fibers that can generate and coordinate
electrical impulses autonomously of the brain or nervous system. The reptilian
heart also possesses adaptations that enable it to withstand harsh conditions
and exhibit apparent lifelessness, such as hibernation, anoxia tolerance, and
decapitation survival. However, these adaptations also pose challenges to the
diagnosis of death in reptiles, and necessitate a comprehensive physical
examination and diagnostic tools such as stethoscope, electrocardiogram,
ultrasound, or necropsy to assess their cardiac status. Reptile cardiology is
an emerging field of veterinary medicine that requires further research and
application to improve the health and welfare of captive reptiles.
Sources:
Kik
MJL, Mitchell MA. Reptile Cardiology: A Review of Anatomy and Physiology,
Diagnostic Approaches, and Clinical Disease. Seminars in Avian and Exotic Pet
Medicine. 2005;14(1):52-60. https://vetmed.illinois.edu/mmitch/pdf/reptilecardiology.pdf
Crossley DA II. Reptilian cardiovascular anatomy and physiology:
evaluation and monitoring (Proceedings). DVM360. 2009. https://www.dvm360.com/view/reptilian-cardiovascular-anatomy-and-physiology-evaluation-and-monitoring-proceedings
Rupprecht C, Kühn C, Witten PE, et al. Reptilian heart
development and the molecular basis of cardiac chamber evolution. Nature.
2009;461(7260):95-98. https://www.nature.com/articles/nature08324
Johnson A, Clinton J, Stevens R. Turtle heart beats five days
after death. Amer Biol Teacher. 1957;19(6):176-177. https://online.ucpress.edu/abt/article/19/6/176/4675/Turtle-Heart-Beats-Five-Days-after-Death
Here are some photos that were taken during a necropsy we did on a turtle with some of our Vet Program students:
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#vetstudent #vetschool #reptilecardio #reptileanatomy #reptilephysiology
#reptilediagnosis #reptiledisease #postmortemheart #reptilecare #reptilelove
