Reptilian hearts differ significantly from those of mammals. Most reptiles possess three chambered hearts, with the exception
of crocodilians. The anatomy of the great vessels is quite different from that of mammals and can be confusing to uninitiated.
Adequate knowledge of normal anatomy and function is paramount in assessing health and performing certain clinical procedures.
Reptile cardiovascular physiology is also significantly different from that of mammals. Reptiles are much less susceptible
to the adverse effects of hypoxia and changes in blood pH, and therefore capable of enduring much wider fluctuations in heart
rate, blood pressure, and oxygenation.
Anatomy and function
The location of the heart within the body cavity varies according to species. In most chelonians the heart lies on midline
just caudal to the thoracic girdle, ventral to the lungs. The heart of some chelonians such as soft-shelled turtles is pushed
to the side of the body cavity in order to accommodate the retracted neck. The heart of most lizards lies within the thoracic
girdle, with the exception of some species such as monitors and tegus (as well as crocodilians) in which the heart lies farther
back in the coelomic cavity. Cardiac location varies in snakes according to species, but usually is found at the junction
of the first and second quarter of the animal's body length. Typically arboreal snakes' hearts are found more cranially in
the body than in terrestrial animals. Snake's hearts are fairly mobile within the coelomic cavity helping to facilitate the
ingestion of large prey items.
The cardiac structure of reptiles is significantly different from that of mammals. Please note that the following descriptions
are very general, and that significant variation exists between species. Most reptiles have three chambered hearts with two
atria and one common ventricle. The right atrium receives blood returning from the systemic circulation via the sinus venosus,
which is formed by the confluence of the right and left precaval veins and the single postcaval vein. The walls of the sinus
venosus contain cardiac muscle and the pacemaker of the heart. The left atrium receives oxygenated blood from the lungs via
the pulmonary vein(s). The atrioventricular valves are bicuspid, membranous structures. Under normal conditions the three
chambered heart functions much like a four chambered structure, therefore relatively little mixing of oxygenated and de-oxygenated
blood occurs. Three cavities exist within the ventricle and can be functionally separate; the cavum venosum, cavum arteriosum
and the cavum pulmonale. These cavities are partially separated by two muscular ridges found within the ventricle. These ridges
vary in prominence in different species, but are generally well-developed in chelonians. The muscular ridge divides the cavum
pulmonale and the cavum venosum. The vertical ridge divides the cavum venosum and cavum arteriosum. The cavum pulmonale receives
blood from the right atrium through the cavum venosum and directs flow into the pulmonary circulation. The cavum arteriosum
receives blood from the pulmonary veins and then directs oxygenated blood to the cavum venosum. The paired aortic arches arise
from the cavum venosum and lead to the systemic circulation. The right and left aortic arches come together to form a single
aorta at variable distances caudal to the heart. Differential blood flow and separation of oxygenated and de-oxygenated blood
is maintained by pressure differences of the outflow tracts and the muscular ridges that partially divide the ventricle. In
most non-crocodilian reptiles the ventricle function as a single pump, meaning that the same pressures are generated by both
the cavum pulmonale and cavum venosum. This is not the case in monitor lizards and at least one species of python in which
significantly higher pressures are generated in the cavum venosum and thus the systemic arches.
Due the unique anatomy, both right to left and left to right shunts are possible in the reptilian heart. Shunting does occur
during apnea, though all the details regarding the exact purpose of shunts are unclear. Multiple theories exist regarding
the purpose of right to left cardiac shunting in reptiles including the conservation of cardiac energy, facilitation of warming,
reduction of plasma filtration into the lungs, reduction of carbon dioxide flux into the lungs and the metering of oxygen
stores from the lung(s) during apnea. Theories to explain the purpose of left to right shunting include facilitation of carbon
dioxide elimination from the lung(s), minimization of ventilation/perfusion mismatches and improvement of systemic oxygen
transport. In times of oxygen deprivation (diving in some reptiles, consumption of large prey in snakes), reptiles can shunt
blood away from the lungs. Right to left cardiac shunting in the non-crocodilian heart can be facilitated by an increase in
pulmonary vascular resistance and action of the muscular and vertical ridge. Resumption of breathing results in a decrease
in pressures within the pulmonary vasculature and restoration of pulmonary blood flow.
Crocodilians are the only reptiles which possess four chambered hearts comparable to mammals. Even so, crocodilian cardiac
anatomy is quite different from what is seen in birds and mammals. Crocodilians possess two aortas; the right arising from
the left ventricle and the left from the right ventricle. Both aortas route blood to the systemic circulation. The right and
left aortas are connected near the base of the heart by the foramen of Panizza. The foramen allows blood from the right ventricle
to bypass the pulmonary circulation when necessary. A valve exists at the opening of the pulmonary artery which has interdigitating
muscular projections, hence the commonly used name "cog-wheel valve". When the animal holds its breath, the cog-wheel valve
closes and blood that would have normally entered the pulmonary circulation is diverted into the left aorta. It should be
noted that most veterinary texts incorrectly report that the location of the foramen of Panizza is in the ventricular septum
or atrial septum.
Heart rate of reptiles depends on species, size, temperature and activity/level of metabolic function. An equation employing
metabolic scaling for determination of the "appropriate" heart rate in reptiles has been proposed: Heart rate = 33.4(Weight
in kilograms-0.25 ). This equation assumes that the reptile is within its preferred optimal temperature zone.
