Lizard and Snake Cardiovascular Physiology

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The oxygen carrying capacity is at its maximum when reptiles are within their POTZ. This has obvious implications for husbandry practices. The haemoglobin molecule is responsible for oxygen uptake.

Blood flow in the heart

Oxygenated blood enters the heart in the left atrium and deoxygenated blood enters in the right. The atria contract shortly after one another. Thus part of the heart is filled with oxygenated blood, the greater part with mixed blood and the other part with deoxygenated blood. During the ventricular systole, the blood is directed into the large arteries, at the base of the heart, so that oxygenated blood is directed to the brain; mixed blood is directed to the body and deoxygenated blood to the lungs. The atrioventricular valve prevents backflow of blood from the heart and three semilunar valves prevent this from the arteries to the heart.

• During normal respiration, the flow of blood tends to create a left to right shunt based upon pressure differentials. This is the recirculation through the pulmonary circulation of oxygenated blood.

• During diving or other instances in which the pulmonary resistance and pressure are elevated, a right to left shunt occurs. It represents the recirculation of systemic blood through the systemic arterial circulation. Pulmonary bypass means the blood is not re-oxygenated.

• The autonomic nervous sytem and endocrine and local factors regulate the shunting (Overgaard et al., 2002). The mechanisms responsible for this process are controversial but the two main hypotheses are pressure and washout shunting.

Heart rate

The heart rate of reptiles is dependent upon numerous variables including body temperature, body size, respiratory rate and sensory stimulation. Based on allometric scaling of bodyweight a simple formula for heart rate is 33.4kg-0.25. Reptiles that are warming up in the morning have higher heart rates than those cooling down later in the day.

Control

The heart is under neuroendocrine control, with the vagal parasympathetic (cholinergic) innervation being the principle mode of neural modulation of chronotopy and inotropy in reptiles. The sympathetic nervous system exerts mild excitatory adrenergic control but non-adrenergic non-cholinergic systems (NANC) may also contribute to cardiovascular control.

• The renin-angiotensin system and counter-regulatory natriuretic peptides appear to play a part in cardiovascular physiology of reptiles (Uva and Vallarino, 1982; Chiu et al., 1986; Breno et al., 2001).

References

• Girling, S.J. and Raiti, P. (2004). BSAVA Manual of Reptiles. British Small Animal Veterinary Association, 2nd Edition. pp. 350. ISBN 978-0905214757