Ectotherm and many poikilotherm species are animals that can have varying core temperatures based on the ambient or environmental temperature. Ectothermic animals generally produce no or very little metabolic heat. Poikilothermic species can have thermal homeostatic abilities and go through varying temperatures throughout the day, whereas ectotherms living in temperature-constant environments do not experience this fluctuation.

Amphibians and reptiles have many similarities, but are distinguished by a few main differences. Amphibians begin their life as an egg, hatch into a larva, and then go through a metamorphosis into their adult state with smooth to semi-lumpy skin, which can be used for gas exchange. Some species of amphibians retain select features of their larval form (e.g., gills) and are called neotenic species. Amphibians, with a few exceptions, also must have environmental sources of water for external fertilization during reproduction.

Reptiles begin their life as an egg and hatch into a miniature version of their adult form. Reptiles have scales or plated skin; with the exception of some soft-shelled turtles, these cannot be used for gas exchange. Reptiles reproduce by internal fertilization in wet or dry environments (O’Malley 2005).

Despite decades of research on reptilian and amphibian anatomy and physiology, clinical modalities to treat these critically ill patients are still in their infancy compared to small animal and human medicine. Much of the diverse and varying inter-species physiological processes and functions are still unknown to many scientists. Little research on transfusion medicine exists for reptilian species and almost none exists for amphibians, leaving most information anecdotal. All of the techniques described in this chapter for collecting, administration, and monitoring pertain directly to reptilian species, but are translatable to amphibian species with the exception of the sedation protocols.

Reptile red blood cells (RBCs) are much larger than avian and mammalian RBCs. They are similar in morphology to those of birds because they are elliptical and contain a nucleus. Chelonian species have the largest RBC size in comparison to other reptile species (Campbell 2006). The chromatin is dense and can have irregular margins. Mild anisocytosis and poikilocytosis are not uncommon in reptile hemograms, but moderate anisocytosis and poikilocytosis might suggest a regenerative response. Similarly, polychromasia or large numbers of immature cells can be indicative of a regenerative anemia. Less mature cells are regularly seen in young reptiles or those in ecdysis and are more round shaped (Thrall et al. 2004). Reptiles have circulating thrombocytes that function similarly to mammalian platelets. The lifespan of a reptile RBC is significantly long, approximately 600–800 days (Pollock 2013). Regeneration can take up to 60 days due to the long RBC lifespan (Saggese 2009).

The blood volume of reptiles is estimated to be 5–8% of body weight (Thrall et al. 2004). Maximum blood oxygen-carrying capacity in reptiles is directly related to the species’ preferred optimal temperature zone (POTZ). An increase or decrease in temperature outside of the POTZ can reduce RBC oxygen-carrying capacity by as much as 40%. This effect reverses when the reptile returns to the species’ POTZ. This emphasizes the importance of proper husbandry of reptiles (Pough 1976).