respiratory portion is located between the heart and the cranial pole of the liver, while in most viperids and elapids there is a respiratory portion cranial to the heart. The right lung is always larger than the left. The left lung is well developed in boids and vestigial in colubrids. Multichambered lungs are found in the snake families Anomalepididae (primitive blind snakes), Typhlopidae (blind snakes), and Acrochordidae (primitive aquatic snakes from Australia and Indonesia).
Instead of alveoli, reptiles have faveolae (deeper than they are wide) and ediculae (wider than they are deep), which are exchange chambers that have a similar function to alveoli. Both are present in the lung tissue but absent in the air sacs. Reptiles have a large lung volume but only 10–20% of the surface area when compared with mammals of similar size. Ventilation is triphasic with a cycle of expiration, inspiration, and relaxation (breath holding) that creates a constant fluctuation of oxygen concentration in the lungs. Oxygen tension and temperature have a significant role in ventilation/respiration. At higher temperatures, there is an increased demand for oxygen, which leads to an increase in tidal volume. Respiratory rate increases at low oxygen tensions and decreases at high oxygen tension.
DIGESTIVE SYSTEM
Reptilian teeth are composed of enamel, dentine, and cement, but lack a periodontal membrane. There are three types of dentition. Acrodont teeth attach to the surface of the bone and are not replaced. These are present in many lizards (bearded dragons, chameleons, water dragons). Pleurodont teeth attach to the labial aspect of the bone and are replaced. Pleurodont teeth can be found in snakes and iguanid lizards. Thecodont teeth attach via deep bony sockets and are continuously replaced, often throughout life. Thecodont teeth are present in crocodilians and some consider snake teeth as modified thecodonts.
Snakes and lizards hatched out of eggs possess an egg tooth, a modified premaxillary tooth to help break through the egg and its membranes. Chelonians and crocodilians have an egg caruncle—a horny tissue in that serves the function of the egg tooth.
Snake dentition is further classified based on presence or absence of fangs. Aglyphous snakes lack fangs (boids). Opisthoglyphous species are rear fanged. Some Colubridae, have modified parotid glands (hognose and boomslangs). Solenoglyphous species (Viperidae, i.e., copperheads, pit vipers, etc.) have rotating front fangs and venom glands behind the eyes. Proteroglyphous snakes (Elapidae, i.e., cobras, sea snakes) have fixed front fangs and venom glands behind the eyes.
Oral secretory glands (palatine, sublingual, mandibular) are present in many species for lubrication of prey items. In some snakes, these are modified into venom glands like the Duvernoy’s gland in colubrids. Duvernoy’s glands are posterior to the eye and some think they are analogous to venom glands in vipers and elapids while others consider them to be distinct glands. Helodermatids (Gila monsters and beaded lizards) have labial venom glands in the mandible, while snakes have maxillary venom glands.
Gastroliths may be seen radiographically and are thought to be functionally ingested only in crocodilians, with other species having accidental ingestion. Insectivorous species are thought to secrete chitinase by the stomach and pancreas to help digestion of the exoskeleton of insects.
As in birds, the cloaca is composed of the coprodeum, urodeum, and proctodeum. The colon empties into the coprodeum, while the genital and urinary openings enter the urodeum. The proctodeum is the most caudal common chamber, which receives feces and urine before an evacuation occurs.
HEPATIC SYSTEM
The liver is surrounded by a Glisson’s capsule and is a paler color in young and neonates. The right lobe is significantly larger in chelonians, which leads to the majority of urinary bladder stones being left sided, as the larger lobe pushes them to the left. The gallbladder is found within the right lobe. Biliverdin reductase is thought to be absent in most species, but may be present in crocodilians. Kupffer cells serve as sinusoidal macrophages. Melanomacrophages may be numerous in some species and function as macrophages and free radical scavengers. Their brown pigment (as seen with hematoxylin and eosin stain) consists of iron and melanin granules. These cells may increase in size and number during disease states. Ito cells for storage of vitamin A may also be numerous in some species.
URINARY SYSTEM
Reptilian kidneys are metanephric, lack loop of Henle, renal pelvis, and pyramids. The nephron is composed of glomerulus, long and thick proximal convoluted tubule, short and thin intermediate segment, and a short distal tubule. The sexual segment, a terminal segment of the kidney in male snakes and lizards, becomes pale and enlarged during reproduction.
Osmoregulation in reptiles presents unique challenges. A reptile’s body mass is approximately 70% water, similar to mammals but lower than amphibians (75–80%). Total sodium and potassium is similar to mammals but variation exists in larger species. Because of the lack of a loop of Henle, reptiles are unable to concentrate urine beyond their plasma osmolality. Methods of water conservation include the secretion of uric acid, presence of salt glands, fluid reabsorption from the cloaca, the renal portal system, and the ability to tolerate decreases in glomerular filtration rate. All reptiles produce and excrete uric acid, but some may also excrete ammonia and/or urea. Uric acid is the primary or only excretory product of terrestrial species. Uric acid is excreted via the renal tubules, so dehydration does not stop its excretion. It precipitates out of solution in the bladder or cloaca to form urates, which can be composed of potassium salts (herbivores) or sodium salts (carnivores). Uric acid is not a sensitive indicator of renal disease because over 60% of renal function must be lost in order to cause hyperuricemia; thus, dehydration is a more common cause of hyperuricemia. Ammonia and urea excretion occurs primarily in aquatic species. Crocodilians are ammoniotelic while freshwater turtles are ureotelic. Sea turtles excrete variable amounts of urea and ammonia. The cloaca, colon, and urinary bladder are significant sites of osmoregulation and allow active ion transport and passive water absorption through their walls. The bladder actively absorbs sodium while secreting potassium and urates.
The renal portal system is an afferent blood supply from the renal artery to the glomerulus. The renal portal vein (arising near epigastric and iliac veins) bypasses the glomerulus and enters at the tubules; thus, during dehydration the renal portal system perfuses the tubules to prevent necrosis. For this reason, the effects of the renal portal system on drug metabolism is of primary consideration for pharmaceuticals excreted by the tubules and not as critical for those excreted by glomerular filtration. Effects of the renal portal system on drug metabolism and excretion are variable according to the metabolic status of the animal and the pharmacokinetics of the drug.
REPRODUCTION
Reproduction is regulated by the pineal gland, hypothalamus, and pituitary glands, which process environmental stimuli into hormonal changes. In temperate species, gonadal stimulation occurs by increased temperature and longer days while in tropical species increased food availability and rainfall provide gonadal stimulation.
Sex determination may be genotypic (ZZ are males, ZW are female) or temperature dependent. Temperature sex determination is not reported or well known in snakes but occurs in all the other reptile groups. In chelonians, high temperatures yield mostly females while low temperatures yield a majority of males. In most the lizards, temperature sex determination is opposite to chelonians. Some turtles and lizards will have a predominance of females at high and low temperatures and males at intermediate temperatures. For crocodilians, females occur at low temperature (28–30 degrees C) in all species. In caimans and alligators, male and females occur at 31–32 degrees C while males occur at 32–34 degrees C. In crocodiles, more males than females will occur at 31–33 degrees C, while the ratio is reversed at 33–34 degrees C.
The right gonad is located close to vena cava and connected to it by small vessels, while the left is associated
