RESPIRATORY SYSTEM

Upper respiratory tract

In lizards the internal nostrils lie rostrally so that incoming air passes through the mouth on the way to the larynx. Snakes, however, have evolved a method of protruding the glottis and trachea out of the mouth while feeding.

The glottis in reptiles is situated quite rostrally, making them a relatively easy animal to intubate. The glottis remains closed at rest, opening only for respiration by the action of a glottis dilator muscle. Vocal cords are absent and sounds like hissing are produced by rapidly expelling air (Liem et al. 2001).

CLINICAL NOTE

Gular gulping or fluttering is not respiration but an olfactory process in chelonians and is used for courtship and territorial display in some lizard species. It is also used for evaporative cooling by vibrating the throat tissues to cool the blood passing through (Gans & Hughes 1967; McCutcheon 1943; Wood & Lenfant 1976).

Lower respiratory tract

Apart from gaseous exchange the lungs also play a role in display, buoyancy and vocalization (Perry & Duncker 1978). The lining of the respiratory tract has a primitive mucociliary apparatus, resulting in reptiles being poor at clearing inflam­matory exudates from their lungs (Murray 1996c). In com­parison to mammals, the lung volume of reptiles is quite large but they have only about 1% of the lung surface area of a mammal of equal size (Wood & Lenfant 1976). In aquatic species this larger lung volume may aid in buoyancy and act as an oxygen reservoir.

Reptiles have no diaphragm and so the combined pleuro­peritoneal cavity is called the celom. More advanced species have a postpulmonary septum, which is a membrane that divides the pleural cavity from the peritoneal cavity.

Reptile lungs are classified into three anatomical types according to the degree of partitioning of the lungs. The most primitive single-chambered (unicameral) lungs are found in snakes and some lizards. Paucicameral, found in iguanas and chameleons, have a few chambers but lack an intrapulmonary bronchus. The most advanced are the multichambered (multicameral) lungs found in monitor lizards, chelonia and crocodiles (Perry 1989; Perry & Duncker 1978). Many reptiles also have smooth muscles in the lung wall, which helps them inspire and expire air (Perry & Duncker 1978; Seymour 1982).

CLINICAL NOTE

The normal respiration rate is 10 to 20 breaths per minute, but this will vary with temperature and the reptile's size.

ACCESSORY RESPIRATORY SURFACES

(See Bellairs 1969c; Bennett & Dawson 1976; Seymour 1982; Wood & Lenfant 1986.)

In most species gas exchange is through the alveolar epithelium of the lungs but some reptiles can also breathe through accessory respiratory surfaces.

• Skin - some soft shell aquatic turtles (e.g., Trionychidae) can absorb oxygen through their skin and shell during submergence.

• Buccal-pharyngeal mucosa - used by many species of lizard.

• Tracheal lung - many snakes have a unique saccular diverticulum that acts in gas exchange.

• Cloacal bursae - in some freshwater turtles these have a highly vascular lining, allowing a high rate of oxygen intake from the water (Liem et al. 2001).

Ventilation

Despite their lack of a diaphragm reptiles draw air into their lungs by negative pressure breathing (Liem et al. 2001). A feature of all reptiles is a triphasic respiratory cycle of expiration, inspiration, and relaxation, which means oxygen concentrations in the lungs are constantly fluctuating. The relaxation or breathholding phase can be very long in aquatic species, lasting from 30 minutes up to 33 hours (Pough et al.

Reptiles can survive considerable periods at low oxygen levels as they are capable of converting to anaerobic metabolism while they breathhold. This tolerance to hypoxia seems to depend on the myocardium and ability to buffer lactic acid (Murray 1996c).

CLINICAL NOTE

Reptiles suffering from pneumonia and hypoxia will tend to seek lower temperatures to reduce the demand for oxygen.

Control of respiration

In mammals the acid base balance and PCO₂ are essential in controlling respiration. However, as reptiles are very tolerant of both anoxia and acid base change it is temperature that is the controlling factor. A rise in temperature increases the demand for oxygen, stimulating increased tidal volume. High oxygen tension decreases the respiration rate (Bennett & Dawson 1976; Murray 1996c; Wood & Lenfant 1976).

CLINICAL NOTE

As reptiles can survive long periods using anaerobic metabolism it is possible to revive patients with cardiopulmonary arrest by ventilating them at least once per minute with oxygen. Overventilation should not be performed because you run the risk of raising the PO₂ and depressing respiration further. Reptiles should be kept at their POTZ to trigger spontaneous respiration.

KEY POINTS

• Capacity for anaerobic metabolism

• Tolerance to acid-base imbalance

• Ability to breathhold for long periods

• Interventricular blood shunting

• Respiration rate varies with temperature and not PCO₂