Cardiovascular System. The Adult Caecilian. The Adult Salamander

The cardiovascular system includes the arterial, venous, and lymphatic systems. The lymphatic system has unique structures, variously termed lymph sacs, lymph hearts, or lymph vesicles, which serve to restrict flow of lymph unidirectionally so that it returns to the heart. The lymph hearts beat in synchrony at 50-60 beats per minute, independent of the cardiac rate (Conklin, 1930). Given the rapidity of the lymph heart rate, it is sometimes difficult to obtain lymph from these sites.

Illness should be expected whenever large volumes of lymph accumulate in any lymph sac. The volume of lymph moved by this system is astounding. One study determined that fluid leakage from the plasma was over 50 times the total plasma volume in a 24-hour period, and this leakage was re- covered and circulated by these lymphatics (Isayama, 1924 as cited in Boutilier et aI., 1992; Churchill et aI., 1927). Well hydrated terrestrial amphibians may be able to route fluid absorbed through the skin directly into the lymphatics (Boutilier et ai., 1992), bypassing the arterial blood.

Since this fluid is then eliminated by the kidneys, this could have a profound effect on the absorption and distribution of pharmacological agents administered cutaneously. The lymph consists of the components of blood, except for erythrocytes. Variation between the components of plasma versus lymph appears to be poorly documented.

The Adult Caecilian. Caecilians possess the typical amphibian heart, consisting of the right and left atria and one ventricle. The left atrium is usually smaller than the right, and the atrial septum is fenestrated.

Due to the lack of limbs, the circulatory system of the caecilian lacks many of the major peripheral arteries and veins found in other amphibians. The routing of blood through the caecilian is largely undocumented, but extrapolating from the patterns established in salamanders and anurans, it is reasonable to assume that the renal and hepatic portal vein systems receive blood from the caudal half of the caecilian (Simons, 1959). Until pharmacokinetics studies document otherwise, it is best to avoid administering drugs that are metabolized or excreted either renally or hepatically in the caudal half of a caecilian. Caecilians may have over 200 lymph hearts subcutaneously which deliver lymph into the intersegmental veins.

The Adult Salamander. The three-chambered heart is generally found beneath the confluence of the pectoral girdle and sternum. A ventricular septum is present in the lesser siren, Siren intermedia, and mudpuppy, Necturus maculosus (Putnam, 1975; Putnam & Dunn, 1978). The left atrium is usually smaller than the right. The interatrial septum is fenestrated except in sirens, Siren spp., and the hellbender, Cryptobranchus alleganiensis (Putnam, 1977; Putnam & Parkerson, 1985). Lungless salamanders (Plethodontidae) have a modified interatrial septum that allows both atria to receive blood from the sinoatrial aperture and prevent stagnation of the flow through the left atria (Putnam & Kelly, 1978). If gills are present,

the branchial vasculature is prominent. In darkly pigmented salamanders the superficial vasculature is difficult to discern unless there is obvious hyperemia. However, many salamanders possess white-colored or translucent abdominal skin which may allow direct visualization of the midline abdominal vein. The caudal tail vein runs immediately ventral to the caudal vertebrae, and again may be visible if the skin is translucent.

Both renal and hepatic portal vein systems are present in the caudal half of the salamander. Blood passing through the renal portal vein system passes through the kidneys before entering the postcaval vein, whereas blood passing through the hepatic portal vein system passes through the liver prior to entering the vena cava. It appears that the renal portal system receives a major portion of the blood drainage from the tail, but the factors that determine the routing of blood in the caudal half of the body are largely undocumented. Until pharmacokinetic studies document otherwise, it is best to avoid administering drugs that are metabolized or excreted either renally or hepatically in the hind limbs or tail of salamanders.

The lymphatic system of the European fire salamander, Salamandra salamandra, has been extensively described (Francis, 1934). Intestinal lymphatics empty into the subclavian veins, whereas the other lymphatics may empty into the cutaneous, cardinal, subvertebral, or lingual veins. The number of lymph hearts varies with species, but in this medium-sized salamander there are around 11 lymph hearts located in the head and coelom, and four lymph hearts that occur caudal to the sacrum.

The Adult Anuran. The heart of the anuran has been extensively studied (Kumar, 1975). The three-chambered heart is generally found beneath the confluence of the pectoral girdle and sternum. The left atrium is usually smaller than the right, and the interatrial septum is complete, unlike that of the other two orders of amphibians. In some anurans, the ventricular trabeculae are numerous and thick (Plate 3.11). In darkly pigmented anurans the superficial vasculature is difficult to discern unless there is obvious hyperemia.

However, many anurans possess white-colored or translucent abdominal skin which may allow direct visualization of the midline abdominal vein. The vasculature of the hindlimb webs and hind limbs are readily identified upon a cursory examination. An extensive lingual venous plexus is present on the underside of the tongue in most species of anurans.

Both renal and hepatic portal vein systems are present in the caudal half of the anuran. Blood passing through the renal portal vein system passes through the kidneys before entering the postcaval vein, whereas blood passing through the hepatic portal vein system passes through the liver prior to entering the vena cava. The factors that determine the routing of blood in the caudal half of the anuran body are largely undocumented. Until pharmacokinetic studies document otherwise, it is best to avoid administering drugs that are metabolized or excreted either renally or hepatically in the hind limbs of anurans.

The lymphatic system of two European ranids has been extensively described (Haslam, 1971), and the structure and function of the lymph hearts of anurans has been reviewed (Carter, 1979). Anurans have few lymph hearts when compared to other amphibians.

There may be from 1-5 or more lymph hearts located along the coccyx, and one pair located subscapularly. Enlarged subcutaneous lymphatic spaces occur in varying places on anurans. A prominent pair may be found dorsally on either side of the urostyle in many terrestrial anurans, and this is a convenient site to obtain samples of lymph fluid. Collection of lymph is enhanced in frogs paralyzed with curare (Conklin, 1930).