role of cardiac shunts in the regulation of arterial blood gases, The

American Zoologist, Feb 1997 by Tobias Wang, Egle H Krosniunas, James W Hicks

SYNOPSIS. The pulmonary and systemic circulations are not completely separated in reptiles and amphibians, so oxygen-rich blood returning from the lungs can mix with oxygen-poor blood returning from the systemic circuit (cardiac shunts). In these animals, the arterial blood gas composition is determined by both lung ventilation and the cardiac shunt. Therefore, changes in cardiac shunting patterns may participate actively in the regulation of arterial blood gases. In turtles the cardiac shunt pattern changes independently of ventilation and the cardiac R-L shunt (pulmonary bypass of systemic venous blood) is reduced under circumstances where the demands on efficient gas exchange are high (hypoxia, hypoxemia or exercise). We propose, therefore, that the size of cardiac shunts is regulated independently of ventilation and hypothesize that there exist at least two groups of peripheral chemoreceptors with different reflex roles.

The Role of Cardiac Shunts in the Regulation of Arterial Blood Gases1

INTRODUCTION

The pulmonary and systemic circulations of reptiles, amphibians and many airbreathing fish are not completely separated, and blood flows to the lungs and body can be altered independently (e.g., Johansen and Burggren, 1980). As a result, systemic venous blood returning from the body can bypass the pulmonary circulation (right-to-left shunt), whereas blood returning from the lungs can recirculate into the pulmonary circulation (left-to-right shunt). Although the functional significance of this cardiovascular design remains largely unknown (cf., Burggren, 1987; Hicks and Wang, 1996), numerous studies attest that the blood flows change in a predictable fashion. In particular, large increases in pulmonary blood flow during ventilation have been characterized for many different species (e.g., Johansen et al., 1970; Shelton, 1970; Shelton and Burggren, 1976; West et al., 1992; Wang and Hicks, 1996a). The underlying control of these changes is not well understood.

In the presence of central vascular shunts, arterial systemic blood is a mixture of systemic venous blood and blood returning from the lungs. Arterial blood gas composition therefore is determined by both lung gases and the degree of admixture. This is in contrast to mammals and birds, where arterial blood gases closely resemble those within the lung and can be regulated exclusively by means of ventilation. The main focus of this chapter is to discuss the possible role of cardiac shunts on arterial blood gas regulation. Following a brief review of the anatomical basis for cardiac shunts and their effects on blood gases, we will discuss the possible neural regulations of blood flows. Although most of this chapter is based on our own recent data on turtles, we will, albeit selectively, draw comparisons to previous studies on other reptiles and amphibians.

THE ANATOMICAL BASIS FOR CARDIAC SHUNTS IN NON-CROCODILIAN REPTILES

In all non-crocodilian reptiles (and amphibians), central vascular shunts result from the incomplete anatomical separation of the pulmonary and systemic circuits within the ventricle of the heart. These shunts are consequently referred to as cardiac shunts. The cardiac shunts can be defined as right-to-left cardiac shunt (R-L shunt) and left-to-right cardiac shunt (L-R shunt), where R-L shunt refers to systemic blood that bypasses the lungs and L-R shunt refers to pulmonary venous blood that reenters the pulmonary circulation.

Two dominating hypotheses (pressure shunt and washout shunt) have been advanced to explain the mechanisms of cardiac shunts in non-crocodilian reptiles. These hypotheses differ regarding the intraventricular flow patterns during systole (Heisler and Glass, 1985; Hicks and Malvin, 1995; Fig. 1). The ventricle is divided into two main chambers (the cavum pulmonale and a larger dorso-lateral chamber, the cavum dorsale) by a septum-like structure called the muscular ridge. In most species, the cavum dorsale is further subdivided into the cavum arteriosum and the cavum venosum. The pulmonary artery emerges from the cavum pulmonale, whereas the two aortic arches arise from the cavum venosum. During diastole, O^sub 2^ poor blood from the right atrium enters the cavum venosum and flows to the cavum pulmonale, while O^sub 2^ rich blood enters the cavum arteriosum directly from the left atrium. During systole, blood is ejected into the systemic arteries from the cavum venosum and into the pulmonary arteries from the cavum pulmonale. The washout hypothesis proposes that the muscular ridge effectively separates the cavum pulmonale and the cavum venosum at the onset of systole and that the R-L shunt results from the O^sub 2^ poor blood residing in the cavum venosum at the end of diastole. Similarly, the O^sub 2^ rich blood remaining in the cavum venosum following systole is washed into the cavum pulmonale during diastole account for the L-R shunt. The pressure shunt hypothesis proposes that the muscular ridge does not separate the cavum pulmonale and the cavum venosum during systole. According to this view, blood will flow between these cava if differences exist in the outflow resistances.