Many reptiles, particularly diving species, display characteristic car
diovascular changes associated with lung ventilation (cardiorespirator
y synchrony), Previous studies on freshwater turtles show that heart r
ate and pulmonary blood flow rate (Q(pul)) increase two- to fourfold d
uring ventilation compared with breath-holding, and some studies repor
t concomitant decreases in systemic blood flow rate (Q(sys)). The prim
ary aim of this study was to provide a detailed description of cardior
espiratory synchrony in free-diving and fully recovered turtles (Trach
emys scripta), During breath-holds lasting longer than 5 min, Q(pul) a
veraged 15 ml min(-1) kg(-1) and increased more than threefold to a ma
ximum value of 50 ml min(-1) kg(-1) during ventilation. Q(sys), also i
ncreased during ventilation compared with during breath-holds lasting
longer than 5 min (from 44 to 73 ml min(-1) kg(-1) during ventilation)
. Neither Q(pul) nor Q(sys) was affected by the number of breaths in t
he ventilatory periods, Changes in Q(pul) and Q(sys) were accomplished
entirely through a significant increase in heart rate during ventilat
ion, while total stroke volume (systemic + pulmonary) remained constan
t, Irrespective of the ventilatory state, Q(sys), exceeded Q(pul) by 2
0-30 ml min(-1) kg(-1). Nevertheless, because Q(pul) increased relativ
ely more than Q(sys), during ventilation, Q(pul)/Q(sys) increased from
0.29 during apnoea to 0.80 during lung ventilation. This study confir
ms cardiorespiratory synchrony in the turtle Trachemys scripta but, in
contrast to earlier studies, a net right-to-left cardiac shunt prevai
led regardless of ventilatory state.