192
s. Egginton and J.C. Rankin
In contrast, capillaries from icefish show a large degree of isotropy, due
largely to the presence of numerous capillary loops (Fig. 4).
These represent true divergent and convergent capillary branches, and
not the arcade microvascular network observed in vertebrate mesentary.
The capillary loops thus form a loose tube around each muscle fibre,
thereby maximizing the contact surface area for a low capillary density.
Discussion
The oxygen content of arterial blood in icefish (Chaenocephalus aceratus)
is only one-twentieth that of a sympatric red-blooded species (Notothenia
coriiceps), reflecting the lack of respiratory pigment [11]. In order to
support adequate tissue metabolism, peripheral oxygen transport relying
on physical solubility must therefore utilize a high rate of tissue perfusion.
Given the observed low blood pressure in icefish this may best be
achieved by an appropriate reduction in peripheral resistance to blood
flow. Despite the abnormally large heart of icefish, involving chamber
dilation to accommodate the large venous return, the maximal in vitro
power output of the myocardium is unremarkable [12]. This suggests there
is indeed a low afterload on the in situ working heart. We have therefore
examined vascular adaptations that may underlie this condition.
The dual arterial supply to the locomotory (pectoral) muscles is made
possible by retention of the primitive condition of a hypobranchial artery
[6,9]. A reduced hypobranchial system, giving rise to the coronary arteries
where these are present, is found in other teleosts [l3]. Based on
measurements of external diameters, Eastman [1] estimates flow capacity
of the hypobranchial system to be some 30% of cardiac output in icefish,
while among the red-blooded nototheniids it was of smaller calibre and
was formed by union of fewer efferent branchial arteries, suggesting its
flow capacity (and hence functional relevance) is greatly reduced. This is
consistent with little obvious filling of the hypobranchial system with
casting compound in specimens of N coriiceps, with both main dorsal
aortic branches filling the pectoral muscle mass and adjacent body
musculature up to the medial division (Egginton, unpublished). It is
unclear whether this structure is related to labriform locomotion, where a
high blood flow (i.e. low resistance vascular bed) to the pectoral muscles
is required to support aerobic swimming, rather than an endemic
specialization of Antarctic fishes.
Total peripheral resistance (TPR) is given as mean arterial blood
pressure (MABP; mmHg)/cardiac output (CO; ml/sec). Using published
values of 10/1.98 for Chaenocephalus aceratus [4] and 23.6/0.50 for
s. Egginton and J.C. Rankin
In contrast, capillaries from icefish show a large degree of isotropy, due
largely to the presence of numerous capillary loops (Fig. 4).
These represent true divergent and convergent capillary branches, and
not the arcade microvascular network observed in vertebrate mesentary.
The capillary loops thus form a loose tube around each muscle fibre,
thereby maximizing the contact surface area for a low capillary density.
Discussion
The oxygen content of arterial blood in icefish (Chaenocephalus aceratus)
is only one-twentieth that of a sympatric red-blooded species (Notothenia
coriiceps), reflecting the lack of respiratory pigment [11]. In order to
support adequate tissue metabolism, peripheral oxygen transport relying
on physical solubility must therefore utilize a high rate of tissue perfusion.
Given the observed low blood pressure in icefish this may best be
achieved by an appropriate reduction in peripheral resistance to blood
flow. Despite the abnormally large heart of icefish, involving chamber
dilation to accommodate the large venous return, the maximal in vitro
power output of the myocardium is unremarkable [12]. This suggests there
is indeed a low afterload on the in situ working heart. We have therefore
examined vascular adaptations that may underlie this condition.
The dual arterial supply to the locomotory (pectoral) muscles is made
possible by retention of the primitive condition of a hypobranchial artery
[6,9]. A reduced hypobranchial system, giving rise to the coronary arteries
where these are present, is found in other teleosts [l3]. Based on
measurements of external diameters, Eastman [1] estimates flow capacity
of the hypobranchial system to be some 30% of cardiac output in icefish,
while among the red-blooded nototheniids it was of smaller calibre and
was formed by union of fewer efferent branchial arteries, suggesting its
flow capacity (and hence functional relevance) is greatly reduced. This is
consistent with little obvious filling of the hypobranchial system with
casting compound in specimens of N coriiceps, with both main dorsal
aortic branches filling the pectoral muscle mass and adjacent body
musculature up to the medial division (Egginton, unpublished). It is
unclear whether this structure is related to labriform locomotion, where a
high blood flow (i.e. low resistance vascular bed) to the pectoral muscles
is required to support aerobic swimming, rather than an endemic
specialization of Antarctic fishes.
Total peripheral resistance (TPR) is given as mean arterial blood
pressure (MABP; mmHg)/cardiac output (CO; ml/sec). Using published
values of 10/1.98 for Chaenocephalus aceratus [4] and 23.6/0.50 for