370
depends on the elastic properties of the protein elastin and is independent of body
mass (Greenwald et al. 2001 ).
Different species of marine mammals vary in diameter of their aortic arch that is
enlarged relative to the descending aorta. Here, some data concerning specialization
reported by Shadwick and Gosline in their impressive works:
– “The ratio of maximal diameter of the arch to that of the thoracic aorta is about
2.3 in the harbour seal ( Phoca vitulina ), 3.6 in the Weddell seal ( Leptonychotes
weddelli ) and 3.2 in the fi n whale ( Balaenoptera physalus ); compared with only
1.4 in the dog;
– in the harbour seal, more than 80 % of the volume change in the entire thoracic
aorta that results from a pressure pulse occurs in the bulbous arch, and this is
more than 90 % in the Weddell seal and fi n whale. The enhanced capacitance of
the arch in the harbour seal is primarily due to its larger diameter, as the relative
wall thickness and elasticity of the arch and thoracic aorta are the same.
– the arch might be somewhat less stiff than the thoracic aorta. In addition to being
greatly expanded, the aortic arch of the fi n whale is also much more distensible
than the relatively thin-walled and much stiffer descending aorta. At the estimated mean blood pressure, the elastic modulus of this vessel is 12 MPa, or 30
times that of the aortic arch,” (Shadwick and Gosline 1995 );
– “the aortic arch and the descending aorta in the fi n whale ( Balaenoptera physalus ) are structurally and mechanically very different from comparable vessels in
other mammals. Although the external diameter of the whale’s descending thoracic aorta (approximately 12 cm) is similar to that predicted by scaling relationships for terrestrial mammals, the wall thickness:diameter ratio in the whale
(0.015) is much smaller than the characteristic value for other mammals (0.05),”
(Gosline and Shadwick 1996 );
– “chemical composition studies indicate that the elastin:collagen ratio is high in
the aortic arch (approximately 2:1), and that this ratio falls in the thoracic
(approximately 1:2) and abdominal (approximately 1:3) aortas. However, the
magnitude of the change in composition does not account for the dramatic difference in mechanical properties. This suggests that there are differences in the
elastin and collagen fi bre architecture of these vessels,” (Gosline and Shadwick
1996 );
– “the descending aorta contains dense bands of tendon-like, wavy collagen fi bres
that run in the plane of the arterial wall. They form a fi bre-lattice that runs in
parallel to the elastin lamellae and reinforces the wall, making it very stiff. The
aortic arch contains a very different collagen fi bre-lattice, in which fi bres appear
to have a component of orientation that runs through the thickness of the artery
wall. This suggests that the collagen fi bres may be arranged in series with elastincontaining elements. This difference in tissue architecture could account for both
the lower stiffness and the extreme extensibility of the whale’s aortic arch. Thus,
both the structure and the mechanical behaviour of the lamellar units in the aortic
arch and aorta of the whale have presumably been modifi ed to produce the
unusual mechanical and haemodynamic properties of the whale circulation,”
(Gosline and Shadwick 1996 ; see also Shadwick and Gosline 1994 ).
10 Marine Elastin
depends on the elastic properties of the protein elastin and is independent of body
mass (Greenwald et al. 2001 ).
Different species of marine mammals vary in diameter of their aortic arch that is
enlarged relative to the descending aorta. Here, some data concerning specialization
reported by Shadwick and Gosline in their impressive works:
– “The ratio of maximal diameter of the arch to that of the thoracic aorta is about
2.3 in the harbour seal ( Phoca vitulina ), 3.6 in the Weddell seal ( Leptonychotes
weddelli ) and 3.2 in the fi n whale ( Balaenoptera physalus ); compared with only
1.4 in the dog;
– in the harbour seal, more than 80 % of the volume change in the entire thoracic
aorta that results from a pressure pulse occurs in the bulbous arch, and this is
more than 90 % in the Weddell seal and fi n whale. The enhanced capacitance of
the arch in the harbour seal is primarily due to its larger diameter, as the relative
wall thickness and elasticity of the arch and thoracic aorta are the same.
– the arch might be somewhat less stiff than the thoracic aorta. In addition to being
greatly expanded, the aortic arch of the fi n whale is also much more distensible
than the relatively thin-walled and much stiffer descending aorta. At the estimated mean blood pressure, the elastic modulus of this vessel is 12 MPa, or 30
times that of the aortic arch,” (Shadwick and Gosline 1995 );
– “the aortic arch and the descending aorta in the fi n whale ( Balaenoptera physalus ) are structurally and mechanically very different from comparable vessels in
other mammals. Although the external diameter of the whale’s descending thoracic aorta (approximately 12 cm) is similar to that predicted by scaling relationships for terrestrial mammals, the wall thickness:diameter ratio in the whale
(0.015) is much smaller than the characteristic value for other mammals (0.05),”
(Gosline and Shadwick 1996 );
– “chemical composition studies indicate that the elastin:collagen ratio is high in
the aortic arch (approximately 2:1), and that this ratio falls in the thoracic
(approximately 1:2) and abdominal (approximately 1:3) aortas. However, the
magnitude of the change in composition does not account for the dramatic difference in mechanical properties. This suggests that there are differences in the
elastin and collagen fi bre architecture of these vessels,” (Gosline and Shadwick
1996 );
– “the descending aorta contains dense bands of tendon-like, wavy collagen fi bres
that run in the plane of the arterial wall. They form a fi bre-lattice that runs in
parallel to the elastin lamellae and reinforces the wall, making it very stiff. The
aortic arch contains a very different collagen fi bre-lattice, in which fi bres appear
to have a component of orientation that runs through the thickness of the artery
wall. This suggests that the collagen fi bres may be arranged in series with elastincontaining elements. This difference in tissue architecture could account for both
the lower stiffness and the extreme extensibility of the whale’s aortic arch. Thus,
both the structure and the mechanical behaviour of the lamellar units in the aortic
arch and aorta of the whale have presumably been modifi ed to produce the
unusual mechanical and haemodynamic properties of the whale circulation,”
(Gosline and Shadwick 1996 ; see also Shadwick and Gosline 1994 ).
10 Marine Elastin