Dog Feet Have Special Blood Vessels
That Protect Them From Cold
Woof, it's cold out there! So how come dogs' paws don't freeze up
when they pad around on snow and ice - and what keeps dogs from minding
the cold?
Japanese scientists say they've discovered why. First of all, they say in a new study published in the journal Veterinary Dermatology,
dogs' pads contain lots of fatty tissue - which doesn't freeze as
easily as other tissues. And the blood vessels in dogs' feet are
arranged in a way that lets them act like living heat exchangers:
arteries in the paws are very close to networks of tiny veins
(venules), facilitating the transfer of heat from venous to arterial
blood.
When a paw is cooled by contact with frozen ground, warmth from the arteries in the paw is transferred to the venules. This helps keep the paw at a tolerable temperature. In addition, it warms the blood before it flows back to the body - thus helping keep the dog's body temperature from falling uncomfortably low.
The so-called "counter-current" heat exchange mechanism sounds like a good one, and it's not unique to canines. Similar systems are seen in other animals, including penguins and foxes, physorg.com reported. The finding suggests that dogs may have evolved in cold environments.
Research has shown that dogs' feet are protected from freezing even in temperatures as low as -35 Celsuis, according to the BBC.
The research, carried out by Dr. Hiroysho Ninomiya and colleagues at Tokyo's Yamazaki Gakuen University, was conducted with the help of an electron microscope and four willing dogs.
Abstract
Dogs are well adapted to cold
climates and they can stand, walk and run on snow and ice for long
periods of time. In contrast to the body trunk, which has, dense fur,
the paws are more exposed to the cold due to the lack of fur insulation.
The extremities have a high surface area-to-volume ratio, so they lose
heat very easily. We offer anatomical evidence for a heat-conserving
structure associated with dog footpad vasculature. Methylmethacrylate
vascular corrosion casts for scanning electron microscopy, Indian
ink-injected whole-mount and histological specimens were each prepared,
in a series of 16 limbs from four adult dogs. Vascular casts and Indian
ink studies showed that abundant venules were arranged around the
arteries supplying the pad surface and formed a vein–artery–vein triad,
with the peri-arterial venous network intimately related to the
arteries. In addition, numerous arteriovenous anastomoses and
well-developed venous plexuses were found throughout the dermal
vasculature. The triad forms a counter-current heat exchanger. When the
footpad is exposed to a cold environment, the counter-current heat
exchanger serves to prevent heat loss by recirculating heat back to the
body core. Furthermore, the arteriovenous anastomoses shift blood flow,
draining blood to the skin surface, and the venous plexuses retain warm
blood in the pad surface. Hence, the appropriate temperature for the
footpad can be maintained in cold environments.
Résumé
Les
chiens sont bien adaptés aux climats froids et ils peuvent rester,
marcher et courir sur la glace pendant de longues périodes.
Contrairement au tronc, muni d’une fourrure dense et épaisse, les pieds
sont plus exposés au froid à cause de l’absence de fourrure isolante.
Les extrémités ont un rapport surface/volume élevé et perdent ainsi de
la chaleur très facilement. Nous offrons une preuve anatomique des
structures de conservation de la chaleur liée à la vascularisation des
coussinets plantaires. Des préparations vasculaires au
méthylméthacrylate pour microscopie électronique, des injections à
l’encre Indian et des préparations histologiques ont été réalisées sur
une série de 16 membres de 4 chiens adultes. Les moulages vasculaires et
les colorations à l’encre Indian, ont montré que de nombreuses veinules
étaient organisées autour des artères irrigant la surface du coussinet
et formaient une triade veine-artère-veine, le réseau veineux
peri-artériel étant intimement relié aux artères. En outre, de
nombreuses anastomoses artério-veineuses (AVAs) et des plexus veineux
bien développés ont été trouvés tout au long de la vascularisation
dermique. La triade forme un échangeur de chaleur à contre-courant.
Lorsque le coussinet plantaire est exposéà un environnement froid,
l’échangeur à contre-courant permet de prévenir la perte de chaleur par
un retour du courant circulatoire chaud vers le centre du corps. De
plus, les AVAs réorientent le flux sanguin, drainent le sang à la
surface cutanée et les plexus veineux retiennent le sang tiède à la
surface du coussinet. Ainsi, une température appropriée pour le
coussinet plantaire peut être maintenue dans les environnements froids.
Functional anatomy of the footpad vasculature of dogs: scanning electron microscopy of vascular corrosion casts
Article first published online: 28 MAR 2011
