Wednesday, March 17, 2010

Blood flow through the ophthalmic veins during exercise in humans

Blood flow through the ophthalmic veins during exercise in humans
Masami Hirashita 1, Osamu Shido 2, and Minoru Tanabe 2
1 Kanazawa College of Economics, Gosho-machi, Kanazawa 920, Japan
2 Department of Physiology, School of Medicine, Kanazawa University, Takara-machi, Kanazawa 920, Japan

Summary. The blood from the face flows into the intracranium
through the ophthalmic veins when human
subjects become hyperthermic. To investigate a possible
mechanism underlying this change in direction of
flow, five young men were subjected to either passive
body warming or exercise on a cycle ergometer, in a
climatic chamber whose air temperature and relative
humidity were 28 ° C and 40%. Tympanic (Try) and oesophageal
temperatures, forehead sweat rate (rhsw), skin
blood flow. (Qsk) and blood flow through the ophthalmic
vein (Qov) were measured, and the mean skin (Tsk)
and mean body (Tb) temperatures were computed. Passive
body warming was induced by a box-shaped body
warming unit enclosing all but the subject's head. Exercise
was performed either at an intensity of 60% maximal
oxygen consumption or with the intensity increasing
in increments. During both tests, rhsw and Qsk
started to increase shortly after the imposition of the
heat load. The Qov began to change with the venous
blood flowing from the face into the intracranium and
a complete reversal in the direction of Qov (from the
face to the intracranium), came significantly later than
the increases in rh~w and Qsk. The Tty at the time of flow
reversal was the same in both tests. The T~k (and hence
Tb) at flow reversal was, however, significantly higher
during passive body warming than during .exercise. The
mechanism for switching the direction of Qov appeared
to have been triggered by a high temperature in the
brain, and not by thermal input from the periphery of
the body. In a febrile subject who volunteered for this
study, the direction of Qov was consistently inwards
even when sitting quietly. From these results, we suggest
that there are elements within the brain that control
the mechanisms for switching the direction of venous
flow through the emissary veins to keep the brain
cool during hyperthermia.
Key words: Passive body warming - Cycle exercise -
Emissary veins - Flow reversal - Tympanic temperature
- Oesophageal temperature
Offprint requests to: M. Hirashita
Brain tissues are more vulnerable to heat than the other
organs in the body (Bowler and Tirri 1974; Shibolet et
al. 1976). When body core temperature increases, venous
blood cooled in the upper respiratory tracts and
the surface of the face enters into the cavernous sinus
through the ophthalmic veins (OV), which can keep
brain temperature lower than the other core temperature.
Such a phenomenon, called selective brain cooling,
has been shown to exist in many species of animals
including human subjects (Baker 1982; Cabanac and
Caputa 1979a, b; Caputa 1980; Caputa et al. 1983).
During normothermia, however, venous blood from the
nasal mucosa has been found to drain into the caval
veins through the facial veins and bypass the cavernous
sinus (Johnsen et al. 1987). Venous blood from the
nasal mucosa could flow either through the facial veins
or through the angularis oculi veins which connect with
the OV. Thus, some mechanism may exist that switches
the direction of venous flow to the facial veins or to the
angularis oculi veins, according to the thermal state of
the body. Johnsen et al. (1987) have shown that in reindeer,
a segment of the facial vein strongly constricts
during heat stress resulting in the return of cool venous
blood from the nasal mucosa, routed directly to the
cavernous sinus through the OV. This switching of direction
in venous blood flow has also been suggested in
human subjects. Caputa et al. (1978) have reported that
venous blood from the face flows through the OV to the
cavernous sinus during hyperthermia and from the intracranium
to the face during hypothermia. The observation
is consistent with the report made by Cabanac
and Brinnel (1985).
Muscle work produces a great amount of heat in the
body and subsequently elevates body temperature. The
occurrence of selective brain cooling during exercise
has also been postulated in animals (Cabanac 1986;
Caputa et al. 1983) and human subjects (Cabanac and
Caputa 1979a, b; Hirata et al. 1987). The hypothalamic
temperature of guinea pigs was shown to become lower
than the trunk temperature by 1.5-1.6°C during exercise.

S. Aizawa and M. Cabanac ,
Department of Physiology, Faculty of Medicine, Laval University, Qu├ębec, Canada, G1K 7P4
Received 20 September 1999;
accepted 15 October 1999.
Available online 29 March 2000.

The normal negative temperature gradient within the skin of the cheek was reversed by simultaneously heating the skin externally with an infrared lamp and cooling it internally, inside the mouth, with ice. Cutaneous blood flow was measured locally under four different conditions: negative and positive gradient of local skin temperature in hypothermia and hyperthermia. There were no significant differences between negative and positive skin temperature gradients. Cutaneous blood flow depended on the core body temperature. These results show that the local skin temperature gradient can not induce vasomotor responses.
Author Keywords: Temperature gradient; Hypothermia; Hyperthermia; Cutaneous blood flow; Temperature sensor; Skin temperature; Oral temperature; Human
Article Outline

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