Thermoregulatory anatomy of pronghorn (Antilocapra americana)

We have found that pronghorn (Antilocapra americana) use external heat exchange with the environment and internal heat exchange between the carotid artery rete and cavernous venous sinus blood to regulate body temperature. Now we have investigated the relationship between the histological structure of the skin, cephalic veins, and carotid rete–cavernous sinus system and the physiological mechanisms pronghorn use, and whether their thermoregulatory anatomy has adaptive advantages. We harvested tissue samples of skin, three veins (i.e., angularis oculi vein, dorsal nasal vein, and facial vein), and the carotid rete–cavernous sinus system from four pronghorn, two culled in summer and two in winter, and examined each histologically. The three veins had the typical structure of veins with large lumina and thin walls. The carotid rete consisted of small (0.1–0.5 mm) arterioles with a density of ~10/mm², intertwined with veins (~2/mm²), enclosed within the cavernous sinus; a structure ideal for heat exchange. We concluded that the main function of the dorsal nasal and facial veins is to return cold blood to the body to effect whole body cooling. The cavernous sinus is supplied with warm blood by the palatine veins in winter and cold blood by the deep facial veins in summer, an arrangement different to that in other ungulates, such as sheep, in which the angularis oculi vein supplies the cavernous sinus. Pronghorn skin is richly supplied with blood vessels that facilitate convective heat loss in summer. In winter, the number of coarse and fine hairs per square millimeter increases more than in European deer to form a thick pelage that minimizes heat loss. In summer, the pelage is shed because hair follicles involute. Unlike in other ungulates, pronghorn skin has little adipose tissue. The number of apocrine glands increases in winter rather than in summer. We concluded that the glands have a reproductive/social function rather than a thermoregulatory one. In summary, our study shows that the thermoregulatory anatomy is consistent with our physiological data and has adaptive advantages that help explain the survival of pronghorn in an arid habitat characterized by extreme temperature variation and sparse vegetation.     

Temperature difference between the body core and arterial blood supplied to the brain during hyperthermia or hypothermia in humans

 A vascular heat transfer model is developed to simulate temperature decay along the carotid arteries in humans, and thus, to evaluate temperature differences between the body core and arterial blood supplied to the brain. Included are several factors, including the local blood perfusion rate, blood vessel bifurcation in the neck, and blood vessel pairs on both sides of the neck. The potential for cooling blood in the carotid artery by countercurrent heat exchange with the jugular veins and by radial heat conduction to the neck surface was estimated. Cooling along the common and internal carotid arteries was calculated to be up to 0.87 °C during hyperthermia by high environmental temperatures or muscular exercise. This model was also used to evaluate the feasibility of lowering the brain temperature effectively by placing ice pads on the neck and head surface or by wearing cooling garments during hypothermia treatment for brain injury or other medical conditions. It was found that a 1.1 °C temperature drop along the carotid arteries is possible when the neck surface is cooled to 0 °C. Thus, the body core temperature may not be a good indication of the brain temperature during hyperthermia or hypothermia. Received: 10 January 2002 / Accepted: 7 May 2002 This research was supported by a UMBC Summer Faculty Fellowship.     

In-vitro heat exchange at the rete of the Boer goat under specified laboratory conditions

1. Each half rete from five Boer goats was perfused with water at 38 °C and flow rate of 2 ml min⁻¹ while simultaneously perfusing the cavernous sinus with water at different temperatures and flow combinations and recording temperatures across the rete.

2. The minimum temperature difference across the rete was recorded at a cavernous sinus perfusion temperature of 37.8 °C and flow rate of 2 ml min⁻¹.

3. Slopes of heat exchange increased threefold when the flow was increased four times.

4. These results support the idea that the rete is an obligate heat exchanger.

Guttural pouches, brain temperature and exercise in horses

Selective brain cooling (SBC) is defined as the lowering of brain temperature below arterial blood temperature. Artiodactyls employ a carotid rete, an anatomical heat exchanger, to cool arterial blood shortly before it enters the brain. The survival advantage of this anatomy traditionally is believed to be a protection of brain tissue from heat injury, especially during exercise. Perissodactyls such as horses do not possess a carotid rete, and it has been proposed that their guttural pouches serve the heat-exchange function of the carotid rete by cooling the blood that traverses them, thus protecting the brain from heat injury. We have tested this proposal by measuring brain and carotid artery temperature simultaneously in free-living horses. We found that despite evidence of cranial cooling, brain temperature increased by about 2.5 degrees C during exercise, and consistently exceeded carotid temperature by 0.2-0.5 degrees C. We conclude that cerebral blood flow removes heat from the brain by convection, but since SBC does not occur in horses, the guttural pouches are not surrogate carotid retes.     

Orbital rete and red muscle vein anatomy indicate a high degree of endothermy in the brain and eye of the salmon shark

The salmon shark has been ranked as the most endothermic lamnid shark based upon geographical range, extent of slow twitch muscle, supra-hepatic rete size, and limited temperature measurements, yet its anatomy has remained largely undescribed, and measurements of brain or eye temperatures have not been reported. In this study, four specimens are examined to determine if the morphological requirements for warming the brain and eyes are present. A well-developed arterial orbital rete lies within a venous sinus on both sides of the cranium. Cool, oxygenated blood from the gills can pass through the vessels of this exchanger before reaching the brain or eyes. Since venous blood in the sinus flows opposite the arterial blood, counter-current heat exchange can occur. A vein originating in the red swimming muscle likely contributes to the warmth of the venous sinus by supplying blood directly from the warmest region of the shark. Before collecting in the orbital sinus, this red muscle vein bathes the brain in warm blood. These morphological data suggest the salmon shark has a significant capacity to warm the brain and eyes.     

Selective Brain Cooling Reduces Water Turnover in Dehydrated Sheep

In artiodactyls, arterial blood destined for the brain can be cooled through counter-current heat exchange within the cavernous sinus via a process called selective brain cooling. We test the hypothesis that selective brain cooling, which results in lowered hypothalamic temperature, contributes to water conservation in sheep. Nine Dorper sheep, instrumented to provide measurements of carotid blood and brain temperature, were dosed with deuterium oxide (D₂O), exposed to heat for 8 days (40◦C for 6-h per day) and deprived of water for the last five days (days 3 to 8). Plasma osmolality increased and the body water fraction decreased over the five days of water deprivation, with the sheep losing 16.7% of their body mass. Following water deprivation, both the mean 24h carotid blood temperature and the mean 24h brain temperature increased, but carotid blood temperature increased more than did brain temperature resulting in increased selective brain cooling. There was considerable inter-individual variation in the degree to which individual sheep used selective brain cooling. In general, sheep spent more time using selective brain cooling, and it was of greater magnitude, when dehydrated compared to when they were euhydrated. We found a significant positive correlation between selective brain cooling magnitude and osmolality (an index of hydration state). Both the magnitude of selective brain cooling and the proportion of time that sheep spent selective brain cooling were negatively correlated with water turnover. Sheep that used selective brain cooling more frequently, and with greater magnitude, lost less water than did conspecifics using selective brain cooling less efficiently. Our results show that a 50kg sheep can save 2.6L of water per day (~60% of daily water intake) when it employs selective brain cooling for 50% of the day during heat exposure. We conclude that selective brain cooling has a water conservation function in artiodactyls.     

Angiology of the brain of the baboon Papio ursinus, the vervet monkey Cercopithecus pygerithrus, and the bushbaby Galago senegalensis

The investigation was undertaken to compare the blood supply and venous drainage of the brain of the baboon P. ursinus, the vervet monkey C. pygerithrus, and the bushbaby G. senegalensis with that of man, because these animals are extensively used as research models. The blood supply of the three primates was found to be similar in each case. Like man they have a complete circulus arteriosus; but they have a single anterior cerebral artery, whereas man has paired anterior cerebral arteries. The arterial supply to the cerebellum in the primates is similar to that in man, the main difference being a "common inferior cerebellar artery" which bifurcates to form the anterior inferior cerebellar and posterior inferior cerebellar arteries. In man, these arteries arise separately from the basilar artery and vertebral arteries, respectively. The dural venous drainage was also found to be similar in these primates but was far more extensive than in man. The primates have additional sinuses--the more important of these being the "basisphenoid sinus" and the petrosquamous sinus. The former drains the basilar sinus and is itself drained via the vertebral venous plexus and internal jugular vein. The latter drains via the petrosquamous foramen into the retromandibular vein. The petrosquamous sinus has a rostral extension which drains through the foramen ovale and two lateral and medial connecting sinuses which drain the cavernous and basilar sinuses, respectively. These sinuses are not found in man.     

Structure of the brain and eye heater tissue in marlins, sailfish, and spearfishes

Marlins, sailfish, and spearfishes have a heat-producing tissue beneath the brain and adjacent to the eyes. This tissue warms the brain and eyes while the rest of the body remains at water temperature. The heater tissue is derived from the superior rectus eye muscle. Only a portion of this eye muscle contains normal skeletal muscle tissue; the rest consists of the modified muscle tissue that is associated with heat production. The heat-producing portion is supplied with blood through a countercurrent heat exchanger that originates from the carotid artery. The vascular rate prevents the heat being produced by the tissue from being dissipated at the gill. An unusual circulatory supply to the eyes and brain is associated with the presence of the heater tissue in these fishes.     

Luteinizing hormone and prolactin are not retrograde transferred in perihypophyseal vascular complex in ewes

The objective of the study was to determine whether luteinizing hormone (LH) and prolactin (PRL) can access the brain by way of transfer from the venous blood of the cavernous sinus to the arterial blood supplying the brain and hypophysis. Studies were performed on heads of 22 mature sheep isolated during different phases of the estrous cycle and perfused with autologous blood. We were not able to demonstrate any transfer of LH and PRL in the investigated periods. This suggests that molecular weight of hormone may be a main factor determining the permeation and transfer of hormones in the perihypophyseal vascular complex.     

Blood vessels and the occurrence of arteriovenous anastomoses in cephalic heat loss areas of mallards,Anas platyrhynchos (Aves)

Summary 1. The blood supply to cephalic heat loss areas (nasal and oropharyngeal mucosa, bill, eyelids) was studied in mallards by using plastic corrosion casts. The structure and organization of the blood vessels, as well as the occurrence of arteriovenous anastomoses (AVAs), were examined by scanning electron microscopy of vascular casts and by paraffin sections.2. Submucosal venous plexuses (cavernous tissue) are present in the nasal cavity, tongue, and lateral margins of the palate. These plexuses receive blood from post-capillary venules, but may also receive a non-nutritive component via numerous AVAs.3. High densities of AVAs were found in the eyelids and in the tip of the bill. In the tongue and nasal mucosa, the AVAs decreased in number caudally. The reason for regional differences in the density of AVAs is discussed in relation to variation in mechanical and thermal stimulation of the tissues.4. The connection of the different heat loss areas with the Rete ophthalmicum, which is a countercurrent heat exchanger important for brain cooling, is pointed out. The vascular pattern of the head suggests that sphincteric veins are involved in regulating the venous return from the evaporative surfaces of the nasal cavity and palate. One of these veins had, in addition to the normal circular smooth muscle fibres, a conspicuous component of longitudinally arranged, subendothelial, smooth muscle fibres.     

Temperature gradients in the nasal cavity of the rabbit

1. 1.|Temperatures at four sites along the ventral nasal concha were recorded in four unrestrained rabbits exposed to ambient temperatures from 0 to 35°C.

2. 2.|The nasal temperatures decreased and temperature gradients from proximal to distal parts of the concha increased in cold-exposed rabbits.

3. 3.|The temperature gradients increased also during panting in heat-stressed rabbits.

4. 4.|The ventral nasal concha is suggested to be an efficacious heat exchanger both in cold and hot ambient, due to its geometry and vascularization.

Local vascular pathway for progesterone transfer to the brain after nasal administration in gilts

In the present study we examined whether local transfer of intranasally administrated tritiated progesterone (3H-P4) would increase its concentration in blood supplying the brain and hypophysis in comparison with other organs. Additionally, the effect of estrous cycle on the P4 transfer was evaluated on isolated gilts' heads. In the first experiment 3H-P4 was instilled into the nasal cavities of anaesthetized, immature pigs (n=10). Simultaneous blood samples were collected for radioactivity measurement every minute from the same occluded carotid artery through two catheters; one catheter was pointed towards the head, the other one towards the heart. In eight animals the ratio calculated between the 'head' and 'heart' samples was significantly (p<0.05) higher than 1 and reached a mean (+/- SEM) level of 3.23 +/- 0.81. In two animals a much higher ratio was observed. A head/heart ratio>1 indicates an existence of local transfer of 3H-P4 from venous blood to the carotid blood. In the second experiment, heads of 26 mature, cycling gilts were perfused through the right carotid artery with autologous blood. The outflow from the left carotid artery was collected as 1 min samples. 3H-P4 was infused into the angularis oculi veins. Transfer of 3H-P4 from the venous blood into the arterial blood reached the mean (+/- SEM) level of 4.11 +/- 1.08 pg/ml on days 2-4, 3.2 +/- 0.70 on days 17-21 and 0.94 +/- 0.22 pg/ml on days 15-16 of the estrous cycle. No 3H-P4 transfer was observed on days 9-11. These findings demonstrate that nasally administered progesterone can reach the brain in locally higher concentration through the vascular pathway. Moreover, the between-vessel transfer of P4 is significantly affected by the stage of the estrous cycle.     

Anatomy of the vascular system of the head and neck of the helmeted guineafowl Numida meleagris

The vascular anatomy of the head and neck of eight adult helmeted guineafowl ( Numida meleagris ) was investigated by latex injections and dissection, resin casting, and lipidol injections and X-ray photography. The vascular anatomy of these regions is similar to that of the domestic fowl Gallus domesticus , the main differences being in the helmet, wattle and cere vascularization, and the presence of a nape-cheek rete in N. meleagris . It is postulated that five vascular arrangements in the head and neck are important in brain temperature regulation. These arrangements are: the nape-cheek rete, the temporal rete, fine arteriovenous networks in the wattles and cere, and the cavernous sinus-intercarotid association. All but the last of these arrangements require pathways of blood flow to the brain other than the most direct route. Such pathways are discussed.     

The contribution of carotid rete variability to brain temperature variability in sheep in a thermoneutral environment

The degree of variability in the temperature difference between the brain and carotid arterial blood is greater than expected from the presumed tight coupling between brain heat production and brain blood flow. In animals with a carotid rete, some of that variability arises in the rete. Using thermometric data loggers in five sheep, we have measured the temperature of arterial blood before it enters the carotid rete and after it has perfused the carotid rete, as well as hypothalamic temperature, every 2 min for between 6 and 12 days. The sheep were conscious, unrestrained, and maintained at an ambient temperature of 20-22 degrees C. On average, carotid arterial blood and brain temperatures were the same, with a decrease in blood temperature of 0.35 degrees C across the rete and then an increase in temperature of the same magnitude between blood leaving the rete and the brain. Rete cooling of arterial blood took place at temperatures below the threshold for selective brain cooling. All of the variability in the temperature difference between carotid artery and brain was attributable statistically to variability in the temperature difference across the rete. The temperature difference between arterial blood leaving the rete and the brain varied from -0.1 to 0.9 degrees C. Some of this variability was related to a thermal inertia of the brain, but the majority we attribute to instability in the relationship between brain blood flow and brain heat production.     

Arteriovenous difference of the blood density in the coronary circulation

The mechanical oscillator technique permits determining blood density continuously with high accuracy. Using this technique arteriovenous density gradients were recorded in the coronary vascular bed of anesthetized dogs. It was found that the coronary sinus blood has a higher density than arterial blood due to the loss of filtered fluid in the microcirculation. The amount of fluid loss corresponds to the lymph flow in the myocardium. Increase of venous pressure leads to an increase of the density gradient. Intermittent coronary sinus occlusion (ICSO) surprisingly leads to a reduction of the density gradient. Injection of osmotically hypertensive fluids influences the arteriovenous gradient by shifting extravascular fluid into the blood. The method permits the determination of filtration coefficients and to estimate the tissue volume available for fluid exchange.     

Comparison of Partial Volume Effects in Arterial and Venous Contrast Curves in CT Brain Perfusion Imaging

Purpose

In brain CT perfusion (CTP), the arterial contrast bolus is scaled to have the same area under the curve (AUC) as the venous outflow to correct for partial volume effects (PVE). This scaling is based on the assumption that large veins are unaffected by PVE. Measurement of the internal carotid artery (ICA), usually unaffected by PVE due to its large diameter, may avoid the need for partial volume correction. The aims of this work are to examine i) the assumptions behind PVE correction and ii) the potential of selecting the ICA obviating correction for PVE.

Methods

The AUC of the ICA and sagittal sinus were measured in CTP datasets from 52 patients. The AUCs were determined by i) using commercial CTP software based on a Gaussian curve-fitting to the time attenuation curve, and ii) by simple integration of the time attenuation curve over a time interval. In addition, frames acquired up to 3 minutes after first bolus passage were used to examine the ratio of arterial and venous enhancement. The impact of selecting the ICA without PVE correction was illustrated by reporting cerebral blood volume (CBV) measurements.

Results

In 49 of 52 patients, the AUC of the ICA was significantly larger than that of the sagittal sinus (p = 0.017). Measured after the first pass bolus, contrast enhancement remained 50% higher in the ICA just after the first pass bolus, and 30% higher 3 minutes later. CBV measurements were significantly lowered when the ICA was used without PVE correction.

Conclusions

Contradicting the assumptions underlying PVE correction, contrast in the ICA was significantly higher than in the sagittal sinus, even 3 minutes after the first pass of the contrast bolus. PVE correction might lead to overestimation of CBV if the CBV is calculated using the AUC of the time attenuation curves.     

Clinical anatomy of the blood spaces and blood vessels surrounding the siphon of the internal carotid artery

The anterior blood space of the cavernous sinus is situated anterolateral to the carotid siphon in 70%, anterior to it in 15%, and lateral to it in 15%. Its height, depth, and mediolateral breadth were measured. The mean distance between the carotid siphon and the skin at the supraorbital foramen was measured with 63 (52.4-71.4) mm. The drainage of the orbital veins was studied and described as well as the area of origin and first course of the ophthalmic artery and its clinical importance.     

Vascular morphology of the bovine spermatic cord and testis

Summary The highly coiled testicular artery within the bovine spermatic cord has a constant luminal diameter but a continuously decreasing mural thickness. The pampini form plexus is composed of three interconnected venous networks differing in mesh sizes and calibres. The large veins of the first network display pouches and permanent constrictions, which may serve as throttle devices. The constitutents of the third network are venules or venous capillaries with diameters between 10 and 20 m; they favor a periarterial position or even occupy the media-adventitia border of the testicular artery. All plexus veins are devoid of valves. The existence of true arteriovenous anastomoses between smaller branches of the testicular artery and plexus veins was established by serial sections. The vascular morphology of the spermatic cord is discussed with special attention to a postulated venous-arterial steroid transfer in this region.Supported by the Deutsche Forschungsgemeinschaft and the Stiftung zur Förderung der wissenschaftlichen Forschung an der Universität Bern     

Swimming muscle helps warm the brain of lamnid sharks

Summary Lamnid sharks are known to have warm red muscle and warm brains. We describe a large vein in lamnid sharks that provides a route for transfer of warm blood from the red muscle to the central nervous system. This red muscle vein runs longitudinally in the red muscle and is valved to direct blood flow anteriorly. It joins the myelonal vein in the neural canal, thus providing a route for blood flow from the red muscle to the brain. Temperature profiles along the neural canal of freshly caught mako sharks show that warm blood enters the myelonal vein from the red muscle vein. Experiments with heat generation by model brains indicate that the metabolic heat produced by the brain is probably not sufficient to cause the temperature elevations observed. Metabolic heat imported from the red swimming muscle may be a valuable addition to the heat budget of the head.