Peripheral thermoregulation: Haematologic or vascular adaptations

Arctic homeotherms maintain their feet near O°C while standing on much colder surfaces by blood-borne heat input. At 5°C in vivo blood viscosity increased approximately 5 times in arctic wolves and wolverines, and for comparison, in non-cold acclimated man. Such increase in blood viscosity in vivo would reduce perfusion; however, specialized arteriovenous plexuses have evolved in foot pad cutaneous tissues. These augment heat delivery and protect tissues from frost bite.     

Optimal hematocrit theory during activity in the bullfrog (Rana catesbeiana).

1. There is an exponential relationship between blood viscosity (cP) and hematocrit (%) for the bullfrog; eta = 1.81 e0.033Hct. The in vitro optimal hematocrit calculated for blood flow through tubes, from this relationship for bullfrog blood, is 30%. 2. Amphibian blood is a non-Newtonian fluid with viscosity dependent on shear rate. It has a finite yield shear stress of about 1.5 dynes cm-2. 3. Hematocrit of bullfrogs was increased from 27% (control) to 57% by isovolemic erythrocythemia (constant volume blood-doping). There was a slight increase in systolic, diastolic and venous blood pressure with elevated hematocrit. 4. Systemic arch blood flow rate was inversely related to blood viscosity for erythrocythemic bullfrogs. The decrease in systemic arch blood flow at high hematocrits was due primarily to reduced pulse volume rather than reduced heart rate. 5. Systemic arch blood flow, when standardised between individuals, was inversely related to blood viscosity; Qbl = 0.185 + 3.73 eta -1. This relationship was significantly different from that predicted by the Poiseuille-Hagen flow formula. The in vivo optimal hematocrit calculated from this relationship was 41%. 6. Optimal hematocrit theory appears to be generally applicable for Rana catesbeiana in vitro and in vivo. Most individuals had an in vivo optimal hematocrit, but the absence of a clear optimal hematocrit for some individuals could reflect methodological variability, or in vivo physiological compensation for the increased blood viscosity at high hematocrit.     

Comparisons of blood viscosity between amphibians and mammals at 3°C and 38°C

(1) The range of temperature exposure of endotherms is narrow compared to ectotherms that can experience daily and seasonal temperature fluxes. (2) Comparison of the blood viscosity of amphibians (bullfrog, Woodhouse's toad, and marine toad) and mammals (horse, dog, and rat) at 3°C and 38°C was undertaken to determine if the effect of temperature on blood viscosity was diminished in amphibians relative to mammals. (3) Mammals did not consistently show greater changes in blood viscosity, plasma viscosity, or relative viscosity with decreasing temperatures relative to the amphibians in this study. (4) These data do not support our hypothesis that blood viscosity of amphibians is less affected by temperature than mammalian blood.     

Is there an optimal haematocrit for rainbow trout,Oncorhynchm mykiss (Walbaum)? An interpretation of recent data based on blood viscosity measurements

The viscosity of blood from rainbow trout was measured following manipulation of haematocrit by bleeding, hypoxia. exercise, and anaesthesia. Blood viscosity when measured at high shear rate (225 s ¹) was proportional to haematocrit, but the dependence of viscosity on shear rate was far less for swollen erythrocytes from exercised and anaesthetized trout. Erythrocyte swelling was most marked in exercised and anaesthetized trout, and is a confounding factor when considering the effect of haematocrit on viscosity.
The viscosity of blood with variable haematocrit, but constant mean cell Hb concentration, indicated that the relative oxygen transport capacity in trout was optimal at a haematocrit of 30%. Data from this, and earlier studies show that haematocrit in trout is variable and labile, yet none of the haematocrit values following manipulations are less than 85% of optimal. Optimal haematocrit is however, significantly higher than measured values from either cannulated or acutely venesected resting trout.     

Strength of asymmetric competition between predators in food webs ruled by fluctuating prey: the case of foxes in tundra

In food webs heavily influenced by multi‐annual population fluctuations of key herbivores, predator species may differ in their functional and numerical responses as well as their competitive ability. Focusing on red and arctic fox in tundra with cyclic populations of rodents as key prey, we develop a model to predict how population dynamics of a dominant and versatile predator (red fox) impacted long‐term growth rate of a subdominant and less versatile predator (arctic fox). We compare three realistic scenarios of red fox performance: (1) a numerical response scenario where red fox acted as a resident rodent specialist exhibiting population cycles lagging one year after the rodent cycle, (2) an aggregative response scenario where red fox shifted between tundra and a nearby ecosystem (i.e. boreal forest) so as to track rodent peaks in tundra without delay, and (3) a constant subsidy scenario in which the red fox population was stabilized at the same mean density as in the other two scenarios. For all three scenarios it is assumed that the arctic fox responded numerically as a rodent specialist and that the mechanisms of competition is of a interference type for space, in which the arctic fox is excluded from the most resource rich patches in tundra. Arctic fox is impacted most by the constant subsidy scenario and least by the numerical response scenario. The differential effects of the scenarios stemmed from cyclic phase‐dependent sensitivity to competition mediated by changes in temporal mean and variance of available prey to the subdominant predator. A general implication from our result is that external resource subsidies (prey or habitats), monopolized by the dominant competitor, can significantly reduce the likelihood for co‐existence within the predator guild. In terms of conservation of vulnerable arctic fox populations this means that the likelihood of extinction increases with increasing amount of subsidies (e.g. carcasses of large herbivores or marine resources) in tundra and nearby forest areas, since it will act to both increase and stabilize populations of red fox.     

Factors Driving Potential Ammonia Oxidation in Canadian Arctic Ecosystems: Does Spatial Scale Matter?

Ammonia oxidation is a major process in nitrogen cycling, and it plays a key role in nitrogen limited soil ecosystems such as those in the arctic. Although mm-scale spatial dependency of ammonia oxidizers has been investigated, little is known about the field-scale spatial dependency of aerobic ammonia oxidation processes and ammonia-oxidizing archaeal and bacterial communities, particularly in arctic soils. The purpose of this study was to explore the drivers of ammonia oxidation at the field scale in cryosols (soils with permafrost within 1 m of the surface). We measured aerobic ammonia oxidation potential (both autotrophic and heterotrophic) and functional gene abundance (bacterial amoA and archaeal amoA) in 279 soil samples collected from three arctic ecosystems. The variability associated with quantifying genes was substantially less than the spatial variability observed in these soils, suggesting that molecular methods can be used reliably evaluate spatial dependency in arctic ecosystems. Ammonia-oxidizing archaeal and bacterial communities and aerobic ammonia oxidation were spatially autocorrelated. Gene abundances were spatially structured within 4 m, whereas biochemical processes were structured within 40 m. Ammonia oxidation was driven at small scales (<1m) by moisture and total organic carbon, whereas gene abundance and other edaphic factors drove ammonia oxidation at medium (1 to 10 m) and large (10 to 100 m) scales. In these arctic soils heterotrophs contributed between 29 and 47% of total ammonia oxidation potential. The spatial scale for aerobic ammonia oxidation genes differed from potential ammonia oxidation, suggesting that in arctic ecosystems edaphic, rather than genetic, factors are an important control on ammonia oxidation.     

Habitat utilization by sympatric arctic charr Salvelinus alpinus L. and brown trout Salmo trutta L. in Lake Atnsjø, south-east Norway

SUMMARY. 1. Habitat utilization, as well as inter- and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south-east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June-October 1985.
2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.
3. Larger (>230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2-3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.
4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.
5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.     

Rheology of embryonic avian blood

Shear stress, a mechanical force created by blood flow, is known to affect the developing cardiovascular system. Shear stress is a function of both shear rate and viscosity. While established techniques for measuring shear rate in embryos have been developed, the viscosity of embryonic blood has never been known but always assumed to be like adult blood. Blood is a non-Newtonian fluid, where the relationship between shear rate and shear stress is nonlinear. In this work, we analyzed the non-Newtonian behavior of embryonic chicken blood using a microviscometer and present the apparent viscosity at different hematocrits, different shear rates, and at different stages during development from 4 days (Hamburger-Hamilton stage 22) to 8 days (about Hamburger-Hamilton stage 34) of incubation. We chose the chicken embryo since it has become a common animal model for studying hemodynamics in the developing cardiovascular system. We found that the hematocrit increases with the stage of development. The viscosity of embryonic avian blood in all developmental stages studied was shear rate dependent and behaved in a non-Newtonian manner similar to that of adult blood. The range of shear rates and hematocrits at which non-Newtonian behavior was observed is, however, outside the physiological range for the larger vessels of the embryo. Under low shear stress conditions, the spherical nucleated blood cells that make up embryonic blood formed into small aggregates of cells. We found that the apparent blood viscosity decreases at a given hematocrit during embryonic development, not due to changes in protein composition of the plasma but possibly due to the changes in cellular composition of embryonic blood. This decrease in apparent viscosity was only visible at high hematocrit. At physiological values of hematocrit, embryonic blood viscosity did not change significantly with the stage of development.     

Glucose turnover and defense of blood glucose levels in Arctic fox (Alopex lagopus)

1. Glucose utilization was assessed in fed and fasted arctic fox, maintained on a diet similar in composition to food available in the wild. 2. Fasted (24 hr) glucose concentration was not significantly different from the fed level (134 mg/dl). 3. Fasting was associated with a significant reduction in glucose space, pool size, total entry rate, and irreversible loss which suggests a decline in gluconeogenesis. 4. Glucose recycling was not significantly different between the fed and fasted states. 5. We suggest that, in the arctic fox, the mechanism for defending blood glucose levels during fasting is based on restricting blood glucose to tissues with a high glucose dependency.     

Effect of microrheology of blood on the apparent flow instability in a rotational viscometer

Flow instability (formation of vortices and a concurrent increase in the apparent viscosity) was studied in the rotational rhombospheroid viscometer of 3 degrees, 5 degrees and 10 degrees gaps over a range of speeds from 10 to 300 r.p.m.. Comparisons between different blood systems were carried out mainly at 250 r.p.m. Experiments were carried out on blood samples obtained directly from human subjects, or from the Blood Bank, or from horses. Reconstituted suspensions of red cells in albumin or dextran were also used. Apparent flow instability was found to be not solely a function of blood viscosity, but a multiple function of many viscosity factors or blood subphases, including instability-decreasing factors such as haematocrit and aggregation of red cells; and instability-increasing factors such as rigidity of red cells; and thus specific to and characteristic of individual blood samples. Apparent instability can be described by multiple regressions as a function, Z, of red cell rigidity, Tk, blood viscosity, napp, and aggregation of red cells, AG; for example: Z = -28.29 + 26.24 Tk + 0.109 napp (r = 0.816; P less than 0.001), or Z = 5.90 - 0.0165 AG - 0.752 napp (r = 0.573; P less than 0.05). The apparent instability can be seen only in one-third of blood samples obtained from horses, and in more than half of blood samples obtained from human donors; majority of human donors shows apparent instability below 3 per cent.     

Effects of alloxan-induced diabetes on hemorheology in rabbits.

Plasma viscosity (PV), apparent whole blood viscosity (WBV), relative blood viscosity (RV) and erythrocyte deformability (filterability) (EDF) were determined in 13 New Zealand White (NZW) rabbits with alloxan induced-diabetes (AID) and 8 normal NZW rabbits, matched for age, sex and weight. AID rabbits were divided into two groups depending on the duration of hyperglycemia (long-term, greater than 6.0 months (n = 7), and short-term, less than or equal to 3.0 months of hyperglycemia, n = 6). Comparing long-term AID rabbits to normal animals, we found significant increases in WBV (P less than 0.001, 0.005 for high and low rates of shear, respectively), and a marked reduction in EDF (P less than 0.001). There was no significant difference in PV between long-term AID and normal rabbits. Conversely, PV was significantly increased in rabbits with short-term diabetes (P less than 0.01) while there was a concurrent significant increase in WBV measured at high and low rates of shear (P less than 0.001, 0.001, respectively). No difference was detected in EDF between normal and short-term AID rabbits. Furthermore, in long-term AID rabbits there was a strongly positive correlation between RV and reduced erythrocyte deformability (r = 0.94, P = 0.006) while WBV strongly correlated with PV (r = 0.92, P = 0.004) in the short-term AID subgroup. We conclude from these data: (1) elevated blood viscosity in long term AID rabbits is associated with reduced erythrocyte filterability; and (2) elevated WBV in short-term AID rabbits is associated with increased PV.     

High viscosity plasma expanders: Volume restitution fluids for lowering the transfusion trigger

Hemorheological studies lead to the axiom that high plasma viscosity is detrimental and that it is beneficial to lower blood viscosity, a precept embodied in the practice of hemodilution, where improved perfusion is attributed to the lowering of blood viscosity. Hemodilution is limited by the transfusion trigger, hemoglobin content of blood of about 7-8 g/dl, which indicates when further volume replacements must restore oxygen carrying capacity with red blood cells (RBC). However, oxygen consumption and delivery are not compromised upon passing this landmark. The reduced blood viscosity does not transmit adequate pressure to the capillaries, causing functional capillary density (FCD) to decrease, jeopardizing organ function through the inadequate extraction of products of metabolism from the tissue by the capillaries. Studies in hemorrhagic shock show that survival is primarily determined by the maintenance of FCD and secondarily by tissue oxygenation. FCD is maintained as hematocrit is reduced beyond the transfusion trigger by increasing plasma viscosity, which transmits systemic pressure to the capillaries and induces vasodilatation through the increased shear stress dependent release of vasodilators. Consequently the transfusion trigger is also a "viscosity trigger" indicating when blood and plasma viscosity are too low. In this condition increasing plasma viscosity is beneficial and extends the transfusion trigger reducing the use of blood transfusions.     

Plasma viscosity and cerebral blood flow

We hypothesized that the response of cerebral blood flow (CBF) to changing viscosity would be dependent on "baseline" CBF, with a greater influence of viscosity during high-flow conditions. Plasma viscosity was adjusted to 1.0 or 3.0 cP in rats by exchange transfusion with red blood cells diluted in lactated Ringer solution or with dextran. Cortical CBF was measured by H(2) clearance. Two groups of animals remained normoxic and normocarbic and served as controls. Other groups were made anemic, hypercapnic, or hypoxic to increase CBF. Under baseline conditions before intervention, CBF did not differ between groups and averaged 49.4 +/- 10.2 ml. 100 g(-1). min(-1) (+/-SD). In control animals, changing plasma viscosity to 1. 0 or 3.0 cP resulted in CBF of 55.9 +/- 8.6 and 42.5 +/- 12.7 ml. 100 g(-1). min(-1), respectively (not significant). During hemodilution, hypercapnia, and hypoxia with a plasma viscosity of 1. 0 cP, CBF varied from 98 to 115 ml. 100 g(-1). min(-1). When plasma viscosity was 3.0 cP during hemodilution, hypercapnia, and hypoxia, CBF ranged from 56 to 58 ml. 100 g(-1). min(-1) and was significantly reduced in each case (P < 0.05). These results support the hypothesis that viscosity has a greater role in regulation of CBF when CBF is increased. In addition, because CBF more closely followed changes in plasma viscosity (rather than whole blood viscosity), we believe that plasma viscosity may be the more important factor in controlling CBF.     

Thin-blooded Antarctic fishes: a rheological comparison of the haemoglobin-free icefishes Chionodraco kathleenae and Cryodraco antarcticus with a red-blooded nototheniid, Pagothenia bernacchii

The icefishes (family Channichthyidae) comprise a unique group of teleost fishes endemic to Antarctic and sub-antarctic seas. All members of the family totally lack haemoglobin. Haematological parameters and viscosity were determined for blood from 11 specimens of two channichthyid species (Chionodraco kathleenae Regan, 1914; Cryodraco antarcticus Dollo, 1900), and 14 specimens of a red-blood Antarctic nototheniid species (Pagothenia bernacchii (Boulenger, 1902)), captured near the Italian research station at Terra Nova Bay, Ross Sea, Antarctica. Channichthyid blood contained only a small number of non-pigmented cells (10 000-40 000 cells μI^?1, depending on species) in contrast to nototheniid blood (360 000-450 000 cells μI^?1 in unstressed specimens). Blood viscosity was measured by cone plate viscometry over a range of shear rates (11.3-450s ^?1), at six temperatures between – 1.8°C and + 15°C. At the ambient Antarctic seawater temperature of – 1.8° C, and at low shear rate (22.5 s^?1), the viscosity of channichthyid blood was relatively low (3.99 ± 0.40 cP) compared with blood taken from unstressed P. bernacchii, which was about 25% more viscous (4.91 ± 0.59 cP). The viscosity of channichthyid blood was almost independent of shear rate, approximating an ideal Newtonian fluid, while the viscosity of nototheniid blood was much more dependent upon both shear rate and temperature, increasing sharply at low shear rates and low temperatures. Viscosity of nototheniid blood varied with haematocrit, which was in turn strongly influenced by stress. Blood samples taken from P. bernacchii under moderate stress induced by handling during acute caudal venepuncture had haematocrit values in the range 15–20% and viscosities of 8-l0cP, while undisturbed specimens sampled through a venous cannula yielded haematocrits of 8–10%. The viscosity of nototheniid plasma did not differ significantly from that of channichthyid whole blood or channichthyid plasma. The higher viscosity of nototheniid blood is attributable to cell content, and in stressed specimens possibly also to adrenergic swelling of erythrocytes. The absence of erythrocytes in channichthyid blood avoids the great increases in viscosity which are induced in corpusculate blood by sub-zero seawater temperatures.     

Systemic and renal hemodynamics after moderate hemodilution with HbOCs in anesthetized rabbits

Hb-based O(2)-carrying solutions (HbOCs) have been developed as red blood cell substitutes for use in patients undergoing hemodilution. Variously modified Hb with diverse solution properties have been shown to produce variable hemodynamic responses. We examined, through pulsed-Doppler velocimetry, the systemic and renal hemodynamic effects of dextran-benzene-tetracarboxylate-conjugated (Hb-Dex-BTC), bis(3,5-dibromosalicyl)fumarate cross-linked (alphaalpha-Hb), and o-raffinose-polymerized (o-raffinose-Hb) Hb perfused in rabbits after moderate hemodilution (30% hematocrit), and we compared the effects of these Hb solutions with the effects elicited by plasma volume expanders. In addition, vascular hindrance (resistance/blood viscosity at 128.5 s(-1)) was calculated to determine whether a moderate decrease in the viscosity of blood mixed with HbOCs may impair vasoconstriction as a result of autoregulation after infusion of cell-free Hb. No changes were observed in renal hemodynamics after hemodilution with reference or Hb solutions. Increase in blood pressure and vascular resistance was found with Hb-Dex-BTC and alphaalpha-Hb (for 180 min) and, to a lesser extent, with o-raffinose-Hb (for 120 min). Furthermore, Hb-Dex-BTC (high viscosity) and o-raffinose-Hb (medium viscosity) induced comparable increases in vascular hindrance (from 0.091 to 0. 159 and from 0.092 to 0.162 cm(-1), respectively) but far less than that produced by alphaalpha-Hb (low viscosity, from 0.092 to 0.200 cm(-1)). These results suggest that maintaining the viscosity of blood by infusing solutions with high viscosity makes it possible to limit vasoconstriction due to autoregulation mechanisms and mainly caused by hemodilution per se.     

Natal den selection by sympatric arctic and red foxes on Herschel Island,Yukon, Canada

In the twentieth century, red fox (Vulpes vulpes) expanded into the Canadian Arctic, where it competes with arctic fox (Vulpes lagopus) for food and shelter. Red fox dominates in physical interactions with the smaller arctic fox, but little is known about competition between them on the tundra. On Hershel Island, north Yukon, where these foxes are sympatric, we focused on natal den choice, a critical aspect of habitat selection. We tested the hypothesis that red fox displaces arctic fox from dens in prey-rich habitats. We applied an approach based on model comparisons to analyse a 10-year data set and identify factors important to den selection. Red fox selected dens in habitats that were more prey-rich in spring. When red foxes reproduced, arctic fox selected dens with good springtime access, notably many burrows unblocked by ice and snow. These provided the best refuge early in the reproductive season. In the absence of red foxes, arctic foxes selected dens offering good shelter (i.e. large isolated dens). Proximity to prey-rich habitats was consistently less important than the physical aspects of dens for arctic fox. Our study shows for the first time that red foxes in the tundra select dens associated primarily with prey-rich areas, while sympatric arctic foxes do not. These results fit a model of red fox competitively interfering with arctic fox, the first detailed study of such competition in a true arctic setting.     

Effects of the non-Newtonian viscosity of blood on flows in a diseased arterial vessel. Part 1: Steady flows

Effects of the non-Newtonian viscosity of blood on a flow in a coronary arterial casting of man were studied numerically using a finite element method. Various constitutive models were examined to model the non-Newtonian viscosity of blood and their model constants were summarized. A method to incorporate the non-Newtonian viscosity of blood was introduced so that the viscosity could be calculated locally. The pressure drop, wall shear stress and velocity profiles for the case of blood viscosity were compared for the case of Newtonian viscosity (0.0345 poise). The effect of the non-Newtonian viscosity of blood on the overall pressure drop across the arterial casting was found to be significant at a flow of the Reynolds number of 100 or less. Also in the region of flow separation or recirculation, the non-Newtonian viscosity of blood yields larger wall shear stress than the Newtonian case. The origin of the non-Newtonian viscosity of blood was discussed in relation to the viscoelasticity and yield stress of blood.     

In silico biophysics and hemorheology of blood hyperviscosity syndrome

Hyperviscosity syndrome (HVS) is characterized by an increase of the blood viscosity by up to seven times the normal blood viscosity, resulting in disturbances to the circulation in the vasculature system. HVS is commonly associated with an increase of large plasma proteins and abnormalities in the properties of red blood cells, such as cell interactions, cell stiffness, and increased hematocrit. Here, we perform a systematic study of the effect of each biophysical factor on the viscosity of blood by employing the dissipative particle dynamic method. Our in silico platform enables manipulation of each parameter in isolation, providing a unique scheme to quantify and accurately investigate the role of each factor in increasing the blood viscosity. To study the effect of these four factors independently, each factor was elevated more than its values for a healthy blood while the other factors remained constant, and viscosity measurement was performed for different hematocrits and flow rates. Although all four factors were found to increase the overall blood viscosity, these increases were highly dependent on the hematocrit and the flow rates imposed. The effect of cell aggregation and cell concentration on blood viscosity were predominantly observed at low shear rates, in contrast to the more magnified role of cell rigidity and plasma viscosity at high shear rates. Additionally, cell-related factors increase the whole blood viscosity at high hematocrits compared with the relative role of plasma-related factors at lower hematocrits. Our results, mapped onto the flow rates and hematocrits along the circulatory system, provide a correlation to underpinning mechanisms for HVS findings in different blood vessels.     

Morphofunctional asymmetry of the microvascular bed of the rabbit concha auriculae

Microvessels of rabbit ears chamber and blood samples drawn from the internal ear vein after 30 min ischemia were studied. In the initial state the length of the left ear microvascular bed was 1.5 times higher, mean microvessel diameter 14.6% less, apparent viscosity of outflowing blood 16.8% less, RBC concentration per volume unity of blood 8% higher, RBC electrophoretic mobility 13.7% higher than those in the right one. It was shown that the left microvessel bed responds to the ischemia with the 13.2% increase of its length and 13.6% increase of the outflowing blood viscosity, the right one--with the 9.7% increase of microvessel diameter and 7.9% decrease of the viscosity.     

Relation between extent of coronary artery disease and blood viscosity.

Blood viscosity (shear rate 100/s) and its major determinants (packed cell volume, plasma fibrinogen concentration, and plasma viscosity) were measured before coronary angiography in 50 men aged 30-55 and related to the extent of coronary artery disease. Twenty-six men had extensive disease (stenosis of two or three major coronary vessels), and 24 had either stenosis of one vessel or no stenosis. The 26 men with extensive disease had significantly higher mean blood viscosity than those with mild or no disease and 25 healthy controls (p less than 0.001). The increased viscosity was due partly to a higher packed cell volume and partly to a higher fibrinogen concentration; plasma viscosity was not significantly increased. These differences could not be explained by smoking history. These results suggest an association between increased blood viscosity and extensive coronary artery disease in men, which merits further investigation.