Comprehensive Physiology Wiley Online Library

Cardiovascular Adjustments to Thermal Stress

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Abstract

The sections in this article are:

1 Critique of Some Methods Employed in Humans
1.1 Total and Regional Cutaneous Blood Flow
1.2 Cardiac Output
1.3 Central Blood Volume
1.4 Regional Blood Flow
1.5 Measurement of Temperature
2 General Features of the Cutaneous Circulation: A Brief Review
2.1 Cutaneous Blood Flow
2.2 Cutaneous Blood Volume
2.3 Innervation of Cutaneous Arterioles
2.4 Local Control of Skin Blood Flow: Interaction With Reflexes
2.5 Reflex and Local Control of Cutaneous Veins
3 Circulatory Adjustments to Heat Stress in Resting Humans
3.1 Changes in Cardiac Output, Blood Pressure, and Blood Volume
3.2 Distribution of Cardiac Output in Heat‐Stressed Resting Humans
3.3 Cardiac Output and Its Distribution in Heat‐Stressed Resting Subhuman Species
3.4 Distribution of Blood Volume in Heat‐Stressed Resting Humans
3.5 Mechanisms of Blood Flow Redistribution in Rest
3.6 Acclimatization in Rest
4 Interaction Between Thermoregulatory and Nonthermoregulatory Reflexes in Control of Skin Blood Flow
4.1 Cutaneous Vascular Responses to Baroreflexes
4.2 Cardiopulmonary (Low‐Pressure) Baroreflex
4.3 Postural Influences: Hydrostatic Problem
4.4 Upright Posture in Heat Stress: Interaction Between Reflex Vasoconstriction and Vasodilation
4.5 Exercise
4.6 Possible Modes of Reflex Interaction: Unsolved Problems
5 Overall Circulatory Adjustments to Heat Stress and Exercise
5.1 Distribution of Blood Volume
5.2 Redistribution of Blood Flow During Exercise
5.3 Central Circulation
5.4 Adaptation to Heat Stress and Exercise: Acclimatization
Figure 1. Figure 1.

Comparison of simultaneous measurements of splanchnic blood flow (SBF) by constant‐dye‐infusion technique and by electromagnetic flowmeters on hepatic artery and portal vein (solid and open circles) or by direct flow measurement (crosses) .

From Rowell , reproduced by permission of Grune & Stratton
Figure 2. Figure 2.

Simultaneous measurement of renal blood flow by p‐aminohippurate clearance (dashed line, left kidney) and electromagnetic flowmeter (solid line, right kidney) during graded hemorrhage, which lowered mean arterial pressure (MAP; top panel) and increased renal nerve activity (second panel).

Adapted from Selkurt
Figure 3. Figure 3.

Schematic illustration of blood volume distribution and venous pressures in upright human compared with those in the dog (and most other animals).

Adapted from Folkow and Neil
Figure 4. Figure 4.

Relationship between venous pressure in leg veins and volume of blood contained at 3 different leg skin temperatures (Tsk). Rise in local Tsk increases the blood volume contained in cutaneous veins at a given venous distending pressure; i.e., venous compliance increased.

Redrawn from Gauer and Thron ; from Rowell
Figure 5. Figure 5.

Right and left forearm blood flows in 1 subject with congenital absence of sweat glands (anhidrotic ectodermal dysplasia) during direct whole‐body heating (forearms were not heated). Forearm blood flows showed almost no increase in response to a 1.5°C increase in Tc. In normal skin this would raise blood flow to 20–30 ml · min−1 · 100 ml−1 of forearm, the increment being confined to skin.

From Brengelmann, Rowell, et al.
Figure 6. Figure 6.

Cardiac output rose ∼6 liters/min, and forearm blood flow rose to 20 ml per 100 ml of forearm per min in 1 subject who was indirectly heated (legs in hot water) for 100 min, which raised body core temperature by 2.6°C. Heart rate, HR; stroke volume, SV.

From Koroxenidis et al.
Figure 7. Figure 7.

Circulatory responses to whole‐body heating to limits of thermal tolerance in a representative subject. Elevation in skin temperature (Ts) by water‐perfused suit raised rectal temperature (Tr) and right atrial blood temperature (Tb), cardiac output (CO), HR, SV, and central venous pressure (CVP). Note time courses of changes in aortic mean and pulse pressures (Ao BP) and right atrial mean pressure (RA‐MP). Total peripheral resistance, TPR.

From Rowell et al.
Figure 8. Figure 8.

Average circulatory responses in directly heated subjects. Initial and final values and approximate time course are shown for Ts and right atrial blood temperature (Tblood) and each cardiovascular variable. Changes in cardiac output and splanchnic, renal, and muscle blood flow contributing to increased skin blood flow are shown in separate boxes.

From Rowell
Figure 9. Figure 9.

Time course of changes in splanchnic blood flow (SBF) and splanchnic vascular resistance (SVR) (open triangles) during prolonged whole‐body heating (as in Figs. and ) in 7 subjects . Solid line in top panel, approximate time course of Tra (Tb) during heating.

From Rowell
Figure 10. Figure 10.

Left, estimated distribution of cardiac output between skin and other major organs at rest in normothermia; right, during severe hyperthermia (Tc greater than 39°C).

Figure 11. Figure 11.

Open bars, distribution of blood flow (radioactive microspheres) in 8 conscious baboons during quiet, seated rest in normothermic and hyperthermic (Tc increased 1.5°C‐2.0°C) conditions (cross‐hatching). Note insignificant rise in brain blood flow with heating. One SE and significance of change (*P < 0.05, **P < 0.01) are shown. Central nervous system, CNS.

Figure 12. Figure 12.

Blood flow (by microsphere technique) to various cutaneous regions during normothermic control period (solid bars) during moderate hyperthermia (Tc rose 1.5°C‐2.0°C) in resting, awake baboons (open bars, 0.5 SE shown where discernible).

From Hales, Rowell, and King
Figure 13. Figure 13.

Schematic illustration of altered distribution of blood volume in a heat‐stressed man. In contrast most species have small hydrostatic influence and small cutaneous vasodilation. Substantial vasodilation occurs in the dog tongue.

Figure 14. Figure 14.

Left, Krogh's simplified model of the cardiovascular system divided into 2 circuits, 1 compliant (long time constant) and 1 noncompliant (short time constant). Right, electrical equivalent of a generalized parallel‐compartment model of the peripheral circulation. Arterial resistance, Ra; venous compliance, Cv; venous resistance, Rv (Rv2 and Cv2 illustrate variable resistance and compliance of cutaneous veins); arterial compliance, Ca. Venous time constant for a particular circuit (τ) is RvCv.

Left, adapted from Caldini et al. ; right, adapted from Mitzner and Goldberg
Figure 15. Figure 15.

Hydraulic model contrasting effects of increasing blood flow through noncompliant (panel A) versus compliant circuits (panels B, C, D). Vasodilation of compliant circuits with long time constants (RvCv, in Fig. ) depletes the volume (vol) of central reservoirs [central blood volume (CBV) or heart and pulmonary vessels]. Vasoconstriction (VC) of compliant circuits passively displaces their volumes to partially restore CBV.

Figure 16. Figure 16.

Schematic illustration of how volume may be mobilized passively from splanchnic veins as cutaneous veins fill up during heat stress. Fall in CVP creates a pressure gradient from right atrium to hepatic veins. Constriction of splanchnic arterioles further lowers splanchnic venous pressure and translocates more volume to the heart. Because of steep splanchnic venous pressure‐volume curve (right), a small decrease in venous pressure displaces a large volume of blood. GI, gastrointestinal tract.

Figure 17. Figure 17.

Relationship between heart rate and plasma norepinephrine concentration in cancer patients treated with hyperthermia (to 41.5°C).

From Kim et al.
Figure 18. Figure 18.

Relationship between SBF as percent of resting SBF and heart rate (HR) during exercise in neutral and hot environments and before (open squares) and after (solid squares) physical conditioning . Data are compared with renal blood flow (RBF) during exercise . Regression lines for SBF versus HR in resting human stressed by heat and lower‐body negative pressure (LBNP) are displaced to the left (cf. norepinephrine data in Fig. ) .

Adapted from Rowell
Figure 19. Figure 19.

Relationship between increase in plasma norepinephrine (NE; ng/ml) and change in HR during quiet standing (open and closed triangles) and during supine exercise (closed circles). Note the rightward displacement on HR axis during exercise (cf. Fig. ).

Adapted from Christensen and Brandsborg
Figure 20. Figure 20.

Average responses to gradual decline in right atrial pressure (RAP) induced by slow ramp (‐1 mmHg/min) of lower‐body negative pressure (LBNP). Note that aortic mean pressure (MP), aortic pulse pressure (PP), and HR (and also aortic dP/dt, not shown) remained constant during the first 20 min of LBNP, whereas RAP fell to 0 mmHg, SBF decreased 10%, and FBF decreased by 35%. Results suggest that falling RAP, which unloaded cardiopulmonary stretch receptors, elicited regional vasoconstriction.

From Johnson, Rowell, et al. , by permission of the American Heart Association, Inc
Figure 21. Figure 21.

Relationship between percentage increase in splanchnic vascular resistance (δSVR %) and increase in plasma renin activity (APRA; ng of angiotensin I generated per 100 ml per 3 h) during prolonged whole‐body heating (as in Figs. and ; ref. ).

Figure 22. Figure 22.

Results from 1 subject in whom body skin temperature (Ts) was driven in different temporal patterns to achieve periods of separation between changes in Ts (top panel) and central temperature [right atrial blood temperature (Tra.) and esophageal temperature (Tes); second panel]. Note close correspondence between rising Tra, sweat rate (SR), and forearm skin blood flow (FBF). Separate influences of Tra and T. on FBF, SR, and heart rate (HR) were determined mathematically. Influence of Tra on FBF was 20 times greater than that of Ts. per °C.

From Wyss, Rowell, et al.
Figure 23. Figure 23.

Results from 1 subject in whom body skin temperature (Ts) was controlled and driven in different patterns to reveal influences of body Ts, rate of change of Ts (, top panel), and central body temperature [esophageal temperature (Tes) and right atrial blood temperature (Tra)] on heart rate (HR), sweat rate (SR), and FBF. Note small effect of increasing Ts on FBF. Most increases in FBF and all the increase in SR attended increased Tra; Ts and had no influence on FBF when altered after elevation of Ts to 38°C for 30 min. Note also maintained high FBF long after SR had fallen. Central and cutaneous influences on FBF were determined by multiple linear regression for FBF, HR, and SR against Tra or Tes Ts, and during periods A, B, and C (top). In periods A and B, influence of Ts and on FBF was minimal.

From Wyss, Rowell, et al.
Figure 24. Figure 24.

Schematic illustration of effects of skin vasodilation on preventricular volume sumps for right (RV) and left (LV) ventricles. As skin veins fill, venous return is transiently reduced so that preventricular sumps are depleted. Central or thoracic blood volume is represented by CBV.

Figure 25. Figure 25.

Skin vasoconstriction in response to lower‐body negative pressure (LBNP) in heat‐stressed human. Top panel shows when 5‐min periods of LBNP were applied before and after whole‐body skin temperature (Ts) was raised to increase body temperature (Tr, second panel) and increase forearm skin vascular conductance (FVC, bottom panel). Note arrow at 85 min and marked fall in mean arterial pressure (MAP, fourth panel). This subject could no longer elicit sufficient skin vasoconstriction to maintain MAP.

From Johnson, Rowell, et al.
Figure 26. Figure 26.

Regressions of forearm blood flow (FBF) vs. esophageal temperature (Tes) from a representative subject during supine rest (SR, open circles), supine exercise (SX, solid circles), upright rest (UR, open triangles), and upright exercise (UX, solid triangles). Under all conditions, skin temperature was held constant at 38°C to raise Tes and FBF during upright and supine rest. Note that, at any given Tes, upright rest and supine exercise decreased FBF below that at supine rest. Upright exercise caused the greatest decrease.

Adapted from Johnson, Rowell, and Brengelmann
Figure 27. Figure 27.

Response of total FBF in right and left arms (RFBF and LFBF) in 1 subject during 1 h of exercise at 750 kpm/min at 24°C ambient. Bottom panel from a separate experiment shows increase in FBF was confined to skin. Forearm muscle blood flow (MBF; 125I‐antipyrine clearance) fell during exercise.

Adapted from Johnson and Rowell
Figure 28. Figure 28.

Forearm blood flow (FBF) measured in left arm (solid circles) and right arm (open circles) during 60 min of prolonged exercise at 24°C ambient.

From Johnson and Rowell
Figure 29. Figure 29.

Forearm blood flow (FBF) vs. esophageal temperature (Tes) in 6 subjects during 17–30 min of moderate (86–147 W) upright exercise. Body skin temperature was maintained at 38°C throughout exercise. The 2 line segments in each panel are from linear regression analysis of data above and below Tes = 38°C.

From Brengelmann, Rowell, et al.
Figure 30. Figure 30.

Raising body skin temperature (Ts; top panel) abolishes forearm cutaneous venoconstrictor responses to exercise (at 3 min) and to local changes in arm Ts between 25°C and 40°C. Cycling body Ts (see bottom 3 panels) between 30°C and 37°C causes rapid changes in venous tone (pressure in occluded forearm veins) when local arm Ts is neutral. Note slow changes in venous tone when local arm Ts changed (11–14 min) while body Ts was 30°C.

From Rowell et al.
Figure 31. Figure 31.

Diagram of known inputs to cutaneous vasoconstrictor and vasodilator systems. The only factor known to influence the vasodilator system is changing Tc, but the vasoconstrictor system receives input from many receptors. The key unanswered question is whether these nonthermoregulatory reflexes also influence vasodilator outflow.

Figure 32. Figure 32.

Effects of upright posture and of upright exercise on pressure (and volume) in dependent veins. Exercise causes rapid venous emptying that restores CBV and CVP. Rapid emptying occurs during exercise in hot environments as well, but because of skin blood flow increases, veins refill so rapidly that average pressure (and volume) rises, resulting in reduced CBV and CVP.

Adapted from Henry and Gauer
Figure 33. Figure 33.

A: decrements in SBF versus percent of maximum O2 consumption () required during exercise in neutral (25.6°C) and a hot (43.3°C) environment. Heat stress reduced SBF at any given percent of (and at absolute as well). B: HR was increased at any given and percent of by heat stress, but the normal relationship between SBF and HR was unchanged (cf. Fig. ).

From Rowell
Figure 34. Figure 34.

Summary of cardiovascular responses to graded exercise in hot (43.3°C, open triangles) and neutral (25.6°C, solid circles) environments. Arrows show direction of change in each variable. Cardiac output, CO; aortic mean pressure, AoMP; total peripheral resistance, TPR. Average data from 6 men.

Adapted from Rowell et al.
Figure 35. Figure 35.

Overall circulatory responses to rapid changes in skin temperature (Ts) (by water‐perfused suit) during upright exercise. Note reduction in Tra, SV, CBV, and RA MP (right atrial mean pressure) with sudden elevation of Ts at 30 and 90 min (some changes are more obvious at 90 min). Note sudden reversal of these effects when Ts was lowered at 60 min and stability of SV, Ao MP (MAP, aortic), and RA MP caused by keeping Ts low from 60 to 90 min.

Adapted from Rowell et al.
Figure 36. Figure 36.

Circulatory responses to prolonged mild (1 liter O2/min) upright exercise in 1 subject before (solid circles) and after (open triangles) 14 days of acclimatization to heat. On the 1st day the subject was severely stressed; HR reached 200 beats/min and Tre reached 40°C. Reductions in Tc, Ts, and HR and increase in SV (at same ) after acclimatization were striking.

Adapted from Rowell et al.
Figure 37. Figure 37.

Acclimatization (postheat) tends to shift relationship between FBF and Tes leftward. Physical training (pre‐ vs. postexercise) had no effect.

From Roberts et al.
Figure 38. Figure 38.

Dehydration (squares) markedly altered relationship between FBF and Tes so that SkBF was much lower at a given Tes. Note fall in SkBF above Tes of 38°C.

From Nadel et al.


Figure 1.

Comparison of simultaneous measurements of splanchnic blood flow (SBF) by constant‐dye‐infusion technique and by electromagnetic flowmeters on hepatic artery and portal vein (solid and open circles) or by direct flow measurement (crosses) .

From Rowell , reproduced by permission of Grune & Stratton


Figure 2.

Simultaneous measurement of renal blood flow by p‐aminohippurate clearance (dashed line, left kidney) and electromagnetic flowmeter (solid line, right kidney) during graded hemorrhage, which lowered mean arterial pressure (MAP; top panel) and increased renal nerve activity (second panel).

Adapted from Selkurt


Figure 3.

Schematic illustration of blood volume distribution and venous pressures in upright human compared with those in the dog (and most other animals).

Adapted from Folkow and Neil


Figure 4.

Relationship between venous pressure in leg veins and volume of blood contained at 3 different leg skin temperatures (Tsk). Rise in local Tsk increases the blood volume contained in cutaneous veins at a given venous distending pressure; i.e., venous compliance increased.

Redrawn from Gauer and Thron ; from Rowell


Figure 5.

Right and left forearm blood flows in 1 subject with congenital absence of sweat glands (anhidrotic ectodermal dysplasia) during direct whole‐body heating (forearms were not heated). Forearm blood flows showed almost no increase in response to a 1.5°C increase in Tc. In normal skin this would raise blood flow to 20–30 ml · min−1 · 100 ml−1 of forearm, the increment being confined to skin.

From Brengelmann, Rowell, et al.


Figure 6.

Cardiac output rose ∼6 liters/min, and forearm blood flow rose to 20 ml per 100 ml of forearm per min in 1 subject who was indirectly heated (legs in hot water) for 100 min, which raised body core temperature by 2.6°C. Heart rate, HR; stroke volume, SV.

From Koroxenidis et al.


Figure 7.

Circulatory responses to whole‐body heating to limits of thermal tolerance in a representative subject. Elevation in skin temperature (Ts) by water‐perfused suit raised rectal temperature (Tr) and right atrial blood temperature (Tb), cardiac output (CO), HR, SV, and central venous pressure (CVP). Note time courses of changes in aortic mean and pulse pressures (Ao BP) and right atrial mean pressure (RA‐MP). Total peripheral resistance, TPR.

From Rowell et al.


Figure 8.

Average circulatory responses in directly heated subjects. Initial and final values and approximate time course are shown for Ts and right atrial blood temperature (Tblood) and each cardiovascular variable. Changes in cardiac output and splanchnic, renal, and muscle blood flow contributing to increased skin blood flow are shown in separate boxes.

From Rowell


Figure 9.

Time course of changes in splanchnic blood flow (SBF) and splanchnic vascular resistance (SVR) (open triangles) during prolonged whole‐body heating (as in Figs. and ) in 7 subjects . Solid line in top panel, approximate time course of Tra (Tb) during heating.

From Rowell


Figure 10.

Left, estimated distribution of cardiac output between skin and other major organs at rest in normothermia; right, during severe hyperthermia (Tc greater than 39°C).



Figure 11.

Open bars, distribution of blood flow (radioactive microspheres) in 8 conscious baboons during quiet, seated rest in normothermic and hyperthermic (Tc increased 1.5°C‐2.0°C) conditions (cross‐hatching). Note insignificant rise in brain blood flow with heating. One SE and significance of change (*P < 0.05, **P < 0.01) are shown. Central nervous system, CNS.



Figure 12.

Blood flow (by microsphere technique) to various cutaneous regions during normothermic control period (solid bars) during moderate hyperthermia (Tc rose 1.5°C‐2.0°C) in resting, awake baboons (open bars, 0.5 SE shown where discernible).

From Hales, Rowell, and King


Figure 13.

Schematic illustration of altered distribution of blood volume in a heat‐stressed man. In contrast most species have small hydrostatic influence and small cutaneous vasodilation. Substantial vasodilation occurs in the dog tongue.



Figure 14.

Left, Krogh's simplified model of the cardiovascular system divided into 2 circuits, 1 compliant (long time constant) and 1 noncompliant (short time constant). Right, electrical equivalent of a generalized parallel‐compartment model of the peripheral circulation. Arterial resistance, Ra; venous compliance, Cv; venous resistance, Rv (Rv2 and Cv2 illustrate variable resistance and compliance of cutaneous veins); arterial compliance, Ca. Venous time constant for a particular circuit (τ) is RvCv.

Left, adapted from Caldini et al. ; right, adapted from Mitzner and Goldberg


Figure 15.

Hydraulic model contrasting effects of increasing blood flow through noncompliant (panel A) versus compliant circuits (panels B, C, D). Vasodilation of compliant circuits with long time constants (RvCv, in Fig. ) depletes the volume (vol) of central reservoirs [central blood volume (CBV) or heart and pulmonary vessels]. Vasoconstriction (VC) of compliant circuits passively displaces their volumes to partially restore CBV.



Figure 16.

Schematic illustration of how volume may be mobilized passively from splanchnic veins as cutaneous veins fill up during heat stress. Fall in CVP creates a pressure gradient from right atrium to hepatic veins. Constriction of splanchnic arterioles further lowers splanchnic venous pressure and translocates more volume to the heart. Because of steep splanchnic venous pressure‐volume curve (right), a small decrease in venous pressure displaces a large volume of blood. GI, gastrointestinal tract.



Figure 17.

Relationship between heart rate and plasma norepinephrine concentration in cancer patients treated with hyperthermia (to 41.5°C).

From Kim et al.


Figure 18.

Relationship between SBF as percent of resting SBF and heart rate (HR) during exercise in neutral and hot environments and before (open squares) and after (solid squares) physical conditioning . Data are compared with renal blood flow (RBF) during exercise . Regression lines for SBF versus HR in resting human stressed by heat and lower‐body negative pressure (LBNP) are displaced to the left (cf. norepinephrine data in Fig. ) .

Adapted from Rowell


Figure 19.

Relationship between increase in plasma norepinephrine (NE; ng/ml) and change in HR during quiet standing (open and closed triangles) and during supine exercise (closed circles). Note the rightward displacement on HR axis during exercise (cf. Fig. ).

Adapted from Christensen and Brandsborg


Figure 20.

Average responses to gradual decline in right atrial pressure (RAP) induced by slow ramp (‐1 mmHg/min) of lower‐body negative pressure (LBNP). Note that aortic mean pressure (MP), aortic pulse pressure (PP), and HR (and also aortic dP/dt, not shown) remained constant during the first 20 min of LBNP, whereas RAP fell to 0 mmHg, SBF decreased 10%, and FBF decreased by 35%. Results suggest that falling RAP, which unloaded cardiopulmonary stretch receptors, elicited regional vasoconstriction.

From Johnson, Rowell, et al. , by permission of the American Heart Association, Inc


Figure 21.

Relationship between percentage increase in splanchnic vascular resistance (δSVR %) and increase in plasma renin activity (APRA; ng of angiotensin I generated per 100 ml per 3 h) during prolonged whole‐body heating (as in Figs. and ; ref. ).



Figure 22.

Results from 1 subject in whom body skin temperature (Ts) was driven in different temporal patterns to achieve periods of separation between changes in Ts (top panel) and central temperature [right atrial blood temperature (Tra.) and esophageal temperature (Tes); second panel]. Note close correspondence between rising Tra, sweat rate (SR), and forearm skin blood flow (FBF). Separate influences of Tra and T. on FBF, SR, and heart rate (HR) were determined mathematically. Influence of Tra on FBF was 20 times greater than that of Ts. per °C.

From Wyss, Rowell, et al.


Figure 23.

Results from 1 subject in whom body skin temperature (Ts) was controlled and driven in different patterns to reveal influences of body Ts, rate of change of Ts (, top panel), and central body temperature [esophageal temperature (Tes) and right atrial blood temperature (Tra)] on heart rate (HR), sweat rate (SR), and FBF. Note small effect of increasing Ts on FBF. Most increases in FBF and all the increase in SR attended increased Tra; Ts and had no influence on FBF when altered after elevation of Ts to 38°C for 30 min. Note also maintained high FBF long after SR had fallen. Central and cutaneous influences on FBF were determined by multiple linear regression for FBF, HR, and SR against Tra or Tes Ts, and during periods A, B, and C (top). In periods A and B, influence of Ts and on FBF was minimal.

From Wyss, Rowell, et al.


Figure 24.

Schematic illustration of effects of skin vasodilation on preventricular volume sumps for right (RV) and left (LV) ventricles. As skin veins fill, venous return is transiently reduced so that preventricular sumps are depleted. Central or thoracic blood volume is represented by CBV.



Figure 25.

Skin vasoconstriction in response to lower‐body negative pressure (LBNP) in heat‐stressed human. Top panel shows when 5‐min periods of LBNP were applied before and after whole‐body skin temperature (Ts) was raised to increase body temperature (Tr, second panel) and increase forearm skin vascular conductance (FVC, bottom panel). Note arrow at 85 min and marked fall in mean arterial pressure (MAP, fourth panel). This subject could no longer elicit sufficient skin vasoconstriction to maintain MAP.

From Johnson, Rowell, et al.


Figure 26.

Regressions of forearm blood flow (FBF) vs. esophageal temperature (Tes) from a representative subject during supine rest (SR, open circles), supine exercise (SX, solid circles), upright rest (UR, open triangles), and upright exercise (UX, solid triangles). Under all conditions, skin temperature was held constant at 38°C to raise Tes and FBF during upright and supine rest. Note that, at any given Tes, upright rest and supine exercise decreased FBF below that at supine rest. Upright exercise caused the greatest decrease.

Adapted from Johnson, Rowell, and Brengelmann


Figure 27.

Response of total FBF in right and left arms (RFBF and LFBF) in 1 subject during 1 h of exercise at 750 kpm/min at 24°C ambient. Bottom panel from a separate experiment shows increase in FBF was confined to skin. Forearm muscle blood flow (MBF; 125I‐antipyrine clearance) fell during exercise.

Adapted from Johnson and Rowell


Figure 28.

Forearm blood flow (FBF) measured in left arm (solid circles) and right arm (open circles) during 60 min of prolonged exercise at 24°C ambient.

From Johnson and Rowell


Figure 29.

Forearm blood flow (FBF) vs. esophageal temperature (Tes) in 6 subjects during 17–30 min of moderate (86–147 W) upright exercise. Body skin temperature was maintained at 38°C throughout exercise. The 2 line segments in each panel are from linear regression analysis of data above and below Tes = 38°C.

From Brengelmann, Rowell, et al.


Figure 30.

Raising body skin temperature (Ts; top panel) abolishes forearm cutaneous venoconstrictor responses to exercise (at 3 min) and to local changes in arm Ts between 25°C and 40°C. Cycling body Ts (see bottom 3 panels) between 30°C and 37°C causes rapid changes in venous tone (pressure in occluded forearm veins) when local arm Ts is neutral. Note slow changes in venous tone when local arm Ts changed (11–14 min) while body Ts was 30°C.

From Rowell et al.


Figure 31.

Diagram of known inputs to cutaneous vasoconstrictor and vasodilator systems. The only factor known to influence the vasodilator system is changing Tc, but the vasoconstrictor system receives input from many receptors. The key unanswered question is whether these nonthermoregulatory reflexes also influence vasodilator outflow.



Figure 32.

Effects of upright posture and of upright exercise on pressure (and volume) in dependent veins. Exercise causes rapid venous emptying that restores CBV and CVP. Rapid emptying occurs during exercise in hot environments as well, but because of skin blood flow increases, veins refill so rapidly that average pressure (and volume) rises, resulting in reduced CBV and CVP.

Adapted from Henry and Gauer


Figure 33.

A: decrements in SBF versus percent of maximum O2 consumption () required during exercise in neutral (25.6°C) and a hot (43.3°C) environment. Heat stress reduced SBF at any given percent of (and at absolute as well). B: HR was increased at any given and percent of by heat stress, but the normal relationship between SBF and HR was unchanged (cf. Fig. ).

From Rowell


Figure 34.

Summary of cardiovascular responses to graded exercise in hot (43.3°C, open triangles) and neutral (25.6°C, solid circles) environments. Arrows show direction of change in each variable. Cardiac output, CO; aortic mean pressure, AoMP; total peripheral resistance, TPR. Average data from 6 men.

Adapted from Rowell et al.


Figure 35.

Overall circulatory responses to rapid changes in skin temperature (Ts) (by water‐perfused suit) during upright exercise. Note reduction in Tra, SV, CBV, and RA MP (right atrial mean pressure) with sudden elevation of Ts at 30 and 90 min (some changes are more obvious at 90 min). Note sudden reversal of these effects when Ts was lowered at 60 min and stability of SV, Ao MP (MAP, aortic), and RA MP caused by keeping Ts low from 60 to 90 min.

Adapted from Rowell et al.


Figure 36.

Circulatory responses to prolonged mild (1 liter O2/min) upright exercise in 1 subject before (solid circles) and after (open triangles) 14 days of acclimatization to heat. On the 1st day the subject was severely stressed; HR reached 200 beats/min and Tre reached 40°C. Reductions in Tc, Ts, and HR and increase in SV (at same ) after acclimatization were striking.

Adapted from Rowell et al.


Figure 37.

Acclimatization (postheat) tends to shift relationship between FBF and Tes leftward. Physical training (pre‐ vs. postexercise) had no effect.

From Roberts et al.


Figure 38.

Dehydration (squares) markedly altered relationship between FBF and Tes so that SkBF was much lower at a given Tes. Note fall in SkBF above Tes of 38°C.

From Nadel et al.
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Loring B. Rowell. Cardiovascular Adjustments to Thermal Stress. Compr Physiol 2011, Supplement 8: Handbook of Physiology, The Cardiovascular System, Peripheral Circulation and Organ Blood Flow: 967-1023. First published in print 1983. doi: 10.1002/cphy.cp020327