RESPONSESOF THE VEINS ANDARTERIOLES OF THE FORE-ARMTO WALKINGDURINGACCLIMATIZATION TO

HEAT IN MAN*

By J. EDWINWOODt AND DAVID E. BASS:

(From the Physiology Branch, Quartermaster Research and Engineering Center, Natick, Mass.;the Evans Memorial and Massachusetts Memorial Hospitals; and the Department

of Medicine, Boston University School of Medicine, Boston, Mass.)

(Submitted for publication August 18, 1959; accepted February 4, 1960)

When a man attempts to exercise for the firsttime in a hot environment he may experience pal-pitation, weakness, headache, nausea, vomiting,dizziness or even syncope. These difficulties oftenprevent him from completing work that could beperformed easily in a cool environment. Thisrelatively benign syndrome, heat exhaustion, hasbeen ascribed in part to the inability of the heartto maintain an adequate flow of blood simultane-ously through the dilated blood vessels of theperiphery and the exercising muscles and to thecentral nervous system (1). If the man attemptsto repeat the task in the heat several times daily,a marked improvement in performance occurswithin four days, that is, he no longer experi-ences heat exhaustion and he can complete thework with relative ease (2). It has been shownthat the pulse rate during a given task becomesprogressively slower as this acclimatization toheat occurs (2, 3). The elevated rectal tempera-ture induced by a standard exercise in the heatdecreases in a similar pattern (4). These ob-servations suggest that during the first few daysof such experiments the cardiovascular system re-sponds to work in a progressively more effectivemanner. This improved response could resultfrom an increased cardiac output or from a re-distribution of the cardiac output. An increasedstroke output might be facilitated by an increasedpressure in the right atrium; this, in turn, could

* This study was supported by a grant from the LifeInsurance Medical Research Fund. Presented in partat the Annual Meeting of the American PhysiologicalSociety, April 15, 1959.

tFormerly, Special Research Fellow of the NationalHeart Institute. Present address: Department of Medi-cine, Medical College of Georgia, Augusta, Ga.

tPresent address: Physiology Branch, EnvironmentalProtection Division, Quartermaster Research and Engi-neering Center, U. S. Army, Natick, Mass.

be due either to constriction of the peripheralveins (5-7) or to an increased blood volume. Ithas been found, however, that an increase in bloodvolume need not accompany acclimatization to heatin man (8). Regarding cardiac output, an altereddistribution might result from a reduction of theflow of blood in the skin and skeletal muscles.The purpose of the present study was to investi-gate responses of the veins and arterioles of theforearm with respect to their roles in the processof acclimatization of man to heat.

METHODS

Seven volunteer soldiers were acclimatized by repeatedexercise in a heated chamber. The age of the subjectsranged from 20 to 23 years. The medical histories,physical examinations and laboratory tests which in-cluded hemoglobin determinations, urinalyses and X-rayexaminations of the chest indicated that the men werenormal. They had been stationed in Massachusetts forat least one year. The men had been subjects for ex-periments which were similar to those of the presentinvestigation; however, they had not been exposedto a hot environment for at least 3 months prior to thepresent study, with the exception of Subject 1 who par-ticipated in both groups of experiments described below.The subjects were accustomed to work of the type re-quired for these experiments.

Two separate studies were performed. Four of thesoldiers served as subjects for the first study and fourwere used in the second study. Subject 1 participatedon both occasions. The first study took place duringAugust 4th through August 15th. In this study the men(Subjects 1, 2, 3, 4) were acclimatized to heat for 9consecutive days. The second study was conductedfrom October 7th through October 13th. This study(Subjects 5, 6, 7, 1) was similar in every respect to thefirst except that the period of acclimatization to heatwas 6 days. Control measurements were made for 2and 1 days, respectively, before the two studies. Thecontrol conditions (the control environment) were:770 + 1° F, relative humidity 40 to 50 per cent. Heatexposures were begun on the day following the re-spective control periods. The conditions in the heat

825

J. EDWIN WOODAND DAVID E. BASS

FOREARMVENOUS

VOLUME130ml /lOOmI

FOREARMBLOODFLOW

ml /lOOmi /mint

6-

4-

2-)'0- os C -46

n4

2 -

10 min 30min 15

OI I =6 lb iO 30

TIME IN MINUTES40

FIG. 1. INDIVIDUAL VALUES OF FOREARMVENOU

UME [30] AND FOREARMBLOOD FLOW OBTAINED ONJECT 6 ON DAY 4 IN THE HOT ENVIRONMENT. Thewhen these values were obtained before (pre-during (walk), and after (post-walk) walking (

treadmill for this subject, as well as for the othejects, are shown.

(the hot environment) were: 1200 + 10 F dry bu+ 20 F wet bulb. The standard exercise consiswalking for 30 minutes at 3.5 miles per hourtreadmill with a 5 per cent grade. Each subjecformed this task 4 times daily, twice in theing and twice in the afternoon. The subj ectsarmy "fatigue" uniforms in the control environmthat they would not feel cool immediately after w

In the hot environment they wore only shorts.

soldiers lived at prevailing climatic temperaturesthey were not walking in the constant tempchamber. They were required to be in bed byeach night. They were not allowed to drink albeverages.

The pressure-volume curves of the veins in tharm and the rates of the flow of blood in the f(of each subject were obtained just before (2 detetions within 10 minutes), during (3 determination10, 20, and 30 minutes of walking, respectivelyimmediately after (5 determinations in 15 minuteof the 4 walks on each day (Figure 1). Measurof rectal temperature (by thermocouple), pulse rapalpation) and arterial blood pressure (by ausculwere obtained before starting and just prior to tlclusion of this walk. The subjects were studiedsame sequence and at the same time each daysubject rested on a cot outside the constant tempchamber for a period of 45 minutes at an ambierperature of 70 to 750 F just prior to the walk inthese measurements were to be made. The subjestood oni the treadmill for 10 to 15 minuttes while I

was being l)laced in a plethysmnograplh. He w

courage(d to move hiis legs interimiittently duririods of standing unless plethysmographic tracingbeing obtained. Water was permitted ad lib.

-WALK The pressure-volume curves of the veins of the fore-arm were obtained with a previously described plethys-mographic method (6). This method allows measure-ment of the change in volume of the veins of the fore-arm which accompanies a change of transmural pressureof these veins of 1 to 31 mm Hg. This change ofvolume of the veins in milliliters per 100 ml of forearmtissue that resulted from a change of effective venouspressure of 30 mmHg is referred to hereafter as ve-nouts volumiie [30]. The initial linear rate of change ofthe venous pressure-volume tracing could be recalculatedin terms of milliliters per 100 ml of forearm tissue perminute and is referred to hereafter as blood flow.

min In these experiments the distal chamber of the two-I chambered plethysmograph was 16.6 rather than 8.5 cm

5'0 long as originally described (6). Changes of the vol-ume of the forearm were recorded from a calibrated

FS VOL- Brody bellows. When the subject was walking and aT SUB- plethysmographic record was being obtained, the bellows! times system was damped to the extent that was necessary to

-walk), prevent recording of artifacts produced by randomon the movements of the forearm. This was accomplished byr sub- adjusting a clamp placed on the hose leading to the

bellows.Arterial occlusion was applied to the wrist prior to

ilb, 800 each measurement. The temperature of the water insted in the plethysmograph was kept at 920 F in all experiments.

on a The volume of the segment of the forearm enclosed inct per- the distal chamber of the plethysmograph was deter-morn- mined by displacement of water. This was measured

wore as soon as the subject removed his arm from thelent so plethysmograph.'alking. Venous pressure was measured in the control environ-

The ment and in the hot environment in the first of the twowhen studies in all four subjects. Venous pressure was

erature measured from an antecubital vein with the arm de-li p.m. pendent before, during, and after a walking period inIcoholic which plethysmographic measurements were not per-

formed. The pressure within the vein was measurede fore-orearm

2rmina-

is aftery) and:s) one

rementsLte (byItation)he con-

in the. Theoeraturent tem-

whichct thenhis arm

vas en-

Lng pe-

Is were

TIME IN DAYS

FTG. 2. FOREARMVENOUS VOLUME [30] VALUES FOR

SEVEN MNTE'N WHOWEREACCLIMATIZED TO HIEAT ON EIGHT

OCCASIONS. The averaged data that were obtained before,(lurinig an(d after walking on the treadmill in the controlenvironment (770 F) and in the hot environmeint (1200F) are shown.

POST-

*l--

WALKPRE-WALK

*-0

826

ACCLIMATIZATION TO HEAT IN MAN

with a Statlham straini gauge and(l was recorde(l witha Saniborn (lirect writing oscillograp)l. The suibject was

upright durinig these measuremenits. Tlhe referenicepoint for the pressure was kept conistanit relative to thelevel of the treadmill and was at about the level of thlesubject's fifth intercostal space aniteriorly.

On other and separate days venious pressures were

measured with the subj ect recumbent immediately be-fore and after walking. Pressures measured in the re-

cumbent position were referred to a point 10 cm anteriorto the back.

Probability and standard error were calculated fromthp original data rather than from the average valuesgiven in Tables I and II.

RESULTS

General observations. With one minor ex-

ception, the men completed all of the exercise pe-

riods in the hot environment. One subject was

unable to complete the 30 minute period of walk-ing on the first day of the second study. Hewalked for 23 minutes before becoming too weakto continue. M\easurements were obtained justprior to the conclusion of the walking in thisinstance. All of the subjects reported dizziness,palpitation, weakness or nausea during the firsttwo days of walking in the heat. In the days thatfollowed they became acclimatized to heat to theextent that they no longer suffered these symp-

toms and completed the period of walking withrelative ease.

Responses of the peripheral veins. Venous vol-ume [30] during exercise in both environmentswas significantly lower (p < 0.01) than was thecorresponding reading obtained just prior to thebeginning of exercise (Figure 2). The changeof venous volume [30] began within 3 to 5 min-utes after the initiation of exercise and reachedits greatest intensity within 10 minutes. Thisvenous volume [30] then persisted with some

random variation for the remaining 20 minutes ofwalking.

The greatest change of venous volume [30]during exercise occurred on the third and fourthdays in the hot environment. These values were

significantly (p < 0.01) lower than those ob-served during exercise in all days in the controlenvironment and on all other days in the hotenvironment. On the other hand, there were no

significant differences for venous volume [30]during exercise among all other days measured,i.e., all control days, and Days 1, 2, 5 and 6 in the

7-

6 *STANDING

FOREARM WALKING

LOOD 125RECOVERY'

cc /100ccImin

TIME IN DAYS

FIG. 3. FOREARMBLOOD FLOWVALUES FOR EIGHT MEN

WHOWEREACCLIMATIZED TO HEAT ON EIGHT OCCASIONS.

The averaged data that were obtained before, during andafter walking on the treadmill in the control environment(770 F) and in the hot environment (1200 F) are

shown.

heat, as well as Days 7, 8 and 9 in the first seriesof heat exposures (Figure 2, Table I).

Blood flow and arterial blood pressuire. Theblood flow of the forearm just prior to walkingwas higher in the hot environment than in thecontrol environment. Walking resulted in an in-crease in blood flow of the forearm in both en-

vironments. The increase of flow with walkingin the hot environment was greater than that ob-served in the control environment (Figure 3).Blood flows during exercise were significantlylower (p < 0.01) on the third and fourth daysin the heat than on the first and second, fifth andsixth, or (for the four subjects who participated)Days 7, 8 and 9 in the hot environment (TableII).

The systolic and diastolic blood pressure of allof the subjects tended to be higher during walk-ing in the hot environmtent than during walking inthe control environment. Pulse pressure was

highest during exercise on the third and fourthdays in the hot environment (Table III).

Pulse rate and rectal temiperature. The mean

pulse rates and rectal temperatures during exer-

cise decreased as the heat phase progressed(Table III). The major portion of the totaldecrease in the pulse and rectal temperature hadoccurred by the fourth day in the hot environment.

77- F l l | 120F [a 0 a

827

J. EDWIN WOODAND DAVID E. BASS

TABLE I

Average venous volume [30] for each subject on each day before walking, during walking and after walking in thecontrol (770 F) and hot (1200 F) environments

First study Second study

Environmental Venous volume [30] Venous volume [30]condition Subject Subjectand day no. Pre-walk Walk Post-walk no. Pre-walk Walk Post-walk

1 3.02 2.83 2.84 2.9

Mean 2.91 3.72 3.43 3.44 3.3

Mean 3.5SE*

1 3.62 3.93 3.54 3.8

Mean 3.71 3.22 4.53 3.24 3.6

Mean 3.6SE

1 1.42 3.23 2.64 2.2

Mean 2.41 2.32 4.13 4.14 3.9

Mean 3.6SE

1 4.72 4.53 3.44 3.4

Mean 4.01 3.92 4.03 3.14 3.8

Mean 3.7SE

1 2.52 2.93 4.04 4.5

Mean 3.51 5.12 5.13 4.94 4.6

Mean 4.91 4.12 4.83 4.44 4.1

Mean 4.4

ml/loo ml2.92.62.11.8

2.42.93.02.52.7

2.8

3.43.02.52.5

2.92.33.82.51.2

2.5

1.02.01.80.9

1.41.11.62.52.8

2.0

3.33.33.02.6

3.13.53.71.92.1

2.8

2.62.82.22.8

2.63.13.62.52.6

3.03.13.63.51.4

2.9

2.43.32.42.5

2.72.93.62.93.4

3.2

2.63.52.53.0

2.92.14.03.00.9

2.5

1.33.11.81.3

1.90.92.83.23.7

2.7

3.34.22.61.1

2.83.15.02.23.6

3.5

2.43.72.93.0

3.03.24.93.53.4

3.82.44.83.81.6

3.2

5 2.46 5.27 4.21 2.6

3.6

5 2.9 2.6 2.26 4.47 3.41 2.3

3.35 2.46 3.97 3.81 3.0

3.3

5 2.56 4.37 4.01 3.2

3.55 3.06 4.97 3.71 4.0

3.9

5 2.66 3.07 4.01 2.0

2.95 1.86 4.77 3.51 3.6

3.4

* Standard error calculated from the data obtained in both studies, during walking, on the preceding two days.

ml/100 ml1.83.63.32.5

2.8

2.44.03.52.2

3.0

Control1

2

Hot1

2

3

4

5

6

7

8

9

4.43.03.0

3.32.74.63.52.8

3.4

2.33.42.62.9

2.82.63.92.63.1

3.1

2.84.03.53.7

3.52.94.03.22.9

3.3

4.03.31.9

2.93.14.13.82.2

3.3

2.03.13.62.3

2.82.94.93.33.6

3.7

2.74.53.92.7

3.52.85.53.72.3

3.6

0.10

0.13

0.13

0.10

828

ACCLIMATIZATION TO HEAT IN MAN

TABLE II

Average blood flow for each subject on each day before walking, during walking and after walking in thecontrol (770 F) and hot (120° F) environments

First study Second study

Environmental Blood flow Blood flowcondition Subject - Subjectand day no. Pre-walk Walk Post-walk no. Pre-walk Walk Post-walk

1 2.92 1.33 2.04 1.7

Mean 2.01 2.62 1.73 2.94 1.6

Mean 2.2SE*

1 6.72 4.83 6.24 7.3

Mean 6.31 6.32 5.73 5.74 8.4

Mean 6.5SE

1 2.72 6.53 2.94 2.7

Mean 3.71 4.52 3.33 4.34 5.5

Mean 4.4SE

1 4.82 5.23 5.44 5.1

Mean 5.11 5.62 5.43 5.84 5.0

Mean 5.5SE

1 5.42 4.23 3.84 6.0

Mean 4.91 5.42 4.43 4.24 4.9

Mean 4.71 4.92 3.43 3.84 4.8

Mean 4.2

ml/100 mlJmin3.52.63.52.3

3.04.43.13.52.9

3.5

8.94.89.09.4

8.08.9

10.711.4

5.09.0

4.05.75.15.0

5.04.73.48.58.7

6.3

10.07.69.57.2

8.69.1

12.19.17.8

9.5

8.16.07.58.6

7.610.9

6.98.98.2

8.77.96.8

12.03.3

7.3

3.42.32.03.1

2.73.22.23.23.7

3.1

7.15.97.59.1

7.45.67.95.64.0

5.8

4.73.84.95.1

4.64.93.87.69.7

6.5

8.37.9

10.84.0

7.88.57.08.1

10.48.5

6.46.98.5

10.58.1

8.56.18.08.7

7.84.05.78.24.6

5.6

5 1.66 2.97 1.71 1.7

2.0

5 3.26 8.27 5.31 3.7

5.15 3.96 4.87 5.81 5.1

4.9

5 3.76 7.27 4.51 4.7

5.05 3.66 5.07 4.71 6.5

5.0

5 3.26 3.17 3.71 5.3

3.85 3.36 4.67 4.21 5.0

4.3

* Standard error calculated from the data obtained in both studies, during walking, on the preceding two days.

2.34.33.32.6

3.1

Control1

2

Hot1

2

3

4

5

6

7

8

9

ml/100 ml/min3.05.45.93.3

4.4

7.010.3

9.19.4

9.07.29.9

11.47.9

9.1

5.68.57.0

10.47.9

5.710.3

7.59.8

8.3

7.09.8

10.99.6

9.37.9

12.58.38.9

9.4

3.97.35.45.9

5.65.56.26.85.5

6.0

4.28.16.86.9

6.54.49.26.07.3

6.7

4.77.75.45.8

5.95.28.76.35.7

6.5

0.22

0.33

0.35

0.35

829

J. EDWIN WOODAND DAVID E. BASS

TABLE III

* Rectal temperature (T.), pulse rate (P.), and arterial blood pressure (B.P.) just prior to the conclusion of theperiod of walking in the control (770 F) and hot (1200 F) environments

First study Second studyEnvironmental

condition Subject T. P. B.P. Subject T. P. B.P.and day no. no.

0 F beats/min mmHg 0 F beats/min mmHg

1234

1234

1 101.32 101.63 100.64 101.5

101.0

1 101.22 100.63 100.64 101.2

101.0

1 100.52 101.03 99.54 101.4

100.6

1234

1 100.62 99.634 100.2

1 100.42 100.43 100.04 100.8

100.4

124144128128

131

128128128136

130

160184172180

174

164172168172

169

168176148164

164

144148156156

151

144140156152

148

5 100.26 99.87 100.21 100.4

100.2

130/80128/64130/60120/64

127/67

130/60144/60138/60138/64

138/61

160/50130/50170/50130/60

148/52

156/50138/55194/70138/70

156/61

140/56115/50120/40185/60

140/54

146/54132/45158/60130/40

142/50

136 145/50130/50

156130/50

5 101.06 100.27 100.71 101.5

100.8

5 101.16 99.67 101.61 101.6

101.0

5 100.46 99.67 100.21 101.2

100.4

5 100.36 99.37 100.51 100.8

100.2

5 99.66 99.87 100.41 100.5

100.1

5 100.66 99.67 100.21 100.8

100.3

152 130/60128 150/70136 126/50132 140/60

137 136/60

180164176184

176

188160168164

170

188156164172

170

168148160164

160

168160160156

161

168140164164

159

135/60130/50130/40148/50

136/50

144/60140/45138/70153/50

144/56

154/60152/60140/50146/50

148/55

140/70130/60144/50146/50

140/58

130/50144/60128/40136/50

134/50

120/60130/68138/50150/60

134/60

1 100.12 99.63 100.54 100.7

100.2

1 100.22 99.634 100.3

1 100.42 99.63 99.84 100.2

100.0

152 136/50120/45

140 164/60120/50

135/51

144 154/54144 122/50136 150/60

110/60134/56

144 142/60144 110/50

120/40155/55

132/51

Control1

Mean

2

Mean

Hot1

Mean

2

Mean

3

Mean

4

Mean

5

Mean

6

Mean

7

Mean

8

Mean

9

Mean

830

ACCLIMATIZATION TO HEAT IN MAN

Venous pressure. The mean venous pressuresobtained on two days in the control environmenton the four subjects were: 12 (range 8 to 20)mmHg before walking; 10 (range 6 to 17) mmHg during walking, and 11 (range 9 to 15) mmHg after walking. The average venous pressuresmeasured in this way on the second day in the hotenvironment were 13 (range 12 to 16), 9 (range8 to 12) and 13 (range 10 to 15) mmHg, re-spectively. These pressures on the fourth day inthe hot environment were 11 (range 10 to 12), 9(range 8 to 1(0) and 11 (range 8 to 14) mmHg,respectively.

All venous pressures obtained with the sub-ject recumbent before and after walking on oneday in the control environment and on the thirdday in the hot environment were below 12 mmHg.

DISCUSSION

Acclimatization to a hot environment by inter-mittent work is a well defined process that iseasily observed in man (9). It is possible to in-duce this state by repeated performance of a taskat high environmental temperatures even if thesubject lives in a cool environment the rest of thetime (2). As the subject becomes acclimatizedto heat the symptoms of heat exhaustion duringwork diminish. His ability to perform the stand-ard task improves. Weakness of the exercisingextremities, and dizziness or syncope no longeroccur. These subjective evidences of acclimatiza-tion to heat were virtually complete by the thirdday of the studies described herein. The di-minishing increment of the pulse rate with exer-cise in the heat continued after the third day inthese studies however.

The responses of the forearm veins and arteri-oles observed in this study were among the earlyevidence of acclimatization to heat. In addition,they coincided with the early subjective improve-ments. It seems reasonable, therefore, to inferthat the peripheral vasomotor responses wereimportantly associated with the acclimatizationprocess. On the other hand, the disappearanceof these vasomotor adaptations on later heat daysin the face of continued acclimatization encouragesa search for otlher mechanisms that maintain thecirculation during later days in the heat.

Previous studies have shown that decreases in

venous volume [30] are indicative of venocon-striction (6). Infusions of epinephrine or nor-epinephrine did not alter the volume of the fore-arm when the transmural pressure of the localveins was 1 mmHg as a result of the externalpressure of the water in the plethysmograph.Since most of the blood of the forearm is in theveins (10, 11) it can be assumed that these veno-constricting stimuli (12) did not change the vol-ume of the veins before the pressure-volume curveswere obtained. The venous pressure-volume curvesrecorded during the venoconstrictor stimulus werelower than those obtained before the stimulus wasapplied (6). The vasodilating stimulus of amarked increase in environmental temperaturedid not alter the venous pressure-volume curve(6). Thus, the starting point of a venous pres-sure-volume curve is one of a small, constant ve-nous volume and the smaller the volume of bloodthat these veins will accept at a transmural pres-sure of 30 mmHg the greater the degree of veno-constriction. Further, if venous pressure is un-changed and the venous pressure-volume curveis lowered by some stimulus, then it may be con-cluded that the naturally occurring volume of theveins of the forearm is lower (6). When theblood volume is unchanged, as was true of thebasal blood volume in experiments similar tothose of the present study (8), the further con-clusion that blood has shifted from the veins ofthe forearm to some other area is reasonable.The results of the present experiments indicatethat-exercise resulted in constriction of the veinswith movement of blood out of the forearm.This response was more intense on the third andfourth days of acclimatization to heat than onother days in the hot environment. The failureto find marked changes in peripheral venous pres-sure does not necessarily indicate that there wereno changes in the effective pressure of the bloodin the right atrium (7). The movement of bloodout of the forearm with exercise may have con-tributed to maintaining the filling pressure of theright ventricle especially on the third and fourthdays in the hot environment, as is suggested byBass and Henschel (13).

Acclimatization to a hot environment is alsocharacterized by a gradual improvement in theability of the body to rid itself of the heat producedby exercise. Two important factors that deter-

831

J. EDWIN WOODAND DAVID E. BASS

mine the rate of heat loss in an environmentwhose temperature exceeds that of the bodyare the rate of peripheral blood flow, particularlyto the skin, and the rate of evaporation from theskin. If the blood flow in the forearm is even apartial reflection of the state of the arterioles nearthe surface of the body then the high blood flowsduring the first, second, fifth, sixth and subsequentdays in the hot environment favored the loss ofheat. Blood flow of the forearm is primarily thatof muscle while heat dissipation is more dependentupon the flow of blood in the skin. Thus, theimportance of the observations of blood flow inthe forearm relative to the rate of heat dissipationis limited. The rate of evaporation with exerciseis high in a hot environment and has been foundto become even higher after several days (14).

The relationship between peripheral venomotorresponses and the dissipation of heat remains tobe evaluated. There is somiie evidence that themajor proportion of heat that is lost from theblood occurs when the blood is flowing throughthe subpapillary venous system of the skin (15).If this be true, then the rate at which a unit vol-ume of blood passes through the peripheral veinsmight affect the amount of heat lost to the en-vironment. This would mean that the greaterthe velocity of the flow of blood through theperipheral veins the smaller the amount of heatthat would be lost per unit volume of blood solong as the circumstances were such that the tem-perature of the blood did not come into completeequilibrium with the temperature at the surfaceof the body, regardless of the velocity of the flowof blood in the veins. Although the importanceof this consideration remains obscure it can bepointed out that the effects of the peripheralvenomotor responses with respect to the loss ofheat would parallel the effects of the peripheralarteriolar responses since their relative states ofdilatation were much the same from day to dayduring walking in the hot environment.

SUMMARYANDCONCLUSIONS

1. Seven normal young men were acclimatizedto heat on eight occasions by walking on a tread-mill for 30 minutes four times a day at an environ-mental temperature of 1200 F dry bulb and 80° Fwet bulb. Four subjects were acclimatized to

heat in this manner for nine days in the summerand four subjects were acclimatized to heat in anidentical study for six days in the fall.

2. The responses to exercise of the veins andarterioles of the forearm during acclimatization tothe heat were measured by plethysmographictechniques.

3. The forearm veins constricted with walkingin a control environment (770 F) as well as withwalking in the hot environment. The degree ofvenoconstriction with walking was greatest onthe third and fourth days of exposure to the hotenvironment.

4. The forearm arterioles dilated during walk-ing at normal environmental temperatures. Thisdilatation with walking was greater in the hotenvironment. The degree of dilatation was lesson the third and fourth days during walking inthe hot environment than on the first and secondor on the final days in this environment.

5. The disappearance of the symptoms of heatexhaustion with exercise (dizziness, weaknessand nausea) coincided with the onset of maximalvenoconstriction and minimal arteriolar dilatationon the third day of the acclimatization process.The maximal venoconstriction and minimal ar-teriolar dilatation with walking did not persistafter the fourth day in the hot environment.Subjective and objective evidences of acclimatiza-tion to heat were nevertheless well maintained forthe remainder of each of the studies.

6. The results of these studies support the con-cept that important adaptations in acclimatizationof man to exercise in a hot environmiient take placein the cardiovascular system. The peripheralvascular adaptations as observed in the forearmblood vessels appear to be an important aspect ofthe cardiovascular responses to exercise in theheat.

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