Body Fluid

1
Body Fluid Temperature Regulation as a Function
of Age

• Introduction
• Much knowledge comes from epidemiological data
  (I.e. heat waves morbidity mortality
• Hear things like The elderly are less heat
  tolerant, or Fluid intake is diminished with
  aging
• Key Question
• Does aging, independent of concomitant changes in
  aerobic capacity, body composition, disease
  prevalence, etc. result in physiological changes
  that impact on thermoregulation and fluid
  balance? (Kenney, W.L. (1995). Perspectives in
  Exercise Science and Sports Medicine 8, 305 - 351.

2
Epidemiology Heat Waves

• High mortality rates among adults over the age of
  60 during heat wave are common
• Excess mortality reported from all causes
• 60 year old group constitutes the largest at-risk
  group when exposed to high ambient temp.
• 10 - 12 fold greater risk of developin gheat
  stroke than do persons under 65
• Study from 3 Los Angeles heat waves indicate
• Increase in heat-related mortality (temp stays
  below 35C) is negligible
• Very few heat related deaths in those less than
  50
• Death risk increases progressively - in those age
  50-90

3
Baseline Temperature Fluid Balance in Older
Adults

• Reported that baseline body core temp (Tc)
  decreases with increasing age
• Are healthy older men and women chronically
  hypohydrated?
• General conclusion Healthy, active older adults
  are hyperosmotic (3-6 mOsmols/kg) and hypovolemic
• Spontaneous fluid ingestion in healthy older
  adults
• Enough fluid consumed in response to
  nonregulatory stimuli (meals)

4
Age Response to Dehydration

• Evidence that older subjects have more difficulty
  defending their fluid balance in response to mild
  dehydration (water deprivation) than young
• Renal Fluid Conservation - blood vessels,
  glomeruli, tubules, loss of functioning nephrons
• Decreased ability of kidneys of older adults to
  concentrate urine and minimize free-water
  clearance after water deprivation (a nephron
  issue)

5
Thirst

• Several studies demonstrate older subjects (67-75
  yrs) are less thirsty and replace less of fluid
  deficit over a 2-7 hour time period
• Visual-analog scales used to assess How thirsty
  do you feel now?
• Reduced thirst due to
• lower thirst sensitivity to hypertonicity
• non-thirst factors fluid availability
  palatability
• Orange juice consumed in larger volumes on single
  occasions

6
Heat Exposure Exercise

• Like fluid deprivation, passive exposure to hot
  environments for long periods and exercise in
  warm conditions cause hyperosmolar hypohydration.
• Following 4h resting exposure to 45o C, 25 rh
  subjects (older 61-67, younger 21-29y) drank ad
  libitum for 1 hr
• older had greater increase in rectal temp,
  despite similar loses of body weight
• Older men had lower thirst rating despite
  chronically higher plasma osmolalities

7
Mack, et al., 1994

• Studied gt65 and lt 28 y men 2.5 loss of body
  weight subjects exercised on cycle ergometer
  (36o C) dry environment for 105 minutes 3-h
  rehydration period followed.
• Results 1) plasma osmolality threshold for
  increased thirst was elevated in older men 2)
  Less fluid consumed by older men 3) an
  attenuated renal response in older subjects -
  production of less concentrated urine during
  dehyration less dilute urine during rehyration

8
Potential Contributing Factors

• Drug Effects
• Most sedatives mild tranquilizers affect both
  thirst and synthesis and release of AVP
• Lithium inhibit action of AVP at renal tubules
• Diuretics used for hypertension or congestive
  heart failure needs must be monitored to prevent
  hypohydration

9
Age Response to Exercise in Warm Environments

• Dynamic exercise presents many challenges to
  regulatory processes that maintain homeostasis
• Warm environment adds to the challenge
• Delivery of oxygen to active muscle - local
  decrease in vascular resistance
• Presents a challenge to blood flow delivery which
  is met by increases in cardiac output and
  adjustments in vascular resistance (active and
  non-active muscles)
• Present other challenges such as blood pressure
  and core temperature

10
Control of Skin Blood Flow (SkBF)

• Skin blood flow determines rate which heat is
  convected from core to periphery (skin)
• Cutaneous arterioles fully dilated deliver 8
  L/min
• Competition between skin and exercise muscle for
  the limited cardiac output is formidable
• SkBF - threshold - steep rise - attenuated rise -
  there appears to be a difference in slope
• Possible mechanism for lower skin blood flow is
  structural change in cutaneous blood vessels
  limit degree of vasodilation

11
Sweat Gland Function

• Local sweating rates are lower in older subjects
• Most studies report no significant age-related
  difference in sweating
• Older subjects maintain a lower mean skin
  temperature thus facilitating a greater
  convective heat transfer from core to skin due to
  larger thermal gradient

12
Exercise Training Effects on Thermoregulation