Energy/Endurance

Exercise Induced Changes in Body Fluids

Of all the exercise-induced body fluid changes during exercise, decreases in vascular blood volume during rhythmic or endurance exercise are of greatest concern. Types of exercises that make use of large muscle groups for a period of time (e.g., running, cycling, walking, swimming) have been the most well researched, with varying results. During some types of endurance exercise, it appears that the vascular blood volume undergoes a reduction that can be almost completely attributed to a decrease in plasma volume.

This is particularly true for cycling done on a cycle ergometer. However, results are inconsistent for treadmill running and walking, in which some investigators showed a plasma volume reduction, others an increase, and others reported no changes McMurray conducted an investigation to evaluate the plasma volume response during distance swimming and found similar plasma volume reductions to those observed during cycling at the same intensity.

Plasma volume is maintained by a combination of opposing forces. These forces are termed Starling forces and consist of hydrostatic pressure, osmotic pressure in the interstitial space and their opposing pressures, osmotic pressure in the capillaries, and the tissue pressure surrounding the capillaries. Water exchange between the intravascular and interstitial spaces occurs via the capillaries. Hydrostatic and osmotic pressures in the interstitial space forces water to move from the vascular space to the interstitial space at the arterial end of the capillary.

Capillary osmotic pressure and pressure from the tissues surrounding the capillaries force water to return to the vascular space at the venous end of the capillary. Compared with the arterial end of the capillary, the venous end has greater surface area and permeability, which allows for greater return of fluids to the vascular space-an action favoring plasma volume conservation.

During exercise, especially aerobic exercise, blood pressure is elevated to supply adequate blood flow to working muscles. This causes an increase in hydrostatic pressure, which drives fluids out of the capillaries and into the interstitial spaces. Because solutes are released as a result of the energy metabolism of contracting muscle, osmotic pressure in the interstitial space rises.

Also, capillary surface area is significantly increased to more efficiently nourish the active muscle mass. All of these actions create an environment more favorable to the reduction of vascular fluid from the plasma portion of the blood. This results in decreased blood volume.

However, there are limits to the amount of fluid that can be lost from vascular plasma. During exercise, the plasma solute concentration will increase for several reasons. First, sodium and potassium are released into the plasma from the skeletal muscle as byproducts of increased metabolic activity during exercise.

In addition, glucose and lipids released into plasma from glycogen and fat-tissue breakdown add to the solute add Water lost in sweat creates a shift in the osmotic pressure gradient favoring the movement of fluids from the plasma to the interstitial spaces. But, as fluid moves from the plasma, plasma osmolarity increases and in doing so, the osmotic pressure shifts back toward moving fluids into the plasma.

Protein oncotic pressure may also playa role in stabilizing blood volume during exercise. It is speculated that during exercise, proteins move at a faster rate out of the interstitial space into the blood via the lymphatic system than they can move into interstitial via pinocytosis.

The presence of proteins in the intravascular space exerts a higher oncotic pressure, resulting in the movement of water into the blood. Lastly, the skeletal muscle cells surrounding the capillaries impose tissue pressure and structurallimits upon the movement of fluids out of the plasma. Greenleaf et al found that the upper limit of plasma volume loss due to exercise was approximately 20% of the resting plasma volume.

The magnitude of plasma volume loss appears related to the body position before exercise and the time spent in this position, as well as the mode of exercise (e.g., cycle versus running), intensity of exercise and hydration status. In short-term exercise, plasma volume decreases result in hemoconcentration, which enhances oxygen-carrying capacity per unit of blood.

The Starling forces, along with the vascular volume and the concentration of osmotically active particles in plasma return to normal within minutes after the cessation of short-term exercise. Plasma volume deficits, however, may continue if the exercise has been lengthy anchor in the heat, combined with moderate-to-severe sweating and dehydration.

Body fluid balance becomes a primary concern for the exercising subject when an endurance exercise in any form is carried on for long enough, especially in the heat. Respiratory water loss begins to make an impact because of the overall increase in ventilation and energy expenditure accompanying exercise. However, the greatest and potentially most serious body fluid loss during exercise relates to increased sweat production for thermo regulation.

As mentioned before, only 20% of the energy expended during exercise is used for actual mechanical work, while the rest is released in the form of heat. This heat must be dissipated before the body’s core temperature (i.e., the temperature of the cranium, thorax, abdomen, pelvis, and deeper muscle masses) is elevated to a dangerous level. Thermo regulation may be seriously impaired if the core temperature exceeds 106°F (41°C). Increased body temperature initiates vasodilation through sympathetic neural control, causing body heat to be transported from the body core to the shell. Under cool and breezy conditions, most of this heat can then be dissipated through convection and radiation, also called dry heat exchange. However, this mechanism diminishes in efficiency as exercise intensity, environmental temperature, and or humidity increase. When vasodilation is no longer a useful means to dissipate body heat, the secretion and evaporation of sweat becomes the foremost avenue of heat removal.

Sweat gland activity is stimulated when the temperature of the blood flowing through the anterior hypothalamus increases. The hypothalamus also receives impulses from temperature receptors in the skin For heat to dissipate, secreted sweat on the skin must be evaporated into the surrounding air. About 0.58 kcal are lost for each gram of water evaporated, thus, approximately 580 kcal of heat are removed through the evaporation of every liter of sweat.

The sweat rate may be affected by many factors, including physical activity level, environmental conditions (ambient temperature, humidity, air velocity, and radiant load), individual aerobic fitness, heat acclimatization, and clothing (insulation and moisture permeability). Therefore, great inter-and intra individual differences may exist in body water loss through sweat. Individuals wearing protective clothing commonly have sweating rates of 1 to 2 L/hr while performing light-intensity exercise 35 Athletes performing high-intensity exercise in the heat can have sweating rates up to 2.5 L/hr .

The immediate source of water for sweat is from the interstitial fluid, even though each sweat gland is served by capillaries. Because the fluid lost through sweat has a lower solute concentration (hypotonic) than interstitial fluid and plasma, osmotically active particles must be left behind in the cutaneous interstitial space. The osmotic gradient from plasma to the cutaneous interstitial space increases, which results in the movement of fluid from the plasma into the interstitial space. This movement of fluid into the interstitial space increases the osmolarity in the plasma, which in turn shifts the osmotic pressure gradient toward movement of water from the intracellular compartment to the blood.

Research examining the relative contribution of the various body compartments to the fluid lost in sweat has accounted for 30-50% as coming from the intracellular compartment. The amount varies depending on the hydration status of the subject. The interstitial fluid contributed 40-60% of the lost water, while plasma contributed approximately 10%. The extracellular fluid is the initial source of water loss from sweating when hydration status is normal; however, the contribution from the intracellular fluid increases as hydration levels decrease.

To compensate for this markedly increased water out­put through sweat, urine production tends to decrease. Also, renal blood flow is reduced during exercise in response to sympathetic nervous system activity, which leads to a reduction in urine formation and to a conservation of fluid during exercise. As a function of increased energy expenditure, water is produced through the metabolic oxidation of macronutrients and released from muscle glycogen. Although in most cases these additions to total body water supply during exercise are not adequate for fluid replacement, their importance should not be totally ignored. Nevertheless, the maintenance of body fluid balance predominantly relies on fluid ingestion. Studies have shown that ad libitum drinking often only replaces 25-35% of the volume lost as sweat .

Exercise, especially when accompanied by heat stress, exerts an intense strain on processes for maintaining normal plasma volume and total body fluid balance. Reductions in plasma volume from water lost in sweat are aggravated when fluid replacement is not adequate. Reduced plasma volume and increased plasma osmolarity due to inadequate hydration status have been well documented as producing adverse effects on thermoregulation, cardiovascular responses, and exercise performance.

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