Of all the exercise-induced body fluid changes during exercise, decreases in vascular blood volume during rhythmic or endurance exercise are the most concerning.
Resistance Training
Types of exercises that utilize large muscle groups for some time (e.g., running, cycling, walking, swimming) are the most researched, with varying results.
During some types of resistance exercise, it seems the vascular blood volume decreases almost entirely due to a decrease in plasma volume. This applies to cycling done with a cycle ergometer. However, the results were inconsistent for running and walking on a treadmill.
Some researchers showed a decrease in plasma volume, others an increase, and others reported no change. McMurray examined the plasma volume response during distance swimming and found that it was similar to that observed during cycling at the same intensity.
Opposing forces work together to maintain plasma volume. 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 force 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 a greater return of fluids to the vascular space action favoring plasma volume conservation.
Other types of exercise
During exercise, especially aerobic, 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, the 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.
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.
In addition, glucose and lipids released into plasma from glycogen and fat-tissue breakdown add to the solute 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 play a role in stabilizing blood volume. It is thought, proteins move at a faster rate out of the interstitial space into the blood via the lymphatic system than they can move into the 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 structural limits 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 resulting 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 the lengthy anchor in the heat, combined with moderate-to-severe sweating and dehydration.
Body Fluid Balance
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 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.
Thermoregulation can be seriously impaired if temperature cores exceed 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 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 become 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, although 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 the 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. In addition, lower renal blood flow during exercise in response to sympathetic nervous system activity. This leads to reduced urine formation and fluid conservation during exercise.
As an energy, water is produced by the metabolic oxidation of macronutrients and released from muscle glycogen.
In many cases, increasing the water supply in the body during exercise is not enough for fluid replacement! However, maintaining body fluid balance depends on fluid intake. However, maintaining body fluid balance depends on fluid consumption.
Studies have shown that ad libitum drinking often only replaces 25-35% of the volume lost as sweat.
High-intensity exercise produces massive pressure to retain plasma volume and overall body fluid balance. The adverse impact of reducing plasma volume through sweat can occur if replacement fluids are insufficient.
Reduced plasma volume and increased plasma osmolarity due to inadequate hydration status can have unhealthy effects on thermoregulation, cardiovascular responses, and exercise performance.
