Hormonal regulation of fluid and electrolyte balance during exercise

Fluid balance during exercise is critical for optimal metabolic, cardiovascular, and thermoregulatory function. At the onset of exercise, water is shifted from the plasma volume to the interstitial and intracellular spaces. This water shift is specific to the amount of muscle that is active and the intensity of effort. Metabolic by-products begin to accumulate in and around the muscle fibers, increasing the osmotic pressure there. Water is then drawn into these areas by diffusion. Also, increased muscular activity increases blood pressure, which in turn drives water out of the blood(hydrostatic forces). In addition, sweating increases during exercise. The combined effect of these actions is that the muscles and sweat glands gain water at the expense of plasma volume. For example, running at approximately 75% of VO₂max decreases plasma volume by 5% to 10% . Reduced plasma volume decreases blood pressure and the amount of blood flow to the skin and muscles. Both of these effects can impede athletic performance.

The endocrine system plays a major role in monitoring fluid levels and correcting imbalances, along with regulating electrolyte balance, especially that of sodium. The two major hormones involved in this regulation are antidiuretic hormone released from the posterior pituitary and aldosterone, a mineralocorticoid released from the adrenal cortex. The kidneys are the primary target organ for both of these hormones.

Posterior pituitary

The pituitary’s posterior lobe is an outgrowth of neural tissue form the hypothalamus. For this reason, it is also reffered to as the neurohypophysis. It secretes two hormones; antidiuretic hormone(ADH; also called vasopressin or arginine vasopressin) and oxytocin. Both of these hormones are actually produced in the hypothalamus. They travel through the neural tissue and are stored in vesicles within nerve endings in the posterior pituitary. These hormones are released into capillaries as needed in response to neural impulses from the hypothalamus.

Of the two posterior pituitary hormones, only ADH is known to play an important role during exercise. Antidiuretic hormone promotes water conservation by increasing the water permeability of the kidneys’ collecting ducts. As a result, less water is excreted in the urine, creating an “antidiuresis”.

Muscular activity and sweating cause electrolytes to become concentrated in the blood plasma. This is called hemoconcentration, and it increases the plasma osmolality(the ionic concentration of dissolved substances in the plasma). This is the primary physiological stimulus for ADH release. The increased osmolality is sensed by osmoreceptors in the hypothalamus. A second and related stimulus for ADH release is a low plasma volume. In response to either stimuli, the hypothalamus sends neural impulses to the posterior pituitary, stimulating ADH release. The ADH enters the blood, travels to the kidneys, and promotes water retention in an effort to dilute the plasma electrolyte concentration back to normal levels. This hormone’s role in conserving body water minimizes the extent of water loss and therefore the risk of severe dehydration during periods of heavy sweating and hard exercise. Picture below illustrates this process. 

Adrenal cortex revisited

The mineralocorticoids, secreted from the adrenal cortex, maintain electrolyte balance in the extracellular fluids, especially that of sodium(Na⁺) and potassium(K⁺). Aldosterone is the major mineralocorticoid, responsible for at least 95% of all mineralocorticoid activity. It works primarily by promoting renal reabsorption of sodium, thus causing the body to retain sodium. When sodium is retained, so is water: thus, aldosterone, like ADH, results in water retention. Sodium retention also enhances potassium excretion, so aldosterone plays a role in potassium balance as well. For these reasons, aldosterone secretion is stimulated by many factors, including decreased plasma sodium, decreased blood volume, decreased blood pressure, and increased plasma potassium concentration. 

Kidneys

Although the kidneys are not typically considered major endocrine organs, they release a hormone called erythropoetine. Erythropoetin(EPO) regulates red blood cell(erythrocyte) production by stimulating bone marrow cells. The red blood cells are essential for transporting oxygen to the tissues and removing carbon dioxide, so this hormone is extremely important in our adaptation to training and altitude. The kidneys also release renin, a hormone and enzyme involved in blood pressure control and fluid and electrolyte balance.

The kidneys have a strong regulatory influence on blood pressure that also allows them to regulate fluid balance. Plasma volume is a major determinant of blood pressure: when plasma volume decreases, so does blood pressure. Blood pressure is monitored by specialized cells within the kidneys. During exercise, these cells can be stimulated by decreased blood pressure, decreased blood flow to the kidneys through increased sympathetic nervous activity accompanying exercise, or direct stimulation from the sympathetic nerves.

Figure shows the mechanism involved in renal control of blood pressure, the renin-angiotensin-aldosterone mechanism. The kidneys respond to decreased blood pressure or blood flow by forming an enzyme and hormone called renin. Renin, in turn, converts a plasma protein called angiotensinogen into an active form called angiotensin I. In the blood, angiotensin I is converted to angiotensin II when it encounters the enzyme angiotensin converting enzyme(ACE) in the lungs. Angiotensin converting enzyme inhibitors are a class of drugs used in the treatment of high blood pressure. They lower blood pressure by blocking, or inhibiting, the conversion of angiotensin I to angiotensin II. Angiotensin II acts in two ways. First, it is a potent constrictor of blood vessels. Through this action, peripheral resistance increases, which raises the blood pressure. The second job of angiotensin II is to trigger aldosterone release from the adrenal cortex.

Recall that aldosterone’s primary action is to promote sodium reabsorption in the kidneys. Because water follows sodium, this renal conservation of sodium causes the kidneys to also retain water. The net effect is to conserve body’s fluid content, thereby minimizing the loss of plasma volume while keeping blood pressure near normal. Figure below illustrates the changes in plasma volume and aldosterone concentrations during 2h of exercise.

The hormonal influences of ADH and aldosterone persist for up to 12 to 48h after exercise, reducing urine production and protecting the body from further dehydration. In fact, aldosterone’s prolonged enhancement of Na⁺ reabsorption may cause the body’s Na⁺ concentration to increase above normal following an exercise bout.  

As shown in the figure on the right side, individuals who are subjected to three repeated days of exercise and dehydration show a significant increase in plasma volume that continues to increase throughout the period of activity. This increase in plasma volume appears to parallel the body’s retention of dietary Na⁺. When the daily bouts of activity are terminated, the excess Na⁺ and water are excreted in urine.

Most athletes involved in heavy training have an expanded plasma volume, which dilutes various blood constituents. The actual amount of proteins and electrolyte(solutes) within the blood remains unaltered, but the substances are dispersed throughout a greater volume of water(plasma), so they are diluted and their concentration decreases. This phenomenon is called hemodilution.

“Physiology of sport and exercise”, fourth edition; Jack H. Wilmore, David L. Costill, W. Larry Kenney