13.6: Water Balance

Regulation of Water Intake

Plasma osmolality measures the ratio of solutes to water in the blood. An individual's plasma osmolality value reflects their hydration status. A healthy body regulates plasma osmolality within a narrow range by deploying multiple mechanisms that manage both water intake and output.

Short-term water regulation involves fluid shifts between ICF and ECF and is influenced by baroreceptor activation. In contrast, long-term water regulation relies on hormonal control, impacting thirst stimulation and management of fluid balance by the kidneys.

Dehydration occurs due to a net loss of water, resulting in inadequate water levels in the blood and other tissues. The water exiting the body, such as through exhaled air, sweat, or urine, is primarily withdrawn from the plasma. As the blood becomes more concentrated, a series of physiological processes trigger the thirst response (Figure 3). Osmoreceptors, sensory receptors in the thirst center of the hypothalamus, monitor the blood's solute concentration (osmolality). When blood osmolality rises beyond optimal levels, the hypothalamus transmits signals that raise conscious awareness of thirst, prompting the individual to drink water. Changes related to aging, such as shifts in total body water, the perception of thirst, regulation of renal tubular concentration, and variations in ADH and aldosterone levels, may contribute to dehydration vulnerability.⁵

​Figure 3 | The Thirst Mechanism in Response to Dehydration |⁷ This diagram illustrates the physiological processes triggered by insufficient water (H₂O) in the body, leading to the sensation of thirst. A decrease in body water results in both decreased blood volume and increased blood osmolality. Decreased blood volume leads to decreased blood pressure and subsequently an increase in angiotensin II. Increased blood osmolality is detected by osmoreceptors in the hypothalamus. Both increased angiotensin II and signals from the osmoreceptors, along with the sensation of dry mouth, stimulate the thirst center in the hypothalamus. This stimulation increases the feeling of thirst, prompting the intake of H₂O. The consumption of H₂O then leads to a decrease in blood osmolality, restoring fluid balance in the body.

In a dehydrated individual, the hypothalamus triggers the release of ADH via the posterior pituitary gland (see Chapter 7). ADH carries two primary effects: it constricts the arterioles in the peripheral circulation, lessening blood flow to the extremities and increasing blood supply to the body's core. Additionally, ADH prompts the insertion of aquaporins within the epithelial cells lining the renal collecting tubules. As a consequence, the water permeability of these cells increases significantly, facilitating a substantial passage of water from the urine through the walls of the collecting tubules, leading to the increased reabsorption of water back into the bloodstream. As the concentration of the blood plasma reduces and the ADH level decreases, aquaporins are removed from the cell membranes of the collecting tubules. Consequently, the movement of water from urine to the bloodstream diminishes.⁸

During dehydration, the hypothalamus communicates via the sympathetic nervous system to the salivary glands in the mouth. This signal prompts a reduction in watery, serous output while increasing the production of stickier, thicker mucus. These alterations in secretions lead to a dry sensation in the mouth and trigger the feeling of thirst.

As water loss causes a decline in blood volume, two additional responses occur. Firstly, baroreceptors in the aortic arch and carotid sinus sense the decrease in blood pressure due to the reduced blood volume, which prompts the heart to increase its rate and the strength of contractions to offset the lowered blood pressure.⁹ Secondly, the renin-angiotensin-aldosterone system is activated, leading to the release of the hormone aldosterone from the adrenal cortex. Aldosterone plays a role in enhancing not only thirst but also the reabsorption of sodium ions in the distal tubules of the nephrons within the kidneys. This reabsorption of sodium causes water to follow, being reabsorbed back into the bloodstream.¹⁰

Regulation of ECF Volume and Osmolality

The body regulates extracellular volume by monitoring changes in volume and osmolality. If an individual consumes a moderate amount of salt, absorption leads to an increase in plasma and extracellular osmolality. This change prompts the movement of water from the intracellular to the extracellular space, following osmotic principles. The resulting shrinkage of cells in the hypothalamic osmoreceptors triggers the release of ADH from the posterior pituitary, increasing water absorption at the renal tubule. As a result, urine osmolality rises, and urine volume decreases, contributing to an increase in extracellular volume while maintaining osmolality.¹¹

The increase in ECF volume is rectified by another mechanism. Atrial receptors respond to atrial stretching, stimulating the release of ANP. ANP production decreases aldosterone secretion, causing an increase in the loss of sodium ions. Additionally, changes in renal perfusion, such as reduced blood flow, are detected by juxtaglomerular cells in the kidney, triggering the release of renin and stimulating the production of angiotensin II, which, in turn, stimulates aldosterone secretion. Tables 5 and 6 outline conditions causing changes in volume and osmolality of body fluids and factors contributing to edema, respectively.¹²

The body’s fluid and electrolyte balance adjusts dynamically depending on specific changes in volume and osmolarity, influencing both ECF and ICF compartments. In isotonic volume expansion, as seen with an isotonic NaCl infusion, ECF volume increases without affecting ICF volume or ECF osmolarity since the solution is isotonic, leaving sodium levels unchanged and causing no osmotic fluid shifts. Conversely, isotonic volume contraction, such as in diarrhea, leads to a reduction in ECF volume while maintaining stable osmolarity and ICF volume because both compartments remain isotonic relative to each other. In cases of hyperosmotic volume expansion, such as with high NaCl intake, ECF volume and osmolarity rise as sodium draws water from the ICF to the ECF, resulting in decreased ICF volume and increased sodium concentration. Conditions like sweating, fever, or diabetes insipidus lead to hyperosmotic volume contraction, where water loss causes decreases in both ECF and ICF volumes. With sodium retained relative to water, ECF osmolarity and sodium concentration increase, creating a hyperosmotic environment.

Hyposmotic volume expansion occurs in Syndrome of Inappropriate Antidiuretic Hormone (SIADH), where excess ADH leads to water retention, increasing both ECF and ICF volumes as water shifts into cells. This dilutes the ECF, reducing osmolarity and sodium concentration. In contrast, hyposmotic volume contraction is seen in adrenal insufficiency, where aldosterone deficiency results in sodium loss, lowering both ECF volume and osmolarity. As water shifts into the ICF to balance osmolarity, ICF volume increases while sodium concentration and ECF osmolarity decrease. Together, these mechanisms demonstrate the body’s intricate processes for maintaining fluid and electrolyte equilibrium across compartments.⁵

Edema develops when excess fluid accumulates in the interstitial spaces, and this process can arise through several mechanisms, each associated with specific underlying etiologies. One common cause of edema is elevated capillary hydrostatic pressure, where increased pressure within the capillaries pushes fluid out into the surrounding tissue. This mechanism is often seen in conditions like renal failure and heart failure, where fluid retention and blood pooling raise pressure within the capillaries, leading to fluid leakage.

Another pathway to edema is low plasma oncotic pressure, which normally helps retain fluid within blood vessels. When oncotic pressure is decreased, fluid more readily moves into the interstitial space. This decrease often occurs in cases of liver failure, malnutrition, and nephrotic syndrome. In liver failure, the liver’s reduced protein production (such as albumin) diminishes oncotic pressure, while in malnutrition, the overall lack of protein in the diet 

contributes to lower plasma protein levels. Nephrotic syndrome similarly leads to protein loss through urine, reducing oncotic pressure and predisposing individuals to edema.

Lymphatic obstruction is one of the mechanisms contributing to edema. When lymphatic flow is blocked or compromised, fluid that would normally be drained by the lymphatic system accumulates in tissues, resulting in lymphedema. This condition is often seen with infections, surgeries, or certain cancers that affect the lymphatic system.

Sodium retention is another critical factor in edema development, as excess sodium draws water into the vascular and interstitial spaces. Sodium retention can occur in conditions like renal hypoperfusion, where decreased kidney blood flow triggers mechanisms to retain sodium, and with excessive salt intake, which directly increases sodium levels in the body and can lead to fluid overload and edema.

Inflammation can also cause edema, as inflammatory processes increase capillary permeability, allowing fluid and proteins to escape into surrounding tissues. This process is prominent in systemic inflammatory responses such as sepsis, where widespread inflammation throughout the body can result in significant fluid leakage into tissues, contributing to generalized edema.⁵