Physiologic Changes During Dehydration
When you become dehydrated, your body experiences a cascade of fluid and electrolyte shifts that trigger cardiovascular strain, impaired thermoregulation, and altered blood composition—all stemming from decreased plasma volume and increased blood osmolality.
Fluid Compartment Changes
Plasma volume decreases proportionally to total body water loss, creating a hypovolemic state that is the foundation for most downstream physiologic consequences 1.
Plasma osmolality increases as water is lost, creating a hyperosmolar state that triggers compensatory mechanisms 1.
Intracellular fluid becomes the primary source of water loss when dehydration progresses beyond mild levels, as the body attempts to preserve extracellular volume 2.
Hemoconcentration occurs, affecting red blood cell morphology and potentially increasing red cell distribution width (RDW) values 3.
Cardiovascular Alterations
Cardiac filling decreases due to reduced blood volume, directly challenging the heart's ability to maintain adequate stroke volume 1, 4.
Blood pressure regulation becomes compromised, with postural hypotension developing as a clinical manifestation of significant water depletion 4, 5.
Heart rate increases as a compensatory mechanism to maintain cardiac output despite reduced stroke volume 1.
Sympathetic nervous system activity increases in response to hypovolemia and reduced blood pressure 6.
Blood viscosity increases due to hemoconcentration, potentially impairing microvascular perfusion 3.
Thermoregulatory Dysfunction
The core temperature threshold for vasodilation and sweating increases, meaning your body tolerates higher temperatures before initiating cooling mechanisms 1.
Sweat rate decreases for any given core temperature due to plasma hyperosmolality, reducing evaporative heat loss capacity 1, 4.
Heat storage accelerates because impaired sweating and reduced skin blood flow limit heat dissipation 1, 4.
Hormonal and Renal Responses
Arginine vasopressin (antidiuretic hormone) release increases to promote water retention by the kidneys 6.
The renin-angiotensin-aldosterone system activates to restore intracellular and extracellular fluid stores through sodium and water retention 6.
Thirst sensations intensify as a behavioral mechanism to promote fluid intake 5, 6.
Neurological Manifestations
Cognitive function deteriorates at fluid losses of just 1-3% of body weight, with effects potentially more severe in vulnerable populations 3.
Confusion, non-fluent speech, and extremity weakness develop with significant volume depletion 4, 5.
Mental performance declines alongside physical performance impairment 7.
Performance and Metabolic Impact
Aerobic endurance performance decreases, particularly when body water deficits exceed 2% of body mass 1, 7.
Perceived exertion increases for the same workload due to cardiovascular and thermoregulatory strain 7.
Reliance on carbohydrate as a fuel source may increase during exercise, though this effect requires further study 7.
Metabolic processes become disrupted, negatively impacting overall health and performance 8.
Critical Thresholds
Body mass losses of <1% are considered euhydrated, with plasma osmolality <290 mmol/kg and urine specific gravity <1.020 1.
Dehydration of ≥2% body mass represents the threshold where significant physiological impairment and performance decrements consistently occur 1, 4.
Dehydration exceeding 15-16% body weight loss (approximately 18.6% total body water loss) overwhelms compensatory mechanisms, leading to severe plasma volume reduction 2.
Common Pitfalls in Assessment
When interpreting elevated RDW or other hematological changes, always consider recent hydration status as a contributing factor before attributing changes solely to pathological processes 3.
Skin turgor assessment can be used to evaluate dehydration status, though it may be less reliable in elderly patients 3.
Daily body mass monitoring (post-void nude weight in the morning) provides the most practical assessment method, with changes >1% indicating inadequate hydration 1, 4.