Dehydration: More Than Just Feeling Thirsty 💧

Water is essential for life, and maintaining fluid balance is a core component of homeostasis. The body works continuously to keep its internal environment stable, but illness, exertion, heat exposure, or inadequate intake can quickly disrupt this balance.

Dehydration exists on a spectrum — from mild physiological stress to life-threatening hypovolaemic shock — with progressively worsening signs and symptoms as circulating volume falls.

🚨 What Happens When the Body Loses Water?

🔹 Mild Dehydration (1–3% fluid loss)

• ✔ Increased thirst — the body’s earliest warning system
  ✔ Dry mouth and slightly darker urine due to renal conservation of water
  ✔ Subtle cognitive changes (mild fatigue, reduced concentration)
  ✔ Slight tachycardia as early cardiovascular compensation

🔹 Moderate Dehydration (4–6% fluid loss)

• ⚠ Dizziness, postural hypotension due to reduced circulating volume
• ⚠ Reduced skin turgor (tenting effect when pinched)
• ⚠ Oliguria (markedly reduced urine output) as kidneys conserve fluid via ADH and RAAS activation
• ⚠ Irritability or confusion from reduced cerebral perfusion

🔹 Severe Dehydration (>7% fluid loss)

• 🛑 Hypotension, marked tachycardia, weak pulses → Signs of hypovolaemic shock
• 🛑 Cold, mottled extremities → Peripheral vasoconstriction to maintain central perfusion
• 🛑 Sunken eyes, severe lethargy → severe extracellular fluid depletion
• 🛑 Anuria (no urine output)  → Kidneys unable to sustain effective filtration

🩺 How Do We Assess Hydration Status?

🔍 Adults (general assessment)

• ✅ Skin turgor test (tenting effect) - delayed recoil suggests dehydration
• ✅ Capillary refill time (<2 sec normal, prolonged in dehydration)
• ✅ Orthostatic BP changes (drop of >20 mmHg systolic or >10 mmHg diastolic)
• ✅ Heart rate trends (resting tachycardia or poor recovery)

🔍 Paediatrics & older adults

• ✅ Sunken fontanelles in infants
• ✅ Tear production ( reduced or absence indicates dehydration)
• ✅ Dry mucous membranes
• ✅ Behavioural changes (lethargy, irritability, decreased  interaction)

🔍 Athletes & Heat-Exposed Populations

• ✅ Urine colour chart (darker shades signal dehydration)
• ✅ Weight loss tracking (acute water depletion post-exercise)
• ✅ Core temperature monitoring (in heat-related dehydration)

💡 Key Takeaway

Dehydration isn’t just about thirst — it represents a progressive disturbance of circulatory homeostasis. If unrecognised or untreated, it can progress to shock and organ dysfunction. Early identification and appropriate rehydration (oral or intravenous) are critical in both clinical and everyday settings.

🧠 Case vignettes

🏃 Case 1: The marathon runner

A 28-year-old woman presents to the medical tent after completing a marathon on a warm day. She reports dizziness when standing, nausea, and fatigue. She has not urinated since early morning.

On examination:

• Heart rate: 108 bpm
• Blood pressure: 110/70 mmHg lying → 88/60 mmHg standing
• Skin turgor is reduced
• Urine sample is dark yellow

Teaching focus:
This case illustrates moderate dehydration driven by excessive fluid loss through sweat. The postural hypotension and tachycardia reflect reduced circulating volume and compensatory cardiovascular responses.

🧸 Case 2: The child with gastroenteritis

A 2-year-old child is brought to the GP with a 2-day history of vomiting and diarrhoea. The parents report poor oral intake and fewer wet nappies.

On examination:

• Dry lips and oral mucosa
• Sunken eyes
• Reduced tear production when crying
• Lethargic but rousable

Teaching focus:
Children dehydrate more rapidly due to higher fluid turnover and limited physiological reserve. Reduced urine output and behavioural changes are key early warning signs.

💭 Reflect and discuss

Which clinical sign of dehydration do you think is most reliable — and which can be misleading? How does the context (age, environment, activity) change your assessment?