BODY FLUID COMPARTMENTS

"Total body water amounts to 80% body weight in infants & 60% in adults"

The body contains water distributed between two main compartment

1. Intracellular fluid (ICF) – Fluid present inside the cells . It accounts for  ~2/3 of total body water & ~40% of total body weight.
2. Extracellular fluid (ECF) –
   • Fluid present outside the cells.
   • It accounts for ~1/3 of total body water,
   • It includes:
     1. Interstitial fluid – between cells
     2. Intravascular fluid – plasma (liquid part of blood)
     3. Transcellular fluid – CSF, pleural, peritoneal, pericardial fluid, etc

Important Concepts in Body fluid transport mechanisms

1. Diffusion 
   • Movement of solutes (like Na⁺, K⁺, glucose) from high to low concentration
   • Example: O₂ & CO₂ gas exchange, electrolyte balance.
2. Osmosis:
   • Movement of water molecules across a semi-permeable membrane from low solute concentration (more water) to high solute concentration (less water)
   • Maintains fluid balance between compartments.
3. Homeostasis
   • Refers to Stability of the internal environment.
   • Key regulators:
     • Kidneys: Control fluid & electrolyte levels
     • Adrenal glands secrete aldosterone which regulates sodium reabsorption, thereby controls ECF volume.
     • Pituitary gland release ADH which regulates osmotic pressure by controlling water reabsorption in kidneys.

Important measures in body fluid transport

1. Hydrostatic Pressure
   • Force exerted by fluid against vessel walls
   • In capillaries, it pushes fluid out into interstitial space. Eg. BP
2. Osmotic Pressure / Oncotic Pressure
   • Pressure created by solutes (mainly plasma proteins like albumin).
   • It sucks water into a compartment.
   • In capillaries, plasma proteins draw water from interstitial space into blood vessels.
3. Osmolality(mOsm/kg)
   • Number of solute particles per kg of solvent (water).
   • Normal plasma osmolality: 275–295 mOsm/kg.
   • Urine osmolality: 50–1200 mOsm/kg (varies with hydration
     1. High osmolality (hyperosmolality): Concentrated body fluids 
     2. Low osmolality (hypoosmolality): Diluted body fluids 

• Occurs when fluid intake is insufficient to meet the body’s needs.
• Not the same as dehydration: Dehydration is loss of pure water only, without sodium loss whereas FVD is loss of fluids and electrolytes together.

Causes

• Inadequate oral fluids, Vomiting, diarrhea, NG suction,
• High fever, polyuria
• Hemorrhage
• Third-space Shifts (Burns, peritonitis)
  

Manifestations

• General: Thirst, dry mucous membranes, Poor skin turgor, Delayed capillary refill.
• Change in Vitals:  Tachycardia, weak thready pulse, High respiratory rate, Low CVP, Postural hypotension, Shock(severe)
• Renal: Decreased urine output, concentrated urine.
• Weight loss.: Weakness, dizziness, seizures, coma(severe)

Types of FVD - [Isotonic, Hypotonic & Hypertonic]

1. Isotonic Dehydration (Hypovolemia)
   • Equal loss of water and electrolytes, Most common..
   • Leads to decreased circulating blood volume → poor tissue perfusion.
2. Hypertonic Dehydration
   • More water loss than electrolyte loss.
   • Water shifts from cells → plasma → cells shrink.
3. Hypotonic Dehydration
   • More electrolyte loss than water loss.
   • Water moves into cells → cell swelling, plasma volume deficit.

Collaborative Nursing Care

Main Management - Oral rehydration (ORS) & IV fluids.

Fluid choice:

• Isotonic dehydration → Isotonic fluids (0.9% NS, LR).
• Hypertonic dehydration → Hypotonic fluids (0.45% NS).
• Hypotonic dehydration → Hypertonic fluids (3% NS).

Other measures:

1. Blood transfusion if blood loss is the cause.
2. Strict I&O records & Daily weight monitoring (1 kg = 1 L fluid)
3. Assess skin turgor (sternum, abdomen, forearm).
4. Monitor Urine specific gravity:>1.025 = concentrated (dehydration); <1.010 = dilute.
5. For NG suction, irrigate nasogastric tubes with isotonic saline, not water, to prevent electrolyte loss.
6. Medications - Antidiarrheals, antimicrobials, antiemetics, antipyretics as needed.
7. Prevent further losses & Restores normal fluid/electrolyte balance.

• Condition where fluid intake or retention is greater than the body’s needs.
• Also called fluid overload or overhydration.

Causes

• Excessive IV fluids (isotonic or hypotonic)
• Excessive water intake (primary polydipsia)
• Heart failure, Kidney failure
• SIADH (too much Anti Diuretic Hormone)
• Long-term corticosteroid use

Signs & Symptoms

• • Peripheral edema - Swelling in arms, legs
  • Jugular vein distension(JVD)
  • Bounding, strong pulse
  • High blood pressure and increased CVP
  • Frequent urination (if kidneys work well)
  • Shortness of breath, crackling sounds in lungs - Pulmonary edema (fluid in lungs)
  • Weight gain
  • Confusion or seizures (cerebral edema).

Colloborative Nursing Care

• Assess for symptoms and manage underlying condition (e.g., heart or kidney failure).
• Restrict fluids and sodium if necessary.
• Give diuretics as ordered.
• Inspect skin, protect edematous areas from injury or pressure sores.
• Track urine output & maintain strict Intake and Output chart
• Weigh patient daily at the same time with same clothes.
• Monitor labs closely (BUN, creatinine, sodium, potassium).
• Pay special attention to patients with acute or chronic kidney disease.