Fluid Compartments of the body

70 kg man is composed of 60% water = 42litres

• Intracellular space (2/3): 28L
• Extracellular space (1/3): 14L = Plasma (3) + Interstitial (10) + Transcellular (1)

Transcellular fluid includes: ocular fluid, CSF, synovial fluid
NB. Circulating blood volume = 5l (70mls/kg), which is composed of plasma (ECF) and red cells (ICF)

Input sources

• Food: 800mls
• Water: 1500mls
• Metabolic oxidation: 200mls

Output sources

• Urine: 1500mls
• Faeces: 200mls
• Skin/respiration (insensible): 800mls

Internal water balance

1. Balance between osmolarities of two compartments
2. [Microcirculation]

External water balance (important in Shock)

• Reduced circulating volume results in reduction of blood pressure

1. Detected by carotid sinus/aortic arch [high pressure] baroreceptors: Sympathetic response
   • Catecholamine response - vasoconstriction to maintain BP, increase FOC, increase cardiac output
   • Stimulation of B2 adrenoceptors in kidneys kicks off RAS response
2. Decrease in renal blood flow / renal perfusion pressure: Renin-Angiotensin-Aldosterone response
   • B2 stimulation releases renin; converts angiogensinogen to angiotensin I
   • angiotensin I converted to angiotensin II by ACE (in the lungs, also degrades bradykinin)
   • Angiotensin II potent vasoconstrictor
   • Angiotensin II stimulates the release of aldosterone (from zona glomerulosa) which promotes Na/water resorption from DCT
3. Stress hormone release - corticosteroids from adrenal cortex
   • Salt/water retention
4. Increase in plasma osmolarity: ADH (produced in paraventricular and supraoptic nuclei) response
   • Osmoreceptors detect a rise in osmolarity (from loss of volume)
   • Stimulates the release of vasopressin (aka anti-diuretic hormone) - potent vasoconstrictor
   • ADH (via increase cAMP, aquaporin) stimulates resorption of water from DCT/CCD
5. Reduced renal perfusion stimulates EPO production (long term)

• Increase in fluid volume

1. Distention of cardiac atria [low pressure receptors] - leads to release of ANP: promotes diuresis
2. Increase in brain naturetic peptide (BNP increased in "cardiac failure")

Assessment of state of hydration

1. Clinical exam
   • Skin turgor
   • Dry mouth
   • Sunken eyes
   • Urine concentration
2. Charts
   • Tachycardia
   • Weights
   • Urine output
   • CVP measurements

Basal water requirements = 30-40ml/kg/day

Aim of fluid therapy

• Satisfy basal water requirement
• Replace fluids lost beyond basal requirement
• Support arterial pressure