Fluid and electrolyte balance

Lecture 6

• Your body is 66% water• Not evenly distributed – separated into

compartments• Able to move back and forth thru the cell

membranes to maintain an equilibrium

• Fluid balance – The amount of water gained each day equals the

amount lost• Electrolyte balance – The ion gain each day equals the ion loss

• Acid-base balance – H+ gain is offset by their loss

• 70 Kg adult man– 42 L of water• 2/3 is ICF (28 L)• 1/3 is ECF (14 L)– Plasma (3.5L)– ISF (10.5 L)

• Homeostatic mechanisms respond to changes in ECF

• No receptors directly monitor fluid or electrolyte balance– Respond to changes in plasma volume or osmotic

concentrations– All water moves passively in response to osmotic

gradients– Body content of water or electrolytes rises if intake

exceeds outflow

Assessment, Measurement and Monitoring

Parameter Significance

Dehydration Overhydration

Pulse Increased Normal

Blood pressure Decreased Normal / increased

Skin turgor Decreased Increased

Eye balls Soft / Sunken Normal

Mucous membranes Dry Normal

Urine output Decreased May be normal or Decreased

Consciousness Decreased Decreased

Electrolytes

• Concentration – Ratio of two variables amount of solute and

amount of water.– Changes can occur either because of solute or

water.

Osmolality

• mmol of solute per Kg of solvent– Osmolality of ECF = ICF– 285 mmol/Kg– Can be measured directly or calculated

– Serum osmolality= 2 × serum sodium

Oncotic pressure

• Capillary membrane separates interstitial fluid from intravascular fluid

• Small molecules move freely but not plasma proteins so they exert colloidal osmotic (oncotic pressure)

• Balance between oncotic pressure and hydrostatic pressure across the capillary membrane is responsible for maintaining fluid in the capillaries

• Total water is 42 L• If 2 L is lost from total ?• If 2 L is lost from intravascular compartment ?• Humans deprived of fluid die after few days

because of circulatory collapse – Impaired blood circulation– Lack of oxygen and nutrients– Failure to remove waste

Water

• Water intake – ½ L/ day --------5 L/day

Water losses kidney 0.5 – 4 L/day

GIT50 ml /day

Insensible loss500– 850 ml/day

AVP (ADH)

• Speciliazed cells in hypothalamus sense difference in osmolality

• Stimulate posterior pituitary to release ADH.– Water conservation by kidney.– Dehydration– 0.5ml/Min

• Increased osmolality stimulate secretion of ADH and decreased osmolality switches off the secretion

Sodium

• 3700mmol (75 % exchangeable)• ECF 140mmol/L• Sodium intake 100– 300 mmol/day• Sodium losses – Sweat 5mmol/day– In diseases GIT losses are important clinically • Infantile diarrhoea (death can occur due to salt and

water depletion)

Sodium regulation

• Aldosterone • Atrial natriuretic peptide

Case history

• A man is trapped in a collapsed building after an earthquake. He has sustained no serious injury. He has no excess to food and water until he is rescued after 72 hrs.– What will have happened to his body fluid

compartments?