1. Fluid and electrolyte balance Lecture 6

2. 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

3. 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

4. 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)

5. 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

6. Assessment, Measurement and Monitoring

7. ParameterSignificance DehydrationOverhydration PulseIncreasedNormal Blood pressureDecreasedNormal / increased Skin turgorDecreasedIncreased Eye ballsSoft / SunkenNormal Mucous membranesDryNormal Urine outputDecreasedMay be normal or Decreased ConsciousnessDecreased

8. Electrolytes

9. Concentration – Ratio of two variables amount of solute and amount of water. – Changes can occur either because of solute or water.

10. 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

11. 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

12. 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

13. Water Water intake – ½ L/ day --------5 L/day Water losses kidney 0.5 – 4 L/day GIT 50 ml /day Insensible loss 500– 850 ml/day

14. 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

16. 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)

17. Sodium regulation Aldosterone Atrial natriuretic peptide

20. 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?