1. Fluid and electrolyte balance
Lecture 3

2. Not evenly distributed – separated into compartments
Human body is 66% water
Not evenly distributed – separated into compartments
Able to move back and forth through 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

3. Electrolytes are present in the human body, and the balance of the electrolytes in our bodies is essential for normal function of our cells and our organs. Common electrolytes that are measured by doctors with blood testing include sodium, potassium, chloride, and bicarbonate.
Fruit and vegetables are good sources of electrolytes
Electrolytes include sodium, potassium, calcium and bicarbonate
Elderly people are particularly susceptible to electrolyte imbalance
Electrolytes

4. 42 L of water 1/3 (14 L) is extracellular fluid (ECF)
A normal 70 Kg adult man have
42 L of water
2/3 of this water (28 L) is intracellular fluid (ICF) while
1/3 (14 L) is extracellular fluid (ECF)
ECF is further divided into
Plasma (3.5L)
interstitial fluid (ISF) (10.5 L)
Interstitial fluid. Interstitial fluid or tissue fluid is a solution that bathes and surrounds the tissue cells of multicellular animals. It is the main component of the extracellular fluid.

5. Water tank model of body fluid compartments
Intracellular Extracellular
Water tank model of body
fluid compartments
Dehydration and Overhydration

6. 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

7. Assessment, Measurement and Monitoring

8. May be normal or Decreased
Parameter
Significance
Dehydration
Overhydration
Pulse
Increased
Normal
Blood pressure
Decreased
Normal / increased
Skin turgor
Eye balls
Soft / Sunken
Mucous membranes
Dry
Urine output
May be normal or Decreased
Consciousness

9. Electrolytes

10. Concentration
Ratio of two variables, amount of solute and amount of water.
Changes can occur either because of solute or water
E.g. sodium concentration of140 mmol/L may become 130mmol/L due two reasons
Either sodium is decreased
Or water volume is increased

11. Osmolality
An osmole (osmol) is the amount of a substance that dissociates in solution to form one mole of osmotically active particles. A milliosmole (mosmol) is 0.001 osmol.
The difference between osmolality and osmolarity is negligible because body fluids typically are dilute aqueous solutions.
mmol of solute per Kg of solvent is osmolarity
Osmolality of ECF = ICF
In other words there is isotonic solution in the two compartments
the normal osmolarity of all body fluids except urine is 285 mmol/Kg
Can be measured directly or calculated
Serum osmolality= 2 × serum (sodium)

12. This simple formula is applied when in serum the concentration of urea and glucose are within the reference intervals.
If either or both are abnormally high, the concentration of one or both (in mmol/L) must be added in to give the calculated osmolality.
Sometime there is a clear difference in the measured and actualy calculated osmolality and this is called osmolal gap.

13. Osmolality vs Osmolarity

14. Isotonic solution is one in which its effective osmole concentration is the same as the solute concentration of a cell. In this case the cell neither swells nor shrinks because there is no concentration gradient for water across the cell membrane.

15. 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

16. Water Total water is 42 L (average 70 kg man or car petrol tank)
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

17. Water Water losses Water intake ½ L/ day --------5 L/day
kidney 0.5 – 4 L/day
Water loss by kidney is controlled by arginine vasopressin (VAP, also know as antidiuretic hormones, ADH)
GIT
50 ml /day
Insensible loss
500– 850 ml/day
Water

18. Water intake
0.5 to 5 L/day
mL/min
Usually
0.5 – 15 L/day
Insensible loss
mL/day
50 mL/day

19. Speciliazed cells in hypothalamus sense difference in osmolality in the intracellular osmolality and ECF
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
AVP (ADH)

22. 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

23. Aldosterone
Atrial natriuretic peptide
Sodium regulation

26. A man is trapped in a collapsed building after an earthquake
A man is trapped in a collapsed building after an earthquake. He has sustained no serious injury. He has no access to food and water until he is rescued after 72 hrs.
What will have happened to his body fluid compartments?
Case history