Body fluids Flashcards
What is the average input of fluid a day and by what mechanisms?
2.5L/day - drinking, eating, metabolism of carbohydrates
What is the average urinary output a day?
1.5L/day but may vary depending on regulation
What happens to the remaining volume of water?
0.9L is lost through lungs, skin and faeces - exercise, metabolism, temp, disease
How much fluid is in the intracellular compartment?
28L (inside of cells)
How much fluid is in the extracellular compartment?
14L
Which areas contribute to the extracellular compartment?
Interstitial and plasma
What is transcellular fluid?
Fluid that must cross an epithelial barrier such as CSF, plural, periotneal, synovial
How much water is a 70kg person?
42L
What is the equilibrium of a substance depend on when moving through compartments?
Permeability
pH of the compartment
Binding capacity
Fat solubility
What is the volume of distribution?
Vd = Q / Cp
Q = total amount of substance
Cp = Concentration of substance in plasma
It is the volume of fluid needed to contain the administered substance at the same concentration as in the plasma.
How can the plasma be measured?
Use a substance that binds strongly to plasma proteins - heparin, insulin, Evan’s blue
How is extracellular fluid measured?
Use a strong polar molecule as it can’t cross the membrane
How can fat or water be measured?
Ethanol or radioactive H for water
What is the concentration of Na both intracellular and extracellular?
Intra - 15mM
Extra - 140mM
What is the concentration of K both intracellular and extracellular?
Intra - 140mM
Extra - 4mM
What is the concentration of Ca extracellular?
2.4mM as half being bound and half free
What is the concentration of Cl both intracellular and extracellular?
Intra - 4mM
Extra - 110mM
What is the concentration of Hco3 both intracellular and extracellular?
Intra - 12mM
Extra - 25mM
What is Ca bound to?
Albumin
How does a change in albumin concentration affect the concentration of Ca?
A change in albumin conc will change the total Ca conc without affecting the conc of free Ca as the free Ca is regulated.
How does a patient appear in hypoalbuminaemia?
Appear hypocalcaemic as the total conc of Ca falls even though the free Ca conc remains the same.
Ascites and oedema - Fall in albumin reduces oncotic pressure so less absorption to oppose the hydrostatic pressure.
What is hypoalbuminaemia caused by?
Hepatic failure - reduced production
Renal failure - increased loss
What is an osmole?
The number of molecules a compound dissociates into when dissolved, affecting osmotic forces.
Define osmolality.
Number of osmoles per mass of solvent in osm.kg
Define osmolarity.
Number of osmoles per volume of solution in osm.L
What is the osmolarity of both the intracellular and extracellular compartments at equilibrium?
285 osm.L
What will alter water movement across cells?
A change in osmolarity.
Why is Na considered impermeable?
Its gradient is maintained by Na/K ATPase so has no effect on concentration
What is osmotic pressure?
The pressure applied to a solvent to PREVENT movement via osmosis. Driven by the interstitial protein concentration = ABSORPTION
What is hydrostatic pressure?
The pressure applied to PROMOTE movement. Driven by blood pressure = FILTRATION
When will a solvent stop moving?
When osmotic and hydrostatic pressures are at equilibrium
What is oncotic pressure?
A type of osmotic force driven by PROTEINS. Ions are at equilibrium between fluid and blood and so only the proteins have an effect.
What is the name given to two solutions with the same osmolality?
ISOTONIC - same solute concentration present
What is the name given to two solutions with the same osmotic pressure?
ISOSMOTIC
Give an example of a substance that can be isosmotic but not isotonic.
Urea
What is the use of Mannnitol?
It is a sugar alcohol that distributes in the extracellular compartment to increase osmalilty and drive water from intracellular. Used to reduce ICP after a haemorrhage or as a diuretic.
What is given to a dehydrated Pt?
Glucose saline, an isotonic solution. The glucose is absorbed intracellularly, taking the water with it to restore intracellular fluid.
Salts can be given alongside to replace the salts lost
From which compartment will an hypotonic solution move water?
Move water from extra to intra as the hyptonic solution is reducing concentration in the extracellular compartment. May cause cell LYSIS if in excess.
From which compartment will a hypertonic solution move water?
Water will move from intra to extra as the hypertonic solution is raising the concentrate of osmoles in the extracellular compartment. May cause cells to shrink
What is the effect of a hypertonic solution on the renal system?
It only distributes in the extracellular compartment. It will reduce resorption in the nephron by decreasing aldosterone. The high osmolality inhibits RAS for salt and water excretion to reduce the blood volume. As move fluid is drawn from intra to extra, BP rises.
The high osmolality also stimulates ADH release so RAS>ADH for excretion to occur.
Does the body correct BP or osmolality first?
BP
What happens to ADH if blood volume falls?
More ADH is released for reabsorption
How can it be distinguished if a drug has been sequestered intracellularly or extracellularly?
If sequestered intra, plasma concentration would be low.
If sequestered extra, Vd would be around 14L.
