Body fluids Flashcards
What is the distribution of body fluids?
- 60% water - total weight of body is 60% water, which of this is
- 40% body weight intracellular water
- 20% body weight extracellular water
What are the typical volumes of body fluids for each compartment in a 70kg male?
- 28L - Intracellular water
- 11L - Interstitium (extracellular fluid)
- 3L - Plasma water ( this is also extracellular fluid)
Why is fluid composition inside and outside of the cell different?
Intracellular has lots of potassium, phosphate
Extracellular has more sodium and chloride
This gradient is important for generating electric fields and action potentials
Movement of molecules:
- Depends on pressure
- Ionic pressure
- Osmotic pressure
- Depends on nature of barrier
- Cell membrane - semi-permeable, allows water only
- Capillary wall - more permeable (capillaries one cell thick), allows water, electrolytes but not proteins (too big)
Describe the cell membrane barrier
- Lipid bilayer with proteins
- Impermeable to most ions and proteins; has pumps
- Water allowed to move freely
- Osmotic pressure drives movement of water
Describe the capillary wall barrier
- Endothelial cells, semi permeable pores
- Only large molecules (e.g. proteins) can exert an osmotic pressure across it
- Hydrostatic pressure drives movement of water
Define diffusion
Movement of molecules from high to low concentration due to random motion
Define osmosis
Net diffusion of water across selectively permeable membrane from region of high concentration to low concentration
What is osmotic pressure?
Pressure exerted on cell membrane due to differences in solute concentrations between cell and its environment
Osmotic pressure will decrease as water moves into area of lower water concentration from an area of higher water concentration
What is the osmole?
Unit of measurement to describe the concentration of solute particles
1 osmole = 1 ionic particle, so Na2SO4 = 3 osmoles, due to 2Na+ and 1 SO42- particle being what Na2SO4 is comprised of
What is tonicity?
The effect of a solution (and solutes that cannot cross) on osmotic movement of water across cell membrane
Describe the tonicity of NaCl, Glucose and Urea
NaCl - 150mOsm , it’s isosmotic and isotonic
Glucose - 300mOsm, it’s isosmotic, NOT isotonic
Urea - 300mOsm, it’s isosmotic, NOT isotonic
What does isosmotic mean?
Isosmotic refers to the situation of two solutions having the same osmotic pressure
What does isotonic mean?
Isotonic refers to a solution having the same solute concentration as in a cell or a body fluid
Describe 3 methods that can be used to measure volumes in the different compartments
-
Dilution method
- Concentration = amount/volume
- Volume = (amount added - amount lost)/concentration
-
Directly measured volumes
- Total body water: Radioactive water, distributed ALL COMPARTMENTS
- EXTRACELLULAR VOLUME: Radioactive Insulin, excluded from cells
- PLASMA VOLUME : Radioactive albumin, stays in plasma only
-
Indirectly measured volumes
- INTRACELLULAR VOLUME- TBW (total body water) - extracellular volume
- INTERSTITIAL VOLUME - Extracellular volume - plasma volume
Describe the osmotic pressures of ICF and ECF
Osmotic pressures of both tend to be balanced
Describe the osmotic pressure gradient
The difference in concentration between two solutions on either side of a semipermeable membrane
How does osmolality of a solution influence cell size?
Hypertonic solution - too much solute, not enough water, lots of water moves out of cells, cells become shrunken, crenated
Hypotonic- too much solute inside cell, water moves in to balance, cells begin to lyse, and if too much water moves in, cells begin lysing (exploding basically)
What does a solution having high osmolarity mean?
It has a high concentration of solutes, therefore, water would move out of cells in an attempt to create equilibrium of osmotic pressures
What does a solution having low osmolarity mean?
It has a low concentration of solutes, therefore, water moves into cells from the extracellular compartment in order to maintain the osmotic pressures in both fluid compartments
How does fluid in the interstitium cause oedema?
- Increased capillary fluid pressure
- Decreased capillary Osmotic press
- Leaky membrane (Coefficient)
- Blocked lymphatics
Define isosmotic
A solution in which the osmotic pressure of the ICF & ECF
compartments are the same, therefore there is no net movement of water.
Define isotonic
A solution in which the osmolarity of intracellular and
extracellular fluids are the same, therefore there is no net movement of water.
