Body Fluids Flashcards
Why is water a vital molecule?
Water is a vital molecule because:
•Water is the medium for all metabolic processes
•Water is involved in diffusion of nutrients/waste products
•Universal solvent
•Stable volume, BP and composition of the body fluids is essential for metabolism (homeostasis)
Water content of body fluid is reduced in the elderly and the overweight
How is body fluid distributed?
Body fluid is divided into 3 compartments
60% of body weight is water
40% of body weight is intracellular water (ICF)
20% of body weight is extracellular water (ECF)
The ECF is subdivided into the plasma (4% is water) and interstitial fluid (16% is water)
Osmosis determines water movement between ICF + ECF
How would body fluids be distributed in a 70kg human?
60% of 70= 42L of body fluid altogether
40% of 70=28L
20% of 70= 14L (11+3 in the ECF)
Describe the composition of body fluids in the ECF and ICF
composition of the fluids is so different to create the electrical gradient for muscle and nerve potential
Why and how is body fluid composition different in the ECF and ICF?
The v different composition of the ECF and ICF are vital for cellular functions, eg depolarisation of excitable cells
The plasma membrane maintains these compositional differences. It has a v low electrolyte permeability as most electrolytes are polar, so can’t pass through the non-polar centre of the phospholipid bilayer
Barrier between ICF & ECF = The Cell Wall
How much fluid do we need?
What does the movement of molecules depend on?
Depends on pressure
Ionic pressure
Osmotic pressure
Depends on the nature of the barrier
Cell membrane
Capillary wall
What is the difference between the cell membrane barrier and the capillary wall barrier?
Cell membrane barrier: Lipid bilayer w proteins.
Impermeable to most ions and proteins; has pumps and highly specified voltage gated channels.
Only water can move freely, driven by osmotic Pa
Capillary wall barrier: Endothelial cells, w semi permeable pores
Allows water and electrolytes, but not proteins
Large molecules can’t get through the barrier so it pulls water in
Only large molecules (e.g. proteins) can exert an osmotic pa across it. Hydrostatic pa drives water movement
Easy diffusion between interstitial and plasma means they have nearly identical ion composition.
What is an osmole?
Osmoles are a unit describing the conc of a solute in body fluids
1 Osm= 1mol of osmotically active particles (6.02x10^23)
Osmoles depend on the dissociation of the solute
For example, glucose doesn’t dissociate, therefore 1mol of glucose is equal to 1 Osm
NaCl on the other hand does dissociate into Na and Cl, therefore 1mol of NaCl is equal to 2 Osm
Describe osmolarity and osmolality
Osmolality and osmolarity are used interchangeably. They describe the conc of body fluids but have technical differences:
Osmolality= conc of osmotically active particles per kg of water (Osm/kg)
Osmolarity is the concentration of osmotically active particles per litre of water (Osm/L).
Osmolarity changes w temp as water expands/contracts.
The osmolarity of the ECF and the ICF is very similar (~300mOsm/L) to prevent large fluid shifts between the compartments
What is hydrostatic and therefore osmotic pressure?
Hydrostatic pressure= pa exerted on a membrane by a stationary fluid. As water diffuses into a compartment, volume in the compartment ⇡, which ⇡ hydrostatic Pa
Osmotic pressure= the hydrostatic pa needed to prevent the inward osmosis of water. It reflects the tendency of water to move passively down a conc gradient
How are hydrostatic and osmotic pressure linked?
They are linked via Starling’s principle of fluid exchange. This states that hydrostatic pa drives filtration of fluid out of a capillary, whilst the osmotic pa drives reabsorption
Osmosis is driven by an osmotic pa/osmolality gradient. Water moves from an area of low osmolality (dilute solution) to high osmolality (conc solution) (this is from a low to high osmotic pa)
Osmosis stops when hydrostatic + osmotic pa reach eqm
State the calculation for osmotic Pa
State the definition for tonicity
Tonicity = effect of a solution [and solutes that cannot cross] on osmotic movement of water across cell membrane
What is the difference between a hyper and hypotonic solutions?
A hypotonic solution has a lower solute conc than the intracellular fluid – this causes water to move into the cell, causing it to expand
A hypertonic solution has a higher solute conc than inside the cell – this draws water into the ECF, causing the cell to shrink