Notes

Water, fluids, spaces, and compartments:
Two primary compartments: ICF and ECF. Volume fo compartment is determined by the amount of solutes.
Disturbances of body fluid balance:
- oedema: abnormal accumulation of fluid beneath the skin.
- dehydration scenarios: vomiting or diarrhoea, severe haemorrhage
- overhydration scenarios: infusion therapy, psychogenic polydisplasia
55%-60% (women/men) of bodyweight is water, 30-42 liters
Fluid volumes
- 60-40-20 rule: 60% of body weight is water, 40% is ICF, 20% is ECF. 3/4 of ECF is ISF, 1/4 is intravascular .
-Mass = Conc. X Volume
Haematocrit: Fraction of blood that is red blood cells - usually 40% for women and 45% for men. Blood volume = Plasma volume/1-Hct.
Water intake and output:
-solutes and water enter or leave the body via the ECF. Average daily intake and output match (2850ml). The body maintains steady-state water balance by ensure the amount of water added each day is exactly balanced that excreted. Only kidneys excrete water. Urine amount varies between 0.5 and 18L/day

How kidneys regulate water excretion:
- water diffuses across the membrane via aquaporins. Type and amount vary between cell membranes.
-expression altered by ADH
- kidneys can alter expression of aquaporins in dehydration and overhydration scenarios.
Differing compositions of compartments is due to activities of barriers separating them. Cell membrane separating ICF and ISF is a lipid bilayer. ISF and IVF separated by fenestrated capillary wall.
Movement between ICF and ISF: Most substances that are biologically important in ICF cannot penetrate lipid membrane. Only small hydrophobic molecules can pass membrane. Proteins are needed for movement of large and small, uncharged, polar molecules, and ions

The main drivers of solute movement between intracellular and interstitial fluid compartments are osmotic pressure and electrochemical gradients The main drivers of solute movement between ISF and plasma compartments is hydrostatic pressure and oncotic pressure.
Osmotic pressure: the pressure that must be applied to a solution to prevent the net flow of water into it. Water follows the osmoles.
Electrochemical graident: the separation of electrical charge across the membrane provides the electrical forces that drives positive ions into the cell and negative ones out.
Hydrostatic pressure: pressure exerted by a fluid due to the forces of gravity
Oncotic pressure: large plasma proteins cannot cross capillary walls and tend to pull water into capillaries (albumin accounts for 80%)

Osmotic pressure of a body fluid is generally expressed as its osmolality ie moles/solvent mass. Osmolality of fluid compartments at equilibrium is 290mOsm. Hyperosmotic >290mM. Hypoosmotic <290mM. Water distribution of mody compartments depends on concentration of electrolytes in the body compartments.

Tonicity: effect on the volume of a cell e.g. hypertonic, isotonic, hypotonic
An ineffective osmole is one that can easily distribute across body compartments so it doesn't case fluid shifts

Isosmotic Dehydration:
- caused by blood loss, diarrhoea, vomiting, burns
- volume of ECF decreases, osmolality of ECF does not change as fluid lost was isotonic
- no change in volume or osmolality of ICF
- water does not move between compartments if osmolality stays the same

Hyperosmotic Dehydration:
- loss of water in excess of solute
- causes: decreased water intake, diabetes insipids, diabetes mellitus, alcoholism, fever
- Volume of ECF decreases, osmolality of ECF decreases
- Water will leave the ICF and move into ECF
- osmolalities of ECF and ICF equalize at a higher level
- volume of both compartments decreases

Hyposmotic Dehydration
- loss of solute in excess of water
- loss of NaCl from plasma; plasma osmolality decreases
- fluid moves from ECF to ICF
- volume of ICF increases
- osmolalites of ECF and ICF equalize at lower level