Abnormalities of Salt and Body Water
The differences between the control of sodium and water balance
The concept of body water balance with intake of water being needed
to replace losses
Why osmoregulation is important
How in conditions of water deficit antidiuretic hormone (ADH arginine
vasopressin AVP) acts on the distal convoluted tubule and collecting
tubules & ducts to produce a concentrated urine
How in conditions of water surfeit the kidneys produce a hypotonic
urine
The main factors that control ADH secretion from the posterior lobe of
the pituitary
The major factors involved in the control of thirst (hypothalamic
osmoreceptor stimulation, angiotensin II, oropharyngeal and
gastrointestinal receptors)
The definition of the terms hyponatraemia and hypernatraemia
We are about 60% water by weight, that body water is divided between what is inside
the cells (intracellular, ICFV) which is 2/3rds and extracellular (ECFV) which is 1/3rd.
The extracellular compartment is divided between what is in the blood (the plasma)
and what is bathing the cells - the interstitial volume.
This could be summarised by the 60-40-20 rule.
For a 70kg male, he would be 60% water by weight so his total body water would be
42L. Of this, about 2/3rds will be intracellular (28L) and about 1/3 rd extracellular
(14L).
The balance of water between the two, it is essentially osmosis that dictates this. The
EC compartment can be divided though between plasma and interstitial volume and
here Starling’s forces will play an important role in balance between the two.
, Of the extracellular fluid volume, the majority of it will be in the interstitial
compartment and less than 10% will be in the circulation.
This has clinical significance because the only compartment we have easy access to
intravenously is the plasma volume, so is this representative of what is going on in the
rest of the body in terms of composition of fluids? The answer is actually yes!
The barriers that separate the compartments are very important, the barrier
separating the intracellular and extracellular compartment is the cell membrane.
The cell membrane is essentially impermeable to solutes and electrolytes, unless there
are special channels for them.
The barrier between the plasma and interstitial compartment is very different, it is the
capillary wall/endothelial cells which is quite permeable to solutes with the exception
of plasma proteins.
For this reason the composition of a plasma sample will be very similar to that of the
interstitial volume, but the intracellular will be different.
K+ is the dominant cation intracellularly while Na+ extracellularly.
Abnormalities in Water Balance
To understand how abnormalities of water balance arise, it is important to understand
the effect of changes in water intake and excretion on:
-The osmolality of the ECF
-The plasma Na+ concentration
Why is there a link between water a balance and plasma Na +?
The differences between the control of sodium and water balance
The concept of body water balance with intake of water being needed
to replace losses
Why osmoregulation is important
How in conditions of water deficit antidiuretic hormone (ADH arginine
vasopressin AVP) acts on the distal convoluted tubule and collecting
tubules & ducts to produce a concentrated urine
How in conditions of water surfeit the kidneys produce a hypotonic
urine
The main factors that control ADH secretion from the posterior lobe of
the pituitary
The major factors involved in the control of thirst (hypothalamic
osmoreceptor stimulation, angiotensin II, oropharyngeal and
gastrointestinal receptors)
The definition of the terms hyponatraemia and hypernatraemia
We are about 60% water by weight, that body water is divided between what is inside
the cells (intracellular, ICFV) which is 2/3rds and extracellular (ECFV) which is 1/3rd.
The extracellular compartment is divided between what is in the blood (the plasma)
and what is bathing the cells - the interstitial volume.
This could be summarised by the 60-40-20 rule.
For a 70kg male, he would be 60% water by weight so his total body water would be
42L. Of this, about 2/3rds will be intracellular (28L) and about 1/3 rd extracellular
(14L).
The balance of water between the two, it is essentially osmosis that dictates this. The
EC compartment can be divided though between plasma and interstitial volume and
here Starling’s forces will play an important role in balance between the two.
, Of the extracellular fluid volume, the majority of it will be in the interstitial
compartment and less than 10% will be in the circulation.
This has clinical significance because the only compartment we have easy access to
intravenously is the plasma volume, so is this representative of what is going on in the
rest of the body in terms of composition of fluids? The answer is actually yes!
The barriers that separate the compartments are very important, the barrier
separating the intracellular and extracellular compartment is the cell membrane.
The cell membrane is essentially impermeable to solutes and electrolytes, unless there
are special channels for them.
The barrier between the plasma and interstitial compartment is very different, it is the
capillary wall/endothelial cells which is quite permeable to solutes with the exception
of plasma proteins.
For this reason the composition of a plasma sample will be very similar to that of the
interstitial volume, but the intracellular will be different.
K+ is the dominant cation intracellularly while Na+ extracellularly.
Abnormalities in Water Balance
To understand how abnormalities of water balance arise, it is important to understand
the effect of changes in water intake and excretion on:
-The osmolality of the ECF
-The plasma Na+ concentration
Why is there a link between water a balance and plasma Na +?
