Chapter 25; the body fluid compartments:
extracellular and intracellular fluids; edema
Renal function depends on the balance between fluid input and output.
Water intake through food and drinks is about 2100 ml/day and water
generated through metabolism of carbohydrates is about 200 ml/day. This
is a total of 2300 ml/day of water input.
Water loss that we are not consciously aware of is called insensible water
loss. This includes water lost by diffusion through skin (300-400 ml/day)
and via the respiratory tract (300-400 ml/day). This loss is increased in
cold weather due to a lower atmospheric vapor pressure.
Water lost through both sweat and feces is 100 ml/day each.
Water lost through urine varies depending on the condition and the
intake, but is usually around 1400 ml/day. This adds up to a total water
loss of 2300 ml/day.
The job of the kidneys is to match the output with the input.
Fluid compartments Percentage of body Weight
of average 70 kg weight
man
Total 60% 42 L
ICF 40% 28 L
ECF 20% 14 L
Interstitial fluid ¾ of ECF 11 L
Plasma ¼ of ECF 3L
In the ECF, fluid is continually exchanged between the plasma and
interstitial fluid, with the exception of proteins as they are too large to
pass through the capillary membrane. Due to this, the negatively charged
proteins tend to attract cations into the plasma and repel anions into the
interstitial fluid. However, their ionic concentrations are considered to be
almost equal.
ECF contains more sodium, chloride, and bicarbonate ions whereas ICF
contains more potassium, phosphate, magnesium and sulfate ions as well
as more proteins than ECF.
, The indicator dilution method is used to determine volume of fluid
compartments. Different substances are used for different compartments.
These substances need to disperse completely in their respective
compartments and not get metabolized or excreted in order for the
measurement to be accurate.
3
H2O, 2H2O and antipyrine are used to measure total body water.
22
Na, I-iothalamate, thiosulfate and inulin are used to measure ECF.
125
ICF = Total body water – ECF
I-albumin and Evans blue dye (T-1824) is used to measure plasma
125
volume as these substances do not pass through the capillary membrane
and so can be used to determine the volume of plasma.
Interstitial fluid volume = ECF – Plasma volume
51
Cr-labelled RBCs are used to measure blood volume.
The following formula is used in this principle:
Volume A ×Conc . A=Volume B ×Conc . B
Where volume and conc. A belong to the indicator BEFORE being injected.
Conc. B is the concentration after the indicator has dispersed in the
compartment. Using these 3 values, we can find the volume of the
compartment, volume B.
Osmolarity refers to the osmoles per liter of a solution. Units used for
solutes in the body are mOsm (milliosmoles).
The osmotic pressure exerted by 1 mOsm of a solute that the cell
membrane is IMPERMEABLE to is about 19.3 mmHg.
The normal osmolarity of plasma is 282 mOsm/L and for interstitial fluid
and ICF is 281 mOsm/L. This value is slightly higher in plasma due to the
osmotic effect of plasma proteins.
Therefore, the total osmotic pressure = 282 x 19.3 = 5441 mmHg in
plasma and 281 x 19.3= 5423 mmHg in ICF and interstitial fluid.
Solutions of 282 mOsm/L are isotonic, such as 0.9% NaCl or 5% glucose
solution. These solutions, which match the body’s tonicity, are
administered when needed so as not mess with the body’s natural
osmotic equilibrium. Solutions of osmolarities higher than this are
hypertonic and lower are hypotonic.
When talking about isotonic, hypertonic, or hypotonic solutions, the
solutes involved are ALWAYS impermeant (cannot pass through cell
membranes), whereas they can be permeant when using the terms
isosmotic, hyperosmotic and hypo-osmotic. An example of this is urea.
extracellular and intracellular fluids; edema
Renal function depends on the balance between fluid input and output.
Water intake through food and drinks is about 2100 ml/day and water
generated through metabolism of carbohydrates is about 200 ml/day. This
is a total of 2300 ml/day of water input.
Water loss that we are not consciously aware of is called insensible water
loss. This includes water lost by diffusion through skin (300-400 ml/day)
and via the respiratory tract (300-400 ml/day). This loss is increased in
cold weather due to a lower atmospheric vapor pressure.
Water lost through both sweat and feces is 100 ml/day each.
Water lost through urine varies depending on the condition and the
intake, but is usually around 1400 ml/day. This adds up to a total water
loss of 2300 ml/day.
The job of the kidneys is to match the output with the input.
Fluid compartments Percentage of body Weight
of average 70 kg weight
man
Total 60% 42 L
ICF 40% 28 L
ECF 20% 14 L
Interstitial fluid ¾ of ECF 11 L
Plasma ¼ of ECF 3L
In the ECF, fluid is continually exchanged between the plasma and
interstitial fluid, with the exception of proteins as they are too large to
pass through the capillary membrane. Due to this, the negatively charged
proteins tend to attract cations into the plasma and repel anions into the
interstitial fluid. However, their ionic concentrations are considered to be
almost equal.
ECF contains more sodium, chloride, and bicarbonate ions whereas ICF
contains more potassium, phosphate, magnesium and sulfate ions as well
as more proteins than ECF.
, The indicator dilution method is used to determine volume of fluid
compartments. Different substances are used for different compartments.
These substances need to disperse completely in their respective
compartments and not get metabolized or excreted in order for the
measurement to be accurate.
3
H2O, 2H2O and antipyrine are used to measure total body water.
22
Na, I-iothalamate, thiosulfate and inulin are used to measure ECF.
125
ICF = Total body water – ECF
I-albumin and Evans blue dye (T-1824) is used to measure plasma
125
volume as these substances do not pass through the capillary membrane
and so can be used to determine the volume of plasma.
Interstitial fluid volume = ECF – Plasma volume
51
Cr-labelled RBCs are used to measure blood volume.
The following formula is used in this principle:
Volume A ×Conc . A=Volume B ×Conc . B
Where volume and conc. A belong to the indicator BEFORE being injected.
Conc. B is the concentration after the indicator has dispersed in the
compartment. Using these 3 values, we can find the volume of the
compartment, volume B.
Osmolarity refers to the osmoles per liter of a solution. Units used for
solutes in the body are mOsm (milliosmoles).
The osmotic pressure exerted by 1 mOsm of a solute that the cell
membrane is IMPERMEABLE to is about 19.3 mmHg.
The normal osmolarity of plasma is 282 mOsm/L and for interstitial fluid
and ICF is 281 mOsm/L. This value is slightly higher in plasma due to the
osmotic effect of plasma proteins.
Therefore, the total osmotic pressure = 282 x 19.3 = 5441 mmHg in
plasma and 281 x 19.3= 5423 mmHg in ICF and interstitial fluid.
Solutions of 282 mOsm/L are isotonic, such as 0.9% NaCl or 5% glucose
solution. These solutions, which match the body’s tonicity, are
administered when needed so as not mess with the body’s natural
osmotic equilibrium. Solutions of osmolarities higher than this are
hypertonic and lower are hypotonic.
When talking about isotonic, hypertonic, or hypotonic solutions, the
solutes involved are ALWAYS impermeant (cannot pass through cell
membranes), whereas they can be permeant when using the terms
isosmotic, hyperosmotic and hypo-osmotic. An example of this is urea.
