Movement of water between body compartments

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Water is the key to life - It has very unique properties like being an amazing solvent, which means that it’s easy for solutes to dissolve into water.
As a result, water can carry essential nutrients to our cells as well as toxins or waste products away from our cells to be excreted out of our system.
Total body water can be subdivided into two major compartments, intracellular fluid (ICF) and extracellular fluid (ECF).
On average total body water in a person is about 60% of their body weight. From the total body water, 2/3 of that, or 40% of body weight is intracellular fluid.
The other 1/3 or 20% of body weight is extracellular fluid. This is also known as the 60-40-20 rule.
Intracellular fluid is the fluid which is inside the cell and extracellular fluid is the fluid outside of the cell. Extracellular fluid can be further subdivided into interstitial fluid, which is the fluid surrounding the cell and plasma which is the fluid that circulates within blood vessels.
Extracellular fluid is the first to be lost and makes up fluids like gut fluids, sweat and other secretions. The extracellular fluid is made up of different solutes, the major cation being sodium (Na+) and the major anions being chloride (Cl-) and bicarbonate (HCO3-).
Each compartment has a specific solute concentration measured in mOsm/L or osmolarity, which is the number of osmoles within a liter of solution.
Now remember that an osmole refers to the individual ions within a solution. So for example, NaCl splits apart in water to become Na+ and Cl-, so a solution of 1 mmol/L of NaCl is actually 2 mOsm/L.
Normally, osmolarity in the intracellular fluid and extracellular fluid is equal. If either side ever has a few more solutes, than water will flow in that direction to lower the concentration slightly and maintain the balance.
This process is called osmosis. Now, some solutes like NaHCO3 (sodium bicarbonate) as well as large sugars like mannitol, are too large to cross cellular membranes and they’re basically trapped in the extracellular fluid.
Usually, the ECF is more easily affected by physiologic processes, so we’ll look at the volume and concentration of the ECF.
In terms of volume, contraction means a decrease in ECF volume, and expansion means an increase in ECF volume. In terms of concentration, an isosmotic disturbance means that there’s no change in ECF osmolarity, a hyperosmotic disturbance means that there’s an increase in ECF osmolarity, and a hyposmotic disturbance means that there’s a decrease in ECF osmolarity.
We’ll go through six common scenarios to see how they affect the body’s fluid compartments, and we’ll use a two step approach for each scenario.
First, we’ll identify any changes in the ECF osmolarity like adding or removing solutes or gaining or losing water. Second, if there is a change in ECF osmolarity, we’ll figure out how water must shift to re-establish balance between ECF and ICF osmolarity.
The first situation is diarrhea. Someone with diarrhea will lose a lot of fluid and solutes from the gastrointestinal tract which ultimately comes from the ECF.
And the osmolarity of diarrhea is similar to that of the ECF. In other words, losing fluid in the form of diarrhea, means that there’s less ECF fluid, but it’s osmolarity is unchanged.
With no change in osmolarity, there will be no water shift and ICF volume stays the same. This means that there’s an isosmotic volume contraction.
Consequences include a decrease in plasma volume leading to a decrease in arterial pressure as well as an increased plasma protein concentration and an increased hematocrit, which is the portion of blood that’s occupied by red blood cells.
The second situation is a person running a marathon and losing a ton of sweat which contains both Na+ and Cl- ions and water.
Interestingly, sweat is actually hyposmotic relative to ECF so it contains relatively more water than solute. When a hyposmotic fluid is lost from the ECF, the volume of the ECF decreases, and the osmolarity increases.
When the osmolarity in the ECF rises compared to ICF, water moves by osmosis from the ICF into the ECF. Once steady state is established, volume decreases and osmolarity increases in both compartments.
This means that there’s a hyperosmotic volume contraction. Consequences include an increased plasma protein concentration due to loss of volume but an unchanged hematocrit, since the red blood cells lose volume as well.
The third situation is someone with adrenal insufficiency, where there’s a deficiency in several hormones including aldosterone.
Aldosterone plays a big role in Na+ reabsorption in the kidneys, so in adrenal insufficiency, excess Na+ is lost in the urine.
Since Na+, a major component of ECF is lost, the ECF osmolarity decreases. This causes water to shift from the ECF to the ICF by osmosis until both compartments have the same osmolarity.
Once steady state is established, both ECF and ICF osmolarities will be lower and the ECF volume decreases and ICF volume increases.
This means that there’s a hyposmotic volume contraction. Consequences include an increase in both plasma protein concentration and hematocrit due to loss of volume but also because water will also shift into the red blood cells.
The fourth situation is a person in the hospital getting an infusion of isotonic NaCl, perhaps after a bout of intense diarrhea.
Now, Na+ may leak across the cell membrane and pass from the ECF into the ICF but the Na/K ATPase pump actively pushes Na+ into the ECF and K+ into the ICF.
Chloride on the other hand stays around sodium so that the charges are balanced. So ultimately, all of the isotonic NaCl solution gets added only to the ECF.
This causes an increase in volume of the ECF but doesn’t change the osmolarity. Since the solution added is isotonic, it does not change the osmolarity so there will be no water shift.
This means that there’s an isosmotic volume expansion. Consequences include a decrease in both plasma protein concentration and hematocrit due to dilution.
The fifth situation is someone who eats a bag full of salty chips. The extra Na+ and Cl- ions in the body increases osmolarity of the ECF.
This causes water to move from the ICF to the ECF in order to raise osmolarity in the ICF. Once steady state is reached, both ECF and ICF osmolarities will be higher but the ECF volume will have increased and the ICF volume will have decreased.
This means that there’s a hyperosmotic volume expansion. Consequences include a decrease in both plasma protein concentration and hematocrit due to the increase in ECF volume.
Additionally, hematocrit will be further decreased from the water shift out of the red blood cells. The sixth situation is someone with syndrome of inappropriate antidiuretic hormone or SIADH.
In this condition, inappropriately high levels of antidiuretic hormone is released causing the kidney to increase water reabsorption.
When there are abnormally high levels of ADH, too much water is reabsorbed and the excess water is distributed throughout the total body water in a 1/3 ECF to 2/3 ICF ratio.
As a result, both ECF and ICF volume increases and that causes the osmolarities to decrease. This means that there’s a hyposmotic volume expansion.
Consequences include a decrease of the plasma protein concentration due to dilution and no change in hematocrit due to the concentration of the red blood cells decreasing from dilution but red blood cell volume increasing because water has moved into them.
Alright, as a quick recap, total body water can be broken down into 1/3 ECF and 2/3 ICF. Changes in solutes or water volume in these compartments causes a compensatory reaction where water flows in order to balance out both compartments until they have the same osmolarity and reach a new steady state.
As a consequence of water shifts, both plasma protein concentration and hematocrit will be affected.