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The terms osmolality and osmolarity are easily confused by scholars. They have different meanings, and hence it is advisable for the scholars to take keen interest in understanding these two terms. It is evident that the two terms are derived from a biological term, osmosis. Osmolality can be referred to as the establishment of the concentration of urine depending on the atoms in it. It is measured in terms of milliosmoles/kilogram (mOsm/Kg) of the water. This is a term that is widely used in Kidney tests where urine is manufactured. Blood of the human beings is said to possess osmolality, and for an adult (normal), the osmolality should range between 28 –295 mOsm/kg of water. Osmolality can further be subdivided into two; Serum osmolality and Urine osmolarity.

Serum osmolality refers to the determination of the particles dissolved in the serum per kilogram of water. In a given solution, the less the solute particles in relation to the solvent, the lowly concentrated is the solution under scrutiny. The term low serum osmolality is used to refer to a serum that has fewer particles dissolved in it in relation to the solvent (level of water) and goes hand in hand with over hydration. A high serum osmolality indicates that there are more particles dissolved in the solvent i.e. there is a shortage of the solvent. Serum osmolality gives very crucial information regarding the hydration status around the body cells, since there must be harmony of the equilibrium between the membranes of the cell. It is paramount to note that serum osmolality indicates the condition of the hydration of the body. The standard value for this type of osmolality ranges between 270-300 mOsm/kg of water. Urine osmolality, on the other side, measures the particles dissolved in urine per Kilogram of water.

Renal disorders employ urine osmolality in their diagnosis and assessment of the hydration status. The standard value for this type of osmolality ranges between 500-800 mOsm/kg of water.

Osmolarity refers to the osmotic concentration of a given solution; it is objectively expressed as milliosmoles of solute for every liter of the given solution. Osmolarity measures the osmotic pressure itself. From the foregoing discussion, it is evident that the two terms are employed to discuss solution only that the takes different perspectives when it comes to analyzing each and every term keenly.  There is a great similarity between the two terms; they both employ the term osmoles (a mole of any undissolvable element).

Despite the aforementioned similarities, the terms exhibit notable differences. These differences range from the obvious differences to the implied ones; osmolality refers to the osmoles of dissolved particles in a kilogram of water while osmolarity refers to the size of the osmoles of the liquid in a liter of the solution. It is, therefore, evident from the definition that the two terms differ right from their definations. Osmolarity deals with osmotic pressure in a solution unlike, osmolality which deals with the measurement of the number of particles in a given solution/fluid. This difference gives the two terms different areas of applicability with the concept of osmolality being employed in the kidney issues (urine osmolality).  The manner in which the two units of measurements are expressed is different; osmolality measured in terms of milliosmoles/kilogram (mOsm/Kg) of the water while osmolarity is expressed as milliosmoles of solute for every liter of the given solution (mOsm/L). Unlike osmolarity osmolality is rather easy to compute.

Usually the units of expressing osmolarity are Osm/L, while Osm/kg is used as units of expressing osmolality. In measuring osmolality an osmometer is required as the instrument of measurement, and it uses the principle of freezing point dejection. In the measurement of the two, a gap exists which is commonly referred to as osmolar gap. Different units are used to present the values (calculated and measured), and this is mainly caused by the dissimilarity in measuring and calculation method. MO is the abbreviation used to represent measured osmolality, while the calculated osmolarity is denoted as CO. Both osmolality and osmolaroty re termed as equivalent based on their low concentration.

Tonicity is a term that is hard to avoid whenever the context at hand is medical. In fluid physiology, it is a term that is highly mistaken and can be defined from three perspectives; Effectiveness of osmolality, the test of the red cells and comparing it with plasma’s osmolality. It should be noted that all these different approaches of defining monotonicity do not exactly mean the same. The most relevant definition is the one that stresses the necessity of defining tonicity in reference to the membrane.  Objectively, tonicity can be defined as osmolar gradient across at any give time that is considered crucial across the membranes.

Basically there are three types of tonicity:-


This is a solution that is said to be highly concentrated; the tonicity of the crystalloids is higher compared to that of blood plasma. Whenever hypertonic crystalloid is being administered there occurs a shift of water molecules to the bloodstream emanating from extravascular space hence, increasing the volume of the intravascular. This shift, osmotic, occurs in the attempt of the body to neutralize the highly concentrated electrolytes in the IV fluid.


This is a solution that is said to be lowly concentrated; the tonicity of the crystalloids is lower compared to that of blood plasma. The administering of the crystalloid in this case causes a shift of water to the space in the extravascular region from the space in the intravascular space, and finally finds its ways into the cells in the tissues.


This is a solution that is said to be equally concentrated; the tonicity of the crystalloids is equal compared to that of blood plasma. A shift of water (occurs between the cells and blood vessels) is not caused when administration to a normal patient (hydrated) is carried out. Basically in an isotonic solution the process of osmosis does not take place.

According to the medical dictionary, an osmole is defined as the number of particles that dissolves in a solution to make up a mole of particles which are osmotically active. It is a term that is highly employed in the field of biochemistry and the medical field. In chemistry, the term is used to define the constituent moles in a compound (chemical). It is a very important unit of measuring osmotic pressure of a given solution .


This is a type of solute that dissolves in the solvent; it conducts electric current in the solution. Thais types of electrolytes can further be categorized into two: strong electrolytes and weak electrolytes.

In strong electrolytes, the solutes dissolve completely and hence the solution is the best in conducting electricity e.g. strong acids; Hydrochloric and Hydriodic. In weak electrolytes, on the other hand, the solutes do not dissolve completely but just a fraction. This solution can be referred to a semiconductor of electricity.

Example of electrolyte solute

NaCls                    Na+aq+Cl-aq

Colligative properties refer to the characteristics of a solution that depend on the molecules present not putting into consideration the kinds of molecules. These properties include; osmotic pressure, depression of the freezing point and elevation of the boiling point. Colligative properties were historically used to determine the molecular weight of a compound (unknown). It is worth noting that colligative property experiments exhibit different average molecular weights. There are four colligative properties to consider:-

  1. Vapor Pressure Lowering (Raoult’s Law)

Vapor pressure of the solvent is the amount of pressure exerted by the vapor in the head space. Raoult’s Law quantifies the amount of vapor pressure lowering observed.

Raoult’s Law:


Where             PA - Partial pressure of the solvent vapor with the solute

                      XA – Mole fraction of the solvent

                       POA–Vapor pressureof the pure solvent

  1. Boiling point Elevation

Boiling point refers to the point at which the pressure of the vapor in the liquid reaches 1 atm. It is said to be proportional to the concentration (molar) of solute particles.

  1. Freezing Point depression

This colligative property builds on the boiling point elevation. The addition of nonvolatile solute results to the solution having lower freezing points than the pure solvent. The freezing point depression has a very small effect, and the technique employed is insensitive.

Source: (Turco, 1980) p.p 46

The size of the freezing point depression is comparative to the number of solute elements and a relationship can be established with the Morality of the solution.

DTf = Kf ? M

 Where            D Tf = freezing point depression

                       Kf  = molar freezing point depression constant

                       M   = molality of the solution

  1. Osmotic Pressure

It appears to be a very convenient method of determining the average molecule weights of the polymer. Practical aspects of the polymer are considered by osmotic pressure.

Osmolality discrimination

In calculation of osmolality (serum) there arises the importance of calculating osmolality discriminant i.e. the variance of the actual (measured using osmometer) from the rated plasma osmolality (concentration of plasma’s ions summed with low molecule substances). Osmolality discriminant enables the approximation of the toxemy level and consequently the correct treatment is selected.

Description of the whole principle of 3WII Advanced osmometer

3WII Advanced osmometer provides quick, accurate, and conveniently obtained information that is needed to a wide range of body problems. This is a gadget used to determine the number average molecular weights of any particular solute whose Dalton counts 20000- 1000000. This gadget works on the principle that whenever a pure solvent in a solution is separated by a permeable membrane then a chemical potential difference is established. This chemical difference or chemical potential causes the solvent molecules to move from the region of higher concentration to the region of low concentration across the permeable membrane. This process continues until the system gets to equilibrium. When the solvent is contained in the fixed volume chamber this movement of the solvent molecules results to reduction of pressure in the solution or solvent chamber until it equates to osmotic pressure of the solution.

For instance, the UIC membrane osmometers have stainless cell that contain two membranes separated compartments, strain gauge and steel diaphragm and stable power supply. These types of osmometers make use of gauge detector whose main goal is to ensure high performance and versatility of the osmometer instruments. Lack of the familiar hydrostatic pressure addition  systems enables the gadget to be more compact and it enables the machine to   be able to give a narrow range of pressure (0-5 cmof water) required for molecular weight determination or even a wider range of pressure like ( 0-100 cm of water) convenient for oncotic  pressure. May be, just to give some simple description of the 3WII Advanced osmometer, it is 0.5% full scale in accuracy, 0.02 cm of water in stability, and its response time is 5-30  minutes.

3WII Advanced osmometer is very useful and finds its application in two major areas; it is used in the emergence room for:-

i.   alcohol intoxication: since ethanol increases the osmolality about 23 mosm/kg,

ii. Drug intoxication screening: even if glucose. BUN, and Na+ are normal, the serum osmolality would be increased.

iii. Head injury and shock: determination of I.V. therapy can be determined by serum or urine osmolality.

iv.  Coma: renal impairment, drug overdose, or diabetes mellitus may result from high serum osmolality.

The other area is that osmometer may be used as a diagnostic machine that reflects the result:-

 i. Renal malfunction Differential Diagnosis: that results in high osmolality.

 ii.  Hyper/hyponatremia Evaluation: that is with lipemic serum.

 iii. Diabetes Insipidus: urine and serum osmolality palys important role in that assessment.

 iv.  Inappropriate ADH Syndrome: osmolality is used whether it is neurogenic or nephrogenic.

 v.  Osmolal Discriminant Evaluation: to figure out which causes the abnormal osmolality whether Na+, urea, or glucose.

Blood osmolality

The blood osmolality can be defined as the concentration of all types of chemical particles found in the blood.  It is the amount of solute concentration per given unit of a given total volume of a certain solution. This concentration, therefore, causes osmotic pressure balances in the blood. Osmotic concentration can be defined simply as concentration of salt that produces certain osmotic pressure. This concentration of salts results from the process called osmoregulation. This is an active process that controls the osmotic pressure in the organism’s body fluids and hence controls the homeostasis   in the organism. This is to mean that the fluids in the organisms are prevented from being so much diluted or so much concentrated. Blood can be defined as the red-coloured fluid that circulates through the heart, arteries, capillaries and veins. This fluid carries with it oxygen and nutrients to all parts of the body tissues. It also carries away all metabolic wastes away from these tissues. Blood contains plasma, which is yellow, portended cellular element.

 The normal value of the blood osmolality ranges from 275 -295 mOsm/kg H2O. (Milliosmoles per kilogramof water). The blood osmolality can also be expressed in relation to osmoles. The value of the blood osmolality may vary from one laboratory to another .This may be because of using different measurement or because of the use of different  samples. However, the doctors should tell the meaning of specific sample test. The test for osmolality is done through a blood test, that is, a sample of the blood is taken.  A person whose blood is to undergo a test should not take any food material or any drugs especially Mannitol (a diuretic) that may interfere with the concentration of the chemical particles in the blood. The test for the blood osmolality is rather crucial as it helps to evaluate the body water content. Moreover, the doctor sometimes orders for this test to be done especially when a patient has a sign of hyponatremia, water loss, and poisoning by methanol and ethanol. Dehydration increases blood osmolality while over hydration reduces blood osmolality. The test can also be done to a person experiencing difficulties in producing urine. To a normally functioning body, increase in the osmolality stimulates releasing of ADH (antidiuretic hormone). This makes the kidney reabsorb more water resulting to more concentrated urine. The water that is reabsorbed by the kidney to the blood makes the blood osmolality fall or reduces to the normal. When the blood has low osmolality, the ADH (antidiuretic hormone) is suppressed and hence the kidney reabsorbs low volume of water. This condition makes the urine be diluted as it gets rid of the excess water content.

Clinical significance of the blood osmolality

Blood osmolality is of great importance and its significance in clinics is evident. In the clinics blood osmolalty is used to monitor the patient in different areas:

                                                                                                                       i.            Post-Operative: I.V. therapy and trauma of surgery

This is a condition that results from electrolyte/metabolite imbalance. Clinical officers are able to establish whether the patients under examination/diagnosis are suffering from this condition by applying blood osmolality.

                                                                                                                                                                            ii.Renal Dialysis:

In renal Dialysis serum osmolality is used and if an increase in serum osmolality is noted renal dialysis is carried out.

                                                                                                                                                                          iii.  ADH Therapy:

The response is measured quickly by measuring the blood osmolality.

                                                                                                                                                                        iv. Insulin Therapy:

Insulin therapy is part and parcel of life of the people with type 1 diabetes. The pancreases of such people fail to produce insulin. Blood osmolality is very significant when it comes to this type of therapy.                                                                                                                                                                    v.  Burn Victims:

In this case serum osmolality is different due to dehydration, medication absorption, and thirst mechanism.

Finally, it is worth to note that osmolality can be used for a quality assurance because it just needs a small volume to get a quick and accurate osmolality.

Regulation of body water volume “formal“ gibbs-duham equation

The amount of water content in the blood or in the body is very important. It ensures proper fluid balance in the body. The whole mechanism of control of the water content or fluid balance in the body is called human homeostasis. It is scientifically proven that the amount of fluid taken in must be equal to the amount of fluid taken out. The body must be maintained at euvolemia. This is the condition of a normal state of the fluid volume. Water is very crucial for human survival. In fact, human being can function for 4-6 weeks without food but can go only for a few days without water. The amount of water required by the body in different people may vary due to either physical exercises or the environment and humidity conditions one is exposed to. For instance, in the United States  of America (USA)  an adult male should take an average of 3.7 litters of water while an adult female should take  2.7 liters a day. This includes even the water contained in the food and other items that one may consume per day. On the other hand, people in sub-Saharan countries are expected to take even double of the amount taken by a person in the USA. This conclusion, therefore, out rules the human conception that one should take two littres of water per day. It is, therefore, conclusive to say that the   amount of water required by a person will depend on the environmental conditions one is exposed to.

 In the process of osmoregulation, Gibbs Duham tried to explain or rather establish the relation between the changes that may occur in the thermodynamic system. The gibbs-duham equation is a criterion for thermodynamic consistency and finds application in chemical equilibria and phase equilibrium. He was able to establish that the properties of the thermodynamics are actually not independent but they are related.

Contributions of different drinks to changing blood osmolality

It is of great importance to note that different drinks have different water content and contribution towards Solute level in the blood. For instance, in sports there is a lot of sweating and therefore loss of many minerals such as chloride calcium and magntisum. This condition if not checked can lead to dehydration of the runner and eventually collapse. The serum osmolality has great importance to clinical services. It is mainly used for two main purposes. One is to establish the osmolar gap while the other is to find out the hyponatraemie condition level. The urine osmolality has a basic importance during testing of renal concentration. In addition, the osmolality in the faecal assist in the diagnosing causes of diarrhea.  As mentioned earlier, the serum osmolality can be  used to establish the cause of the hyponatraemia. That is a patient   with serum sodium less than  1ess 30 mmol/l can also be identified to  have pseudohyponatraemia especially when the concentration of lipids and proteins is high. Moreover, a patient with high osmolality may  have reactive hyponatraemia mainly because of the excess solute  resulting to  low level of water in the cells. On the other hand, the osmolar gap is established through getting the osmolality by calculating and through measurement. The difference between the two results is the osmolality gap. The calculated osmolality can be obtained through the use of the following formulae:

             osmolality = 2 x serum sodium + serum glucose + serum urea

                                    (all the variables have to be in mmol/l

The expected or the normal difference between the calculated and the measured osmolality is supposed to be 10 mmol/l. When the value happens in excess of this expected difference, it may show that there is some existence of the exogenous agent. Presence of ethanol can lead to increased osmolality gap, however, existence of substances like methanol may not sufficiently lead to increased osmolality. When the amount of ethanol has been measured, then the calculated osmolality can be obtained through use of the following of the formulae:

Calculated osmolality = 2 x serum sodium + serum glucose + serum urea + 270 x ethanol

 (All variables have be in mmol/l except  ethanol which have be in mg/dl)

The faecal osmolality can be used to establish faecal osmolar gap. The calculated osmolality in this case must incorporate 2x (Na +k). If the established osmolality gap is more than 100 mmol/l,then the condition will be similar to that of the osmotic diarrhea. IF the established  difference between the calculated and the  measured osmolality  is  a normal faecal osmolality gap, then  it shows  that there is a secretory diarrhea  and can be conclusively be said that there might be damage  or irritation in gastro-intestinal mucosa. This experiment can be done from a faecal sample with a higher fluid content. Another  important use of the osmolality  can be established from the fact that the cell membrane is free permeable  to water,  and  the difference between the osmolality in the excellular and the intracellular fluids  is the same. This would simply mean that the intracellular osmolality can then be established from the plasma osmolality .This information become very important as the change in the osmolality of the intracellular fluids can be as a result of the changes of the osmolality of the excellular fluids. This change may have results to the problem in the normal functioning of the cells and the  volume.


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