ECF Osmolarity

Osmolality

Osmolarity refers to the number of osmoles of solute per liter of solutionA homogeneous mixture of two or more substances..​Explanation: It measures how many particles (osmoles) of solute are present. The measurement considers a given volume of the entire solution, which includes both solute and solvent.​ Osmoles per liter (Osm/L) or milliosmoles per liter (mOsm/L).​

Osmolality refers to the number of osmoles of solute per kilogram of solvent.​ It determines the number of particles (osmoles) of solute in a specific mass of the solvent. The solute’s mass is excluded from this measurement. Osmoles per kilogram (Osm/kg) or milliosmoles per kilogram (mOsm/kg).​
Imagine you’re preparing a fruit punch. You’re interested in how many fruit pieces (solute) are present in each liter of the punch (solution).​ You focus on the number of fruit pieces per kilogram of just the juice. You ignore the volume added by the fruit itself.

In clinical practice, osmolality is often preferred. It is measured based on the mass of the solvent. This makes it less susceptible to changes in temperature and pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange.. This provides a more accurate assessment of a solution’s concentration, especially in physiological conditions.​ Osmolarity, being volume-based, can be influenced by environmental factors that affect volume, such as temperature fluctuations.

Increasing Plasma Osmolarity

Think of yourself eating a round of really salty foods. You’ve put more solute into your blood, increasing the osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments. of your plasmaThe liquid component of blood.. Although you’ve not changed the waterThe universal solvent essential for life. content of your body, you have changed the solute content and thus you osmolarity. There is also another way this could be accomplished. Even if you don’t eat salty foods, you could urinate very dilute urineThe liquid waste excreted by the kidneys.. This can happen under the influence of ADH or a diuretic. By reducing the solvent, you have increased the osmolarity.

Let’s talk about beer and pretzels for a moment. First off, alcohol turns off ADH, making you pee dilute urine. If you eat the nasty bowl of pretzels on the bar, you are affecting your plasma in two ways. You are increasing the solutes and decreasing your solvent.

Dehydration Fluid Shifts

When the body experiences dehydrationA condition in which fluid loss exceeds intake, leading to a decrease in total body water., the first area affected is the extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid. (ECF)—this includes both plasma and interstitial fluidThe fluid surrounding cells within tissues.. Water is lost through sweat, respirationThe process of gas exchange, including ventilation, external and internal respiration., urine, or diarrhea, leading to a decrease in ECF volume. As a result, the osmolarity of the ECF increases. This means there are now more solutes like sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. compared to the amount of water.
Because water moves by osmosis, it flows from areas of lower solute concentration to higher solute concentration. So, in response to the hypertonicA solution with a higher solute concentration than the inside of a cell, causing water to leave th ECF, water shifts out of the intracellular fluidThe fluid inside a cell, primarily composed of cytosol. (ICF) and into the ECF. This helps restore ECF volume temporarily. However, cellsThe basic structural and functional units of life. shrink as a result. Cellular function can become impaired, especially in sensitive tissues like the brain.
To compensate, the body activates hormonal mechanisms. ADH (antidiuretic hormoneADH A hormone that increases water reabsorption in the kidneys to concentrate urine and reduce water) is released to increase water reabsorptionThe process of fluid moving back into capillaries from surrounding tissues due to colloid osmotic p in the kidneys, producing concentrated urine. Thirst is stimulated by osmoreceptorsHypothalamic cells detecting blood osmolarity to regulate ADH. in the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions., prompting the person to drink water. Over time, restoring fluid intake and retaining water helps rebalance fluid compartmentsDivisions of body water: intracellular, extracellular, and transcellular. and lower osmolarity

Consequences of Increased ECF Volume

Thirst can lower your ECF osmolarity, but it also raises your blood volume. We’ve avoided blood volume as a factor for osmolarity, but there are consequences for having too much fluid volume. OK, so you drink some water to lower that osmolarity. Immediately, the kidney stops reabsorbing so much sodium. More sodium is lost in the urine, decreasing your blood’s osmolarity but increasing the osmolarity of your urine. This means that your urine is going to be more concentrated, or have more solute in it. This would increase the color and also increase that specific gravityA measure of urine concentration that compares the density of urine to water. of the urine.

Decreasing Plasma Osmolarity

Your body regulates sodium concentration among your fluid compartments. It does this to maintain the osmolarity of extracellular fluids. If your osmolarity is high, you could always drink water to decrease it. However, thirst is not so simple. It is an emotion, much as hunger is. People avoid and ignore the plea of thirst.

Thirst is triggered easily by having low blood volume and/or low saliva. Baroreceptors in the aorta and carotid artery are also capable of sensing when the blood volume is low. There are also osmoreceptors in the hypothalamus that are monitoring the plasma osmolarity. If the plasma osmolarity goes too high, thirst will be stimulated. If your blood gets too salty, the hormone ADH is also triggered. The easiest way to raise your blood volume is to add more water. Thirst does that, if you respond to the emotion.

Hypotonic Hydration

Hypotonic hydration, also called water intoxication, happens when you ingest or retain excess water. This dilutes body fluids and lowers plasma osmolarity. This creates an osmotic imbalance between the extracellular fluid (ECF)—which includes the interstitial fluid (IF)—and the intracellular fluid (ICF) compartments. Since the ECF becomes hypotonic relative to the ICF, water moves into the cells because it follows the concentration gradientA difference in the concentration of a substance across a space.. This causes the cells to swell.
In terms of fluid compartments: the interstitial fluid (IF) becomes more dilute first, as it is part of the ECF. This reduction in osmolarity causes water to move into cells, expanding the intracellular fluid (ICF) volume. While cells in most tissues may tolerate this swelling to some degree, brain cells are especially sensitive. The swelling of neuronsThe functional cells of the nervous system that transmit signals. can lead to increased intracranial pressure. This can result in symptomsSubjective experiences reported by the patient (e.g., nausea, fatigue). like confusion, nausea, or seizures. In severe cases, it may even lead to coma. Thus, hypotonic hydrationExcess water intake dilutes ECF, causing water to move into cells (ICF), leading to swelling. occurs when water shifts from the IF to the ICF. This is due to an osmotic gradient caused by diluted plasma.