The care of any seriously ill patient’s fluid and electrolyte balance is crucial. The most common biochemical profile is the measurement of serum sodium, potassium, urea, and creatinine, often with bicarbonate, and it provides a wealth of information on a patient’s fluid and electrolyte state, as well as renal function.
Body fluid compartments
Water is the most important component of the human body fluid compartments. A 70 kilogram ‘typical’ individual contains around 42 litres of water in total. This is made up of two-thirds intracellular fluid (28 L (ICF)) and one-third extracellular fluid (14 L (ECF)). Plasma (3.5 L) and interstitial fluid (1.5 L) are two subsets of the ECF (10.5 L). This is illustrated in the image below.
Intravascular, interstitial, and intracellular fluid compartments are the three major bodily fluid compartments. Capillaries transport fluid from the intravascular to the interstitial and intracellular compartments. The endothelial glycocalyx, endothelial cells, and the subendothelial cell matrix form the capillary “membrane,” which separates the capillary intravascular region from the interstitial fluid compartment. Water and small-molecular-weight particles like as electrolytes, glucose, acetate, lactate, gluconate, and bicarbonate pass effortlessly through this capillary “membrane.” Gases such as oxygen and carbon dioxide can freely pass through this membrane and enter or depart the intravascular compartment by following their concentration gradient.
The gap between the capillaries and the cells is known as the interstitial compartment. The matrix and cells within the interstitial space are supported by fluids. A cell membrane separates the intracellular compartment from the interstitial space. Water passes through this membrane freely, but minute and big molecular weight particles do not. Any particle movement between the interstitium and the cell must go through some sort of transport system (eg, channel, ion pump, carrier mechanism).
Fluids are constantly moving over the endothelial membrane of capillaries, across the interstitium, and into and out of cells. The amount of fluid that passes through the capillary “membrane” is determined by several factors, including capillary colloid oncotic pressure (COP), hydrostatic pressure, and permeability, which is determined by the endothelial glycocalyx layer (EGL) and the pore diameters between the cells. Proteins, predominantly albumin but also globulins, fibrinogen, and others, are the natural particles in blood that cause COP. The pressure exerted on the capillary membrane by blood pressure and cardiac output is known as hydrostatic pressure within the capillary. When intravascular hydrostatic pressure exceeds COP, membrane pore size rises, the EGL is disrupted, or intravascular COP falls below interstitial COP, fluid travels into the interstitial space. The EGL is now known to play a significant role in controlling fluid and other molecule (eg, albumin) transport across the capillary layer, with the oncotic pressure of the glycocalyx playing a larger role than the oncotic pressure of the interstitium; various disease processes and therapy (eg, IV fluid administration) can significantly disrupt the EGL, resulting in altered transcapillary movement.
A water tank model with a partition and an inlet and exit is a schematic approach of depicting fluid balance. Fluids received orally or by intravenous infusion are represented by the intake supply, whereas the urinary tract is generally represented by the output. Surface evaporation might be thought of as insensible loss.
Selective fluid loss from each of these compartments results in distinct signs and symptoms. Intracellular fluid loss, for example, results in cellular dysfunction, most notably lethargy, confusion, and coma. The loss of blood, an ECF fluid, causes circulatory collapse, renal failure, and shock. Loss of total body water will have a similar effect in the long run. However, the signs of fluid depletion are not immediately visible because the water loss, while significant, is distributed across both ECF and ICF compartments.
The water tank model depicts the relative volumes of each of these compartments and can be used to help visualize some clinical fluid and electrolyte balance disorders. It is critical to understand that determining the volume of body fluid compartments is not the responsibility of the biochemistry laboratory. Clinically, the patient’s state of hydration, i.e. the volume of the body fluid compartments, is evaluated. The term “dehydration” simply refers to fluid loss from body compartments.
Overhydration occurs when fluid builds up in the body fluid compartment.
When interpreting electrolyte results, this ‘biochemist’s picture’ may be helpful in visualizing what is wrong with the patient’s fluid balance and what needs to be done to correct it. The main characteristics of disordered hydration are depicted in the table below. Clinical evaluations of skin turgor, eyeball tension, and mucous membranes are not always accurate. Skin elasticity decreases with age, and oral mucous membranes may appear dry in patients who breathe through their mouths.
| Feature | Dehydration | Overhydration |
| Pulse | Increased | Normal |
| Blood pressure | Decreased | Normal or increased |
| Skin turgor | Decreased | Increased |
| Eyeballs | Soft/sunken | Normal |
| Mucous membranes | Dry | Normal |
| Urine output | Decreased | May be normal or decreased |
| Consciousness | Decreased | Decreased |
Electrolytes
Na+ is the main extracellular cation and the main intracellular cation is potassium (K+). Protein and phosphate are the most abundant anions within cells, whereas chloride (Cl) and bicarbonate (HCO3) predominate in the ECF.
A request for serum ‘electrolytes’ usually yields values for the concentrations of sodium and potassium ions, as well as bicarbonate ions. Because sodium ions are present in the highest concentration, they contribute the most to total plasma osmolality. Although potassium ion concentrations in the ECF are low in comparison to the high concentrations found inside cells, changes in plasma concentrations are significant and can be fatal.
Urea and creatinine concentrations are indicators of renal function, with higher concentrations indicating a lower glomerular filtration rate.
Concentration
Keep in mind that a concentration is a ratio of two variables: the amount of solute (for example, sodium) and the amount of water. A concentration can change as a result of changes in one or both variables. A sodium concentration of 140 mmol/L, for example, may fall to 130 mmol/L because the amount of sodium in the solution has decreased or the amount of water has increased.
Osmolality
The composition of body fluids varies greatly. While the concentration of substances varies in different body fluids, the overall number of solute particles, or osmolality, remains constant.
Semipermeable membranes allow water to freely flow between body compartments. Osmotic pressure must always be equal on both sides of a cell membrane, and water moves to maintain this osmolality, even if this water movement causes cells to shrink or expand in volume.
osmolality in different body compartments must be equal. This is achieved by the movement of
water across semipermeable membranes in response to concentration changes.
The ICF and ECF normally have the same osmolality. Isotonic solutions are stored in the two compartments.
A solution’s osmolality is measured in millimoles of solute per kilogram of solvent, which is usually water. Serum (and all other body fluids except urine) has an osmolality of around 285 mmol/kg in humans.
The osmolality of a serum or plasma sample can be measured directly or calculated if the concentrations of the major solutes are already known.
The serum osmolality can be calculated using a variety of formulas. Clinically, the simplest is:
This simple formula is only valid if the serum urea and glucose concentrations are within the reference intervals. If either or both of these values are abnormally high, the concentration of either or both (in mmol/L) must be added to the calculated osmolality. There is sometimes a noticeable difference between measured and calculated osmolality. This is referred to as the osmolal gap.
Oncotic pressure
The capillary membrane serves as a barrier between the intravascular and interstitial compartments. Because small molecules move freely through this membrane, they are not osmotically active across it.
In contrast, plasma proteins do not, and they exert a colloid osmotic pressure known as oncotic pressure (the protein concentration of interstitial fluid is much lower than that of blood). If the plasma protein concentration changes significantly, the balance of osmotic and hydrostatic forces across the capillary membrane may be disrupted.
Bibliography
- Anesthesia UK via https://www.frca.co.uk/article.aspx?articleid=289
- Msdvetmanual via https://www.msdvetmanual.com/emergency-medicine-and-critical-care/fluid-therapy/body-fluid-compartments-and-fluid-dynamics-in-animals
- Clinical Biochemistry : AN ILLUSTRATED COLOUR TEXT by Allan Gaw, Michael J Murphy, Rajeev Srivastava, Robert A Cowan and Denis St J O’Reilly
