Introduction

What are we made of?

When we look at the composition of the human body, we see a remarkable balance between solid structures and fluids. As shown in this pie chart, about 40% of our body mass is made up of solids. These include bones, muscles, proteinsLarge molecules made of amino acids with various functions in the body., fats, and other structural components. They provide support and function to our tissues.
The remaining 60% is fluid, divided into three main compartments. The largest share, 33%, is found inside our cellsThe basic structural and functional units of life. as intracellular fluidThe fluid inside a cell, primarily composed of cytosol. (ICF). This fluid is essential for the cellular environment, supporting chemical reactions and maintaining cell structure. Interstitial fluid (IF) makes up 22%. It bathes and cushions our cells. Plasma, the liquid portion of blood, accounts for the smallest portion at 5%. These fluid compartmentsDivisions of body water: intracellular, extracellular, and transcellular. are regulated with care. This ensures that our cells stay hydrated. It also allows nutrients, wastes, and signals to move efficiently throughout the body.

Electrolyte Composition

The sodium-potassium pumpA transport protein that moves sodium out of the cell and potassium into the cell using ATP. (Na⁺/K⁺-ATPase) actively maintains the high intracellular potassium(K⁺): Major ICF cation; essential for muscle and nerve function. and low sodium(Na⁺): Major ECF cation; important for fluid balance, nerve function. concentrations, essential for cell function.

This bar graph highlights the distinct electrolyte makeup of the body’s three major fluid compartments. These compartments are intracellular fluid (ICF), plasmaThe liquid component of blood., and interstitial fluidThe fluid surrounding cells within tissues. (IF).
Starting with the ICF, the dominant ion is potassium (K⁺), seen here at a high concentration of 150 mg/dL. Potassium is essential for generating resting membrane potentials. It is actively transported into cells by the sodium-potassium pump. Also notable in the ICF is the high level of phosphate (HPO₄²⁻). Proteins also play a crucial role as important intracellular buffersSubstances that resist changes in pH.. They are structural components.
In contrast, both plasma and interstitial fluid—which make up the extracellular fluid—have high concentrations of sodium (Na⁺) and chloride (Cl⁻). Plasma has about 125 mg/dL of sodium, while interstitial fluid is slightly higher at 150 mg/dL. These ionsCharged atoms or molecules. help maintain fluid balanceThe maintenance of proper fluid volume and distribution in the body. and nerve signal transmission. Plasma has more proteins than interstitial fluid. Mainly albuminA plasma protein that helps maintain osmotic pressure and transport substances. helps create oncotic pressureThe force exerted by gases in the respiratory system, affecting airflow and gas exchange. to pull waterThe universal solvent essential for life. back into the bloodstream. Bicarbonate (HCO₃⁻) levels are similar in both plasma and IF. This similarity reflects their shared role in buffering The process of stabilizing pH by binding or releasing H⁺ ions. blood pH.
This compartmentalization of electrolytes is crucial for cell function. The selective permeability of membranes and active transport mechanisms maintain these gradients, which allow cells to communicate, generate energyThe capacity to do work or cause change., and respond to environmental changes

Fluid Compartments

Recall from our very first chapter that there are fluid compartments in the body. The intracellular fluid or the ICF is all the fluid contained in all your cells. It’s as if I took each cell and poured out all the contents into a glass. That is your ICF and it makes up about 40% of the fluids in your body. Your ECF or extracellular fluid(ECF) Fluid outside cells, including plasma and interstitial fluid. consists of all the other fluid in the body. This compartment has its own two sub-compartments. Extracellular fluid can be found in the plasma portion of whole blood and makes up about 4% of your fluids. That would be found inside the blood vessels. Extracellular fluid also contains the interstitial fluids, making up about 16% of the fluids in your body. These are the fluids that your cells are bathing in.

Your plasma exchanges with the outside world via your digestive system, respiratory systemThe organ system responsible for gas exchange (oxygen and carbon dioxide)., and urinary systemThe organ system that removes waste and maintains fluid balance.. The plasma then exchanges with your interstitial fluids. The interstitial fluids then exchange with the cells themselves. There are two fluid compartments. Substances like oxygen, carbon dioxide, sodium, and potassium must move through them to reach cells. Gradients have to be set up across the interstitial fluids for things to move. This compartment of interstitial fluids will be very important in this chapter.

Fluid Shifts

Now, the thing is that when your body manages osmolarity—which sodium is the primary contributor to—it uses fluid shifts. It doesn’t do it by adding or deleting electrolytes or solids. It actually does it by moving around water. A fluid shift is a rapid movementA fundamental property of life involving motion of the body or its parts. of water between compartments. A compartment could be plasma, ICF, or IF. So, if you drink a lot of water, you’ll flood your plasma with it. Much of it will then shift into your interstitial fluids. Officially, this is your body adjusting the volume in an effort to control osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments.. So, you have to remember that we control many of these solids by adjusting the volume. You can add water by drinking, or you can lower it by peeing water out.

E+ Electrolytes

In my general biology class, we talk about ions. Here in A&P, we talk about electrolytes. It is important to realize that you have electricity running through your body. A thought is really just the movement of electricity. Move your big toe. Electricity zapped through your spineProminent ridge on the posterior scapula dividing it into supraspinous and infraspinous fossae.. It traveled through your sciatic nerveThe largest nerve in the body, arising from the sacral plexus.. The message reached your toe muscles to contract. Your body’s electricity depends on ions present in your blood. It also relies on ions in your interstitial fluids and your cells. Therefore, we call salts electrolytes, which is a word that denotes the ability to conduct an electrical currentThe flow of electrical charge, as in ions moving across a neuron’s membrane.. When a salt enters your bloodstream from your small intestine, it immediately separates. It splits into its anionA negatively charged ion. and cation. When you ingested the salt at lunch, it immediately split into the sodium cation. It also split into the chloride anion when it hit your blood. This splitting is called dissociation. That is what you see in that flask on the right of the picture. The cations and anions are fully separated because NaCl is a very strong electrolyte. There are also weak electrolytes. As you can see in the picture weak electrolytes don’t dissociate completely in your blood. Anything that isn’t an electrolyte doesn’t dissociate. This molecule in the flask on the left could be an alcohol. It stays intact when you put it into water.

Osmolarity

Osmolarity refers to the number of osmoles of solute per liter of solutionA homogeneous mixture of two or more substances..​ It measures how many particles (osmoles) of solute are present in a given volume. The solution 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 calculates how many particles (osmoles) of solute are present in a given mass of the solvent. The measurement excludes the solute’s mass.​ 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 how many fruit pieces are present per kilogram of just the juice. You ignore the volume added by the fruit itself.

In clinical practice, osmolality is often preferred. It’s measured based on the mass of the solvent. This makes it less susceptible to changes in temperature and pressure. 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.