Overview of Fluid Balance

Fluid balance is the regulation of the body’s fluid compartments to maintain a stable internal environment. Fluid affects essential functions like cellular metabolism, temperature regulation, and the delivery of oxygen and nutrients to the cells. Any change in the volume or concentration of fluid can negatively affect these vital life processes.

Okay, so let’s start by reviewing some basics about fluid balance. First, fluid is found in two major compartments in the body: inside the cells, called intracellular fluid, and outside the cells, called extracellular fluid. The extracellular fluid compartment is further divided into the intravascular space, which is inside the blood vessels; the interstitial space, which is found between cells; and the transcellular space, where fluid, like pericardial, cerebrospinal, and synovial fluid, is contained in epithelial-lined spaces.

Now, the fluid in these compartments is mostly made up of water. In fact, over half of an adult's body weight and volume is water, which is called total body water. In addition, fluid is also made up of small and large solutes. Small solutes can easily cross cell membranes and are found in both intracellular and extracellular spaces. They include gases like oxygen and carbon dioxide, glucose, and electrolytes. On the other hand, large solutes can't easily cross cell membranes and are typically found in the extracellular space. These solutes include plasma proteins and blood components like red blood cells, platelets, and white blood cells.

Alright, fluid is in constant motion as it shifts in and out of cells and between fluid compartments through the processes of osmosis and filtration. First, osmosis is the diffusion of fluid across a semipermeable membrane from an area of low solute concentration, or osmolality, to an area of higher osmolality, until both sides are equalized. The force that pulls water from the area of low osmolality to high is called osmotic pressure. Now, when the osmotic pressure is created by the presence of proteins, like albumin, it's called colloid osmotic pressure or oncotic pressure. It's this oncotic pressure that keeps fluid in the intravascular space.

Next, there’s filtration, which is the movement of fluid that’s driven by the forces of oncotic pressure, along with hydrostatic pressure, which is the pressure exerted by fluid. To understand how this works, let’s zoom in on the capillaries. Now, on the arterial end, blood pressure creates hydrostatic pressure within the capillary that’s higher than the blood’s oncotic pressure, so fluid is pushed through the capillary wall and into the interstitial space. Then, at the venous end, the inward driving oncotic pressure inside the capillary is greater than the capillary’s hydrostatic pressure, so fluid moves back into the capillaries. In this way, fluid is filtered through the capillaries, so nutrients, wastes, and gases can be exchanged between the blood and the tissues. An imbalance between hydrostatic and oncotic pressures can result in edema.