Fluid balance - Overview: Nursing

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.Alright, now let’s move on to how fluid balance is maintained.

Ideally, intake, or the fluid that enters the body, should be balanced with the output, or fluid that leaves the body. Most of the body’s fluid intake comes from drinking fluids, along with the fluid contained in foods, while output is accomplished through urine, feces, sweat, and insensible loss through lungs and skin.Now, fluid balance is monitored through osmoreceptors in the hypothalamus.

When the osmoreceptors detect increased plasma osmolality or decreased plasma volume, the posterior pituitary gland is stimulated to release antidiuretic hormone, or ADH, which triggers the kidneys to retain fluid, so fluid volume increases and osmolality decreases.

Increased osmolality also stimulates the thirst mechanism in the hypothalamus, prompting the desire to drink.Next, fluid volume, along with blood pressure, is regulated by the renin-angiotensin-aldosterone system, or RAAS.

Now, when blood pressure falls, the kidneys release the enzyme renin, which converts angiotensinogen into angiotensin I.

Angiotensin I is then converted into angiotensin II by angiotensin-converting enzyme, or ACE for short. Angiotensin II is a potent vasoconstrictor that causes the muscular walls of small arteries to constrict, increasing blood pressure.

It also triggers the release of the hormone aldosterone from the adrenal cortex, as well as ADH from the posterior pituitary gland, prompting the kidneys to retain sodium and excrete potassium.

The increased sodium causes water to be retained, thereby increasing blood volume and blood pressure.Alright, as a quick recap...Fluid balance is the regulation of the body’s fluid compartments to maintain a stable internal environment.

In order to maintain this balance, fluid is in constant motion as it shifts in and out of cells and between fluid compartments through the processes of osmosis and filtration.

At the same time, the body regulates fluid balance through ADH, the RAAS, and through the thirst mechanism.