Fluids: A look at fluids

Focus Topic: Fluids

Fluids. Where would we be without body fluids? Nowhere. Fluids are vital to all forms of life. They help maintain body temperature and cell shape, and they help transport nutrients, gases, and wastes. Let’s take a close look at fluids and the way the body balances them.

Fluids: Making gains = losses

Focus Topic: Fluids

The skin, the lungs, the kidneys — just about all major organs — work together to maintain a proper balance of fluid. To maintain proper balance, the amount of fluid gained throughout the day must equal the amount lost. Some of those losses can be measured (sensible losses); others can’t (insensible losses).

Fluid compartments



Fluids: Following the fluid

Focus Topic: Fluids

The body holds fluid in two basic areas, or compartments — inside the cells and outside the cells. Fluid found inside the cells is called intracellular fluid (ICF); fluid found outside them, extracellular fluid (ECF). Capillary walls and cell membranes separate the intracellular and extracellular compartments. (See Fluid compartments.) To maintain proper fluid balance, the distribution of fluid between the two compartments must remain relatively constant.

ECF can be broken down further into interstitial fluid, which surrounds the cells, and intravascular fluid, or plasma, which is the liquid portion of blood. In an adult, interstitial fluid accounts for about 75% of the ECF. Plasma accounts for the remaining 25%.

Fluids: A look at electrolytes

Focus Topic: Fluids

Electrolytes work with fluids to maintain health and well-being. They’re found in various concentrations, depending on whether they’re inside or outside the cells. (See Understanding electrolytes.) Electrolytes are crucial for nearly all cellular reactions and functions. Let’s take a look at what electrolytes are, how they function, and what upsets their balance.

Understanding electrolytes



Fluids: Anions and cations

Focus Topic: Fluids

Electrolytes are substances that, when in solution, separate (or dissociate) into electrically charged particles called ions. Some ions are positively charged; others, negatively charged. Anions are electrolytes that generate a negative charge; cations are electrolytes that produce a positive charge. An electrical charge makes cells function normally. Chloride, phosphorus, and bicarbonate are anions; sodium, potassium, calcium, and magnesium are cations.

Fluids: Electrolyte balance

Focus Topic: Fluids

Sodium and chloride, the major electrolytes in ECF, exert most of their effects outside the cell. Calcium and bicarbonate are two other electrolytes found in ECF. Potassium, phosphate, and magnesium are among the most abundant electrolytes inside the cell. Although electrolytes are concentrated in one compartment or another, they aren’t locked or frozen in these areas. Like fluids, electrolytes move about trying to maintain balance and electroneutrality.

Fluids: Fluid and electrolyte movement

Focus Topic: Fluids

Just as the heart beats constantly, fluids and solutes move constantly within the body. That movement allows the body to maintain homeostasis, the constant state of balance the body seeks.




Focus Topic: Fluids

Solutes within the body’s intracellular, interstitial, and intravascular compartments move through the membranes separating those compartments in different ways. The membranes are semipermeable, meaning that they allow some solutes to pass through, but not others. Fluids and solutes move through membranes at the cellular level by diffusion, active transport, and osmosis and through the capillaries by capillary filtration and reabsorption.

Fluids: Diffusion goes with the flow

Focus Topic: Fluids

In diffusion, solutes move from an area of higher concentration to an area of lower concentration, which eventually results in an equal distribution of solutes within the two areas. Diffusion is a form of passive transport because no energy is required to make it happen; it just happens. Like fish swimming downstream, the solutes simply go with the flow. (See Diffusion.)





Fluids: Actively transporting

Focus Topic: Fluids

In active transport, solutes move from an area of lower concentration to an area of higher concentration. Like fish swimming upstream, active transport requires energy to make it happen.

The energy required for a solute to move against a concentration gradient comes from a substance called adenosine triphosphate (ATP). Stored in all cells, ATP supplies energy for solute movement in and out of cells.

Some solutes, such as sodium and potassium, use ATP to move in and out of cells in a form of active transport called the sodium-potassium pump. With the help of this pump, sodium ions move from ICF (an area of lower concentration) to ECF (an area of higher concentration). With potassium, the reverse happens: A large amount of potassium in intracellular fluid causes an electrical potential at the cell membrane. As ions rapidly shift in and out of the cell, electrical impulses are conducted. These impulses are essential for maintaining life.

Other solutes that require active transport to cross cell membranes include calcium ions, hydrogen ions, amino acids, and certain sugars.

Fluids: Osmosis lets fluids through

Focus Topic: Fluids

Osmosis refers to the passive movement of fluid across a membrane from an area of lower solute concentration and comparatively more fluid into an area of higher solute concentration and comparatively less fluid. Osmosis stops when enough fluid has moved through the membrane to equalize the solute concentration on both sides of the membrane. (See Osmosis.)

Fluids: Boy, these walls are thin

Focus Topic: Fluids

Within the vascular system, only capillaries have walls thin enough to let solutes pass through. The movement of fluids and solutes through the walls of the body’s capillaries plays a critical role in fluid balance.

Fluids: The pressure is on

Focus Topic: Fluids

The movement of fluids through capillaries — a process called capillary filtration — results from blood pushing against the walls of the capillary. That pressure, called hydrostatic (or “fluid-pushing”) pressure, forces fluids and solutes through the capillary wall.

When the hydrostatic pressure inside a capillary is greater than the pressure in the surrounding interstitial space, fluids and solutes inside the capillary are forced out into the interstitial space. When the pressure inside the capillary is less than the pressure outside of it, fluids and solutes move back into the capillary.




Fluids: Keeping the fluid in

Focus Topic: Fluids

A process called reabsorption prevents too much fluid from leaving the capillaries no matter how much hydrostatic pressure exists within the capillaries. When fluid filters through a capillary, the protein albumin remains behind in the diminishing volume of water. Albumin is a large molecule that usually can’t pass through capillary membranes. As the concentration of albumin inside a capillary increases, fluid begins to move back into the capillaries through osmosis.

Think of albumin as a “water magnet.” The osmotic, or pulling, force of albumin in the intravascular space is referred to as the plasma colloid osmotic pressure. The plasma colloid osmotic pressure in capillaries averages about 25 mm Hg. (See Albumin.)

Fluids: You’re free to leave the capillaries

Focus Topic: Fluids

As long as capillary blood pressure (the hydrostatic pressure) exceeds plasma colloid osmotic pressure, water and solutes can leave the capillaries and enter the interstitial fluid. When capillary blood pressure falls below plasma colloid osmotic pressure, water and diffusible solutes return to the capillaries.

Normally, blood pressure in a capillary exceeds plasma colloid osmotic pressure in the arteriole end and falls below it in the venule end. As a result, capillary filtration occurs along the first half of the vessel; reabsorption, along the second half. As long as capillary blood pressure and plasma albumin levels remain normal, the amount of water that moves into the vessel equals the amount that moves out.

Occasionally, extra fluid filters out of the capillary. When that happens, the excess fluid shifts into the lymphatic vessels located just outside the capillaries and eventually returns to the heart for recirculation.