Homeostasis is the steady-state of the body by which the body's internal systems maintain a balance. It is maintained by adaptive responses that promote healthy functioning of the body.
One of the primary objectives of I.V. therapy is to maintain or restore the fluid and electrolyte balance in the body. Our bodies are composed largely of fluid. These fluids will account for about 60% of total body weight in an adult male and 45%-50% in an adult woman. In infants, fluids account for about 80% of total body weight and will steadily decrease throughout childhood until it reaches adult percentages at around 8.
Body fluids are composed of water and solutes. Solutes are classified as either electrolytes or nonelectrolytes. The nonelectrolytes are solutes without an electrical charge, including glucose, proteins, lipids, oxygen, carbon dioxide, and organic acids. The electrolytes will be discussed later.
- regulate the body's temperature
- transport nutrients and gases throughout the body
- carry cellular waste products to excretion sites
The body fluids are distributed between two major compartments: intracellular fluid is inside the cells, and extracellular is fluid outside of the cells.
The extracellular fluid occurs in two forms: interstitial fluid surrounding each cell and lymph gland: and intravascular fluid, which is blood plasma. The interstitial fluid comprises the cerebrospinal, pleural, peritoneal, or synovial fluids and the secretions from the salivary glands, pancreas, liver, and sweat glands. The distribution of fluids between these two compartments is constant when the body is healthy. The heart, kidneys, liver, adrenal and pituitary glands, and nervous system all play a part in maintaining fluid balance in the body.
If there is fluid accumulation in a compartment other than the intracellular or extracellular space, it is referred to as third-space fluid shifting. This accumulation happens when a cellular membrane allows water and fluid to enter but not exit. For example, fluids will pool in the burn site with severe burns, causing depletion of the intracellular fluid (I.C.F.) and Extracellular fluid (E.C.F.). If pancreatitis fluids "leak out" into the peritoneal cavity, this causes depletion in the I.C.F. and E.C.F. Patients undergoing long and extensive surgeries will collect third-space fluids and become intravascularly depleted despite the administration of large volumes of I.V. fluids and blood. Third-space fluid shifting can also occur due to acute bowel obstruction, ascites, and sepsis. These patients will experience tachycardia, hypotension, weight gain, low urine output, poor skin turgor, and hyponatremia.
Fluid balance is also affected by:
- fluid volume
- distribution of fluids in the body
- concentration of solutes in the fluid
The distribution of fluids depends on the hydrostatic and colloid osmotic pressures in the capillaries. Fluid volumes and concentration are regulated by interacting with two hormones, antidiuretic hormone (ADH) and aldosterone.
ADH is secreted when plasma osmolarity increases or circulating blood volume decreases, and blood pressure drop. ADH restores blood volume by reducing diuresis and increasing water retention.
Aldosterone is secreted when the serum sodium level is low, the potassium level is high, or the circulation volume of fluid decreases. It causes the kidneys to retain sodium and water.
Osmolarity is the concentration of a solution. Usually, the serum has the same osmolarity as other body fluids, approximately 300 mOsm/L. If your patient has a serum osmolarity lower, they may have a fluid overload. A higher serum osmolarity indicates the patient may be experiencing hemoconcentration of the fluid and dehydration.
Body fluids are in constant motion moving between the fluid compartments through membranes. Homeostasis is maintained when the solutes and fluids are distributed evenly on each membrane side. When there is an imbalance, these molecules will move between the compartments by various routes, including:
- In diffusion, molecules move from areas of higher concentration to areas of lower concentration
- Active transport requires energy for molecules to move from areas of lower
- concentration to areas of higher concentration. These molecules are moved by physiologic pumps (sodium-potassium pump)
- Passive transport, solutes are affected by the electrical potential across cell membranes
- Filtration is the movement of substances from an area of high hydrostatic pressure to an area of lower hydrostatic pressure
- Capillary filtration forces fluid and solutes through capillary wall pores
- Osmosis fluids flow passively from an area of higher water concentration to an area of lower concentration. The process stops when the solute concentrations on both sides of the membrane are equal. Responds to osmolality changes because of osmotic and hydrostatic pressures