Fluids and Electrolytes Demystified, Second Edition

Part I

Foundational Concepts and Assessments

chapter 1

Key Elements Underlying Fluid and Electrolyte Balance

LEARNING OBJECTIVES

At the end of this chapter, the student will be able to

   Describe the process of fluid and substance movement into and out of the cell.

   Contrast the regulatory mechanisms for maintaining fluid balance.

   Distinguish between characteristics of fluid balance and fluid imbalance.

   Contrast electrolyte balance and conditions of electrolyte imbalance.

   Discuss the process for determining the effectiveness of a treatment regimen in restoring fluid and electrolyte balance.

KEY TERMS

Anions

Cations

Diffusion

Electrolytes

Extracellular

Facilitative diffusion

Filtration

Homeostasis

Hypertonic

Hypervolemia

Hypotonic

Hypovolemia

Interstitial

Intracellular

Isotonic

Osmolality

Osmosis

Plasma

Tonicity

1. Overview

The human body is a miraculous machine. It functions, almost totally automatically, to produce energy and motion when supplied with essential fluid, nutrients, and oxygen. Through a delicate process of combining and breaking links between cations (positively charged molecules) and anions (negatively charged molecules), often referred to as electrolytes, chemical reactions are generated that release energy. This energy, in turn, results in mobility at the cellular level with active transport of electrolytes across membranes and tissue and organ mobility, such as a muscle fiber shortening and muscle contraction. This mobility proceeds to system activity, such as heartbeats that send blood throughout the body, and mobility of the entire body, such as in walking or running.

The most incredible mobility occurs at the cellular level when fluids and electrolytes are exchanged across membranes to maintain homeostasis, the balance in the body needed to sustain life. While some of these exchanges are passive and flow freely with little effort, other exchanges are active, energy-exhausting processes designed to maintain a critical balance of fluid and electrolytes on each side of the cell membrane and an environment that is appropriately charged with acids or bases to allow essential chemical reactions to occur. What is fluid balance? What are the electrolytes of life? This chapter will address these questions and other questions beginning with a basic overview of select anatomy and physiology of the human body.

2. The Cell

Cells are the basic unit of structure and function of life. Many organisms consist of just one single cell. This cell performs all the vital functions for that organism. On the other hand, many organisms are multicellular, including humans, whose bodies are composed of about 70 trillion cells in their own environment. Cells make up tissues, tissues form organs, and organs form organ systems, and these all interact in ways that keep this internal environment relatively constant despite an ever-changing outside environment. With very few exceptions, all body structures and functions work in ways that maintain life.

All cells are bounded by a plasma membrane. This membrane is selectively permeable—allowing certain things in and out while excluding others. Useful substances such as oxygen and nutrients enter through the membrane, whereas waste products such as carbon dioxide leave through it. These movements involve physical (passive) processes such as

•   Osmosis—water movement across a membrane from an area of low concentration to an area of high concentration

•   Diffusion—movement of molecules from an area of high concentration to an area of low concentration

•   Facilitative diffusion—movement of molecules from an area of high concentration to an area of low concentration using a carrier cell to accelerate diffusion

•   Filtration—selective allowance or blockage of substances across a membrane, wherein movement is influenced by a pressure gradient

The movement of substances across a membrane also includes physiologic (or active) processes such as

•   Active transport—molecules moving against a concentration gradient with the assistance of energy. Sodium and potassium differ greatly from the intracellular to the extracellular environment. To maintain the concentration difference, sodium and potassium move against the concentration gradient with the help of adenosine triphosphate (ATP), an energy source produced in the mitochondria of cells. This active transport process is referred to as the sodium–potassium pump. Calcium is also moved across the cell membrane through active transport.

•   Endocytosis—plasma membrane surrounds the substance being transported and takes the substances into the cell with the assistance of ATP.

•   Exocytosis—manufactured substances are packaged in secretory vesicles that fuse with the plasma membrane and are released outside the cell.

Figure 1–1 shows the relationship between the cell and its extracellular environment regarding transport of electrolytes across the cell membrane.

FIGURE 1–1 • The relationship between the cell and its extracellular environment regarding transport of electrolytes across the cell membrane.

Functionally, the membrane is active and living. Many metabolic activities take place on its surface, and it contains receptors that allow it to communicate with other cells and detect and respond to chemicals in its environment. Additionally, it serves as a conduit between the cell and the extracellular fluids in the body’s internal environment, thereby helping to maintain homeostasis. If we are to understand many aspects of physiology, it is important that we also understand the mechanism by which substances cross the cell membrane.

If cells are to survive and function normally, the fluid medium in which they live must be in equilibrium. Fluid and electrolyte balance, therefore, implies constancy or homeostasis. This means that the amount and distribution of body fluids and electrolytes are normal and constant. For homeostasis to be maintained, the fluids and electrolytes that enter (input) the body must be relatively equal to the amount that leaves (output). An imbalance of osmolality, the amount of force of solute per volume of solvent (measured in milliosmoles per kilogram—mOsm/kg or mmol/kg), of this medium can lead to serious disorders or even death. Fortunately, the body maintains homeostasis through a number of self-regulating systems, which include hormones, the nervous system, fluid–electrolyte balance, and acid–base systems.

3. Fluid

Water is a critical medium in the human body. The chemical reactions that fuel the body occur in the body fluids. Fluid is the major element in blood plasma that is used to transport nutrients, oxygen, and electrolytes throughout the body. Considering that the human body is composed of 50% (adult females) to 60% (adult males) to 75% (infants) fluids, it is easy to understand that fluid must play an important role in maintaining life. Fluid intake should approximately equal fluid output each day to maintain an overall balance.

Intake of fluids and solid foods that contain water accounts for nearly 90% of fluid intake. Cellular metabolism, which results in the production of hydrogen and oxygen combinations (H2O), accounts for the remaining 10% of water in the body (see Chapter 2). Fluid intake comes from the following sources (approximate percentages):

•   Fluid intake (50%)

•   Food intake (40%)

•   Metabolism (10%)

Solid foods are actually high in fluid content, for example

•   Lean meats—70% fluid

•   Fruits and vegetables—95% or more fluid

Excess fluid intake can result in overload for the heart and lungs and fluid deposits in tissues and extravascular spaces.

Fluid loss can occur from inadequate intake or from excessive loss from the body, most commonly from the kidneys. Fluid loss occurs from

•   Urine (58%)

•   Stool (7.5%)

•   Insensible loss

•   Lungs (11.5%)

•   Skin—sweat and evaporation (23%)

Excess loss through perspiration and respiration or through vomiting or diarrhea may severely reduce circulating volume and present a threat to tissue perfusion.

Fluid is contained in the body in several compartments separated by semipermeable membranes. The major compartments are

•   Intracellular—the area inside the cell membrane, containing 65% of body fluids

•   Extracellular—the area in the body that is outside the cell, containing 35% of body fluids

•   Tissues or interstitial area contains 25% of body fluids.

•   Blood plasma and lymph represents 8% of body fluids.

•   Blood plasma is contained in the intravascular spaces.

•   Transcellular fluid includes all other fluids and represents 2% of body fluids (eg, eye humors, spinal fluid, synovial fluid, and peritoneal, pericardial, pleural, and other fluids in the body).

Thus, most fluid is located inside the body cells (ie, intracellular), with the next highest amount being located in the spaces and tissues outside the blood vessels (ie, interstitial), and the smallest amount of fluid being located outside body cells in the fluid surrounding blood cells in the blood vessels (ie, plasma).

Intracellular fluid balance is regulated primarily through the permeability of the cell membrane. Cell membranes are selectively permeable, allowing ions and small molecules to pass through while keeping larger molecules inside, such as proteins that are synthesized inside the cell.

Some electrolytes are actively transported across the cell membrane to obtain a certain electric charge difference and a resulting reaction. Water moves across the cell membrane through the process of osmosis, flow from a lesser concentration of solutes to a greater concentration of solutes inside and outside the cell. If the extracellular (outside the cell) fluid has a high concentration of solutes, water will move from the cell out to the extracellular fluid, and conversely, if the concentration of solutes inside the cell is high, water will move into the cell. The ability of a solution to effect the flow of intracellular fluid is called tonicity.

•   Isotonic fluids have the same concentration of solutes as cells, and thus no fluid is drawn out or moves into the cell.

•   Hypertonic fluids have a higher concentration of solutes (hyperosmolality) than is found inside the cells, which causes fluid to flow out of the cells and into the extracellular spaces. This causes cells to shrink.

•   Hypotonic fluids have a lower concentration of solutes (hypo-osmolality) than is found inside the cells, which causes fluid to flow into cells and out of the extracellular spaces. This causes cells to swell and possibly burst.

Problems arise if insufficient water is present to maintain enough intracellular fluid for cells to function normally or if excessive water flows into a cell and causes a disruption in function and even cell rupture.

Extracellular fluid balance is maintained through closely regulated loss and retention to ensure that the total level of fluid in the body remains constant. Mechanisms are in place for regulation of water loss, such as secretion of antidiuretic hormone (ADH) to stimulation retention of water in urine, which helps to prevent excessive fluid elimination. The mechanism of thirst (also stimulated by ADH, as well as by blood pressure) is used to stimulate the ingestion of fluids and fluid-containing foods.

Fluid regulation depends on the sensing of the osmolality, or solute concentration, of the blood. As more water is retained in the body solutions, the osmolality is decreased and can result in hypo-osmolar fluid that has a lower amount of solute than water. When water is lost from the body, the osmolality of body fluids increases and can result in hyperosmolar fluid that has a higher amount of solute than water. The body responds to an increase in osmolality by stimulating the release of ADH, which causes the retention of fluid and lowers the osmolality of body fluids.

Fluid exerts a pressure on membranes (ie, hydrostatic pressure), and that pressure serves to drive fluid and some particles out through the membrane while others are held in. Solutes dissolved in fluid exert a pressure as well (ie, oncotic pressure) that pulls fluid toward it. Inside the blood vessels in the arterial system, fluid level is high, and the hydrostatic pressure drives fluid out into the interstitial area (along with nutrients and oxygen). In the venous system, on the other hand, the hydrostatic pressure is low and the osmotic pressure is high because solute (including red blood cells and protein molecules) is concentrated; thus fluid is drawn into the veins along with carbon dioxide and metabolic waste (Figure 1–2). The pressure of the volume and solutes in the blood vessels provides blood pressure needed to circulate blood for perfusion to the tissues.

FIGURE 1–2 • The relationship between hydrostatic pressure and osmotic pressure in the arterial and venous systems.

Fluid volume also plays a part in regulation of fluid levels in the body. Several mechanisms, in addition to ADH, respond to the sensation of low or high fluid volumes and osmolality. Neural mechanisms, through sensory receptors, sense low blood volume in the blood vessels and stimulate a sympathetic response resulting in constriction of the arterioles, which, in turn, result in a decrease in blood flow to the kidneys and decreased urine output, which retains fluid. The opposite response occurs when high blood volume is noted.

•   Arteriole dilation results in increased blood flow to the kidneys.

•   This results in increased urine output and fluid elimination from the body.

The renin–angiotensin–aldosterone mechanism also responds to changes in fluid volume:

•   If blood volume is low, a low blood pressure results.

•   Cells in the kidneys stimulate the release of renin.

•   This results in the conversion of angiotensinogen to angiotensin I.

•   This stimulates sodium reabsorption and results in water reabsorption.

An additional mechanism for regulating sodium reabsorption is the atrial natriuretic peptide (ANP) mechanism:

•   When an increase in fluid volume is noted in the atrium of the heart, ANP is secreted.

•   This decreases the absorption of sodium.

•   This results in sodium and water loss through urine.

When a decrease in volume is noted in the atria, ANP secretion is inhibited. Table 1–1 shows the relationship between fluid volume and renal perfusion.

TABLE 1–1 Relationship Between Fluid Volume and Renal Perfusion

Fluid volume regulation is necessary to maintain life. Decreased and inadequate fluid volume (ie, hypovolemia) can result in decreased flow and perfusion to the tissues. Increased or excessive fluid volume (ie, hypervolemia) can place stress on the heart and cause dilutional electrolyte imbalance. It is clear that the renal system plays a vital role in fluid management. If the kidneys are not functioning fully, fluid excretion and retention will not occur appropriately in response to fluid adjustment needs.

SPEED BUMP 1

4. Electrolytes

As stated earlier, electrolytes are electrically charged molecules or ions that are found inside and outside the cells of the body (intracellular or extracellular). These ions contribute to the concentration of body solutions and move between the intracellular and extracellular environments. Electrolytes are ingested in fluids and foods and are eliminated primarily through the kidneys, as well as through the liver, skin, and lungs. The regulation of electrolytes involves multiple body systems and is essential to maintaining homeostasis.

Electrolytes are measured in units called milliequivalents per liter (mEq/L) rather than in milligram weights because of their chemical properties as ions. The milliequivalent measures the electrochemical activity in relation to 1 mg of hydrogen. Another measure that may be used is the millimole, an atomic weight of an electrolyte. This measure is often equal to the milliequivalent but on some occasions may be a fraction of the milliequivalent measure. Care should be taken when interpreting the value of an electrolyte to ensure that the correct measure is being used and that the normal range for that electrolyte in that measure is known. For example, 3 mEq of an electrolyte cannot be evaluated using a normal range of 3 to 5 mmol/L because you might misinterpret the finding. You must use the normal range in milliequivalents for proper interpretation. Table 1–2 shows the approximate ranges for electrolytes in both milliequivalents and millimoles. These values may vary slightly from laboratory to laboratory, so consult the normal values established at your health care facility.

TABLE 1–2 Major Electrolytes, Their Functions, and Their Intracellular and Extracellular Concentrations

The major cation in extracellular fluid is sodium (Na+). Since sodium has a strong influence on osmotic pressure, it plays a major role in fluid regulation. As sodium is absorbed, water usually follows by osmosis. In fact, sodium levels are regulated more by fluid volume and the osmolality of body fluids than by the amount of sodium in the body. As stated earlier, ANH and aldosterone control fluid levels by directly influencing the reabsorption or excretion of