Learning Objectives
By the end of this section, you will be able to:
- Explain how the components and movement of fluids within the body contribute to homeostasis
- Describe the role of the kidneys, lungs, and endocrine glands in homeostasis
In a healthy body, physiological processes exist to maintain a state of internal equilibrium. If too much of one chemical accumulates, the body can remove the excess chemicals and restore balance. This relatively stable internal state is called homeostasis.
The body must maintain appropriate balances of various fluids, electrolytes, pH, acids, and bases to maintain homeostasis. The body uses several mechanisms to compensate for natural imbalances, including those within the renal, respiratory, and endocrine systems. However, these compensatory mechanisms have limits, and the body can no longer compensate effectively in some conditions. In these cases, medical and nursing interventions may be required to assist the individual in re-establishing the body’s homeostasis. This chapter will focus on how the balance of fluid and electrolytes and acid-base balance contribute to the maintenance of homeostasis.
Fluids in the Body
The body’s balance of fluids, electrolytes, pH, acids, and bases are determined by chemistry. Nurses must familiarize themselves with several foundational concepts to treat patients with an imbalance. Specifically, nurses should understand the components and regulation of body fluids and the characteristics of the different homeostatic compensatory mechanisms in the body. With a solid foundational knowledge of these concepts, nurses will better understand the pathophysiologic changes occurring in the body and possess the skills to intervene and provide appropriate and timely patient care.
Composition of Fluids
To comprehend the medical significance of fluid imbalances, it is important first to have a general understanding of the overall fluid composition in the body. Body fluids consist of water, electrolytes, proteins, blood plasma and component cells, and other soluble particles, each of which is called a solute. These fluids are found in two main areas of the body: intracellular and extracellular compartments. Figure 10.2 illustrates these compartments. Table 10.1 describes the characteristics of both intracellular and extracellular fluid compartments. Note there are three subtypes, or spaces, of extracellular fluid.
| Compartment | Definition | Characteristics |
|---|---|---|
| Intracellular fluid (ICF) | Fluid found inside cells |
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| Extracellular fluid (ECF) | Fluid found outside of cells Three subtypes:
|
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Movement and Regulation of Body Fluid
The movement of body fluid between intra- and extracellular compartments is governed by several mechanisms, including oncotic pressure, hydrostatic pressure, and osmosis. The pressure created by colloid in a fluid is called oncotic pressure which prevents the movement of water from one solution to another. The force exerted by a fluid against a wall is called hydrostatic pressure. It results in the movement of fluid between compartments. For example, the hydrostatic pressure of blood is the pressure exerted by blood against the walls of the blood vessels by the heart’s pumping action. The pressure exerted by plasma proteins, such as albumin and globulinis called osmotic pressure. It acts as a pulling force to keep fluids inside the vessel. Hydrostatic pressure opposes osmotic pressure at the arterial end of capillaries, where it pushes fluid and solutes out into the interstitial compartment. On the venous end of the capillary, hydrostatic pressure is reduced, which allows oncotic pressure to pull fluids and solutes back into the capillary. Figure 10.3 illustrates these opposing pressures inside a capillary.
The movement of water through a semipermeable membrane is referred to as osmosis. The water moves from an area of lesser solute concentration to an area of greater solute concentration, thereby equalizing the concentration of the solute on either side of the membrane. Figure 10.4 illustrates osmosis, in which water has moved to the right side of the membrane to equalize the concentration of solutes on that side with the left side. Being familiar with the concept of osmosis is especially important for understanding how the body maintains its balance of electrolytes.
Routes of Gains and Losses
Fluids and electrolytes are gained and lost from the body through several different routes and mechanisms. Nearly all fluid and electrolyte gains occur through oral intake (e.g., eating and drinking). In a healthy adult, oral intake remains nearly equal to the body’s output to maintain overall homeostasis. With some medical conditions, however, the intake or output of fluid becomes excessive, resulting in an imbalance and disruption to homeostasis. The various routes of fluid output (losses) are listed in Table 10.2.
| Route | Mechanism |
|---|---|
| Urine output |
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| Perspiration |
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| Respiratory losses |
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| Gastrointestinal (GI) losses |
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Evaluating Laboratory Values
Evaluating laboratory values is one of the most important nursing interventions for patients at risk for fluid and electrolyte imbalances. The nurse should be mindful of trends and changes in electrolyte levels that may indicate the patient is developing an imbalance that may threaten homeostasis. This chapter discusses specific characteristics, assessment findings, and treatments for common fluid and electrolyte imbalances in more detail.
Homeostatic Mechanisms
The body has many internal, or feedback, mechanisms in place that will attempt to compensate for fluid, electrolyte, and acid-base imbalances. These mechanisms are housed within major organs, including the kidneys, heart, and lungs, as well as within glands, including the pituitary and adrenal glands. Though these mechanisms are usually effective at compensating for imbalances, certain conditions may cause them to become insufficient and require medical intervention.
Kidney and Adrenal Functions
The kidneys play a major role in maintaining fluid, electrolyte, and acid-base balances. Most importantly, these organs filter the body’s circulating fluids (5–6 liters in total) many times throughout the day, resulting in about 200 liters of daily fluid filtration. During this process, the kidneys actively remove excess toxins and other waste products for excretion as urine and promote the reabsorption of vital nutrients and electrolytes back into the bloodstream. In addition to filtering body fluid, the kidneys also actively maintain the body’s pH within a tight range by excreting or retaining hydrogen ions as needed. Under normal circumstances, this mechanism will result in an adequate acid-base balance in the body, which is vital for most normal physiological processes to occur.
Hydrostatic pressure is especially important in governing the movement of water in the kidneys’ nephrons to ensure proper blood filtering to form urine. As hydrostatic pressure in the kidneys increases, the amount of water leaving the capillaries also increases, and more urine filtrate is formed. If hydrostatic pressure in the kidneys drops too low, as can happen in dehydration, the functions of the kidneys will be impaired, and less wastes and fluid will be removed from the bloodstream. Any dysfunction or injury related to the kidneys can significantly affect the body’s ability to maintain adequate overall fluid, electrolyte, and acid-base balances. For example, the presence of kidney stones can cause blockages that result in severe dysfunction of the kidney and resulting fluid and electrolyte imbalances. This highlights the significance of early intervention and appropriate treatment for conditions that negatively affect the kidneys’ normal functions.
The adrenal glands, located on top of the kidneys, also play a major role in maintaining fluid and electrolyte balance. Specifically, these glands secrete aldosterone, a mineralocorticoid hormone that regulates water and sodium balance in the body. This hormone exerts its effects on the collecting ducts of the kidneys, where it promotes the retention of sodium and water in the bloodstream and increases the excretion of potassium as a waste product in the urine.
Renin-Angiotensin-Aldosterone System
When the kidneys detect low serum sodium levels or hypotension, they secrete a hormone called renin, which activates the renin-angiotensin-aldosterone system (RAAS) (Figure 10.5). This system is one of the most important compensatory mechanisms in the body, not only for fluid and electrolyte balance but also for the maintenance of adequate blood pressure and organ perfusion. The secreted renin acts on the inactive form of a peptide, known as angiotensinogen, released from the liver to create a hormone called angiotensin I. Angiotensin I is then transported to the lungs, where it interacts with angiotensin-converting enzyme (ACE) to synthesize angiotensin II, an active hormone that can exert its effects directly on blood vessels and the adrenal glands. When angiotensin II acts on blood vessels, it causes vasoconstriction, which increases blood pressure. When angiotensin II acts on the adrenal glands, it causes the release of aldosterone, which results in water and sodium retention and potassium excretion. Ultimately, the goals of the RAAS are to increase blood pressure, promote water and sodium retention, and excrete potassium. Thus, this compensatory mechanism plays a key role in maintaining the body’s blood pressure, fluid and electrolyte balance, and overall homeostasis.
Natriuretic Peptides
Acting in direct opposition to the RAAS are natriuretic peptides (NPs). NPs are stored in the heart tissue and released when the body detects high blood pressure and fluid overload. The goals of NPs are to cause vasodilation of blood vessels, which decreases blood pressure, and causes sodium to be excreted as a waste product in the urine. These opposing hormones are part of the endocrine system’s negative feedback loop, which allows the body to continuously maintain an appropriate balance of fluid and electrolytes, assuming the mechanisms are functioning optimally.
Cardiac and Lung Function
The heart and lungs are two other major organs that help the body maintain homeostasis. The heart is responsible for maintaining the systemic circulation of blood throughout the body, so any cardiac dysfunction can result in decreased tissue and organ perfusion. When decreased perfusion happens to the kidneys, the kidneys cannot effectively filter the blood, resulting in potentially serious fluid and electrolyte imbalances. The heart tissue also contains baroreceptors, specialized nerve cells that can detect the “stretch” (i.e., the level of vasoconstriction or dilation) of vessels, which reflects blood pressure. When these receptors detect a decrease in vasoconstriction (low blood pressure), the sympathetic nervous system is stimulated, resulting in increased heart rate and contractility to compensate for the low blood pressure. An example is seen in any shock state where blood vessels are dilated, causing hypotension, and the body responds with tachycardia in an effort to compensate.
The respiratory system plays a vital role in maintaining overall acid-base balance. While breathing in oxygen, the lungs will exhale carbon dioxide (CO2). Within the body, CO2, as a waste product of ventilation, becomes carbonic acid, which acts as an acid inside the body and so lowers blood pH. When the lungs are not functioning optimally, as seen with conditions such as chronic obstructive pulmonary disease (COPD) or an acute respiratory illness, CO2 is retained in the body, resulting in excessive acid levels, or acidosis. On the other hand, in situations where hyperventilation occurs, the patient may be exhaling too much CO2, causing buildup of excessive levels of bases in the body, known as alkalosis. See section 10.4 for more information about acid-base imbalances.
Life-Stage Context
Age-Related Changes That Affect Homeostasis
As people age, they may experience a decrease in the function of many major organs, including the heart, kidneys, and lungs. This suboptimal organ function places older adults at higher risk for fluid, electrolyte, and acid-base imbalances. Specifically, a decrease in cardiovascular contractility due to aging may cause decreased organ perfusion. When the decreased blood volume reaches the kidneys, it may result in an impaired ability of the kidneys to filter the blood and excrete waste products. This predisposes older adult patients to fluid, electrolyte, and acid-base imbalances that threaten overall homeostasis. In a healthy adult, these imbalances may have mild symptoms, but they can quickly become life-threatening in older adults if left untreated.
Children, on the other hand, are especially susceptible to fluid imbalances, specifically dehydration. This is related to their small stature and low body mass, in combination with their increased metabolism.
Pituitary Function
Another important controlling mechanism for maintaining fluid balance is housed in the brain, through the pituitary gland. It is controlled by signals sent to the gland from the hypothalamus, a structure in the brain that acts as a control center for the endocrine system.
The solute concentration of the blood, or blood osmolarity, may change in response to the consumption of certain foods and fluids, as well as in response to disease, injury, medications, or other factors. Blood osmolarity is constantly monitored by osmoreceptors, which are specialized cells in the hypothalamus that are particularly sensitive to the concentration of sodium ions and other solutes. In response to high blood osmolarity, which can occur during dehydration or following a very salty meal, osmoreceptors signal the posterior pituitary to release antidiuretic hormone (ADH), also known as vasopressin, with the goal of restoring blood osmolarity to normal levels (275–295 mOsm/kg) by retaining water in the body. Antidiuretic hormone acts on the kidneys, causing them to hold onto more water in the blood vessels and excrete less fluid as urine. With more water retention, the concentration of solutes is reduced, resulting in normalized osmolarity and better maintenance of homeostasis. Figure 10.6 illustrates the communication between the hypothalamus and pituitary gland that results in the release of ADH.