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Pharmacology for Nurses

4.1 What Is Homeostasis?

Pharmacology for Nurses4.1 What Is Homeostasis?

Learning Outcomes

By the end of this section, you should be able to:

  • 4.1.1 Define homeostasis.
  • 4.1.2 Compare and contrast intracellular fluid and extracellular fluid and their effects on the body’s cells.
  • 4.1.3 Discuss the major cations and anions and their essential functions.

Homeostasis is the ability of the body to maintain a stable and constant internal environment despite changes in the external environment (Billman, 2020). This means that the body can regulate and balance its various physiologic processes, such as body temperature (see Figure 4.2), fluid balance, pH levels, blood sugar levels, and hormone levels, to ensure they remain within a narrow range that is optimal for the body’s functioning.

A flow chart shows how normal body temperature is maintained. If the body temperature rises (the cycle depicted on the right of the chart), blood vessels dilate, resulting in loss of heat to the environment. Sweat glands secrete fluid. As this fluid evaporates, heat is lost form the body. As a result, the body temperature falls to normal body temperature. If body temperature falls (the cycle depicted on the left), blood vessels constrict so that heat is conserved. Sweat glands do not secrete fluid. Shivering (involuntary contraction of muscles) releases heat which warms the body. Heat is retained, and body temperature increases to normal.
Figure 4.2 The human body is maintained by complex mechanisms that keep it in balance to promote physiologic function. (credit: modification of work from Biology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)


Homeostasis is a fundamental process that enables the body to maintain a stable internal environment in the face of constantly changing internal and external conditions. Disruption of homeostatic mechanisms may cause diseases and severe impacts on physiologic well-being. Two key principles govern homeostasis: fluid balance and electrolyte balance.

The first principle of fluid balance involves maintaining the body’s fluid compartments in osmotic equilibrium, except for transient change. This means that the concentration of solutes in each compartment is carefully regulated to prevent water from flowing into or out of cells, which could disrupt cellular function (Koeppen & Stanton, 2023; Valls & Esposito, 2022).

The second principle of electrolyte balance involves ensuring that the numbers of ions (anions or cations) within each compartment of the body are balanced and electrically neutral. Each compartment works to maintain a constant volume of fluid and to replace and exchange ions to maintain this neutrality (Koeppen & Stanton, 2023; Valls & Esposito, 2022). This helps ensure that the body’s pH remains within a narrow range, which is essential for proper physiologic functioning.

Special Considerations

Homeostasis in Children, Pregnant Clients, and Older Adults

Children have unique physiologic characteristics and developmental needs. Children have a relatively higher water requirement compared with adults because of their higher metabolic rate and larger proportion of body water. Children are also more likely than older individuals to become dehydrated because their kidneys are less efficient at conserving water. Electrolyte balance in children may be more susceptible to imbalances because of their immature kidneys, resulting in higher renal losses of sodium and potassium.

Pregnancy brings about significant physiologic changes that impact homeostatic mechanisms. Pregnancy increases blood volume and fluid retention to support the developing fetus. Hormonal changes influence fluid balance, causing the client to be more susceptible to edema and electrolyte imbalances. Increased levels of progesterone can also affect electrolyte regulation. Changes in kidney function and the demands of the developing fetus may impact the delicate balance of sodium, potassium, and calcium.

As individuals age, various physiologic changes occur. Older adults often have a reduced sense of thirst and may not drink enough fluids, leading to an increased risk for dehydration. Age-related changes in the kidneys can also affect water and electrolyte regulation, resulting in a higher risk for imbalances such as hyponatremia (low sodium) and hyperkalemia (elevated potassium).

Homeostasis is a complex mechanism that requires an integrated control system for self-regulation. It is primarily governed by a feedback loop that involves three key components: the receptor, the control center, and the effector (Figure 4.3).

  • The receptor is a sensory organ that detects changes in the internal or external environment of the body. It sends signals to the control center when it detects a change that needs to be corrected.
  • The control center is typically located in the brain or other part of the nervous system. It receives the signals from the receptor and processes them to determine the appropriate response. The control center then signals the effector.
  • The effector is the part of the body that carries out the response. It could be a muscle, gland, or organ that alters its activity to counteract the initial change detected by the receptor.
A multi-colored feedback loop indicates how the receptor (orange circular arrow) signals the control center (green circular arrow), which then signals the effector (blue circle at end).
Figure 4.3 Feedback loops involve a receptor to a stimulus, the control center, and the effector that carries out the control center’s response. (credit: reproduced with permission of Tina D. Barbour-Taylor)

The feedback loop plays a critical role in maintaining a balanced internal environment in the face of changing internal and external conditions (Hannezo & Heisenberg, 2019; Molnar & Gair, n.d.). By detecting and correcting deviations from the set point, the body is able to ensure that its various physiologic processes are functioning optimally, thereby promoting overall health and well-being.

Cellular Compartments

Sixty percent of the total body weight is composed of water. The body separates this water into two main fluid compartments: one that contains intracellular fluid and one that contains extracellular fluid. The fluid compartments work together to maintain fluid and electrolyte balance within the body. The movement of water and electrolytes between the compartments is regulated by various mechanisms, such as osmosis and active transport (Libretti & Puckett, 2023; Molnar & Gair, n.d).

The body strives to maintain a balance between the fluids in the compartment to ensure that the cells are surrounded by an environment that allows them to function optimally, as can be observed in Figure 4.4. The concentration of ions, such as sodium, potassium, and chloride, is carefully regulated in each compartment to maintain the proper osmotic pressure, which is the pressure needed to prevent water from flowing into or out of cells.

Disruptions in the fluid compartments, such as excessive fluid loss or retention, can lead to various health problems, including dehydration or edema. By regulating the equilibrium of body fluids, the body can guarantee the optimal operation of its diverse physiologic functions, thereby enhancing overall health and well-being.

Different nutrients, such as glycolipids and glycoproteins (depicted as green and blue rope-like strands), can permeate the cell membrane in a lipid bilayer (outlined in red with yellow inserts)  to maintain homeostasis between intracellular and extracellular fluid compartments.
Figure 4.4 The cell membrane is a semipermeable membrane that allows for the homeostasis of intracellular and extracellular fluid compartments. (credit: modification of work from Anatomy and Physiology 2e. attribution: Copyright Rice University, OpenStax, under CC BY 4.0 license)

Intracellular Fluid

Intracellular fluid, also known as cytoplasm, refers to the fluid contained within the cells of the body. As shown in Table 4.1, intracellular fluid accounts for approximately 40% of the total body fluid in adults, and it plays a critical role in many physiologic processes.

The composition of intracellular fluid is tightly regulated to ensure that it provides an optimal environment for cellular function. It contains various electrolytes, such as potassium, magnesium, and phosphate ions, as well as proteins and other molecules essential for cell function.

Extracellular Fluid

Extracellular fluid is the fluid that surrounds the cells of the body, including fluid in the blood vessels and the fluid in the spaces between tissues and organs. It accounts for approximately 20% of the total body fluid in adults.

Extracellular fluid contains various electrolytes, such as sodium, chloride, and bicarbonate, as well as proteins, hormones, and other molecules.

The extracellular fluid is divided into three subcompartments. The interstitial compartment, which surrounds the tissue cells, makes up approximately 15% of fluid volume; the intravascular compartment, which contains the plasma and blood, makes up approximately 5% of fluid volume; and the transcellular compartment makes up approximately 1% of fluid but is generally not included in fluid volume calculations.

Transcellular Compartment (Third Space)

The transcellular compartment, also known as the third space, refers to a small volume of fluid that is contained within certain body cavities and structures, such as the pleural cavity, peritoneal cavity, and joint spaces. This compartment is separate from the intracellular and extracellular fluid compartments and is characterized by its limited communication with the rest of the body. The volume of fluid in the transcellular compartment is regulated by various mechanisms, including pressure and transport mechanisms, which ensure that it is in balance with the other fluid compartments. Table 4.1 lists fluid compartments and their volumes and major electrolytes for an adult.

Fluid Compartment Fluid Volume Electrolytes
Major Anions Major Cations
Intracellular fluid ~40% Phosphate Magnesium
Extracellular fluid
  • Interstitial fluid
  • Intravascular fluid
  • Transcellular fluid
~20% total (of the below areas)
  • 15%
  • 5%
  • 1%
Table 4.1 Adult Body Fluid Volumes and Electrolytes


An ion is an atom or molecule that has an unequal number of protons and electrons. This inequality gives the ion an electrical charge, either positive or negative. Ions are critical to maintaining homeostasis within the body.


An anion is a negatively charged ion, meaning it has more electrons than protons. This happens when an atom gains one or more electrons, leaving it with a negative net charge. Chloride (Cl), bicarbonate (HCO3), phosphate (PO4), and sulfate (SO4) are anions.


A cation is a positively charged ion, meaning it has fewer electrons than protons. This happens when an atom loses one or more electrons, leaving it with a positive net charge. Calcium (Ca2+), magnesium (Mg2+), potassium (K+), and sodium (Na+) are cations. Cations are important for acid–base reactions (reactions that affect pH).


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