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fluid electrolytes & acid base

HEAL100 Health Care — exam and revision material · Other

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Health Care exam material, 36 pages. Originally published on StudyMerit.

Pages
36
Academic year
2025/2026
Language
English

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Week 5 PHARM Fluid, Electrolyte, & Acid-Base Balance 1.0 Core Concepts in Fluid & Electrolyte Balance Homeostasis is the body's natural tendency to maintain a stable internal environment. The intricate balance of fluids, electrolytes, and pH is the foundation of this stability. For nurses, a foundational understanding of these concepts is not just academic—it's a critical component of safe and effective patient care. Deviations from normal balance can lead to life-threatening conditions, making this a frequently tested and high-stakes topic on the ATI exam. 1.1 Foundational A&P Review ● Osmolality: Refers to the concentration of a solution, expressed as the total number of solute particles (like sodium, glucose, or urea) per kilogram of fluid. The body works constantly to maintain a normal plasma osmolality of 275–295 mOsm/kg . The greatest contributor to plasma osmolality is sodium , which is controlled by the hormone aldosterone . Changes in osmolality are the primary driver of water movement between body compartments. ● Osmosis: The movement of water across a semipermeable membrane from an area of lower osmolality (fewer solutes, more water) to an area of higher osmolality (more solutes, less water). ● Solution Tonicity: Describes the relative concentration of an intravenous (IV) fluid compared to the body's own plasma. This determines how water will shift when the fluid is administered. When IV fluids are infused, they can cause significant fluid shifts based on their tonicity: ● Hypertonic Fluids: These solutions have a higher osmolality than plasma. When infused, they pull water out of the cells and interstitial space and into the plasma, expanding the intravascular volume. ● Hypotonic Fluids: These solutions have a lower osmolality than plasma. They cause water to move from the plasma into the interstitial fluid and cells, potentially causing cells to swell. ● Isotonic Fluids: These solutions have the same osmolality as plasma. They expand the plasma volume without causing any significant fluid shift between compartments. 1.2 The Body's Regulatory Mechanisms The body has several complex mechanisms to maintain fluid balance:

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● Fluid Intake: The single most important regulator is the thirst mechanism . ● Fluid Output: The kidneys are the primary regulators of fluid output, influenced by key hormonal systems: ○ Renin–angiotensin-aldosterone system (RAAS) ○ Aldosterone ○ Antidiuretic hormone (ADH) When these intricate regulatory systems are overwhelmed by illness or injury, pharmacological intervention with IV fluid therapy becomes necessary to restore homeostasis. 2.0 Intravenous Fluid Therapy: Crystalloids vs. Colloids Intravenous fluid therapy is a cornerstone of modern medicine, used to strategically correct imbalances in body fluids and electrolytes. The primary goal is to replace lost fluids and promote adequate urine output. The two main categories of solutions used to achieve this are crystalloids and colloids. 2.1 Crystalloids Crystalloids are IV solutions that contain electrolytes and other small molecules that closely mimic the body's extracellular fluid. Their primary mechanism is to replace fluids and promote urine output. A key characteristic of crystalloids is their ability to leave the plasma and move into the interstitial and intracellular spaces. The specific compartment they enter is determined by the solution's tonicity. When administering crystalloids, the nurse must constantly monitor for signs of fluid volume excess, particularly pulmonary edema , and should frequently assess lung sounds for crackles . 2.2 Colloids Colloids are solutions containing large molecules (e.g., proteins, starches) that are too large to easily pass through the capillary membranes. Their primary function is to remain in the intravascular space and draw water from the cells and tissues into the plasma, thereby rapidly expanding plasma volume. Examples of colloid solutions include Dextran 40 and 5% albumin . 2.3 Comparison of Common IV Solutions The selection of an IV fluid is a critical decision based on the patient's specific fluid and electrolyte status. IV Solution Tonicity Category Clinical Notes

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Normal saline (0.9% NaCl) Isotonic No significant fluid shift; used for volume expansion. Lactated Ringer’s Isotonic No significant fluid shift; contains multiple electrolytes. Plasma-Lyte 148 Isotonic No significant fluid shift. 5% dextrose in water (D5W) Isotonic Initially isotonic, but dextrose is quickly metabolized, leaving free water. This makes the solution functionally hypotonic and can lead to cellular swelling and increased intracranial pressure. 5% dextrose in 0.2% NS Isotonic No significant fluid shift. Hypotonic saline (0.45% NaCl) Hypotonic Moves water from plasma to interstitial fluid and cells. Plasma-Lyte 56 Hypotonic Moves water from plasma to interstitial fluid and cells. Hypertonic saline (3% NaCl) Hypertonic Moves water from cells and interstitial fluid to plasma. 5% dextrose in normal saline Hypertonic Moves water from cells and interstitial fluid to plasma.

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