Chapter 17
Fluid, Electrolyte, and Acid-Base Imbalances
KEY POINTS
INTRODUCTION
• Body fluids and electrolytes play an important role in maintaining a stable internal environment
of the body.
• A variety of adaptive responses keep the composition and volume of body fluids and
electrolytes within the narrow limits of normal to maintain homeostasis and promote health.
• Many diseases and their treatments could affect fluid and electrolyte balance and alter
homeostasis.
WATER CONTENT OF BODY
• Water is the primary component of the body, accounting for about 50-60% of the body weight
in the adult.
• The 2 major fluid compartments in the body are intracellular fluid (ICF), or inside the cells, and
extracellular fluid (ECF), or outside the cells. ECF is composed of interstitial fluid, plasma, and
transcellular fluids.
ELECTROLYTES
• Electrolyte composition varies between the ECF and ICF, though the overall concentration of
the electrolytes is nearly the same in the 2 compartments.
Copyright © 2023 by Elsevier, Inc. All rights reserved.
, • The measurement of electrolytes is important in evaluating electrolyte balance, as well as in
determining the composition of electrolyte preparations.
MECHANISMS CONTROLLING FLUID AND ELECTROLYTE MOVEMENT
Many different processes are involved in the movement of electrolytes and water between the
ICF and ECF. Some of these include osmosis, simple diffusion, facilitated diffusion, and active
transport. Two forces drive water movement: hydrostatic pressure and osmotic pressure.
• Osmotic pressure is the amount of pressure required to stop the osmotic flow of water.
• Measuring osmolality is important because it indicates the water balance of the body.
• The tonicity of the fluid surrounding cells affects them. Fluids with the same osmolality
compared to plasma are isotonic. Solutions which have a lower osmolality comparatively with
plasma are hypotonic; those with an increased osmolality compared to plasma are hypertonic. •
Hydrostatic pressure is the force of fluid within a compartment and is the major force that
pushes water out of the vascular system at the capillary level.
• Oncotic pressure (colloidal osmotic pressure) is osmotic pressure exerted by colloids in
solution. The major colloids in the vascular system contributing to osmotic pressure are
proteins, such as albumin.
FLUID MOVEMENT IN CAPILLARIES
• The amount and direction of fluid movement between the interstitial space and the capillary is
determined by the interaction of (1) capillary hydrostatic pressure, (2) plasma oncotic pressure,
(3) interstitial hydrostatic pressure, and (4) interstitial oncotic pressure.
• When capillary or interstitial pressures are altered, fluid may abnormally shift from one
compartment to another, resulting in edema or dehydration.
Copyright © 2023 by Elsevier, Inc. All rights reserved.
Fluid, Electrolyte, and Acid-Base Imbalances
KEY POINTS
INTRODUCTION
• Body fluids and electrolytes play an important role in maintaining a stable internal environment
of the body.
• A variety of adaptive responses keep the composition and volume of body fluids and
electrolytes within the narrow limits of normal to maintain homeostasis and promote health.
• Many diseases and their treatments could affect fluid and electrolyte balance and alter
homeostasis.
WATER CONTENT OF BODY
• Water is the primary component of the body, accounting for about 50-60% of the body weight
in the adult.
• The 2 major fluid compartments in the body are intracellular fluid (ICF), or inside the cells, and
extracellular fluid (ECF), or outside the cells. ECF is composed of interstitial fluid, plasma, and
transcellular fluids.
ELECTROLYTES
• Electrolyte composition varies between the ECF and ICF, though the overall concentration of
the electrolytes is nearly the same in the 2 compartments.
Copyright © 2023 by Elsevier, Inc. All rights reserved.
, • The measurement of electrolytes is important in evaluating electrolyte balance, as well as in
determining the composition of electrolyte preparations.
MECHANISMS CONTROLLING FLUID AND ELECTROLYTE MOVEMENT
Many different processes are involved in the movement of electrolytes and water between the
ICF and ECF. Some of these include osmosis, simple diffusion, facilitated diffusion, and active
transport. Two forces drive water movement: hydrostatic pressure and osmotic pressure.
• Osmotic pressure is the amount of pressure required to stop the osmotic flow of water.
• Measuring osmolality is important because it indicates the water balance of the body.
• The tonicity of the fluid surrounding cells affects them. Fluids with the same osmolality
compared to plasma are isotonic. Solutions which have a lower osmolality comparatively with
plasma are hypotonic; those with an increased osmolality compared to plasma are hypertonic. •
Hydrostatic pressure is the force of fluid within a compartment and is the major force that
pushes water out of the vascular system at the capillary level.
• Oncotic pressure (colloidal osmotic pressure) is osmotic pressure exerted by colloids in
solution. The major colloids in the vascular system contributing to osmotic pressure are
proteins, such as albumin.
FLUID MOVEMENT IN CAPILLARIES
• The amount and direction of fluid movement between the interstitial space and the capillary is
determined by the interaction of (1) capillary hydrostatic pressure, (2) plasma oncotic pressure,
(3) interstitial hydrostatic pressure, and (4) interstitial oncotic pressure.
• When capillary or interstitial pressures are altered, fluid may abnormally shift from one
compartment to another, resulting in edema or dehydration.
Copyright © 2023 by Elsevier, Inc. All rights reserved.
