Ch. 4 Fluid & Electrolyte
Amount and Composition of Body Fluids
- 60% of human weight is fluid (water & electrolytes)
- Younger people have higher percentage of body fluid than older people
- Body fluid is located in Intracellular space (ICS) & extracellular space (ECS)
- ⅔ of the body is ICF & is located in skeletal muscle mass
- Extracellular space is divided into intravascular space, interstitial, & transcellular fluid
space
- Intravascular space contains plasma (3.5L of blood is plasma), remaining 2.5L is
erythrocytes, leukocytes, & thrombocytes
- Interstitial space has the fluid between cells, tissues,organs, & blood vessels
(10L) ex. Lymph fluid
- Transcellular space smallest division of ECF 1L ex. Cerebrospinal, synovial,
pericardial, intraocular, pleural, peritoneal cavity
- Body fluid shifts between ICS & ECS to transport gasses, nutrients, wastes while
maintaining overall bodily function
- Electrolytes in body fluids are active chemicals (cations that carry positive charges &
anions that carry negative charges)
- Major Cations: Sodium, Potassium, Calcium, Magnesium, Hydrogen
- Major Anions: Chloride, Bicarbonate, Phosphate, Sulfate, Proteinate Ions
- Electrolyte concentration in the body is expressed in mEq/L
- ECF (plasma): Cations Sodium, Potassium, Calcium, Magnesium, Anions Chloride,
Bicarbonate, Phosphate, Sulfate, Organic acids, Proteinate
- ICF: Cations Potassium, Magnesium, Sodium, Anions Phosphates & Sulfates,
Bicarbonate, Proteinate
- Normal movement of fluid through capillary call into tissues is dependent on 2 primary
forces:
- Capillary hydrostatic pressure generated by cardiac contraction & exerted by
plasma fluids on the walls of the blood vessel
- Plasma oncotic (colloid osmotic) pressure, exerted by plasma proteins
(capillary-impermeable albumin)
Osmosis & Osmolality
- Fluid shifts through the membrane from the region of low solute concentration to the
region of high solute concentration until the solutions are of equal concentration when
they are 2 different solutions separated by impermeable membrane
- Osmosis: movement of water caused by concentration gradient
- Number of dissolved particles contained in a unit of fluid determines the
osmolality of a solution & influences the movement of fluid between fluid
compartments
- Tonicity: ability of all the solutes to cause an osmotic driving force that promotes
water movement from 1 compartment to another
- IV solutions are termed isotonic, hypotonic, hypertonic
- Sodium, mannitol,glucose, sorbitol are effective osmoles
, Diffusion: natural tendency of a substance to move from an area of higher concentration
to one of lower concentration (ex. Exchange of oxygen & carbon dioxide between
pulmonary capillaries & alveoli)
Filtration
- Hydrostatic pressure in capillaries tend to filter fluid out of the intravascular compartment
into interstitial fluid
- Filtration allows the kidneys to filter 180 L of plasma per day
Sodium-Potassium Pump
- Located in cell membrane & actively moves sodium from the cell into ECF
Active Transport: energy must be expended for the movement to occur against a
concentration gradient
Routes of Gains & Losses
Kidneys
- Usual daily urine volume in adults is 1.5 L
- Output of urine is approximately 0.5-1mL of urine/kg of bodyweight per hour (0.5 to
1mL/kg/hr)
Skin
- Sensible perspiration=sweating (chief solutes sodium, chloride, potassium)
- Sweat loss varies from 0-1000mL or more every hour
- Fever increases water loss through lungs & skin
- Sodium content in sweat is low but w/ increased sweat loss sodium deficit & FVD occur
Lungs
- Lungs eliminate water vapor at a rate of 400mL every day (loss is greater w/ increased
respiratory rate or depth in dry climate)
GI Tract
- Usual loss through GI tract is 100mL even though approximately 8 L of fluid circulates
through GI system every 24 hr
- Bulk of fluid is normally reabsorbed in small intestine (diarrhea & fistulas cause losses)
- Infectious diarrhea leading cause of death from hypovolemia
Lab Tests for Evaluating Fluid Status
Osmolality: concentration of fluid that affects the movement of water between fluid
compartments of osmosis (mOsm/kg)
- Measures solute concentration per kg of solvent in blood & urine
- Measure of a solutions ability to create osmotic pressure & affect movement of water
- Serum osmolality primarily reflects concentration of sodium
- Urine osmolality is determined by urea, creatinine, & uric acid
Osmolarity: concentration of solutions is measured in milliosmoles per L (mOsm/L)
- Normal serum osmolality 275-300 mOsm/kg
- Normal urine osmolality 50-1400mOsm/kg random urine test
- 300-900 mOsm/kg in 24 hr urine collection
- Plasma osmolality=2 serum sodium + glucose/18+BUN/2.8
- BUN: made up of urea (end product of protein metabolism) by liver
- Normal BUN 10-20 mg/dL
Amount and Composition of Body Fluids
- 60% of human weight is fluid (water & electrolytes)
- Younger people have higher percentage of body fluid than older people
- Body fluid is located in Intracellular space (ICS) & extracellular space (ECS)
- ⅔ of the body is ICF & is located in skeletal muscle mass
- Extracellular space is divided into intravascular space, interstitial, & transcellular fluid
space
- Intravascular space contains plasma (3.5L of blood is plasma), remaining 2.5L is
erythrocytes, leukocytes, & thrombocytes
- Interstitial space has the fluid between cells, tissues,organs, & blood vessels
(10L) ex. Lymph fluid
- Transcellular space smallest division of ECF 1L ex. Cerebrospinal, synovial,
pericardial, intraocular, pleural, peritoneal cavity
- Body fluid shifts between ICS & ECS to transport gasses, nutrients, wastes while
maintaining overall bodily function
- Electrolytes in body fluids are active chemicals (cations that carry positive charges &
anions that carry negative charges)
- Major Cations: Sodium, Potassium, Calcium, Magnesium, Hydrogen
- Major Anions: Chloride, Bicarbonate, Phosphate, Sulfate, Proteinate Ions
- Electrolyte concentration in the body is expressed in mEq/L
- ECF (plasma): Cations Sodium, Potassium, Calcium, Magnesium, Anions Chloride,
Bicarbonate, Phosphate, Sulfate, Organic acids, Proteinate
- ICF: Cations Potassium, Magnesium, Sodium, Anions Phosphates & Sulfates,
Bicarbonate, Proteinate
- Normal movement of fluid through capillary call into tissues is dependent on 2 primary
forces:
- Capillary hydrostatic pressure generated by cardiac contraction & exerted by
plasma fluids on the walls of the blood vessel
- Plasma oncotic (colloid osmotic) pressure, exerted by plasma proteins
(capillary-impermeable albumin)
Osmosis & Osmolality
- Fluid shifts through the membrane from the region of low solute concentration to the
region of high solute concentration until the solutions are of equal concentration when
they are 2 different solutions separated by impermeable membrane
- Osmosis: movement of water caused by concentration gradient
- Number of dissolved particles contained in a unit of fluid determines the
osmolality of a solution & influences the movement of fluid between fluid
compartments
- Tonicity: ability of all the solutes to cause an osmotic driving force that promotes
water movement from 1 compartment to another
- IV solutions are termed isotonic, hypotonic, hypertonic
- Sodium, mannitol,glucose, sorbitol are effective osmoles
, Diffusion: natural tendency of a substance to move from an area of higher concentration
to one of lower concentration (ex. Exchange of oxygen & carbon dioxide between
pulmonary capillaries & alveoli)
Filtration
- Hydrostatic pressure in capillaries tend to filter fluid out of the intravascular compartment
into interstitial fluid
- Filtration allows the kidneys to filter 180 L of plasma per day
Sodium-Potassium Pump
- Located in cell membrane & actively moves sodium from the cell into ECF
Active Transport: energy must be expended for the movement to occur against a
concentration gradient
Routes of Gains & Losses
Kidneys
- Usual daily urine volume in adults is 1.5 L
- Output of urine is approximately 0.5-1mL of urine/kg of bodyweight per hour (0.5 to
1mL/kg/hr)
Skin
- Sensible perspiration=sweating (chief solutes sodium, chloride, potassium)
- Sweat loss varies from 0-1000mL or more every hour
- Fever increases water loss through lungs & skin
- Sodium content in sweat is low but w/ increased sweat loss sodium deficit & FVD occur
Lungs
- Lungs eliminate water vapor at a rate of 400mL every day (loss is greater w/ increased
respiratory rate or depth in dry climate)
GI Tract
- Usual loss through GI tract is 100mL even though approximately 8 L of fluid circulates
through GI system every 24 hr
- Bulk of fluid is normally reabsorbed in small intestine (diarrhea & fistulas cause losses)
- Infectious diarrhea leading cause of death from hypovolemia
Lab Tests for Evaluating Fluid Status
Osmolality: concentration of fluid that affects the movement of water between fluid
compartments of osmosis (mOsm/kg)
- Measures solute concentration per kg of solvent in blood & urine
- Measure of a solutions ability to create osmotic pressure & affect movement of water
- Serum osmolality primarily reflects concentration of sodium
- Urine osmolality is determined by urea, creatinine, & uric acid
Osmolarity: concentration of solutions is measured in milliosmoles per L (mOsm/L)
- Normal serum osmolality 275-300 mOsm/kg
- Normal urine osmolality 50-1400mOsm/kg random urine test
- 300-900 mOsm/kg in 24 hr urine collection
- Plasma osmolality=2 serum sodium + glucose/18+BUN/2.8
- BUN: made up of urea (end product of protein metabolism) by liver
- Normal BUN 10-20 mg/dL
