Physiology: Body Fluid Compartments Flashcards
What is osmolarity and what are its units for body fluids?
The concentration of osmotically active particles in a solution
Units: osmol/l, or mosmol/l for body fluids as they are weak salt solutions
How is osmolarity calculated?
Molar concentration of the solution
x
Number of osmotically active particles present
What is the osmolarity of 100mM MgCl2?
Molar conc. x no. of osmotically active particles
= 100 x 3 (Mg + Cl + Cl)
= 300 mosmol/l
What is tonicity?
The effect that a solution has on cell volume
Describe how cell volume is affected when submerged in a... - Isotonic - Hypotonic - Hypertonic ... solution
- Isotonic: no overall net change
- Hypotonic: solution contains more water than the cell, water enters the cell by osmosis, cell volume increases, may lead to cell lysis
- Hypertonic: solution contains less water than the cell, water leaves the cell by osmosis, cell volume decreases, the cell shrinks
What 2 factors does tonicity depend upon?
- Osmolarity of the solution
- Permeability of the cell membrane to the solute
Total body water (TBW) makes up what proportion of total body weight in males vs females?
Why is there a difference?
Males: ~60%
Females: ~50%
This difference is due to the higher fat distribution in females - fat contains little water volume
What are the 2 major compartments that make up total body water?
Intracellular fluid (ICF) - ~2/3rd of TBW Extracellular fluid (ECF) - ~1/3rd of TBW
What is the estimated... - Total body water volume - ICF volume - ECF volume ... for a 70kg male?
TBW: 42L (60% of weight)
ICF: 28L (2/3rds of TBW)
ECF: 14L (1/3rd of TBW)
What is extracellular fluid composed of?
Interstitial fluid (~80%)
Plasma (~20%)
Lymph and transcellular fluid (negligible)
How can the body fluid compartments be measured?
By obtaining the volume of distribution of a tracer
Name a suitable tracer for each of the following body fluid compartments: - Total body water - ECF - Plasma ...?
- Total body water: 3H2O (titriated water - a radioactive form of water)
- ECF: Inulin (an exogenous plant protein)
- Plasma: Labelled albumin (a plasma protein)
How can ICF be calculated?
By using tracers to measure TBW and ECF then TBW = ECF - ICF
What is the equation used to work out the distribution volume of a tracer?
Distribution volume = known dose of tracer added (mol or mg) / concentration of the tracer in a sample (mol/litre)
Whole-body fluid homeostasis relies on a balance between input and output of water and salts from the body. What is the main way that water balance is maintained?
By increasing and decreasing water ingestion
Increased and decreased excretion of water by the kidneys alone is insufficient to maintain water balance
How does water imbalance in the body affect the body fluid compartments?
Water imbalance changes body fluid osmolarity
This results in fluid shift between the ECF and ICF compartments by osmosis
What are the main ions of the…
- ICF
- ECF (interstitial fluid + plasma)
…?
- ICF: K+
- ECF (interstitial fluid + plasma): Na+, Cl- and HCO3-
What are the osmotic concentrations of the ECF and ICF?
Both are identical (usually ~300 mosmol/L)
What is fluid shift?
Movement of water between the ICF and ECF in response to an osmotic gradient
Describe fluid shift in response to increased osmolarity of the ECF e.g., during dehydration
- ECF becomes hypertonic
- Water from the ICF leaves the cells to restore the fluid and electrolyte balance between the ICF and ECF
- Osmotic concentrations are identical again
Describe fluid shift in response to decreased osmolarity of the ECF e.g., during excess H2O intake
- ECF becomes hypotonic
- Water is taken into the ICF of the cells to restore the fluid and electrolyte balance between the ICF and ECF
- Osmotic concentrations are identical again
List 3 challenges to fluid homeostasis
- Gain or loss of water
- Gain or loss of NaCl
- Gain or loss of isotonic fluid
How does gain or loss of water disturb fluid homeostasis?
It changes fluid osmolarity so causes fluid shift, this results in a similar change in ECF and ICF (e.g., both increase or both decrease)
How does gain or loss of NaCl disturb fluid homeostasis?
It changes fluid osmolarity as…
- Na+ is excluded from the ICF (as cells are relatively impermeable to Na+)
- Water follows Na+ by osmosis
This results in opposite changes in ECF and ICF e.g., NaCl gain causes increased ECF volume and decreased ICF volume and vice versa
How does gain or loss of isotonic fluid disturb fluid homeostasis?
There is no change in fluid osmolarity but ECF volume increases
e.g., 0.9% NaCl IV infusion
The kidneys alter the composition and volume of the ECF. What is regulation of ECF volume essential for?
Long-term regulation of blood pressure
Why is ECF volume important for regulating BP?
Because plasma is a component of the ECF so increased ECF increases plasma volume and decreased ECF decreases plasma volume
Why is electrolyte balance so important (2)?
- Electrolyte concentrations can affect water balance due to changes in osmolarity
- Concentrations of individual electrolytes can affect cell function
What are the 2 most important electrolytes?
Na+ (>90% for ECF)
K+ (>95% for ICF)
What is the relationship between total body sodium and volume of ECF?
Na+ is the major determinant of ECF volume
ECF volume is determined by the balance of sodium intake (mainly dietary) and sodium output in the urine, as water follows Na+
Why is potassium balance important for cells?
K+ plays a key role for establishing membrane potential
Why must plasma K+ be closely regulated?
Even small changes in membrane potential in cardiac and muscle cells can have severe consequences e.g., paralysis, cardiac arrest
