Fluids & Electrolyte Overview
Last updated 17th March 2019
An understanding of the normal fluid and electrolyte physiology is essential to understanding the many forms of pathology where this can be disturbed.
There is notable interaction between the cardiovascular and renal systems here, but the basic principles will be looked at.
This is a really good introductory video from Armando Hasudungan: https://www.youtube.com/watch?v=raW6b5kQHPY
There is notable interaction between the cardiovascular and renal systems here, but the basic principles will be looked at.
This is a really good introductory video from Armando Hasudungan: https://www.youtube.com/watch?v=raW6b5kQHPY
Body Composition
The overall composition of the body can be separated into different compartments to aid understanding.
We can think of the body as being made up of solids and liquids.
We will use adult values here and a 70kg man as an example.
The body is composed of 60% water (42L).
The remaining 40% is comprised of the different solids:
The water is distributed through nearly every tissue in the body, but there are some key similarities and differences in certain tissues.
As such, we divide the body into a number of compartments.
This is freely permeable to water, but the movement of ions and larger molecules is highly regulated.
As water moves freely, the osmolality of all the compartments is the same, even if the constituents are different.
We can think of the body as being made up of solids and liquids.
We will use adult values here and a 70kg man as an example.
The body is composed of 60% water (42L).
The remaining 40% is comprised of the different solids:
- Proteins
- Lipids
- Minerals
The water is distributed through nearly every tissue in the body, but there are some key similarities and differences in certain tissues.
As such, we divide the body into a number of compartments.
- Intracellular ⅔ - 28L
- Extracellular ⅓ - 14L
This is freely permeable to water, but the movement of ions and larger molecules is highly regulated.
As water moves freely, the osmolality of all the compartments is the same, even if the constituents are different.
Intracellular Fluid
This makes up the majority of the total body water - ⅔ or about 28L.
Although dispersed amongst a huge number of cells, the similarity of the environment, separated by the cell membrane, allows it to be effectively considered as a single compartment.
The constitution is:
The proteins, phosphate and other inorganic anions make up the major anionic component of the intracellular space.
Although dispersed amongst a huge number of cells, the similarity of the environment, separated by the cell membrane, allows it to be effectively considered as a single compartment.
The constitution is:
- Potassium 155 mmol/L - the major cation
- Proteins 4 mOsm/L
- Phosphate and inorganic anions
- Magnesium 20mmol/L
- Sodium 5 mmol/L
- Chloride 4 mmol/L
- Calcium <0.5 mmol/L
The proteins, phosphate and other inorganic anions make up the major anionic component of the intracellular space.
Extracellular Fluid
This makes up about ⅓ of the total body water - about 14L.
The extracellular compartment has further important subdivisions:
The separation of the intravascular space from the other extracellular spaces only has a relatively small significance in terms of electrolytes and water.
This is because the capillaries membranes are highly permeable to these substances.
The major difference is the restricted movement of proteins, which remain primarily within the intravascular space.
Their negative charge produces the Donnan effect, where there is a slightly higher level of cations intravascularly compared to the interstitium.
However, this difference is only a magnitude of about 2% and so the compartments can be considered to be pretty homogenous from an electrolyte perspective.
The constitution of this fluid is:
The extracellular compartment has further important subdivisions:
- Intravascular - 3L
- Interstitial - 9L
- Transcellular - 1L
- Bone/connective tissue - 1L
The separation of the intravascular space from the other extracellular spaces only has a relatively small significance in terms of electrolytes and water.
This is because the capillaries membranes are highly permeable to these substances.
The major difference is the restricted movement of proteins, which remain primarily within the intravascular space.
Their negative charge produces the Donnan effect, where there is a slightly higher level of cations intravascularly compared to the interstitium.
However, this difference is only a magnitude of about 2% and so the compartments can be considered to be pretty homogenous from an electrolyte perspective.
The constitution of this fluid is:
- Sodium 145 mmol/L - the major cation
- Chloride 115 mmol/L - the major anion
- Bicarbonate 26 mmol/L
- Potassium 4 mmol/L
- Calcium 2.5 mmol/L
Intravascular Compartement
This refers to the fluid that is within the blood vessels.
It varies with age and weight and usually about 70-75ml/kg (5L in our example)
It consists of:
The plasma makes up the majority of the intravascular compartment.
The RBCs also contribute a significant volume of about 2L, but this is intracellular fluid.
The plasma is composed of 90% water.
There are a significant number of proteins as well:
As they are solutes which are too large to pass through the vessel wall membranes they contribute an osmotic pressure to the compartment.
It is important to note that this is only a relatively small component of the total osmotic pressure (the majority still being provided by sodium).
This is just about enough to offset the hydrostatic pressure within the vessels, that would otherwise drive fluid out of this space.
It varies with age and weight and usually about 70-75ml/kg (5L in our example)
It consists of:
- Plasma (60%) - 3L
- Water
- Proteins
- Ions
- Water
- Blood cells (40%) - 2L
- RBCs
- WBCs
- Platelets
- RBCs
The plasma makes up the majority of the intravascular compartment.
The RBCs also contribute a significant volume of about 2L, but this is intracellular fluid.
The plasma is composed of 90% water.
There are a significant number of proteins as well:
- Albumin
- Globulins
- Fibrinogen
As they are solutes which are too large to pass through the vessel wall membranes they contribute an osmotic pressure to the compartment.
It is important to note that this is only a relatively small component of the total osmotic pressure (the majority still being provided by sodium).
This is just about enough to offset the hydrostatic pressure within the vessels, that would otherwise drive fluid out of this space.
Interstitial Compartment
This refers to the fluid that surrounds the cells of the body but is outside of the blood vessels.
It is essentially of the same composition as the intravascular compartment but without the proteins.
It forms the majority of extracellular fluid.
It is essentially of the same composition as the intravascular compartment but without the proteins.
It forms the majority of extracellular fluid.
Transcellular Fluid
This refers to fluid that has been secreted but is separate from the interstitial compartment by an epithelial layer.
It is usually about 1L.
For example:
It is usually about 1L.
For example:
- GI tract
- Bladder urine
- CSF
- Aqueous humour
- Bile
Requirements
The daily water requirement of the body is about 2500ml (25-35ml/kg).
This is usually obtained through:
The body loses fluid from a number of sources:
The daily electrolyte requirements are:
The usual dietary sodium intake is well in excess of this at around 450 mmol/day.
Only about 20 mmol is lost in sweat and faeces combined and so much much be excreted by the kidney.
This is usually obtained through:
- Drinking 1200ml
- Food 1000ml
- Metabolic process 300ml
The body loses fluid from a number of sources:
- Insensible losses (skin, lungs) 900ml
- Sweat - variable
- Faeces 100ml
- Urine - adjusted to balance
The daily electrolyte requirements are:
- Sodium 1-1.4 mmol/kg
- Potassium 0.7-0.9 mmol/kg
- Chloride 1.3-1.9 mmol/kg
The usual dietary sodium intake is well in excess of this at around 450 mmol/day.
Only about 20 mmol is lost in sweat and faeces combined and so much much be excreted by the kidney.
Control
This is a good overview video from Armando Hasudungan: https://www.youtube.com/watch?v=FzeL1fcBwnE&index=69&list=PLqTetbgey0ad6pWRVGoRIoj--pneouZQh&t=0s
As with many homeostatic mechanisms, the control of body fluid and electrolytes can be considered in 3 parts:
It is therefore through manipulation of these sites that most regulation will be enacted.
Sensors
The sensory limb of this control is primarily through osmoreceptors in the hypothalamus.
These detect change in the tonicity of the surrounding ECF, which is primarily governed by the sodium concentration.
These receptors are very sensitive and can detect changes of 1-2%.
The osmoreceptor cells are located in the anterior hypothalamus, near the supraoptic nuclei.
With an increase in extracellular osmolality these cells start to shrink, and this shrinking causes the triggering of action potentials.
These feed to cells in the supraoptic nuclei who are responsible for the ADH release.
Controllers
The control pathways lie within the hypothalamus with regards to ADH, as described above.
Effectors
The efferent limb of the homeostatic system can be considered as:
Thirst is the behavioural change to increase oral intake of fluid, described as the conscious desire for water.
In the context of minimal losses of water from the body, this is an essential mechanism to maintain adequate total body water.
The thirst centre is also situated in the hypothalamus, and similarly stimulated be changes in the osmolality of the extracellular fluid causing shrinking of cells.
Additional factors that stimulate thirst include:
This factor and the dry mouth sensation allow an immediate improvement in thirst in response to drinking, which is important given the usual 30-60 minutes that would usually be needed for drinking water to result in a change in osmolality
ADH
Antidiuretic hormone (ADH), also known as arginine vasopressin, is released from the posterior pituitary (although synthesised in the hypothalamus) in response to an increase in tonicity.
It acts at the collecting ducts in the kidneys to increase water reabsorption through the stimulation of the expression of aquaporin type 2 channels.
This results in and increased retention of water in the body.
As other ions continue to be excreted at the same rate, the osmolality of the body returns to the desired set point.
ADH is also a vasopressor.
Its release can also be triggered by cardiovascular stimuli:
This response is less sensitive than with changes in osmolality.
A number of other factors may increase or decrease ADH release.
Increase:
Aldosterone is a mineralocorticoid released from the adrenal cortex.
This is release as an end product of the renin-angiotensin-aldosterone system (RAAS), which is usually sensitive to blood pressure changes.
It can also be released directly from the adrenal cortex in response to an elevation in potassium or a decrease in sodium levels.
It is transported through the blood bound to proteins (particularly albumin) and acts at the distal convoluted tubule.
He it results in increased activity of the eNAC and Na+/K+ ATPase systems.
The end result is an increase in sodium reabsorption (at the expense of K+ loss) and with a subsequent retention of water too.
This is a notably less important mechanism than the first two.
The natriuretic peptides ANP and BNP are released from the heart.
That have opposite effects to the previous 2 hormones.
They are released from the heart in response to stretching, often associated with an increased blood volume.
An important end result of their effect is the inhibition of aldosterone activity.
This is indirectly through the RAAS by reducing sympathetic ANS activity and afferent arteriole vasodilation in the nephron, reducing the release of renin.
They also exert a direct vasodilatory effect.
As with many homeostatic mechanisms, the control of body fluid and electrolytes can be considered in 3 parts:
- Sensors
- Controllers
- Effectors
It is therefore through manipulation of these sites that most regulation will be enacted.
Sensors
The sensory limb of this control is primarily through osmoreceptors in the hypothalamus.
These detect change in the tonicity of the surrounding ECF, which is primarily governed by the sodium concentration.
These receptors are very sensitive and can detect changes of 1-2%.
The osmoreceptor cells are located in the anterior hypothalamus, near the supraoptic nuclei.
With an increase in extracellular osmolality these cells start to shrink, and this shrinking causes the triggering of action potentials.
These feed to cells in the supraoptic nuclei who are responsible for the ADH release.
Controllers
The control pathways lie within the hypothalamus with regards to ADH, as described above.
Effectors
The efferent limb of the homeostatic system can be considered as:
- Thirst
- ADH secretion
- Aldosterone
- Natriuretic peptides
Thirst is the behavioural change to increase oral intake of fluid, described as the conscious desire for water.
In the context of minimal losses of water from the body, this is an essential mechanism to maintain adequate total body water.
The thirst centre is also situated in the hypothalamus, and similarly stimulated be changes in the osmolality of the extracellular fluid causing shrinking of cells.
Additional factors that stimulate thirst include:
- A decrease in blood volume
- A decrease in blood pressure
- An increase in angiotensin
- A dry mouth
This factor and the dry mouth sensation allow an immediate improvement in thirst in response to drinking, which is important given the usual 30-60 minutes that would usually be needed for drinking water to result in a change in osmolality
ADH
Antidiuretic hormone (ADH), also known as arginine vasopressin, is released from the posterior pituitary (although synthesised in the hypothalamus) in response to an increase in tonicity.
It acts at the collecting ducts in the kidneys to increase water reabsorption through the stimulation of the expression of aquaporin type 2 channels.
This results in and increased retention of water in the body.
As other ions continue to be excreted at the same rate, the osmolality of the body returns to the desired set point.
ADH is also a vasopressor.
Its release can also be triggered by cardiovascular stimuli:
- The arterial baroreceptor reflexes
- The cardiopulmonary reflexes
This response is less sensitive than with changes in osmolality.
A number of other factors may increase or decrease ADH release.
Increase:
- Nausea
- Hypoxia
- Morphine
- Nicotine
- Alcohol
- Clonidine
- Haloperidol
Aldosterone is a mineralocorticoid released from the adrenal cortex.
This is release as an end product of the renin-angiotensin-aldosterone system (RAAS), which is usually sensitive to blood pressure changes.
It can also be released directly from the adrenal cortex in response to an elevation in potassium or a decrease in sodium levels.
It is transported through the blood bound to proteins (particularly albumin) and acts at the distal convoluted tubule.
He it results in increased activity of the eNAC and Na+/K+ ATPase systems.
The end result is an increase in sodium reabsorption (at the expense of K+ loss) and with a subsequent retention of water too.
This is a notably less important mechanism than the first two.
The natriuretic peptides ANP and BNP are released from the heart.
That have opposite effects to the previous 2 hormones.
They are released from the heart in response to stretching, often associated with an increased blood volume.
An important end result of their effect is the inhibition of aldosterone activity.
This is indirectly through the RAAS by reducing sympathetic ANS activity and afferent arteriole vasodilation in the nephron, reducing the release of renin.
They also exert a direct vasodilatory effect.
Links & References
- Hasudungan, A. Overview of fluid and electrolyte physiology (fluid compartment). Youtube. 2018. https://www.youtube.com/watch?v=raW6b5kQHPY
- Wilkinson, M. Body water and compartments. e-LFH. 2012.
- Hasudungan, A. Hormones in body fluid homeostasis (ADH/vasopressin, Aldosterone and Natriuretic peptides). Youtube. 2018. https://www.youtube.com/watch?v=FzeL1fcBwnE&index=69&list=PLqTetbgey0ad6pWRVGoRIoj--pneouZQh&t=0s
- Guyton A, Hall J. Textbook of Medical Physiology (11th ed). 2006. Elsevier Saunders.
- Power I, Kam P. Principles of Physiology for the Anaesthetist (2nd ed). 2008. Hodder Arnold.
