Lecture 2: Electrolytes and Fluid Balance I Flashcards
Two main types of cellular fluid compartements
intracellular fluid (ICF) compartment (cytoplasmic compartment bounded by the plasma membrane)
extracellular fluid (ECF) compartment (outside cytoplasm)
Divisions of ECF
interstitial fluid (immediate fluid bathing the cells, also called extravascular fluid)
blood plasma (also known as intravascular fluid)
(see figure)
Does ICF or ECF have more fluid under normal conditions?
ICF, otherwise cells would collapse
What separates the interstitial fluid from the blood plasma?
Capillary wall vessel
Distribution of water among cellular compartments
See figure
What determines water flow between cellular compartments?
Distribution of electrolytes and solutes
Main electrolytes and solutes
Sodium Potassium Calcium Chloride Bicarbonate ion Glucose Plasma proteins, for example, albumin and immunoglobulin, Magnesium ion Inorganic phosphate Hydrogen ion Urea (important for the assessment of renal function)
What is concentration?
measure of the amount of a solute/electrolyte relative to the amount of water
Important for proper disease management
What does higher concentration of solutes create?
Osmotic pressure that drives water in favor of that pressure.
What might happen in a comatose patient or a patient after surgery who is unable to drink water?
Increased concentration of sodium in interstitial fluid leading to dehydration
What are some other ways the body can lose sodium?
During prolonged vomiting
In patients with diarrhea
In patients with intestinal fistulae (abnormal opening that allows the gut, fluid content to leak)
Definition of osmolality
Measure of the number of solutes per kilogram of water in the ICF or ECF compartments.
What is the osmolality of the ICF and ECF under normal conditions?
Water flows between these compartments so that under normal conditions the osmolality of the ICF and ECF are equal (isotonic)
In what organ are osmotic concentrations of the ECF and ICF different?
The kidneys
What is the osmolal gap?
difference between the observed and calculated osmolality
important in the diagnosis of acid-base imbalance
What does it mean if osmolal gap is large?
presence of another solute not considered in the usual formulae used to calculate osmolality
What is oncotic pressure?
aka colloidal pressure
osmotic pressure due to the concentration of proteins in blood plasma
proteins are too large to cross capillary vessels
What happens if capillaries are leaky?
Proteins leak out of plasma into the Interstitial fluid, and there is high oncotic pressure there
Which molecules can pass through capillary membranes?
Glucose
Amino acids (NOT PROTIENS)
triglyerides
What does decrease in oncotic pressure in the plasma do?
(due to abnormally lower plasma protein levels)
causes water movement in the opposite direction (from plasma to interstitial compartment), causing water retention in the interstitial fluid, leading to edema.
What can lead to decrease in oncotic pressure in plasma?
Leakage of capillary blood vessels due to trauma and surgery
Causes increase of oncotic pressure in interstitial fluid, which leads to edema around the cells
What is edema?
accumulation of fluid in the intersitial fluid
due to loss of colloidal/oncotic pressure in the blood plasma because of low albumin production, or cardiac failure leading to reduce effective circulating blood volume
Is the oncotic pressure of the interstitial fluid higher or lower than the oncotic pressure of the blood plasma under normal conditions?
lower
What are the predominant electrolytes contributing to the osmotic pressure in the intracellular fluid?
Potassium (90% is in the ICF)
Magnesium
Hydrogen phosphate
What do changes of the potassium concentration in the ECF do?
Can affect muscles to respond abnormally to stimuli.
What is the cellular uptake of potassium into the ICF stimulated by?
Insulin
What is hyperkalemia associated with?
Acidosis (due to electrical neutrality and acid-base balance)
What are the major electrolytes and solutes contributing to the osmotic pressure in the ECF?
Sodium (major contributor)
Bicarbonate ion
Calcium ion
What are the major electrolytes and solutes contributing to the osmotic pressure in the ECF?
Sodium (major contributor)
Bicarbonate ion
Calcium ion
Chloride ion
Glucose
Urea
Plasma proteins (plasma)
Role of bicarbonate ion in ECF
mainly responsible for buffering acidity
Role of Calcium ion in ECF
muscular contraction
in the relaxed state muscles should contain lower levels of calcium.
Role of chloride ion in ECF
Important for hydrating exocrine glands by the cystic fibrosis trans-conductance regulatory protein (CFTR)
Deficiency in maintaining Cl- concentration in ECF leads to cystic fibrosis
What does an imbalance in the solute concentrations do?
movement of water to restore that balance.
Except in the kidneys, the osmotic concentration of the ICF and ECF are equal under normal conditions
How does water move between ICF and ECF?
Water is permeable through the ICF and ECF compartments. It is not actively transported
Can be transported by facilitated diffusion (not active transport) through aquaporin (protein channels) in the distal collecting tubules of the kidneys.
What does increase in the concentration of the ECF compartment cause? (independent of amount of solute)
Out flow of water from the ICF into the ECF compartment, which causes cellular dysfunction
Stimulation of the hypothalamic thirst center
What does stimulation of the hypothalamic thirst center do?
Release of anti-diuretic hormone (ADH), known as (vasopressin), by the pituitary glands.
ADH acts on the kidney tubules to reabsorb water, leading to concentrated urine.
The movement of water involves the stimulation of aquaporin in the distal tubules by this hormone.
What symptoms does loss of fluid from the ICF lead to?
Lethargy
Unconsciousness and eventually coma
What symptoms does loss of fluid from the ECF lead to?
Reduction in blood pressure which causes, circulatory collapse
Hypoxia (inadequate supply of oxygen) and ischemia (inadequate supply of blood)
Loss of kidney function
Loss of liver function
During trauma or surgery…
loss of blood and hence fluid
the body tries to retain water by secreting vasopressin.
It is necessary to give saline drips to patients to maintain their electrolytes and the ECF and ICF compartment tonicity.
What happens during cholera?
excessive loss of water via the intestinal mucosa
loss of bicarbonate ions
The treatment is to replace ions in the body by giving a glucose/saline drip, usually composed of bicarbonate, sodium, potassium, and glucose and other electrolytes (this shows the importance of ions). Body can more easily retain water with these ions.
What causes edema?
If there is less plasma proteins/low level of albumin (hypoalbuminemia) in the blood plasma , for example, due to liver disease, this causes the oncotic pressure of the blood plasma to decrease.
This also leads to reduced blood volume and pressure.
The net movement of water is from the blood plasma (Intravascular compartment) to the Interstitial compartment (Extravascular).
This retention of water in the interstitial compartment leads to swelling, commonly known as edema
Consider a patient who is vomiting with the loss of sodium, and consequently water loss. Thus, the ECF and ICF volumes will be both low, and this patient will be fluid depleted. Assume that there is no intake of fluids and electrolytes. The body in the immediate term will try to regulate the ECF and ICF volume as follows:
Vasopressin action leads to retention of water by the kidneys.
Aldosterone action leads to reabsorption of sodium by the kidneys, which is followed by the net movement of water (osmosis)
The reabsorption of sodium will cause a rise in the osmolality of the ECF, leading to water flow from the plasma into the interstitial fluid compartment.
Eventually water moves from the ECF to the ICF because although the osmolality of the ECF initially increases, it is still less than that of the ICF, which retained most of the potassium.
What happens during dehydration or deprivation of water intake?
Increase in ECF osmolality independent of amount of sodium/solutes
Causes redistribution of water from ICF to ECF
What happens during dehydration or deprivation of water intake?
Increase in ECF osmolality independent of amount of sodium/solutes
Causes redistribution of water from ICF to ECF
Eventually, increased secretion of aldosterone and vasopressin
What would be the effect of a drug that inhibits aldosterone?
Na+ not reabsorbed, secreted in urine instead
Water follows Na+
Normal conditions and kidney reabsorption of bicarbonate ions
kidney permit reabsorption of bicarbonate ions filtered at the glomeruli into the ECF.
This is accompanied by the exchange of chloride ions to maintain ionic neutrality.
What happens in the kidney during respiratory acidosis?
kidneys secret hydrogen ions and permit normal reabsorption of bicarbonate ions.
What happens in the kidney during respiratory alkalosis?
kidneys respond by reducing the reabsorption of bicarbonate ions.
What does potassium excretion by the kidney depend on?
mainly dependent on the glomerular filtration rate, which is controlled by the kidneys.
The plasma potassium concentration is the most important factor that regulates potassium excretion in the urine.
What kind of transport is used for K+ and Na+ transport?
Active (uses ATP)
What would a defect in the movement of chloride ions into the ECF cause?
limit the movement of water into this compartment.
