Basic Water And Electrolyte Homeostasis Flashcards
Where is water stored in the body?
Outline to distribution in a 70kg man
Intracellular compartment - cytosines and organelles
Extra cellular compartment - vascular, extravascular, plasma, interstitial, CSF, vitreous humour, GI, pleural and pericardial
28L in intracellular compartment
14L in extracellular: 10.5 in interstitial and 3.5 in plasma
What electrolytes are found in the body?
Cations (+ve): sodium, potassium, magnesium, calcium and hydrogen
Anions(-ve): chloride, bicarbonate, phosphate, sulphate, organic acids and proteins
How can electrolytes move between compartments?
Across cell membrane by passive diffusion or active transport
Across capillary membranes civil size exclusion passive diffusion
What is the anion gap?
(Na + K) - (Cl + Bicarbonate)
Should be 10-16
What is electroneutrality?
State of net charge of 0 achieved in any compartment
What three forces control water and solute movement?
Oncotic pressure (colloid osmotic pressure) Osmotic pressure Hydrostatic pressure
What is oncotic pressure?
Form of osmotic pressure exerted by proteins that determines distribution of fluid between blood vessels and intersitial fluid
What is osmotic pressure?
Ability of a solution to causes osmosis
Controls water movement between compartments speared by semi-permeable membranes
Water moves from dilute compartment( low osmotic pressure) to concentrated compartment (high osmotic pressure)
What is difference between osmolality and osmolarity?
Osmolality : amount of solute in 1Kg of water (mmol/Kg)
Osmolarity : amount of solute in 1L of water (mmol/L), is temperature dependent
In water osmolality=osmolarity
In plasma some of the volume taken up by substances : and water volume 6% less then total volume.
Major solutes that contribute: Na, Cl, K, PO4, urea and glucose
What is the calculation of osmolality?
2*(Na+K) + urea + glucose
What is the osmolar gap?
Osmolality - osmolarity
Increased gal suggested presence of other osmotically active solutes in plasma
What is hydrostatic pressure?
Mechanical pressure generated by heart against the fluid membrane
What are the three IV fluids, which compartments do they enter?
5% dextrose crystalloid : plasma, interstitial fluid and intracellular
0.9% saline: plasma and interstitial fluid
Plasma/blood: plasma
How are changes to water balance detected?
1) osmolality : osmoreceptors detect changed in ECF. Sensitive to 1-2% changes to osmolality
2) volume : changes to intravascular column detected by baroreceptors. Low volume = right atrium and veins. High pressure = carotid sinus. Input to hypothalamus. 8-10% changes
What is produced due to changes in water balance?
ADH
Made by hypothalamus
How is sodium balance achieved?
Aldosterone to increase sodium reabsorption
Renin-angiotensin-aldersterone axis
Reunion secreted by decreased BP and sodium levels
What are the natriuretic peptides?
Three isoforms;
Atrial natriuretic peptide (ANP) - stretch in atrium walls
Brain/b type natriuretic peptide (BNP) - stretch in ventricle walls
C-type natriuretic peptide (CNP)
Released in response to increased plasma volume
How do the natriuretic peptides control plasma volume?
Inhibit vasopressin, increase GFR, decrease renin and inhibit aldersterone: cause water and sodium chloride secretion
Decrease blood pressure via medulla oblongata
What effect does aldersterone have on K?
Increase excretion form kidneys
Released due to hyperkalaemia
What occurs in diabetes insipidus?
No effective ADH - water loss
Cranial - no ADH secretion
Nephrogenic - reduced ADH action in kidneys
Treat with DDAVP
What diseases cause disturbances in aldersterone?
Addisons : hypoaldosterism. Sodium loss and potassium excess. Treat with Fludrocortisone
Conn’s: primary hypoaldosterism. Sodium excess and potassium loss (seen in cushing’s - excess cortisol)
What is oedema?
What is caused by?
Water excess independent of sodium
Increase in hydrostatic pressure
Decrease in oncotic pressure
Increased permeability of capillary membrane
Lymphatic obstruction
How is water/sodium disturbance treated?
Treat cause
If deficient replace with IV
If overloaded restrict fluids
How is fluid deficit calculated?
TBW * (actual sodium - normal sodium/actual sodium
TBW=body weight * 0.6
Normal sodium= 140