Water Homeostasis Flashcards
What is the total body water (in a 70kg male)?
What % of this is the body weight?
42L
60% of body weight
What is the total body water (42L) split into?
Intracellular fluid - 28L (40% of body weight)
Extracellular fluid - 14L (20% of body weight)
What is the predominant electrolyte in intracellular fluid?
K+
What are the predominant electrolytes in extracellular fluid?
Na+
Chloride
Bicarbonate
Ca2+ especially in heart & muscle
What is the extracellular fluid split into and measurements?
Interstitial fluid (surrounds cells) - 11L Plasma (in blood vessels) - 3L
What is sensible & insensible loss & examples?
Sensible = measurable - urine, vomit Insensible = immeasurable - sweat, breath
What is the main regulator of water loss?
Kidneys
Where does water intake come from?
Drink, diet and IV fluid
What’s the regulating hormones for water homeostasis?
ADH, aldosterone and Atrial natriuretic peptide
What is osmolality?
Measure of the number of dissolved particles by kg of fluid
Conc/KG solution
What is osmolarity?
Measure of the number of dissolved particles per L of fluid
Conc/Litre solution
What is osmotic pressure?
Pressure applied to a solution, by a pure solvent, required to prevent inward osmosis through a semipermeable membrane
*What is oncotic pressure?
Albumin pressure on Capillery wall keeping fluid in
- what is hydrostatic pressure?
Pressure difference between Capillary blood (plasma) and interstitial fluid –water & solutes move from plasma into interstitial fluid
What happens when water is lost from extracellular fluid?
Increase in solutes / decrease in water
Increase in osmolality in ECF
Change detected by osmoreceptors in the hypothalamus
Results in ADH release from posterior pituitary gland
ADH acts to increase water reabsorbtion in the collecting ducts of the kidney in order to dilute the solute and return water in ECF to normal
What happens when there is decreased renal blood flow?
Decreased water in ECF=decrease in effective circulating volume=decrease in renal blood flow
Renin release from juxtaglomerular cells in kidneys
Raas system- renin converts angiotensinogen to angiotensin I
Angiotensin converting enzyme (ACE) then converts angiotensin I into angiotensin II
Triggers the release of aldosterone from the adrenal cortex above the kidneys
Aldosterone increases Na+ reabsorption in the kidneys (water also reabsorbed) in exchange for potassium or hydrogen excretion
stimulates ADH release
Sodium reabsorption brings water with it to return water in ECF to normal
What is water controlled by?
Changing osmolality (sodium)
What does RAAS stand for and what does it do?
Renin angiotensin aldosterone system
Maintain BP
When is the RAAS system activated and by what cells?
Decreased BP (juxtaglomerular cells in afferent arteriole-kidney) Decreased NaCl (macula densa of DCT)
What happens in the RAAS system?
Renin (from the juxtaglomerular cells in kidneys) turns angiotensinogen (from liver) to angiotensin I
ACE (angiotensin converting enzyme) from the lungs turns angiotensin I to angiotensin II
This stimulates the release of ADH & Aldosterone
Stimulates sympathetic NS too
What does ADH do?
Secreted by posterior pituitary gland
Acts on APII proteins on collecting ducts
Increase collecting duct permeability - reabsorbs more water
Increase extracellular fluid= increased BP
What does aldosterone do?
Secreted by adrenal cortex (suprarenal cortex)
Increased Na+ reabsorbtion in ascending limb of Loop of Henle - excretes K+ in exchange
H2O follows Na+ (obligatory H2O movements)
Increase BP as ECF increases
What can excess water cause?
 oedema
What causes dehydration?
Water deprivation, vomiting, diarrhoea, burns, heavy sweating, diabetes insipidus (urinate a lot since too little ADH) diabetes mellitus, drugs
What are the consequences of dehydration?
Thirst, dry mouth, in elastic skin, sunken eyes, raised haematocrit (viscosity of blood), weight loss, confusion, hypotension
What are the causes and consequences of water excess?
High intake/decreased loss of water, excess ADH
Hyponatraemia (low Na levels), cerebral over perfusion (due to high blood vol and pressure)- causes headaches, confusion & convulsions
What is serous effusion?
Excess water in a body cavity
What is oedema?
Excess water in the intercellular tissue space
What is high & low sodium called?
Hypernatraemia
Hyponatraemia
What is high & low potassium called?
Hyperkalaemia
Hypokalaemia
What is high & low calcium called?
Hypercalcaemia
Hypocalcaemia
