5. Body Fluid Compartments and Water Balance Flashcards
What is water balance?
How is water distributed in the body?
How does composition of extracellular fluid (plasma and interstitial fluid) differ from intracellular fluid?
Water gain and loss should = 0 for body to be in hydration balance. NB: fluid secreted into digestive system around 9L/day. Fluid reabsorbed around 8.9L/day. Faeces around 100ml.
Most in cells = intracellular fluid, need for chemical reactions etc. Some in extracellular fluid (plasma and intersitial fluid). And a small bit in other ECF e.g. lymph.
ECF: plasma and IF look similar (biggest difference is protein present in plasma, not IF). Have HCO3- (imp buffer), Na+, Cl-, very little K+. ICF: lots of K+, little Na+, lot of PO43- (imp buffer), lots of protein.
NB: remember it like an island in sea (NaCl) with banana tree on (rich in K+ and phosphate)
What are the major solutes contributing to osmolality?
What are the 2 classifications of solutes?
Define:
a) osmolarity
b) osmolality
What is normal plasma osmolality?
Na+, Cl-, glucose, urea
Electrolytes (inorganic salts e.g. Na+, K+, Cl-, all acids and bases, some proteins. GREATER OSMOTIC POWER THAN NON-ELECTROLYTES)
Non-electrolytes (e.g. glucose, lipids, creatinine and urea)
a) measure of solute concentration expressed as the number of osmoles (Osm) of solute per litre of solution = Osm/L or mOsm/L. BUT temperature and presure can change volume = change osmolarity so not a stable unit. Instead use:
b) measure of osmoles of solute per kilogram of solvent = Osm/Kg or mOsm/Kg. Not affected by temp/pressure changes.
300mOsm/Kg
What are the 3 fluid ‘zones’ for homeostasis in the body?
What are the different types of membrane transport, and where are they used in terms of the homeostasis ‘zones’?
What are aquaporins?
Changing environment e.g. gut lumen, Buffer zone (plasma and interstitial fluid = ECF, held at equilibrium between plasma and IF), Steady state zone (ICF)
Simple diffusion, facillitated diffusion (via membrane transport protein), primary active transport (via membrane transport protein and ATP). All of them are used between changing env and buffer, and between steady state and buffer, but only 1 and 2 between plasma and IF becuase use hydrostatic pressure etc.
For facilitated diffusion of water - transports H2O across a membrane
What is the predominant extracellular cation?
Why does it hold a central position in fluid and electrolyte balance?
Na+ (major determinant of plasma and ECF osmollity)
Where Na+ goes H2O follows. Thus if plasma volume and osmolality are regulated, then both total H2O and Na+ will be regulated.
Describe how the total body water and total body sodium are regulated by the CNS.
1. CNS: THIRST MECHANISM: structures in hypothalamus and pituitary characterised by extensive vasculature and lack of normal BBB = allows linkage between CNS and peripheral blood flow. NB: if destroyed = lose thirst. If increase in plasma osmolality (1-2%) -> osmoreceptors in hypothalamus -> cerebral cortex -> drink (also if decreased plasma volume (10%) -> decreased BP -> RAAs). Thus plasma osmolality decreases.
MONITORING of plasma osmolality/volume: baroreceptors in heart -> medulla oblongata -> hypothalamus -> posterior pituitary -> ADH via supraoptic and
What are the 3 ways the total body water and total body sodium are regulated?
What factors can trigger ADH release?
Describe how the total body water and total body sodium are regulated by hormones.
1) CNS 2) Hormones 3) Kidneys
Prolonged fever, excessive sweating, vomiting, diarrhea, severe blood loss, traumatic burns
ADH/vasopressin released from post pituitary when increased plasma osmolality and decreased plasma volume (detected by baroreceptors). Makes cells of CD and DCT permeable to H2O = more reabsorption. Targets aquaporins - directly influences how many receptors are on apical membrane.
Aldosterone (main regulator of Na+) released from adrenal cortex when reduced Na+ or increased K+. Causes increased Na+ resoprtion and K+ secretion via upregulating kidney Na+/K+ ATPase and ENaC. BUT Na+ doesn’t belong in there so as soon as it’s pumped back in from urine filtrate, it’s put back into ECF. -ve fb.
ANP released from cardiac atria when increased blood volume and causes Na+ secretion via decreasing aldosterone, ADH and renin + angiotensin II release = decreased BP
List the 4 hormones that regulate plasma osmolality and volume.
Roughly what % of H2O is reabsorbed in the PCT?
Why is it not 100%?
ADH (DCT and CD more permeable to water), angiotensin II and aldosterone (reabsorption of Na+ and Cl- when dehydrated), ANP (excretion of Na+ and Cl- followed by H2O to decrease blood vol)
80% (via active reabsorption of solutes)
Allows for variable reabsorption at end of tubule depending on individual’s water balance status (depending on relative presence/absence of aquaporins and Na+/K+ pumps etc. in kidneys - related to aldosterone and ADH levels)
What is
a) normal 24hr urine osmolality in a healthy person?
b) random urine osmolality in a healthy person?
c) urine osmolality after 12-24 hours of fluid intake restriction?
List 3 failures of body fluid balance.
a) 500 - 800 mOsm/Kg
b) 50 - 1400 mOsm/Kg
c) >800 mOsm/Kg = base for water deprivation test
Dehydration, water intoxication, oedema.
Describe:
a) dehydration
b) water intoxication
c) oedema (and it’s 4 main causes)
a) Water intake insufficient to cover loss over a long period of time, then water lost from plasma and IF -> weight loss, fever, confusion. Excess urine prod can occur in diabetes. Young (kidney not developed) and old (decline of kidney function) are most affected.
b) Too rapid rehydration or complete blockage of urine production = dilution of body fluids. Na+ in ECF reduced = osmotic shift of H2O into cells -> cell swelling (dangerous in brain = coma etc.)
c) tissue swelling due to accumultion of fluid in ECF, 4 main causes:
1. capillary hydrostatic pressure > osmotic pressure = fluid leaves capillary e.g. portal hypertension in cirrhosis
2. loss of plasma proteins = weakens OP to return H2O to blood e.g. ascites
3. obstruction of lymph circulation
4. increased capillary permeability (inflammation) - capillary more permeable to let monocytes through but lets H2O through too
PIC = number 2
Explain why a high salt diet is not good for the body.
What causes lactose intolerence?
Increased Na+ and Cl- in the ‘environment’ zone -> increase in ECF plasma and IF Na+ and Cl-. Water brings Na+ conc back to normal but the Na+ conc overall remains high and there’s extra water volume in the ECF due to ADH release and thirst = increased pressure in cardiovascular system.
The kidney retains water to dilute the blood and there’s an increase in angiotensin II which constricts kidney BVs = consistantly high BP -> kidney failure
Mutations in gene coding for lactase (LCT) or for the Na+ dependant glucose transporter (SLC5A1) = lactose is not digested. Thus lactose goes through digestive tract and takes H2O with it (b/c has osmotic power) -> fermented in colon -> diarrhea.
What happens in hyperglycemia?
Lots of glucose in plasma b/c can’t regulate it properly -> IF also builds up glucose (b/c plasma and IF systems roughly the same) -> normally, glucose brought into cell via glucose receptor (requires working insulin receptor and K+ to be moved intracellularly too) BUT aquaporin lets H2O out of cells b/c glucose in ECF osmotically active -> cells dehydrated. Dilutes electrolytes in blood, and Na+ in ECF. K+ store to ‘top up’ K+ in cell, but not for Na+. Glucose extracted in glomerulus and some not reabsorbed = remains in urine and takes H2O with it = dehydrated.
