Week 4 - Lecture 1b - Altered Fluid Balance Flashcards
fluid compartment
total body water = 60%
2/3 is intracellular
1/3 is extracellular
- plasma 5%
- interstitial fluid - 14% in spaces between cells
- transcellular fluid 1%
Fluid regulation
body fluid volume is regulated by kidneys
water and ions move across the cell membrane of the renal tubules
fluid movement among compartments
exchange and mixing of fluids are continuous regulation
- osmotic pressure
- hydrostatic pressure
- water moves freely along osmotic gradients
- all body fluid osmolality almost always equal
osmosis
osmotic pressure : generated as water moves across the semipermeable membrane
osmole
- an ormolu is the unit of measurement reflecting the osmotic activity of non-diffusable parties exert in pulling water from one side of the semipermeable membrane to the other
number of moles of solute that contribute to osmotic pressure
Osm- osmole
mOsm = milliosmole
osmolarity
the osmolar concentration in 1L of solution (mOsm/L)
Referring to fluids outside of the body, concentration/ volume
osmolality
the osmolar concentration in 1kg of water (mOsm/kg of H=O)
Referring to fluids within the body, concentration/mass
fluid movement among compartments
change in solute concentration of any compartment leads to net water flow
increased extracellular fluid osmolality - water leaves cell
Decreased fluid osmolality - water enters cell
fluid movement among compartments
between plasma and interstitial fluid across capillary walls
- fluid leaks from arteriolar end of capillary, reabsorbed at venule end
- lymphatics pick up remaining and return to blood
between interstitial and intracellular fluid across cell membrane
- two way osmotic flow of water
- ions are selectively pumped across the cell membrane to regulate osmosis
regulation of water intake
the thirst mechanism is the driving force for water intake
governed by the hypothalamic thirst centre
- rise in plasma osmolality - stimulates thirst
- hypothalamic osmoreceptors detect extracellular fluid osmolality
- antidiuretic hormone (ADH) is produced by the hypothalamus and stored in the posterior pituitary. it is released upon positive thirst signals from the hypothalamic osmoreceptors
- ADH increases water retention in the kidney which results in concentrated urine with less output
Additional factors
- the low blood pressure is detected by baroreceptors (stretch receptors)
- these activate a pathway that leads to the kidneys increasing their sodium retention
influence of ADH on regulation of water output
water reabsorption in collecting ducts proportional to ADH release
decreased ADH leads to dilute urine, large volume; allows water to be excreted and hence decreases volume of body fluids
increase in ADH leads to concentrated urine, small volume, reabsoption of water, which increases the volume of body fluids
hypothalamic osmoreceptors sense extracellular fluid solute concentration and regulate ADH accordingly
stretch receptors (baroreceptors) : detecting cell membrane stretch from gaining or losing water - increase of 1-2% can activate these receptors
influence of ADH pt 2
other factors may trigger ADH release
large changes in blood volume or pressure
- decrease BP - increased ADH release
- due to blood vessel baroreceptors (carotid and aorta: high blood pressure; left atrium: low blood pressure)
- Renin-angiotensin-aldosterone system (RAAS)
- leads to sodium retention by the kidneys
Factors lowering blood volume:
- intense sweating
- vomiting, or diarrhoea
- severe blood loss
- traumatic burns
- prolonged fever
Mechanism to promote fluid excretion
diuretics : drugs that increase urine production
- target : kidneys
- decrease reabsorption of sodium/water moves with sodium : water loss
- Various types impact on different segments of the tube
Loop diuretics
reduce sodium reabsorption in thick ascending loop
impair ability to concentrate urine
thiazide diuretics
prevent reabsorption in distal convoluted tubule
coupled with potassium loss
potassium sparing diuretics: aldosterone antagonist
prevent reabsorption in distal convoluted tubule
aldosterone function inhibited, no potassium loss
tonicity
osmotic pressure or tension of a solution
tonicity is determined by solutes that cannot cross the semipermeable membrane
isotonic
solution with same non-penetrating solute concentration as cytosol
hypertonic
solution with higher non-penetrating solute concentration than cytosol
hypotonic
solution with lower non-penetrating solute concentration than cytosol
disorders of water balance
fluid deficiency
- volume depletion
dehydration (negative water balance)
fluid excess
- volume excess
- hypotonic hydration ( water intoxication, positive water balance)
Fluid sequestration (edema)
fluid deficiency
output exceeds intake over a long enough period of time
- volume dehydration
- dehydration
-differ in the relative loss of water and electrolytes and resulting osmolality of extracellular fluid
volume depletion
proportionate amounts of water and sodium are lost without replacement
osmolality remains normal
- haemorrhage
- severe burns
- chronic vomiting or diarrhoea
- Addison disease (aldosterone hypo secretion)
- inadequate Na+ and water reabsorption
total body water declines
Clinical manifestations of hypovolemia
thirst dry mucous membrane weight loss flattened neck veins diminished skin turgor prolonged time for capillaries to refill after blanching decreased urine output increased heart rate decreased blood pressure altered level of consciousness
compensation kicks in to counter clinical manifestations (eg. RAAS pathway)
unsuccessful : multi system failure
dehydration
negative fluid balance
- body eliminates significantly more water than sodium
- extracellular fluid osmolality rises
- water loss due to
- profuse sweating
- water deprivation
- diuretic abuse
- endocrine disturbances
- diabetes mellitus
- ADH hyposecretions ^
dehydration affect all fluid compartments
profuse sweating
sweat glands > water loss
- water from bloodstream to capillaries of sweat glands
- blood osmolality rises - water from interstitial fluid enters bloodstreams
- tissue osmolality increases - water moves out of cells (ICF) into interstitial fluids (IF)
Both intracellular and interstitial fluid compartments lose water (eg. blood plasma)
clinically dehydration is often detected by loss of skin elasticity : turgor
slow return after pinching
tenting - sign of dahydration
Fluid excess
less common
- kidneys are highly effective at compensating for excessive intake by excreting more urine
volume excess
- sodium and water are retained
- extracellular fluid remains isotonic
- can occur due to aldosterone hypersecretion
- renal failure
Fluid excess : hypotonic hydration
cellular over hydration or water intoxication
- more water than sodium is retained
- or rapid excess water ingestion
- occur after large amount of water and salt loss
extracellular fluid osmolality decreases - leads to hyponatremia and a net osmosis into tissue cells
- swelling of cells
- severe metabolic disturbances : nausea, vomiting, muscular cramping, headache, confusion, depression of deep tendon reflexes (biceps, triceps, patellar, achilles)
- possible death
treatment
- limiting water intake or increasing sodium intake
- loop diuretics to increase water elimination
- hypertonic saline to increase water elimination
fluid sequestration : edema
atypical accumulation fluid in interstitial space
leads to tissue swelling (not cell swelling)
caused by
- increased fluid out of blood
- decreased fluid in blood
Increased fluid out of blood
imbalance In colloid osmotic pressures
oncotic pressure refers to pressure in the blood vessels due to protein content
caused by hypoproteinemia
decrease plasma protein levels leads to low colloid osmotic pressure
clinical manifestations of oedema
depend on site of occurrence
joint pain : impaired
brain, lungs : function can be so impaired that may result in death
fluid accumulation in peripheral interstitium exceeds absorption ability
- fluid becomes mobile when pressure is applied to the area
- leaves an indentation : pitting edema
