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Medical Pathophysiology > Week 4 - Lecture 1b - Altered Fluid Balance > Flashcards

Week 4 - Lecture 1b - Altered Fluid Balance Flashcards

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USMLE® Step 1 flashcards
1
Q

fluid compartment

A

total body water = 60%

2/3 is intracellular

1/3 is extracellular

  • plasma 5%
  • interstitial fluid - 14% in spaces between cells
  • transcellular fluid 1%
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2
Q

Fluid regulation

A

body fluid volume is regulated by kidneys

water and ions move across the cell membrane of the renal tubules

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3
Q

fluid movement among compartments

A

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
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4
Q

osmosis

A

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

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5
Q

osmolarity

A

the osmolar concentration in 1L of solution (mOsm/L)

Referring to fluids outside of the body, concentration/ volume

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6
Q

osmolality

A

the osmolar concentration in 1kg of water (mOsm/kg of H=O)

Referring to fluids within the body, concentration/mass

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7
Q

fluid movement among compartments

A

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

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8
Q

fluid movement among compartments

A

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
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9
Q

regulation of water intake

A

the thirst mechanism is the driving force for water intake

governed by the hypothalamic thirst centre

  1. rise in plasma osmolality - stimulates thirst
  2. hypothalamic osmoreceptors detect extracellular fluid osmolality
  3. 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
  4. ADH increases water retention in the kidney which results in concentrated urine with less output

Additional factors

  1. the low blood pressure is detected by baroreceptors (stretch receptors)
  2. these activate a pathway that leads to the kidneys increasing their sodium retention
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10
Q

influence of ADH on regulation of water output

A

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
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11
Q

influence of ADH pt 2

A

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
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12
Q

Mechanism to promote fluid excretion

A

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
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13
Q

Loop diuretics

A

reduce sodium reabsorption in thick ascending loop

impair ability to concentrate urine

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14
Q

thiazide diuretics

A

prevent reabsorption in distal convoluted tubule

coupled with potassium loss

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15
Q

potassium sparing diuretics: aldosterone antagonist

A

prevent reabsorption in distal convoluted tubule

aldosterone function inhibited, no potassium loss

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16
Q

tonicity

A

osmotic pressure or tension of a solution

tonicity is determined by solutes that cannot cross the semipermeable membrane

17
Q

isotonic

A

solution with same non-penetrating solute concentration as cytosol

18
Q

hypertonic

A

solution with higher non-penetrating solute concentration than cytosol

19
Q

hypotonic

A

solution with lower non-penetrating solute concentration than cytosol

20
Q

disorders of water balance

A

fluid deficiency
- volume depletion
dehydration (negative water balance)

fluid excess

  • volume excess
  • hypotonic hydration ( water intoxication, positive water balance)

Fluid sequestration (edema)

21
Q

fluid deficiency

A

output exceeds intake over a long enough period of time

  1. volume dehydration
  2. dehydration

-differ in the relative loss of water and electrolytes and resulting osmolality of extracellular fluid

22
Q

volume depletion

A

proportionate amounts of water and sodium are lost without replacement

osmolality remains normal

  1. haemorrhage
  2. severe burns
  3. chronic vomiting or diarrhoea
  4. Addison disease (aldosterone hypo secretion)
    - inadequate Na+ and water reabsorption

total body water declines

23
Q

Clinical manifestations of hypovolemia

A 
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

24
Q

dehydration

A

negative fluid balance
- body eliminates significantly more water than sodium

  1. 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

25
Q

profuse sweating

A

sweat glands > water loss

  1. water from bloodstream to capillaries of sweat glands
    - blood osmolality rises
  2. water from interstitial fluid enters bloodstreams
    - tissue osmolality increases
  3. water moves out of cells (ICF) into interstitial fluids (IF)

Both intracellular and interstitial fluid compartments lose water (eg. blood plasma)

26
Q

clinically dehydration is often detected by loss of skin elasticity : turgor

A

slow return after pinching

tenting - sign of dahydration

27
Q

Fluid excess

A

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
28
Q

Fluid excess : hypotonic hydration

A

cellular over hydration or water intoxication

  1. more water than sodium is retained
  2. 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
29
Q

fluid sequestration : edema

A

atypical accumulation fluid in interstitial space

leads to tissue swelling (not cell swelling)

caused by

  1. increased fluid out of blood
  2. decreased fluid in blood
30
Q

Increased fluid out of blood

A

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

31
Q

clinical manifestations of oedema

A

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

Medical Pathophysiology (30 decks)

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