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Physiology > Fluid Balance and Administration > Flashcards

Fluid Balance and Administration Flashcards

1
Q

What are the three main components of bodily fluid?

A

Water
Ions
Protein

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2
Q

What forces affect the net movement of fluid through the capillary membrane?

A

Osmotic pressure

Hydrostatic pressure

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3
Q

What percentage of humans is water?

A

Men - 60%
Women - 50% - due to higher body fat
Children - 75%

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4
Q

Definition of total body water?

A

Total volume of water in the body

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5
Q

Definition and components of extracellular fluid?

A

Total volume of fluid outside of cells - intravascular (plasma), interstital (in tissues), lymphatics and transcellular

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6
Q

Definition of intracellular fluid?

A

Total volume of fluid inside of cells (TBW - ECF)

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7
Q

Definition of plasma

A

Blood without cells - contains protein, water and electrolytes

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8
Q

Definition and examples of transcellular fluid?

A

The smallest component of ECF (mostly made up of interstital fluid and plasma). Fluid contained in epithelial lined spaces

  • CSF
  • intraocular fluid
  • pleural
  • synovial
  • digestive secretions
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9
Q

What are third space losses?

A

Fluid ‘lost’ into the transcellular fluid spaces as it can’t be exchanged with rest of ECF

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10
Q

Breakdown of total body water in average 70kg man.

A

45L total

  • 2/3 intracellular (30L)
  • 1/3 extracellular (15L) - plasma 3.5L, lymph 1.5L, interstital 8.5L and transcellular 1.5L
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11
Q

What are the main ions in ECF?

A

Sodium 135 - 145mmol/l

Bicarbonate - 25mmol/l

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12
Q

What are the main ions in ICF?

A

Potassium - 150mmol/l
Magnesium - 40mmol/l
Phosphate - 100mmol/l
Protein - 8mmol/l

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13
Q

By what cellular pump are potassium levels maintained?

A

Na+/K+ ATPase pump

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14
Q

What are the 5 main ways that plasma potassium levels are controlled?

A

1) dietary intake
2) renal excretion
3) plasma pH
4) endocrine hormones
5) medications

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15
Q

Describe how renal excretion can affect plasma potassium levels.

A

Filtration at the glomerular apparatus depends on the plasma concentration
In the collecting duct, sodium reabsorption depends on exchange with potassium (controlled by aldosterone)
- aldosterone is produced in response to low BP, hyponatraemia and/or hyperkalaemia

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16
Q

Describe how plasma pH can affect plasma potassium levels.

A

Hydrogen ions move in and out of cells in exchange for potassium

  • when hydrogen ion levels in plasma rise, they are exchanged with intra-cellular potassium, causing hyperkalaemia
  • when hydrogen ion levels in plasma drop, intra-cellular hydrogen ions are are exchanged with extra-cellular potassium, causing hypokalaemia
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17
Q

Describe how endocrine hormones can affect plasma potassium levels.

A

e. g. insulin, adrenaline and aldosterone stimulate cellular uptake of potassium
- hyperaldosterinism causes hypokalaemia, and can be caused by cirrhosis, renal artery stenosis, heart failure and nephrotic syndrome

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18
Q

Describe how different medications can affect plasma potassium levels.

A

Hyperkalaemia - ACEI, ARBs, ciclosporin

Hypokalaemia - loop + thiazide diuretics, beta2-agonists, corticosteroids, amphotericin and theophylline

19
Q

What effects does plasma sodium have on the body

A

Excess - water retention

Deficiency - neuromuscular dysfunction

20
Q

How is sodium excreted?

A

Freely filtered through glomerular membrane
- around 65% freely reabsorbed in PCT
- remainder actively reabsorbed via sodium-potassium pumps in the ascending loop of Henle
ANP
- stimulated by fluid overload and decreases sodium reabsorption
ADH
- stimulated by increasing plasma osmolality and increases water reabsorption in DCT

21
Q

Describe plasma protein movement within the body

A

Capillary beds are impermeable to proteins, so they remain within the capillary and generate osmotic pressure
Albumin 75% of this osmotic pressure

22
Q

Describe the capillary bed and how this has a role in fluid movement.

A

This is the site of fluid and solute exchange between bloodstream and interstitum
Flow to capillary beds is controlled by pre-capillary sphincter
Capillary wall - single layer of endothelial cells surrounded by a basement membrane
- potential space between adjacent cells regulates permeability

23
Q

Definition of osmotic pressure

A

Difference in concentration between particles in solution on either side of a semi-permeable membrane (can be proteins or ions)
- water is drawn by higher osmotic pressures

24
Q

Definition of capillary osmotic pressure

A

Pressure generated by the plasma proteins in the capillaries

25
Q

Definition of tissue osmotic pressure

A

Pressure generated by the interstitial fluid

  • affected by interstitial protein concentration and capillary wall permeability to proteins
  • the more permeable to capillary wall is to proteins, the higher the tissue osmotic pressure
26
Q

Definition of hydrostatic pressure

A

The difference between capillary pressure and pressure of interstitial fluid within tissues

27
Q

What factors affect the capillary and the tissue hydrostatic pressure?

A 
Capillary hydrostatic pressure
- blood pressure
- pulse pressure
Tissue hydrostatic pressure
- interstitial fluid volume
- tissue compliance
28
Q

Describe the Starling hypothesis

A

Factors affecting the net flow of fluid/solutes across the capillary bed

  • osmotic pressure is normally constant (25mmHg) as there are few proteins in the interstitial fluid and the capillary wall is normally impermeable
  • tissue hydrostatic pressure is constant, so movement of fluid is determined by capillary hydrostatic pressure (so due to blood pressure; net flow of water out of capillary at arterial end, and net flow of water into capillary at venous end)
29
Q

Starling’s equation

A

K = outward pressure - inward pressure

K - filtration constant for capillary membrane
OP - capillary hydrostatic pressure + tissue osmotic pressure
IP - tissue hydrostatic pressure + capillary osmotic pressure

30
Q

Causes of oedema

A 
Increased capillary hydrostatic pressure
- fluid overload
- venous obstruction
Decreased capillary osmotic pressure
- hypoproteinaemia (cirrhosis, nephrotic syndrome)
Increased tissue osmotic pressure
- increased capillary permeability (burns/inflammation)
Decreased tissue hydrostatic pressure
31
Q

Average daily water balance

A 
Input - 2600mls
- oral fluid - 1500mls
- food - 750mls
- metabolism - 350mls
Output - 2600mls
- urine- 1500mls
- faeces - 100mls
- lungs 400mls
32
Q

Average daily electrolyte maintenance requirements (mmol/l)

A 
Sodium - 70-140
Potassium - 70
Chloride - 70
Phosphate - 14
Calcium - 7.0
Magnesium - 7.0
33
Q

How does fever affect daily water requirements

A

20% increase for each 1 degree of temperature increase

- 1L/24 hour extra

34
Q

Average urine output.

A

0.5ml/kg/hr

35
Q

Sources of excess fluid loss in surgical patients

A

Blood loss (trauma, surgery)
Plasma loss (burns)
Intra-abdominal inflammatory fluid loss (pancreatits)
Sepsis
Abnormal insensible loss (fever)
GI fluid loss (vomiting, bowel obstruction, diarrhoea, ilesotomy)

36
Q

What should be taken into account when assessing fluid balance

A 
Patient size and age
Abnormal ongoing losses, pre-existing deficits/excesses, fluid shifts
Renal and CV function
Fluid balance charts
Serum electrolytes
37
Q

How does the body try to prevent fluid lost post-op?

A

Surgical trauma causes ADH and aldosterone release. This promotes water conservation via sodium retention and potassium excretion

38
Q

How to assess fluid depletion

A

History - thirst, fluid loss, fluid balance chart
Examination - dry mucous membranes, decreased capillary refill time, sunken eyes, low skin elasticity, decreased urine output, increased heart rate, decreased blood pressure/pulse pressure and confusion)

39
Q

Methods of fluid replacement

A

Enteral - preferred if GI tract functioning and there isn’t an excessive deficiency
Parenteral
- GI tract non-functioning
- needs rapid fluid replacement

40
Q

Types of parenteral fluid replacement

A 
Crystalloid (isotonic)
- normal saline, 5% dextrose, Hartmann's and dextrose saline (0.18% saline and 4% dextrose)
Colloid
- human albumin 
- gelatins (haemaccel, gelofusion)
Dextrans (40 or 70)
Hetastarch (6% in saline)
Blood
41
Q

Describe an IV regimen to meet basal fluid requirements

A

3L fluid/24 hours - either 2 saline:1 dextrose, or 2 dextrose: 1 saline.

  • plus 60mmol/l potassium
  • doesn’t account for patients requiring fluid resuscitation
42
Q

What to assess when correcting dehydration

A

Which compartment has lost fluid
- e/g/ bowel losses are from ECF
Extent of dehydration

43
Q

Why should no potassium be given in the 24 hours post-op

A

Potassium rises during surgery due to

  • cellular injury
  • blood transfusions
  • decreased renal potassiunm clearance due to transient renal impairment
  • ‘stress hormones’ encourage potassium release from cells

Physiology (7 decks)

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