Fluid Shifts

Question 1

What is interstitial fluid and how much of the body does this make up ?

Answer 1

Acts as the go-between blood and body cells
-Total body water is around 60% of body weight in a 70 kg (i.e. ~ 42 L) young man
-1/3rd of body water is extracellular (i.e. ~ 14 L)
-Around 75% (i.e. ~ 11 L) of extracellular fluid (ECF) is interstitial i.e. bathing the body cells

Question 2

What are the main mechanism driving direction and magnitude of fluid movement ?

Answer 2

Hydrostatic pressure
-Capillary hydrostatic pressure (Pc)
-Interstitial hydrostatic pressure (Pi)
Osmotic pressure (colloid oncotic pressure or oncotic pressure)
-Capillary oncotic pressure (πc)
-Interstitial oncotic pressure (πi)

Question 3

What is oncotic pressure ?

Answer 3

Pressure exerted by plasma proteins in the blood that pulls water into the circulatory system

Capillary oncotic pressure (πc)
-Opposes filtration
-Product of presence of proteins in capillary lumen e.g. albumin, affected by (mal)nutrition

Interstitial oncotic pressure (πi)
-Favours filtration
-Product of presence of proteins in the interstitial fluid

Aka colloid osmotic pressure

Filtration = fluid movement from vessel into interstitial fluid

Question 4

What is hydrostatic pressure ?

Answer 4

Force exerted by blood against the walls of the vessels

Capillary hydrostatic pressure (Pc)
-Favours filtration
-Product of arterial and venous pressures
-Highest at arterial end of capillary and falls along length

Interstitial hydrostatic pressure (Pi)
-Opposes filtration
-Normally single digit values or slightly negative

Filtration = fluid movement from vessel into interstitial fluid

Question 5

What are changes in factors which encourage fluid movement into the interstitium ?

Answer 5

Changes in any one of Starling’s forces alter the direction and magnitude of fluid movement across the capillary wall
-Increased capillary hydrostatic pressure (Pc)
-Decreased capillary oncotic pressure (πc)
-Increased capillary permeability
-Decreased lymphatic drainage

Question 6

What is Oedema and some specific types ?

Answer 6

Fluid accumulation in the interstitial space

Pulmonary oedema – fluid accumulation in the interstitial spaces of the lung. Can cause;
-Hypoxia
-Dyspnoea

Peripheral/systemic oedema– fluid accumulation in the periphery i.e. legs

Both largely managed with diuretics

Can be caused by high venous pressure

Question 7

How are the lungs protected against oedema ?

Answer 7

Pulmonary resistance is only ~10% of that of the systemic circulation
-Pulmonary capillary hydrostatic pressure is low (~ 8-11 mmHg) but capillary osmotic pressure at 25 mmHg. Osmotic pulling in > hydrostatic being pulled out
-Efficient lymphatic drainage remove any filtered fluid thus preventing accumulation of interstitial fluid

Lungs v sensitive to fluid accumulation

Question 8

How can systemic oedema cause orthopnea ?

Answer 8

Fluid accumulates in ankles etc when stood up but when lie down no gravity so fluid redistributes centrally
-Orthopnea = Shortness of breath when lying down

Question 9

What is BNP and what are its uses ?

Answer 9

B-type natriuretic peptide (a hormone)
-Measures stretch of heart; too much volume can increase it
-Increased BNP means heart failure likley
-Released mainly from ventreicular myocardia

Causes natriuresis, diuresis, and vasodilation (counteracts RAAS)