fluid shift across the capillary wall - pulmonary and systemic oedema (CVS 11)

Question 1

interstitial fluid

Answer 1

• total body water is ~60% of body weight in a 70kg young man (ie. ~42L)
• 1/3rd of total body water is extracellular (ie. ~14L)
• ~75% (ie. ~11L) of the extra cellular fluid is interstitial ie. bathing the body cells
• interstitial fluid acts as a go between blood and body cells

Question 2

components of CVS

Answer 2

• heart
• arteries
• arterioles
• capillaries
• veins

Question 3

capillaries

Answer 3

• single layer of endothelial cells
• allow rapid exchange of gases, water and solutes with interstitual fluid
• deliver nutrients and O2 to the cells
• remove metabolites from cells
• blood flow in the capillaries depends on the contractile state of the arterioles

Question 4

what is blood flow in the capillaries dependent on

Answer 4

contractile state of the arterioles

Question 5

regulation of blood flow at capillary bed

Answer 5

• terminal arterioles regulate regional blood flow to the capillary bed (CB) in most tissues
• ‘precapillary sphincters’ regulate flow in a few tissues (eg. mesentery)
• blood flow through CB is very slow to allow adequate time for exchange
• capillaries unite to form venules

Question 6

transport across capillary wall

Answer 6

• small, water soluble substances (ie.hydrophillic) pass through the water-filled pores (Na+, K+, glucose, amino acids)
• lipid soluble substances (ie.lipophillic) pass through the endothelial cells (O2, CO2)
• exchangeable proteins are moved across by vesicular transport
• large molecules eg.plasma proteins cannot generally cross the capillary wall (plasma proteins generally cannot cross the capillary wall)
• fluid movement follows pressure gradient (bulk flow)
• ‘movement of gases and solutes’ follow fick’s law of diffusion (ie. downhill)

Question 7

transcapillary fluid flow

Answer 7

• transcapillary fluid flow is passively driven by pressure gradients across the capillary wall
• it is ultra-filtration (ie. exchange across the capillary wall of essentially protein-free plasma)
• net filtration pressure (NFP) = fources favouring filtration - forces opposing filtration
• a filtration co-efficient (Kf) will also affect net fluid filtration

Question 8

NFP

Answer 8

• net filtration pressure (NFP) = fources favouring filtration - forces opposing filtration
• a filtration co-efficient (Kf) will also affect net fluid filtration

Question 9

forces involved in trascapillary fluid flow (strarlings flow)

Answer 9

• forces favouring filtration:
• > Pc - capillary hydrostatic pressure
• > 3.14/pieI- interstitial fluid osmotic pressure
• forces opposing filtration:
• > pie/3.14c - capillary osmotic pressure
• Pi- interstitual fluid hydrostatic pressure (-ve in some tissues)
• > starlings forces favour filtration at arteriolar end, reabsorption at venular end
• > capillary hydrostatic pressure decreases as you go along capillary (is +ve value when filtration is favoured, -ve value when reabsorption is favoured)

Question 10

NFP at arteriolar and venular ends of skeletal muscle capillaries

Answer 10

• NFP= forces favouring fitration - forces opposing filtration
• NFP = (PC + 3.14i) - (3.14C + Pi)
• NFP arteriolar end = (35+1) - (25+1) = +10mmHg
• NFP venular end = (17+1) - (25+1) = -8mmHg
• > starlings forces favour filtration at arteriolar end, reabsorption at venular end
• > during a day, filtration exceeds reabsorption by 2-4 litres
• > excess fluid is returned to the circulation via the lymphatics as lymph

Question 11

effect of distance along capillary on forces involved in transcapillary fluid flow

Answer 11

capillary hydrostatic pressure decreases as you go along capillary (is +ve value when filtration is favoured, -ve value when reabsorption is favoured)

Question 12

major forces involved in systematic transcapillary fluid flow

Answer 12

• forces favouring filtration = capillary hydrostatic pressure
• forces opposing filtration = capillary osmotic pressure

Question 13

starlings forces in pulmonary capillaries

Answer 13

• pulmonary resistance is only ~10% of that of the systematic circulation
• pulmonary capillary hydrostatic pressure is low (~8-11mmHg)
• capillary osmotic pressure at 25mmHg
• efficient lymphatic drainage remove any filtered fluid thus preventing accumulation of interstitial fluid

Question 14

oedema

Answer 14

• accumulation of fluid in interstitial space

- diffusion distance increases, gas exchange compromised in pulmonary oedema

Question 15

causes of oedema

Answer 15

1. raised capillary pressure
2. reduced plasma osmotic pressure
3. lymphatic insufficiency
4. changes in capillary permeability

Question 16

what causes raised capillary pressure (which is a cause of oedema)

Answer 16

• arteriolar dilation
• raised venous pressure
• > left ventricular failure - pulmonary oedema
• > right ventricular failure - peripheral oedema (eg. ankle, sacral)
• > prolonged standing - swollen ankles

Question 17

what causes reduced plasma osmotic pressure (which is a cause of oedema)

Answer 17

• normal plasma protein concentration = 65-80 g/l
• oedema if malnutrition
• > protein malabsorption
• > excessive renal excretion of protein
• > hepatic failure

Question 18

effect of heart failure (HF) on frank starlings curve

Answer 18

shifts the curve to the right (even with treatment)

Question 19

what causes lymphatic insufficiency (which is a cause of oedema)

Answer 19

• lymph node damage

- filariasis-elephantiasis

Question 20

what causes changes in capillary permeability (which is a cause of oedema)

Answer 20

• inflammation

- histamine which increases leakage of protein

Question 21

pulmonary oedema

Answer 21

• left ventricular failure = pulmonary oedema
• pulmonary oedema is the accumulation of fluid in the interstitial and intra-alveolar lung spaces
• it is manifested clinically by varying degrees of shortness of breath
• chest X-rays shows haziness in perihilar region

Question 22

pitting oedema

Answer 22

• ankles

- sacrum