Capillaries II

Question 1

What is the importance of fluid exchange?

Answer 1

→ Important for normal physiological function

→ Fluid reabsorption from tissues to blood can maintain circulation during haemorrhage

Question 2

Where does fluid move in the capillary wall?

Answer 2

→ moves across the membrane into interstitial space due to blood flow which exerts hydraulic pressure.

Question 3

What kind of pressure do large molecules exert and why?

Answer 3

→ Exert an osmotic pressure termed oncotic pressure

→ because they are too large to pass through the membrane

Question 4

What does oncotic pressure create?

Answer 4

→ A suction force to move fluid into the capillary

Question 5

What does fluid movement across capillary walls depend on?

Answer 5

→ Balance between hydraulic and oncotic pressures across the capillary wall.
These forces aren’t in balance so the net movement is toward fluid being moved out.
The interstitial fluid is drained into lymphatic system that drains fluid back into the capillary

Question 6

What are the four pressures that determine filtration rate?

Answer 6

HYDROSTATIC PRESSURES:
→Pc: capillary blood pressure
→ Pi: interstitial fluid pressure

ONCOTIC PRESSURE:
→ πp: plasma proteins
→ πi: interstitial proteins

Question 7

What is the fluid flux equation?

Answer 7

→Jv = Lp x A {(Pc - Pi) - σ(πp - πi)}

→Lp is the hydraulic conductance of the endothelium, ie. how leaky the endothelium is to the fluid.
→A is the wall area.
→σ is the reflection coefficient = fraction of osmotic pressure that is exerted
→ JV is the movement of fluid or ‘flux’

Question 8

What is the equation for effective osmotic pressure?

Answer 8

→ σ x potential osmotic pressure

σ for plasma protein is 0.9
ie. 10% plasma proteins are conducted across capillary wall into interstitial space

Question 9

What do Starling’s forces normally favour in capillaries?

Answer 9

Starling’s forces favour filtration in capillaries.

Question 10

What is favored filtration or reabsorption?

Answer 10

→Taking into account factors that affect both filtration and reabsorption
→the balance is tipped into filtration
→However, there are other factors involved, so the balance can change.

Question 11

What do well perfused capillaries do?

Answer 11

→Well-perfused capillaries will filter along their entire length.

Question 12

What does the lymphatic circulation do?

Answer 12

→ Returns excess tissue fluid/solutes back to the CVS

Question 13

How many liters per day are filtered by the lymphatic system?

Answer 13

→ About 8 liters per day are filtered

Question 14

What do lymph vessels have?

Answer 14

→ Valves to ensure unidirectional flow and smooth muscle

Question 15

What contributes to lymph flow?

Answer 15

→ Spontaneous contractions of smooth muscle contributes to lymph flow
→ Surrounding skeletal muscle contractions and relaxations also contribute to lymph flow

Question 16

What does lymph contain?

Answer 16

→ Immune cells

→ at the lymph nodes

Question 17

What does control of extracellular fluid balance depend on?

Answer 17

→ Capillary filtration
→ Capillary reabsorption
→Lymphatic system

Question 18

What happens in the capillaries during hypovolemia (decreased blood volume)?

Answer 18

→ A bit of filtration to begin with but then reabsorption
→ Osmotic pressure is now able to overcome the hydraulic pressure
→Caused by sympathetic nerve-induced vasoconstriction of pre-capillary arterioles leading to drop in downstream capillary pressure and hence shift the balance of force for reabsorption.

→ More fluid is absorbed back into circulation
→ Increasing blood volume

Question 19

What does a drop in CO cause?

Answer 19

→ Drop in BP (BP = CO x TPR)
→ so Pc is reduced

Overall balance no longer favours filtration which is good in this situation to increase blood volume

Question 20

What is oedema?

Answer 20

→ Excess of fluid within interstitial space

→ Imbalance between filtration, reabsorption + lymph function

Question 21

What are the causes of oedema?

Answer 21

→ Increased capillary pressure
→ Decreased plasma protein oncotic pressure
→ Inflammatory response- capillaries become leaky and plasma proteins will leave so reduction in oncotic pressure- favouring filtration- oedema

→ Lymphatic problems

Question 22

What are three situations where you would have Increased Capillary Pressure (Pc) ?

Answer 22

→ Dependent gravitational oedema (standing up for a long time)- increased venous pressure
→ Deep venous thrombosis-prevention of venous return due to occlusion that blocks a deep vein- so you are going to get a net filtration into the interstitial space and swelling of the lower limb.

→ Cardiac failure- a build up of pressure in pulmonary artery which backs up to lung causing pulmonary oedema.

Question 23

What is the equation for net filtration?

Answer 23

P = (Pc - Pi) - σ (πp - πi)

Question 24

How can reduced plasma protein concentration cause oedema?

Answer 24

→ reduced plasma oncotic pressure
→ greater influence of Pc and πi. Less of reabsorptive force
→ Fluid efflux from capillaries into the interstitial fluid

Question 25

How can malnutrition cause oedema?

Answer 25

→ Not enough protein intake to make plasma proteins

Question 26

How can nephrotic syndrome cause oedema?

Answer 26

→ Urinary protein loss - replaced by liver production

Question 27

How can liver disease cause oedema?

Answer 27

→ Not enough endogenous albumin produced

Question 28

How does inflammatory mediated oedema arise?

Answer 28

→ Swelling is triggered by local chemical mediators of Inflammation
→ Large increase in capillary permeability
→reduction in reflection coefficient

Question 29

What are the effects of inflammation?

Answer 29

→ Increased Lp

→ Increased protein permeability

Question 30

How is lymphatic obstruction caused?

Answer 30

→Filariasis/elephantitis
→ nematode infestation
→ Larvae migrate to lymph system
→ grow and block lymph drainage

Question 31

What is lymphatic removal?

Answer 31

→ caused by surgery to treat testicular cancer

→ removal of lymphatics

Question 32

What can abnormalities in fluid exchange lead to?

Answer 32

→ Abnormalities in fluid exchange can lead to oedema / tissue swelling

Question 33

What are hydraulic conductance(Lp) and wall are(A) in normal conditions?

Answer 33

In normal conditions, Lp and A are constant

Question 34

Factors promoting filtration and reabsorption…

Answer 34

Factors promoting filtration:
Pc- capillary blood pressure pi i- interstitial proteins

Factors promoting reabsorption
pi p- plasma proteins

Question 35

What happens to Pc along the length of a capillary in well perfused capillaries?

Answer 35

Pc but not pi p alters along length of capillary

This will be opposed by the net osmotic pressure gradient.
Due to blood flow, pressure is lost but on the other hand osmotic pressure is constant because plasma proteins are not lost favouring reabsorption back into capillary

Question 36

What happens to filtration force along the capillary?

Answer 36

Outward filtration force declines along the capillary but its net value remains above force favouring re-absorption so well perfused capillaries filter along their entire length