Capillaries II: Fluid Exchange

Question 1

What is the importance of fluid exchange?

Answer 1

• Important for normal physiological function
• Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage
• Abnormalities can lead to oedema/tissue swelling

Question 2

Describe fluid movement at the capillary wall.

Answer 2

• Moves across membrane into interstitial space due to blood flow which exerts a hydraulic pressure
• Large molecules (eg. plasma proteins) cannot pass through membrane so they exert an osmotic pressure termed oncotic pressure which creates suction force to move fluid into capillary

Question 3

What is fluid movement dependant on?

Answer 3

Balance between hydraulic and oncotic pressures across the capillary wall

Question 4

What 4 pressures determine filtration rate?

Answer 4

Osmotic pressures:
- Pip - plasma proteins
- Pii - interstitial proteins

Hydrostatic pressures:
- Pc = Capillary blood pressure
- Pi = Interstitial fluid pressure

Question 5

What allows fluid to move out of the capillaries and into interstitial fluid?

Answer 5

• Capillary blood pressure > interstitial fluid pressure
• This is why fluid moves out of the capillaries into the interstitial fluid

Question 6

What allows fluid to move back into capillaries?

Answer 6

• Pressure exerted by plasma proteins > pressure exerted by interstitial proteins
• This is why fluid moves back into the capillaries

Question 7

Describe Starling’s principle of fluid exchange.

Answer 7

Jv = LpA { (Pc - Pi) - Theta (Pi p - Pi i) }

Jv = the volume of fluid moved
Lp = hydraulic conductance of the endothelium
Theta = reflection coefficient - fraction of the osmotic pressure that is exerted
A = wall area
(Pc - Pi) = hydraulic pressure difference (capillary blood pressure - interstitial blood pressure)
(pip - pii) = osmotic pressure difference (plasma proteins - interstitial proteins)

Question 8

What is the formula for effective osmotic pressure?

Answer 8

Effective osmotic pressure = σ x potential osmotic pressure
σ for plasma protein is 0.9

Question 9

What factors promote filtration?

Answer 9

Capillary blood pressure and interstitial proteins

Question 10

What factors promote reabsorption?

Answer 10

Plasma proteins

Question 11

What is the effect when the factors promoting filtration and reabsorption no longer become balanced?

Answer 11

EXAMPLE: If there are greater factors promoting filtration, it disrupts this balance and can lead to oedema

Question 12

Explain how well perfused capillaries can filter along their entire length

Answer 12

• Capillary pressure decreases from the arterial end to the venous end
• Osmotic pressure remains constant throughout the length of the capillary hence filtration can occur along the length
• Hydraulic pressure difference decreases along the length but is still present

Question 13

Describe the lymphatic circulation

Answer 13

• Lymphatic circulation returns excess tissue fluid/solutes back to the cardiovascular system
• Lymph vessels have valves and smooth muscle
• Spontaneous contractions of the smooth muscle contributes to lymph flow
• Surrounding skeletal muscle contractions/relaxation also contributes to lymph flow

Question 14

What happens in the capillaries during hypovolemia (decreased blood volume)? PART 1

Answer 14

• Occurs when there is a low extracellular fluid volume e.g. after a haemorrhage
• Blood volume decreases so venous return decreases reducing stroke volume and cardiac output so blood pressure is reduced
• Oncotic pressure stays constant but the arterial pressure has dropped
• Capillary no longer filters along its entire length and favours reabsorption towards the venous end

Question 15

What happens in the capillaries during hypovolemia (decreased blood volume)? PART 2

Answer 15

• More fluid enters the capillaries which is life preserving- it increases central venous pressure which increases return and output - increases blood pressure
• Also aided by sympathetic nerve activity - vasoconstriction of pre capillary arterioles decreases capillary pressure downstream

Question 16

Describe what causes increased venous pressure

Answer 16

• When blood flows through the venous system is impeded there is an accumulation of blood volume at the venous end which increases pressure at the venous end
• Due to the higher hydraulic pressure there is an increase in filtration as the hydraulic pressure difference does not decrease to the same degree as when blood is able to flow freely to the venous end and lose pressure

Question 17

Describe what oedema is.

Answer 17

• Excess of fluid within the interstitial space.
• Imbalance between filtration, reabsorption, lymph function

Question 18

What can cause oedema?

Answer 18

• Increased capillary pressure (Pc)
• Decreased plasma protein oncotic pressure (πP)

Question 19

Give examples of oedema caused by increased capillary pressure

Answer 19

• Gravitational oedema – standing up for long periods
• Deep venous thrombosis

Question 20

Describe DVT

Answer 20

• Prevention of venous return
• Increases venous pressure
• Increased Pc across capillaries and increased filtration

Question 21

What causes decreased plasma oncotic pressure?

Answer 21

• Malnutrition / malabsorption - not enough protein intake to make plasma proteins
• Nephrotic syndrome - urinary protein loss
• Liver disease - not enough endogenous albumin produced

Question 22

What does decreased plasma oncotic pressure lead to?

Answer 22

Greater influence on capillary pressure
- More fluid exits the capillaries leading to oedema

Question 23

Describe how inflammation leads to inflammatory mediated oedema

Answer 23

• Swelling triggered by local chemical mediators of inflammation
• Large increase in capillary permeability
• Protein permeability increases so interstitial proteins increase
• Less fluid is reabsorbed
• More fluid accumulates in the interstitial space

Question 24

State 2 examples of lymphatic problems that cause oedema

Answer 24

Lymphatic obstruction -
Filariasis/elephantiasis - nematode infestation, larvae migrate to lymphatic system grow/mate /form nests - block lymph drainage

Lymphatic removal -
Lymphedema - caused by surgery to treat testicular cancer - removal of lymphatics

Question 25

Describe how standing for long periods of time can cause oedema.

Answer 25

• Venous pressure increases
• Increased Pc across the capillaries
• Increases filtration.
• Greater interstitial fluid in legs

Question 26

Overall control of extracellular fluid balance depends on what three factors?

Answer 26

Capillary filtration
Capillary reabsorption
Lymphatic system

Question 27

What is the difference between filtration and reabsorption?

Answer 27

Filtration: Movement of fluid out of the capillaries to interstitial fluid
Reabsorption: Movement of fluid back into the capillaries from interstitial fluid

Question 28

How is fluid moved into interstitial space?

Answer 28

• Hydrostatic pressure of the blood > hydrostatic pressure of the interstitial fluid
• Creates a pressure gradient which moves fluid down the pressure gradient.
• Pressure moves fluid from the blood (high pressure) to the interstitial fluid (low pressure)