Capillaries II: Fluid Exchange Flashcards
What is the importance of fluid exchange?
- Important for normal physiological function
- Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage
- Abnormalities can lead to oedema/tissue swelling
Describe fluid movement at the capillary wall.
- 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
What is fluid movement dependant on?
Balance between hydraulic and oncotic pressures across the capillary wall
What 4 pressures determine filtration rate?
Osmotic pressures:
- Pip - plasma proteins
- Pii - interstitial proteins
Hydrostatic pressures:
- Pc = Capillary blood pressure
- Pi = Interstitial fluid pressure
What allows fluid to move out of the capillaries and into interstitial fluid?
- Capillary blood pressure > interstitial fluid pressure
- This is why fluid moves out of the capillaries into the interstitial fluid
What allows fluid to move back into capillaries?
- Pressure exerted by plasma proteins > pressure exerted by interstitial proteins
- This is why fluid moves back into the capillaries
Describe Starling’s principle of fluid exchange.
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)
What is the formula for effective osmotic pressure?
Effective osmotic pressure = σ x potential osmotic pressure
σ for plasma protein is 0.9
What factors promote filtration?
Capillary blood pressure and interstitial proteins
What factors promote reabsorption?
Plasma proteins
What is the effect when the factors promoting filtration and reabsorption no longer become balanced?
EXAMPLE: If there are greater factors promoting filtration, it disrupts this balance and can lead to oedema
Explain how well perfused capillaries can filter along their entire length
- 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
Describe the lymphatic circulation
- 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
What happens in the capillaries during hypovolemia (decreased blood volume)? PART 1
- 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
What happens in the capillaries during hypovolemia (decreased blood volume)? PART 2
- 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
Describe what causes increased venous pressure
- 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
Describe what oedema is.
- Excess of fluid within the interstitial space.
- Imbalance between filtration, reabsorption, lymph function
What can cause oedema?
- Increased capillary pressure (Pc)
- Decreased plasma protein oncotic pressure (πP)
Give examples of oedema caused by increased capillary pressure
- Gravitational oedema – standing up for long periods
- Deep venous thrombosis
Describe DVT
- Prevention of venous return
- Increases venous pressure
- Increased Pc across capillaries and increased filtration
What causes decreased plasma oncotic pressure?
- Malnutrition / malabsorption - not enough protein intake to make plasma proteins
- Nephrotic syndrome - urinary protein loss
- Liver disease - not enough endogenous albumin produced
What does decreased plasma oncotic pressure lead to?
Greater influence on capillary pressure
- More fluid exits the capillaries leading to oedema
Describe how inflammation leads to inflammatory mediated oedema
- 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
State 2 examples of lymphatic problems that cause oedema
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
Describe how standing for long periods of time can cause oedema.
- Venous pressure increases
- Increased Pc across the capillaries
- Increases filtration.
- Greater interstitial fluid in legs
Overall control of extracellular fluid balance depends on what three factors?
Capillary filtration
Capillary reabsorption
Lymphatic system
What is the difference between filtration and reabsorption?
Filtration: Movement of fluid out of the capillaries to interstitial fluid
Reabsorption: Movement of fluid back into the capillaries from interstitial fluid
How is fluid moved into interstitial space?
- 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)
