Capillaries II Flashcards
What is the importance of fluid exchange
- Fluid exchange is important for normal physiological function, we need H2O for chemical reactions
- Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage
- Abnormalities in fluid exchange can lead to oedema/tissue swelling
Describe fluid movement at the capillary wall
- Fluid 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
- Fluid movement across capillary walls depends on the balance between hydraulic and oncotic pressures across the capillary wall
What 4 pressures determine filtration rate
- Osmotic pressures:
Pi p - plasma proteins
Pi i - interstitial proteins - Hydrostatic pressures:
Pc = Capillary blood pressure
Pi = Interstitial fluid pressure - In the hydrostatic pressures the capillary blood pressure is higher than the interstitial fluid pressure hence why fluid moves out of the capillaries into the interstitial fluid
- In the osmotic pressures the pressure exerted by plasma proteins is greater than the pressure exerted by interstitial proteins hence 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 - how leaky the endothelium is to fluid
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)
(pi p - pi i) = osmotic pressure difference (plasma proteins - interstitial proteins)
Effective osmotic pressure = σ x potential osmotic pressure
σ for plasma protein is 0.9
State which factors promote filtration and reabsorption and what happens when they are no longer balanced
- Factors that promote filtration are the capillary blood pressure and interstitial proteins
- Factors that promote reabsorption are the plasma proteins
- Volume of fluid moved is a balance between these forces
- When factors that promote filtration or reabsorption are more present it disrupts this balance and can lead to oedema when factors promoting filtration increase
Explain how well perfused capillaries can filter along their entire length
- Capillary pressure decreases from the arterial end to the venous end
- The osmotic pressure remains constant throughout the length of the capillary hence filtration can occur along the length
- The 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)
- Occurs when there is a low extracellular fluid volume e.g. after a haemorrhage
- Blood volume has decreased so venous return decreases reducing stroke volume and cardiac output so blood pressure is reduced
- This means that because the oncotic pressure stays constant but the arterial pressure has dropped the capillary no longer filters along its entire length and favours reabsorption towards the venous end
- This is good because it means 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 flow 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 and what it is caused by
- Excess of fluid within the interstitial space
- Imbalance between filtration, reabsorption, lymph function
- Causes:
Increased capillary pressure (Pc)
Decreased plasma protein oncotic pressure (πP)
Inflammatory response
Lymphatic problems
Give examples of oedema caused by increased capillary pressure
Clinical scenarios:
- Dependent (gravitational) oedema – standing up for long periods
- Deep venous thrombosis
- Cardiac failure
eg. Deep vein thrombosis (DVT) prevention of venous return
Increased venous pressure causes ‘back-up’ of pressure leading to increased PC across capillaries and increased filtration
Describe what causes decreased plasma oncotic pressure and what it leads to
Caused by -
- Malnutrition/malabsorption - not enough protein intake to make plasma proteins
- Nephrotic syndrome - urinary protein loss - replaced by liver production
- Liver disease - not enough endogenous albumin produced
Reduced plasma protein concentration means there is a reduced plasma oncotic pressure so there is greater influence of capillary pressure which means more fluid exits the capillaries leading to oedema
Describe how inflammation leads to inflammatory mediated oedema
- Swelling is triggered by local chemical mediators of inflammation
- There is a large increase in capillary permeability which means protein permeability increases so interstitial proteins increase so less fluid is reabsorbed and more 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
