Capillaries - fluid exchange Flashcards
Capillaries, extracellular matrix and epithelial cells
Importance of fluid exchange
- Fluid exhchange is important for physiological functions - we need water for chemical reactions.
- Fluid re-absorption from tissues to blood can maintain circulation during haemorrhage.
- Abnormalities in fluid exchange can lead to tissue swelling.C
Fluid movement at the capillary wall
- Capillary walls are semi permeable.
- Fluid moves across membrane into interstitial space due to blood flow which exerts a hydraulic pressure.
- Big molecules cant pass through small membrane, they exert oncotic pressure which creates sunction force to move fluid into capillary.
4 pressures determine filtration rate
Starling’s principle of fluid exchange
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Starling’s forces normally favour filtration
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Well perfused capillaries filter along their entire length
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Lymphatic circulation
- Lymphatic circulation returns excess tissue fluid back to the CVS.
- Lymph vessels have valves and smooth muscle.
- Spontaenous contraction of smooth muscle and skeletal muscle contraction/relaxation contributes to lymph flow.
- Lymphs also contain immune cells at lymph nodes.
Fluid balance
Organisation of the lymphatic system
Low capillaru pressure (Pc) - hypovolemia
Increased Pc gives oedema (as in DVT)
Oedema
Excess of fluid within the interstitial space-imbalance between flitration, reabsorption, lymph function.
Caused by:
* Increased capillary pressure
* Decreased plasma protein oncotic pressure (πP)
* Inflammatory response
* Lymphatic problems
Increased capillary pressure (Pc)
Clinical scenarios: Dependent (gravitational) oedema - standing up for long periods of time.
Deep venous thrombosis.
Cardiac failure.
example: Deep vein thrombosis (DVT) prevention of venous return, increases pressure causes backup of pressure leading to increased Pc across capillaries and increased filtration.
Net filtration P = (Pc-Pi)-σ(πp-πi)
(Assuming Lp and A are constant.
Decreased plamsa oncotic pressure (πp)
Low protein oedema:
* Malnutrition/malabsorption: not enough protein intake to make plasma proteins.
* Nephrotic syndrome: Utrinary protein loss - replaced by liver production.
* Liver disease: not enough albumin produced.
Reduced plasma protein conc –> Reduced plasma oncotic pressure - greater influence of Pc and πi –> Fluid efflux from capillaries into the interstitial fluid –> oedema.
Inflammatory-mediated oedema
Swelling is triggered by local chemical mediators of inflammation, large increase in capillary permeability.
Inflammation:
* Increased Lp
* increased protein permeability (increased πi, reduced σ)
* chemicals, insect bite, nettle sting
* Infection/physical trauma/autoimmune disease
Lymphatic problems
Summary
