fluid movement across the memebrane Flashcards
What proportion of the body water is extracellular?
1/3
What proportion of the body water is intracellular?
75%
What makes up capillaries?
A dingle layer of endothelial cells
What do capillaries allow?
rapid exchange of gases, water and solutes with interstitial fluid.
What is the main function of capillaries?
deliver nutrients and O2 to the cells
Removal of metabolites from cells
What does the flow in the capillaries depend on?
contractile state of the arterioles
What regulates regional flow to the capillary bed?
terminal arterioles
What regulates flow in some tissues?
precapillary sphincters
What is the rate of the blood through in the capillary bed and why?
slow to allow adequate time for exchange
How do small water soluble substance cross the capillary?
through pores
How do lipid soluble substances pass through the capillary?
They pass through the endothelial cells
How do exchangeable proteins pass through the capillary?
They are moved across by vesicular transport
How do plasma proteins cross the capillary?
They don’t. They usually stay within the blood
How is trans capillary fluid flow driven?
it is driven by pressure gradients across the capillary wall
What is ultra filtration?
exchange across the capillary wall of essentially protein- free plasma
What is the equation for net filtration pressure?
NFP= sum of forces favouring filtration - sum of forces opposing filtration
What is the filtration coefficient?
How permeable the membrane is to fluid
What are two forces favouring filtration?
- Pc : capillary hydrostatic pressure
- interstitial fluid osmotic pressure
What causes capillary hydrostatic pressure?
BP
What are two forces opposing filtration?
- capillary osmotic pressure
- interstitial fluid hydrostatic pressure
Starling forces favour filtration at arteriolar end and reabsorption at venular end. True/ False?
True
What is the NFP at the venular end?
(-8mmHg), favouring movement from interstsitium back into the capillaries
What is the NFP at the arteriolar end?
(10mmHg), favouring movement from capillary into the interstitium
Where does excess fluid go?
excess fluid is returned to the circulation via the lymphatics as lymph
What is the difference between pulmonary resistance and systemic resistance?
pulmonary resistance is much lower
What is oedema?
Accumulation of fluid in interstitial space
What two things are affected by oedema?
- diffusion distance is increased meaning gas exchange is compromised
- compliance is also affected
What are the causes of oedema?
- raised capillary pressure
- reduced plasma osmotic pressure
- lymphatic insufficiency
- changes in capillary permeability
What causes raised capillary pressure?
- arteriolar dilation
- raised venous pressure ie from LVF, RVF, valvular damage
What effect does HF have on the Frank Starling curve?
It shifts the curve to the right
at which plasma osmotic pressure does oedema occur?
Oedema occurs if pressure is less than 30g/l
What causes a reduced plasma osmotic pressure?
- malnutrition
- protein malabsorption
- excessive renal excretion of protein
- hepatic failure
What causes lymphatic insufficiency?
- lymph node damage
- filariasis
What is the difference between oedema caused by HF than oedema caused by lymphatic insufficiency?
You wont have pitting oedema in lymphatic insufficiency
What causes changes in capillary permeability?
- inflammation
- histamine which increases leakage of protein
What are the clinical signs for pulmonary oedema?
- SOB
- crepitation in auscultations of lung bases
- haziness in peripheral regions on a CXR
