3- fluid shift across capillary wall Flashcards
what % of body & total water is interstitial fluid?
- Total body water is 60% of body weight
- 1/3 of body water is extra cellular
- 75% of extracellular fluid is interstitial (bathing the body cells)
what is the interstitial fluid?
fluid bathing the body cells - acts as the go between blood & body cells
where does interstitial commonly act?
capillaries allows rapid exchange of gases, water & solutes with interstitial fluid
(delivery of O2 & nutrients and removal of metabolites)
what controls blood flow in capillary? what blood flow speed to do you want?
terminal arterioles regulate regional blood flow to capillary bed in most tissues - the blood flow depends in their contractile state
- you want blood flow to be slow to allow adequate time for exchange
- precapillary sphincters regulate flow too
what are the different ways molecules transport across the capillary wall?
→ there are small pores that allow small water soluble substances to pass through
→gases - lipid soluble substances pass through endothelial cells
→exchangeable proteins - moved across by vesicular transport
→plasma proteins can’t pass capillary walls and remain within capillary
what is transcapillary flow?
movement of fluid across capillary wall driven by pressure gradients
what is ultra filtration?
- specific type of filtration process that occurs across the walls of capillaries, the smallest blood vessels in the body
- fluid involved in ultrafiltration is essentially protein-free plasma. Plasma is the liquid component of blood that contains water, electrolytes, and various solutes
what is net filtration pressure?
overall pressure gradient that determines the direction and rate of fluid movement across the capillary walls in the process of ultrafiltration
how do you calculate net filtration pressure?
NFP=(Pc−Pi)−(πc−πi)
NFP = net filtration pressure
Pc = capillary hydrostatic pressure
πc = capillary osmotic pressure
Pi = interstitial fluid hydrostatic pressure
πi = interstitial fluid osmotic pressure
(think - hydrostatic pressure pushes fluid out of blood vessels, while osmotic pressure pulls fluid into them)
what is Pc in net filtration pressure?
Capillary hydrostatic pressure = the force exerted by the blood against the capillary walls. It tends to push fluid out of the capillaries into the interstitial space
what is Pi in net filtration pressure?
Interstitial hydrostatic pressure = the pressure in the tissue surrounding the capillaries. It opposes the movement of fluid out of the capillaries
what is πc in net filtration pressure?
Capillary oncotic pressure = the osmotic pressure due to proteins (e.g., albumin) in the blood. It tends to pull fluid into the capillaries
what is πi in net filtration pressure?
Interstitial oncotic pressure = the osmotic pressure in the tissue. It opposes the movement of fluid into the capillaries
what is the number of hydrostatic pressure at
a) arteriole end?
b) venule end?
a) 35 mmHg
b) 17 mmHg
what starling forces are favoured at each side of capillary bed?
favour filtration (hydrostatic) at arteriolar end, reabsorption (osmotic) at venular end
*filtration= into intertstitium and rebasorption into blood vessels
how are lymphatics involved in capillaries?
during a day, filtration exceeds reabsorption by 2-4 litres - excess fluid is returned to circulation via lymphatics as lymph
(filtration into interstitial fluid and reabsorption into blood)
is pulmonary or systemic resistance stronger?
systemic resistance is higher - pulmonary resistance is only 10% of it (this is important because lower resistance allows for efficient blood flow and oxygenation in the lungs)
what are the starling forces in pulmonary capillaries? (are hydrostatic & osmotic pressures high/low etc)
- pulmonary capillary hydrostatic pressure is low (8-11 mmHg)
- this is beneficial as low hydrostatic pressure means less force pushing fluid out capillaries so helps prevent excessive filtration of fluid out capillaries into lung tissue →reduces risk of pulmonary edema
- capillary osmotic pressure at 25 mmHg(high)
- it’s the osmotic force due to proteins in blood which creates a force pulling fluid back into capillaries, this is high osmotic pressure in capillaries which helps counteract the hydrostatic pressure, preventing excessive fluid loss from capillaries into lung tissue
what is oedema?
accumulation of fluid in interstitial space
what are 4 causes of oedema?
- raised capillary hydrostatic pressure (e.g. high BP)
- reduced plasma osmotic pressure (e.g. low albumin)
- lymphatic insufficiency
- changes in capillary permeability
*3+4 can be caused by various things e.g. infection, trauma, congenital etc
what can cause raised capillary hydrostatic pressure (which leads to oedema)?
- could be because of arteriolar dilation (more fluid in capillaries raising pressure) like high BP
could be due to raised venous pressure:
- if left ventricular failure, pulmonary put back in pulmonary circulation →pulmonary oedema
- right ventricular failure →peripheral oedema (if upright = ankle, sacral)
- prolonged standing = raised venous pressure→swollen ankles
what should normal plasma protein concentration be?
65-80 g/l
- oedema can occur if <30 g/l
(this can cause malnutrition, protein malabsorption, excessive renal excretion of protein, hepatic failure)
what can cause lymphatic insufficiency that can lead to oedema?
- lymph node damage
- filariasis - elephantiasis (super massive legs & feet)
what can cause changes in capillary permeability that can lead to oedema?
- inflammation
- histamine increases leakage of protein
what is pulmonary oedema?
accumulation of fluid in interstitial & intra-alveolar lung spaces
how does pulmonary oedema clinically present?
- it’s manifested clinically by varying degrees of shortness of breath
- clinically there may be crepitations in auscultation of lung bases
- chest X-ray shows haziness in peripheral region
what causes pitting oedema?
Right ventricular failure
in heart failure what results in more fluid retention?
renin-angiotensin-aldosterone-system (RAAS) that is upregulated in heart failure
