Week 6 - Cardiovascular system: The microcirculation Flashcards
Describe the structure of the barrier between blood and interstitial fluid (microcirculation)
Microcirculation is the smallest blood vessels
- inc. capillaries, lymphatic capillaries, arterioles and venules
Capillaries:
- small, narrow, RBC move in single file line = maximises exposure to capillary membrane (endothelium)
- large SA = allows good fluid, solute and has exchange between plasma + interstitial fluid
- thin walls (monolayer of endothelial cells) NO muscle tissue
- has slowest velocity (due to high cross sectional area)
Lymphatic capillaries
- drains fluid from interstitial space and returns it to blood
Plasma = fluid in blood (in the vessels)
Interstitial fluid = fluid surrounding cells (outside vessels)
How does cross section of vessels affect velocity
Small cross section = high velocity
Cross section is larger in smaller vessels e.g. aorta crosse section is smaller than capillaries
Velocity = CO / cross sectional area of vessel
What are the factors governing movement of fluid (H2O) between blood and interstitial fluid (starlings law of filtration - hydrostatic / colloid pressure)
Movement of H2O is determined by 4 forces driven by conc. gradient
4 Forces:
- hydrostatic pressure inside capillaries (pushes wall out)
- hydrostatic pressure in interstitial fluid (pushes capillary walls in)
- osmotic pressure inside capillaries
- osmotic pressure in interstitial fluid
Hydrostatic Pressure:
- is force generated by pressure of fluid on capillary walls (by blood plasma / interstitial fluid)
- always highest in blood vessel than interstitial fluid (as body is always trying to push fluid out)
Colloid Osmotic (Oncotic) Pressure
- driven by conc. gradient of proteins
- ↑protein conc. gradient in plasma = H2O isn’t filtered out (due to osmosis)
- protein draws H2O from interstitial fluid into capillaries
- oncotic pressure = pressure exerted by proteins
-
NOTE: hydrostatic and osmotic pressures are usually in balance
- hydrostatic pushes water out of capillaries
- osmotic draws water into fluid
- direction of flow depends on diff. between the 2
- absorption = fluid moving from interstitial to plasma
- filtration = fluid moving from plasma to interstitial
- Capillary walls sense difference in both hydrostatic pressures = net hydrostatic pressure
- Colloid osmotic pressure = diff. between both pressures
- Net filtration pressure = balance of all 4 pressures
- this determines net flow of fluid across capillary membrane
Why are plasma proteins important
- Prevent vascular system from collapsing + tissue from swelling
- H2O would constantly flow out of plasma into interstitial fluid / extracellular space if NO plasma proteins present
- = they prevent loss of water from plasma (instead draws water from interstitial fluid)
- blood vessel walls are permeable to H2O but impermeable to proteins (= proteins stay in blood) - ## Protein conc. in plasma is higher than in interstitial fluid
What happens to fluid movement in peripheral circulation and how is excess fluid / particles returned to circulation
There is a slight net loss in fluid (peripheral circulation)
- but loss is recovered by lymphatic system (returns excess fluid back into circulation)
How do particles and excess fluid return to circulation (the pathway)
- Only the end of the lymphatic system picks up / transport excess fluid back into circulation
- Have valves = one way flow of blood (from tissue to veins)
- Lymph collectors have smooth muscle, skeletal muscle (aids movement back to heart / pumping action)
- Lymphatic capillary cells are open ended = absorb fluid / molecules
- absorbed fluid forms lymph (clear fluid) - Fluid enters lymphatic capillaries, pumped through the lymph vessels, emptied into large veins which then carry fluid back to heart
- fluid is pumped against gravity to get back into heart - Fluid enters circulation via the lymphatic duct (they empty into large veins)
Explain the 2 pathways that fluid exchange occurs via
- Transcellular Pathway
- movement THROUGH cell
- allows large bulk flow of fluid to move (due to large SA)
Can be transported via:
1. simple diffusion - for small non-polar molecules, cell membrane isn’t a barrier
2. facilitated diffusion - small polar molecules bind to transporter proteins then are transported
3. transcytosis - for large, insoluble molecules e.g. proteins, molecule is endocytose into cell then exocytosed out on other side - Paracellular Pathway
- movement BETWEEN cells (i.e. moves between gap of 2 diff. cells close together)
- small SA = limited flow of fluid
- limited by tightness of junction seal (a.k.a. the “leakiness” of a cell)
- does NOT occur in BBB (due to no tight junction, cells are close together = no gaps)
Can be transported via:
1. simple diffusion - any molecule small, non-polar, large (e.g. proteins( can move this way
Explain the 3 types of capillaries
- Continuous
- is tight (no big gaps), found in muscle nerve
- fluid can go transcellularly or through gap from intracellular cleft (if big enough) - Fenestrated
- is leaky, found in pancreas, intestines
- fluid can go through gaps or through holes (a.k.a. fenestrations)
- has gaps due to movement of hormones (and their need to enter blood) - Sinusoidal
- very leaky, found in liver, bone marrow, spleen
- has big gaps = allows easy flow / secretion of large molecules (which may need to enter blood)
- e.g. RBC need to get out of tissues and into blood = need big space to do this
- intercellular cleft = where 2 cells come together
How does oedema arise
Oedema - swelling caused by buildup of excess fluid in tissues
- High capillary pressure
- causes arteriolar dilation
- occurs when there is a venous blockage (e.g. clot) - Low blood protein
- results in fluid being drawn into interstitial fluid
- low protein level is due to liver failing to produce albumin, kidneys failing to retain it - Lymohatic failure
- system unable to collect excess fluid = builds up
- o
Where might oedema occur
- Ankle oedema caused by left heart failure
- Macular oedema = swelling of the retina
- swelling due to protein deposits
- Elephantiasis = swelling of whole leg
How does a heart failure (left ventricle) oedema occur
Heart failure causes ankle + leg oedema and peripheral oedema
Ankle + Leg:
- Left ventricle is unable to pump well = ↓ CO = ↓ MAP
- MAP = CO x TPR
- Low MAP (pressure) is detected by baroreceptors = tries to restore MAP
- also kidney regulation mechanism try restore
- Results in Na+ / H2O being retained
- Results in ↑ plasma volume = ↑ venous + capillary pressure
= ↑ filtration (fluid pushed out of blood into interstitial fluid)
Pulmonary Oedema
- Left ventricle dysfunction = ↓ CO / ↓ ejection = blood accumulates
- Results in backlog of blood in pulmonary vessels = pulmonary capillary pressure ↑
= fluid is pushed into lungs + filtered into interstitial fluid
