Physiology Of Microcirculation And Lymphatics Flashcards
What makes up the microvascular bed
Terminal arterioles, capillary bed and the post capillary venules
Thoroughfare channels vs true capillaries
Both are found in the capillary bed within the microvascular bed
Thoroughfare = larger arterioles that have continuous flow
True capillaries = smaller capillary where blood flow is intermittent these focus more on nutrient and gas exchange)
At rest, how many capillary beds are fully open?
25%
this number increases during exercise to meet metabolic needs of muscles
Pericytes
Cells found around some capillaries which control vasoconstriction and vasodialation through contractile filaments
General make up of capillary walls
1-3 endothelial cells connected w/ tight junctions
- usually are 50 micrometers in length, 0.2 micrometers thick and 8-10 micrometers in diameter
3 groups of capillaries
Continuous
Fenestrated
Discontinuous
Continuous capillaries
Most common form of capillary
Have no gaps or fenestrations, but do possess clefts (except in BBB)
Found in
- lymph nodes
- all muscles
- the CNS
Function
- feed BBB
- lymphocyte homing
- metabolic exchange
Fenestrated capillaries
Have thin perforated walls with very small circles with a diaphragm called fenestrations
Found in
- endocrine glands
- GI
- choroid plexus
- kidney glomeruli
Function:
- secretion
- absorption
- filtration
Sinusoidal (discontinuous) capillaries
Rarest type of capillaries
Have fenstrations and large gaps between endothelial cells
Found in
- liver
- bone marrow
- spleen
Functions:
- promote haematopoesis
- filter blood cells
- exchange particles
Structural characteristics of capillary flow
Decreased blood pressure compared to other vessels
- lack resistance due to structure
Decreased blood velocity
Increased exchange surface are
- occupy the most surface area of all vessels
Possess vasomotion
all allow for sufficient surface area and time for efficient exchange of gases and nutrients
Epinephrine vaso effects
Small doses produce vasodilation
Large doses produce vasoconstriction
norepinephrine elicits only vasoconstriction regardless of doseage
Endocrine and neural mechanisms that increase peripheral resistance (vasoconstriction)
Epinephrine/norepinephrine
Angiotensin- 2
Increased renin secretion
Sympathetic activity
Endocrine and neural mechanisms that decrease peripheral resistance (vasodialate)
Decreased renin secretion
Parasympathetic innervation
Laplace law
Describes the relationship of tension in vessel wall with respect to transmural pressure.
T = (change in pressure) x (radius)
change in pressure is (intravascular pressure - tissue pressure)
Ways that material are exchanged across the capillary wall
Pinocytosis (larger molecules such as proteins)
Bulk flow via junctions and fenstrations via blood pressure (mild-moderate molecules that aren’t lipid soluble)
Diffusion via junctions “facilitated diffusion” (for small molecules and gasses that aren’t lipid soluble)
Diffusion directly through cells (gasses that are lipid soluble)
Osmosis ( water only)
What are starling force pressures
Capillary Hydrostatic pressure (Blood pressure)
tissue Interstitial pressure (plasma protein content)
Capillary oncotic pressure
these forces determine the rate of blood flow through the capillary wall
Difference between hydrostatic pressure and osmotic pressures = net pressure
Difference between hydrostatic and osmotic pressures
Hydrostatic
- due to fluid pushing against a boundary
- “pushes” the fluid across the boundary
- caused by blood pressure in blood vessels
Osmotic
- due to non diffusable solutes that cant cross a boundary
- “ pulls” the fluid towards the solute to even it out
- caused by plasma proteins in blood vessels
Startling forces equation
JV = Kf x Pnet
- Kf = hydraulic conductance (water permeability of the capillary wall)
- Pnet = net pressure (difference of hydrostatic pressures vs oncotic pressures)
What does excess filtration cause?
Edema
- due to inability to recover filtered fluid and can in return compromise preload and compromise LV preload
Causes 2 changes in body pressures
1) increases capillary hydrostatic pressure (BP)
- can lead to heart failure
2) decreases oncotic pressure or plasma proteins
- can leads to liver failure or kwashiorkor disease
Precapillary sphincters
Found between true capillaries and metarterioles
- control amount of blood flowing into capillaries
What does histamine do to the starling forces?
Increases permeability of capillaries to proteins = increases interstital colloid osmotic pressure
- causes fluid to move out of capillaries easier
Also increases hydraulic conductance
What happens if JV is >0? What happens when it is <0?
JV> 0 = filtration from capillary to interstitial tissue
JV < 0 = resorption from interstital tissue to capillaries
When does interstitial fluid move into lymphatic freely
When the pressure in the interstitum is greater than the pressure in the lymphatic
fluid always moves from high -> low concentrations of pressure
What is the most common cause of interstital fluid accumulation?
Excessive capillary fluid filtration
Three conditions that from intracellular edema
Hyponatremia
Depression of metabolic systems
Lack of nutrition to cells
Two conditions that causes extracellular edema
Abnormal leakage of fluid from plasma into interstitial spaces
Failure of lymphatic is to return fluid from interstitium back into the blood
What drugs may cause edema? What drugs cure it?
Cause:
- steroids
- estrogens
- CCBs
- NSAIDs
- thiazolidinediones
Cure:
- diuretics (flurosemide)
Jugular vein distention (JVD)
Occurs when central venous pressure is above the normal level
If JVD does not fall when inspiration occurs = Kussmaul’s sign and usually indicative of constrictive pericarditis
