Hemodynamics II Flashcards
Compared to veins, arteries have a larger tunica ___ comprised mainly of ____.
media comprised mainly of smooth muscle
___ have valves to ensue unidirectional flow.
___ consist o fonly a thin endothelial layer of cells.
Veins have valves
Capillaries consist of only a thin endothelial layer of cells
Aorta/Large arteries serve to assist blood flow through their compliance.
Arterioles’ function?
Valves - they regulate blood flow to vascular beds & regulate total peripheral resistance via vasoconstriction & vasodilation controlled through neural and humoral input
Metarterioles
Specialized type of arteriole that connects to venules.
When open, they allow blood flow to bypass the capillary bed and flow directly from arteiroles to venules.
Most common in finger tips & ears.
Blood pressure drops most dramaticlaly across what type of vessel?
Arterioles because they function as the valves.
Their vasoconstriction reduces bp downstream of it.
Capillaries
- Smallest - single layer of endothelial cells with pericytes on the outer layer
- 3 types - continuous, fenestrated, sinusoidal
- Some include fenestrations (pores) to facilitate exchange
- Pinocytosis, diffusion, filtration
- Nutrient & waste products are exchanged w/surrounding tissue
- Numerous in metabolically active organs
The bulk of transcapillary exchange is ___, which follows Fick’s law
Diffusion
According to Fick’s law, what factors drive diffusion?
J = -(Permeabilitymolecule)(Surface areacapillary)(Conctissue-Conccapillary)
- Permeability of the molecule
- SA of the capillary
- Difference in teh conc between capillary and surrounding tissue
Two types of diffusion-mediated capillary exchange (from capillary into surrounding tissue)
-
Flow limited: diffusion of small molecules with high permeability coefficients (e.g. water, NaCl, urea, glucose)
- Diffuse rapidly from the capillary to the surrounding cells so that there’s few by the time it reaches the venous end of the capillary bed.
-
Diffusion limited: large molecules with poor permeability OR small molecules with low capillary density or in the presence of edema
- Remain in the capillary
Under conditions that increase distance between the capillary and cells (e.g. edema or low capillary density) what happens?
normally flow-limited molecule sbecome diffusion-limited as the distance between the cells and capillary become too great
Filtration/absorption through capillaries
Small water soluble molecules move through pores/fenestrations, intercellular clefts, and channels in the endothelium.
Occurs the most in the liver capillaries.
Filtration/Absorption is regulated by
a balance hydrostatic and osmotic pressure nin the capillaries & interstitial fluid
Hydrostatic pressure promotes exchange.
Capillary hydrostatic pressure (Pc) =
Tissue hydrostatic pressures (Pi) =
As blood flows from the arterial side to the venous side, what happens to Pc?
Capillary hydrostatic pressure (Pc) = blood pressure determined by arterial & venous pressure; driving force of filtration
Tissue hydrostatic pressures (Pi) = ~0
Pc drops from ~Parterial to ~Pvenous
Oncotic pressure (pi) is the osmotic pressure exerted by plasma proteins (esp albumin) that don’t easily pass the capillary wall.
Compare oncotic pressure from capillaries to tissue
Albumin is high in capillary plasma (arterial or venous), so
capillary oncotic force >> interstitial fluid oncotic force.
Since capillary oncotic force inhibits loss of water, it promotes absorption.
The Starling equation demonstrates how nutrients are delivered on the arterial side and waste uptake occurs on the venous side. How?
If capillary hydrostatic pressure > capillary oncotic pressure, then +Qf and filtration.
As Pc drops across the capillary bed…
capillary hydrostatic pressure < capillary oncotic pressure, then - Qf and absorption.
What effect will vasoconstriction vs vasodilation have on absorption/filatraiton?
Vasoconstriction will decrease pressure on the arteriolar side –> enhanced absorption at the capillary bed
Vasodilation will increase pressure of the arteriolar side –> enhanced filtration/edema
What impact does heart failure or venous clots have on filtration/absorption?
Increases pressure in the venous side –> enhanced filtration & edema
What impact would hemorrhage have on absorption/filtration
Decreased pressure in the venous side –> enhanced reabsorption
Pinocytosis is more common in capillaries within ___ tissue and is more common on the ___ side of the capillary bed
Muscle tissue
Venous side
LaPlace’s law: Wall tension = (Transmural pressure)(radius)
How does this explain how capillary walls can withstand pressure from the arterial side?
Capillaries can withstand high pressures without a mscular wall due to their reduced radius.
Having an small radius (R) reduces the effects of elevated transmural pressure on wall tension
When wall tension becomes elevated, arteries sometimes compensate by
Expand into a spherical structure (aneurysm) because
T =P(R/2) for a sphere
However, as this continues the risk of rupture increases later as the sphere starts resembling a cylinder again
Venous system function
Reservoir for the blood (~70% of blood)
When increased cardiac output is desired, the venous system constricts to increase preload while also partitioning blood o the heart & other organs.
How is the venous system able to serve its reservoir function?
Extremely high compliance of the veins
Valves in veins
- prevent increased ____ pressure caused by standing.
- facilitate return flow to the heart due to ___ and __.
Valves
- prevent increased hydrostatic capillary pressure caused by standing.
- facilitate return flow to the heart due to muscle contraction and respiration.
Lymphatic system collects ___ and ___ from the interstitial space and returns it to the bloodstream via a series of unidirectional vessels and nodes
additional fluid and leaked plasma proteins
