Cardiovascular Lecture 6 Flashcards
Which vessels are the smallest of the body?
Capillaries
Describe the structure of capillaries?
Comprised of a single layer of endothelial cells.
What does blood flow through capillaries depend on?
The contractile state of the arterioles
Can substances cross capillaries?
Yes, they allow the exchange of gases, water, and solutes with interstitial fluid, which is why there is a high density of capillaries in metabolically active organs.
Describe capillary flow.
Quite low velocity due to large cross-sectional area.
How would blood bypass the capillaries?
Via AV shunts or metarterioles
Why can capillaries withstand high internal pressure?
Due to small radii. Explained by law of laplace.
Vessel wall tension (T)=Transmural pressure (P) x Radius (r)
What role does the capillary endothelium play.
A vasoactive role: important source of subtstances that cause contraction and relaxation of vascular smooth muscle. Includes prostacyclin (PGI2), Nitric Oxide, Endothelin, Nitroprusside, and vasodilator agents A passive role via transcapillary exchange of solvents and solutes by: diffusion, filtration, pinocytosis.
Describe the capillary role of PGI2
- Relaxes vascular smooth muscle via an increase in cAMP
- Released due to an increase in shear stress with accelerated blood flow
- Primary function is to inhibit platelet
adherence to endothelium and
platelet aggregation, thus preventing
intravascular clot formation
Describe the capillary role of nitric oxide
- Much more important than PGI2 for endothelium-mediated vascular dilation - Component of the endothelium-derived relaxing factor - Released when endothelium stimulated by acetylcholine / other vasodilator substances (ATP, bradykinin, serotonin, substance P, histamine) - Also can be released due to an increase in shear stress - Activates guanylyl cyclase in vascular smooth muscle to increase [cGMP], which causes relaxation by decreasing myofilament sensitivity to [Ca2+]
Describe the capillary role of endothelin
Potent vasoconstrictor peptide synthesised by endothelium, may be involved in pathological states
Describe the capillary role of nitroprusside
Drug that is not endothelium-mediated, but also acts on
vascular smooth muscle to increase cGMP and cause relaxation
Describe the capillary role of vasodilator agents
Vasodilator agents released from parenchymal tissue also act locally on vascular smooth muscle (i.e., adenosine, H+ , CO2 , K+ )
Describe the capillary role of diffusion
• Under normal conditions, 5000 times more water crosses capillary wall by
diffusion compared to filtration
• Rate of diffusion of water across the capillary wall is 40 times greater than
the rate at which it is brought to the capillaries by blood flow
• Diffusion also governs transcapillary exchange of solutes, gases, and waste
• Described by Fick’s law
• Diffusion of lipid-insoluble molecules restricted to water-filled pores
• Flow limited transport:
- For small molecules (e.g., water, NaCl, urea, glucose), capillary pores offer little
restriction to diffusion (reflection coefficient, 0)
- Only limitation to net movement across capillary wall is rate at which blood flow
transports molecules to the capillary
- Concentration of molecule in blood and interstitial
fluid reaches equilibrium near capillary origin (for
larger molecules, this will be slightly delayed)
- Concentration falls to negligible levels soon after
- If flow large, molecule may still be present further
down the capillary
• Diffusion of lipid-insoluble molecules restricted to water-filled pores
• Diffusion limited transport:
- For larger molecules, diffusion becomes limiting factor
- For molecular weights greater than 60,000,
diffusion becomes minimal
- For small molecules, diffusion only a factor if
distance between capillary and parenchymal cells
is great (e.g., tissue edema or low capillary density)
• Diffusion of lipid-soluble molecules not limited to capillary pores - occurs
directly through lipid membrane
• Thus, move rapidly between blood and tissue (e.g., O2 and CO2
)
Describe the capillary role of filtration
• Most water flows through (~4 nm) intercellular clefts (pores) in the
endothelial walls of capillaries
• Represent only about 0.02% of capillary
wall (absent in cerebral capillaries, where blood-brain barrier blocks entry of
many small molecules)
• Most porous capillaries (e.g., in kidney,
intestine) contain (~20-100 nm)
fenestrations, whereas other
capillaries (e.g., in liver) have
discontinuous endothelium
• These permit the passage of molecules
to large for intercellular clefts
• Direction and magnitude of water movement estimated as sum of
hydrostatic and osmotic pressure across the membrane
• Hydrostatic Pressure (Pc
):
- Favours movement of fluid from vessel lumen to interstitial space
- Principal force in capillary filtration
- Depends on arterial / venous pressure (increase in either increases Pc
) and
resistance of arterioles (increase decreases Pc
) / venules (increase increases Pc
)
- Interstitial fluid pressure (Pi
) opposes it (Pc
- Pi = driving force for filtration)
• Osmotic Pressure ():
- Favours movement of fluid from interstitial space into vessel lumen
- Key factor restraining fluid loss from capillaries is osmotic pressure of plasma
proteins (e.g., albumin), called colloid osmotic pressure / oncotic pressure (p
)
- Due to relative impermeability to capillary wall (reflection coefficient, 1)
Starling hypothesis: defines the relative influence of hydrostatic and osmotic pressure to fluid exchange across capillary endothelium
- when positive, filtration occurs; when negative, absorption occurs
- only small portion of plasma filtered, most is absorbed
- remainder returns to vascular system as lymph fluid
