LEARNING OUTCOMES
DISCUSS THE CONTROL OF BLOOD VESSEL DIAMETER
MECHANISMS OF SMOOTH MUSCLE CELL CONTRACTION AND RELAXATION
DISCUSS THE ROLE OF THE ENDOTHELIUM IN THE MODULATION OF VESSEL
DIAMETER
WHAT IS THE VASCULAR SYSTEM?
The vascular system is a very rich network of blood vessels
The vasculature plays a key role in supplying oxygen and nutrients to our tissues
Our organs are supplied by a very rich network of vessels.
- In particular the heart which is supplied by the coronary circulation
The arteries- specifically the small ones- are able to actively regulate and control the amount of blood that
is supplied to ensure that it is matched to demand via regulating their diameter
Blood vessels need to constrict and dilate promptly to ensure that there is sufficient oxygen and nutrients
reaching
- e.g. for the contraction of the heart muscle myocardium.
STRUCTURAL FEATURES
Three important layers of the smooth muscle cell:
- Tunica media-
o made up of smooth muscle cells that overlie and
external elastic lamina.
o On the inside this is covered by a single delicate layer
of endothelial cells (next to lumen where blood lies)
which overlie an internal elastic lamina
- Tunica adventitia-
o on the outside
o the adventitia layer made up of connective tissue,
elastin and collagen
o also a number of very important cell types that
infiltrate this layer including:
macrophages
fibroblasts
- Tunica intima-
- Innermost layer
- layer of endothelial cells that is in contact with the blood
- Because capillaries are only one cell layer thick, they only have a tunica intima. This ultra-
thin design allows for the exchange of gases and nutrients through the capillary walls.
- consists of a layer of endothelial cells lining the lumen of the vessel, as well as a
subendothelial layer made up of mostly loose connective tissue.
- Often, the internal elastic lamina separates the tunica intima from the tunica media.
, ARTERIES
SMALL ARTERIES:
- It has a thick layer of smooth muscle cells
- Thicker in smaller sized arteries= resistance sized arteries: because they are able to control over
50% of blood perfusion into the tissues
LARGE ARTERIES:
- Larger arteries in particular conduit arteries like the aorta has a layer of elastin
- Vascular function important: elastin property of vessels
o Blood ejected from the heart is initially accommodated by the distention or elasticity of the
aorta and this will help reserve a steady state of blood flow into the tissues
o In disease the elastin is replaced by collagen which is non elastic
- Therefore the contractile state of the smooth muscle cells is controlled by:
o Extrinsic factors:
Circulating hormones
Local release of neurotransmitters from sympathetic nerve endings
The release of mediators from the endothelial cells
o Intrinsic factors:
Contractile state regulated by calcium ion concentration – intracellular ion
concentration
SMOOTH MUSCLE CELLS (SMC)
Membrane contains many receptors, and these include:
- Receptors for agonists like - histamine, noradrenaline and angiotensin
- ion channels: calcium channels, potassium channels and sodium channels
Important in influencing the level of intracellular calcium ions
- when the intracellular calcium ions are high- SMC contracts
- when the intracellular calcium ions are low- SMC relaxes
consequently the vasoconstrictors and vasodilators will influence the contractile state by either:
- increasing or decreasing the intracellular calcium ion concentration
- they can also alter the sensitivity of the contractile machinery to calcium
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