Blood vessels

Vessel wall structure:

  1. tunica intima - inner layer, inc endothelial lining with variable number of elastic fibres. in arteries, thick layer of elastic fibres called internal elastic membrane

    1. allows frictionless pathway for blood

  2. tunica media - middle layer, sheets of smooth muscle with connective tissue.

    1. collagen fibres bind the intima and media together and externa

    2. separated from externa by external elastic membrane

    3. allows control of the internal diameter

  3. tunica externa - protective sheath of connective tissue

    1. in veins generally thicker than media

    2. in arteries contains collagen fibres with bands of elastic fibres

Arteries

Elastic Arteries:

  • tunica media contains high density of elastic fibres

  • during ventricular systole, pressure rises so elastic arteries expand when there is ventricular diastole pressure falls so elastic arteries contract

  • allows continuous blood flow

Muscular Arteries

  • thick tunica media

  • contain more smooth muscle cells than elastic arteries

Arterioles

  • poorly defined tunica externa

  • resistance vessels

Clinical issues associated with arteries:

Arteriosclerosis - thickening and hardening of arterial walls

Atherosclerosis - deposition of lipids in the tunica media that damage the endothelial lining

Focal calcification - deposition of calcium salts following degeneration of smooth muscle in the tunica media

Aneurysm - bulge in a weakened arterial wall

Capillaries

continuous, fenestrated and fenestrated

more capillary networks for skeletal muscle, liver and kidney because of higher metabolic activity

connective tissues have less networks

continuous capillaries

  • endothelium is a complete lining, found in all tissues except epithelia and cartilage

  • restricted permeability

fenestrated capillaries

  • contains pores that penetrate the endothelial lining

  • allows rapid exchange between interstitial fluid and blood

  • found in the brain and the blood vessels in endocrine organs such as hypothalamus and pituitary

sinusoids

  • similar to fenestrated but flattened and irregular shaped

  • discontinuous endothelium

  • basement membrane is thinner or absent

  • blood flow is slow

  • found in spleen, pit gland and adrenal glands

capillary beds

  • precapillary sphincter stops blood flow to one singular capillary

  • collaterals are multiple arteries that supply blood to a capillary bed

  • arterial anastomosis is the fusion of two collateral arteries

  • arteriovenous anastomoses are direct connections between arterioles and venules

    • blood can bypass the capillary bed, regulated by lymphatic system

  • angiogenesis - formation of new blood vessels due to vascular endothelial growth factor (VEGF)

  • angiogenesis is most important in cardiac muscle

Venules

  • direct blood from capillary beds to small veins

  • small - endothelium + basement membrane

  • large - smooth muscle around endothelium

Veins

  • venules - smallest veins that collect blood from capillary beds

  • medium size veins has thin tunica media, thickest layer us tunica externa

  • large veins include superior and inferior venae cavae

  • venous valves

  • more distensible than arteries

  • capacitance vessels because they have high capacitance, acting as blood reservoirs