Blood Vessels D. Matesic

Blood VesselsBranching of arteries and veins and relation to lymphatic

system

Fig. 19.2

Figure 19.1

Blood Vessels

General structure of arteries and veins 3 layers- “tunics”

1.tunica intima-tunica intima-endothelium + loose CT2.2.tunica media-tunica media-smooth muscle + elastic fiber

sheets 3.3.tunica externa-tunica externa-connective tissue (mostly

collagen fibers)

Most arterioles and venules also have 3 layers, but tunica media is thinner and smallest ones lack tunica externa

Capillary structure and transport mechanismsCapillaries allow exchange of materials between the blood and tissues

Comprised of 2 layers: Endothelium, and basal lamina.

pericytes surround and help support the capillaries

Transport of materials across capillaries occurs via: 1. diffusion across endothelial cell membranes 2. intercellular clefts – space between tight junctions 3. fenestrations – pores 4. vesicular transport –

Fig. 19.16

Fig. 19.3 (optional)

Functions of endothelium

1. Physical lining of heart and blood vessels2. Permeability barrier for exchange of substances

between plasma and tissue interstitial fluid3. Secrete paracrine agents that act as vasodilators

and vasoconstrictors4. Mediate angiogenesis (new capillary growth)5. Regulate platelet clumping, clotting and

anticlotting

Capillary Beds –interwoven networks of capillaries that branch from terminal arterioles (via metarterioles)

precapillary sphincters control flow of blood

through the capillary bed

after exchange of nutrients/fluids between the blood in the capillary bed and the tissue, blood flows into the postcapillary venules and ultimately

back to the heart.

Capillary Beds

Fig 19.4Not required

Vascular anastomosesRegions where vessels interconnect; provide alternate routes for blood to reach a given body part