Blood Vessels D. Matesic NOT ON EXAM, ON FINAL

Blood Vessels Branching 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-endothelium + loose CT 2.tunica media-smooth muscle + elastic fiber sheets 3.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 mechanisms Capillaries 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 Physical lining of heart and blood vessels Permeability barrier for exchange of substances between plasma and tissue interstitial fluid Secrete paracrine agents that act as vasodilators and vasoconstrictors Mediate angiogenesis (new capillary growth) 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 19.4 Not required

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