Blood Vessels

1. Blood Vessels Chapter 19

2. Introduction • BVs of the body form a closed delivery system powered by the pumping heart - dynamic structures that pulsate, constrict, and relax - can proliferate according to the changing needs of the body - in an adult stretch for 60,000 miles • Heart contraction forces blood into the large arteries that leave the ventricles - blood moves successively into smaller branches finally reaching the smallest arterioles that feed into the capillaries • Blood leaving the capillaries is collected by venules that merge to form larger veins - ultimately empty into the heart

3. Structure of Blood Vessels • Lumen - central blood-filled space of a vessel Except for the smallest are composed of 3 tunics: • Tunica intima – in contact with the lumen blood - contains the endothelium, the simple squamous epithelium that lines the lumen of all vessels • Tunica media – circular sheets of smooth muscle - contraction causes vasoconstriction - relaxation causes vasodilation • Tunica externa (adventitia) – composed of CT with many collagen and elastic fibers - cells and fibers run longitudinally

4. Structure of Arteries, Veins, and Capillaries Figure 19.1a

5. Arteries • Arteries – carry blood away from the heart - oxygenated blood for the systemic circuits - carry oxygen-poor blood for the pulmonary circuit • Passage of blood through the arteries proceeds from elastic arteries, to muscular arteries, to arterioles

6. Elastic Arteries The largest arteries: • Diameters range from 2.5 cm to 1 cm • Includes the aorta and its major branches • Sometimes called conducting arteries • High elastin content dampens surge of blood pressure Figure 19.2a

7. Muscular (distributing) Arteries ‘Middle-sized’ • Lie distal to elastic arteries • Diameters range from 1 cm to 0.3 mm • Includes most named arteries • Tunica media is thick • Unique features include: - internal and external thick sheets of elastic laminae Figure 19.2b

8. Arterioles Smallest arteries • Diameters range from 0.3 mm to 10 µm • Larger arterioles possess all three tunics – smaller lead into capillary beds basically a single layer of smooth muscle cells • Diameter of arterioles regulated by - Local signaling factors in the tissues - Sympathetic NS Figure 19.2c

9. Capillaries • Smallest blood vessels –diameter from 8-10um - composed of only a single layer of endothelial cells surrounded by a basement membrane - most important because they renew and refresh interstitial fluid - just large enough for RBCs to pass through in single file • Site-specific functions of capillaries: Lungs - oxygen enters blood, carbon dioxide leaves Small intestines – receive digested nutrients Endocrine glands – pick up hormones Kidneys – removal of nitrogenous wastes

10. RBCs in a Capillary Fig 19.3

11. Capillary Beds • Network of capillaries run through almost all tissues, especially loose CT • A terminal arteriole leads to a metarteriole from which branch true capillaries • True capillaries merge into the thoroughfare channel which joins a venule • Precapillary sphincters – smooth muscle cells - wrap around the root of each true capillary where it leaves the metarteriole - regulates the flow of blood to tissues

12. Not all tissues and organs have a rich capillary supply: • Tendons and ligaments – poorly vascularized • Epithelia and cartilage – avascular - receive nutrients from nearby CT

13. When precapillary sphincters relax, blood fills the true capillaries (tissues are functionally active) Figure 19.4a

14. When sphincters contract, they force most flood to flow straight from metarterioles to thoroughfare channels, bypassing the true capillaries (tissues already have adequate O2 Figure 19.4b

15. Capillary Permeability • Endothelial cells of capillaries are held together by tight junctions and occasional desmosomes • Tight junctions block passage of small molecules - but do not surround the whole perimeter of the cells - intercellular clefts, gaps of unjoined membrane allow small molecules to enter and exit • Pericytes – strengthen and stabilize the capillary - external to the epithelium - spider-shaped with thin processes that form widely-spaced networks, do not interfere with capillary permeability

16. 2 types of capillaries • Continuous – most common and lack pores - occurs in most organs such as skeletal muscle, skin, and the CNS • Fenestrated – have pores spanning the endothelial cells - occur only where there are exceptionally high rates of exchange of small molecules - example: in the small intestine, receives digested food and glomeruli of the kidneys which filter blood

17. Cross Section of a Continuous Capillary Figure 19.5a

18. Fenestrated Capillary Figure 19.5b

19. Routes of Capillary Permeability 4 routes into and out of capillaries • Direct diffusion • Through intercellular clefts • Through cytoplasmic vesicles • Through fenestrations

20. Low Permeability Capillaries • Blood-brain barrier - capillaries have complete tight junctions - no intercellular clefts are present - vital molecules pass through highly selective transport mechanims - not a barrier against oxygen, carbon dioxide, and some anesthetics

21. Sinusoids • Some organs contain wide, leaky capillaries called sinusoids or sinusoidal capillaries - have a large diameter and follows a twisted path - has both expanded and narrowed regions - usually fenestrated, their endothelial cells have fewer cell junctions than ordinary capillaries - intercellular clefts are wide open - occur where there is an extensive exchange of large materials ( proteins or cells) - example: bone marrow and spleen

22. Sinusoids Figure 19.5c

23. Veins • Conduct blood from capillaries toward the heart - systemic circuit carry oxygen-poor blood - pulmonary circuit carry oxygen-rich blood • After passing through arteries blood pressure is much lower - walls of veins are thinner • Venules – smallest veins - diameters from 8 – 100 um - smallest venules are called postcapillary venules • Venules join to form veins • Tunica externa – thickest tunic in veins

24. Mechanisms to Counteract Low Venous Pressure • Valves are opened by blood flowing toward the heart and are closed by backflow • The skeletal muscular pump aids venous return: contracting skeletal muscles press against a vein and propel blood toward the heart, forcing valves proximal to the muscles ot open and valves distal to close Figure 19.6

25. Vascular Anastomoses • Where vessels unite or interconnect to form vascular anastomoses • Organs receive blood from more than one arterial source - neighboring arteries form arterial anastomoses - provide alternative pathways or collateral channels - can be 90% occluded before a myocardial infarction occurs • Veins anastomose more frequently than arteries - can see through the dorsal surface of your hand - abundant so occlusion of a vein rarely blocks bloodflow

26. Vasa Vasorum • Walls of BVs contain living cells and require a blood supply of their own - tunica externa of large vessels contain tiny arteries, capillaries, and veins called vasa vasorum - arise as tiny branches from the same vessel or as small branches from other, nearby vessels - nourish outer region of large vessels • Inner half of large vessels receive nutrients by diffusion from luminal blood • Small vessels need no nasa vasorum – entirely supplied by luminal blood

27. Pulmonary Circulation • Pulmonary trunk leaves the right ventricle - divides into right and left pulmonary arteries • Superior an dinferior pulmonary veins - carry oxygenated blood into the left atrium

28. Pulmonary Circulation Figure 19.7

29. Systemic Circulation • Systemic Arteries – carry oxygenated blood away from the heart - aorta, largest artery in the body

30. Major Arteries Figure 19.8a

31. The Aorta • Ascending aorta – arises from the left ventricle - branches into coronary arteries • Aortic arch – lies posterior to the manubrium • Branches include: - Brachiocephalic trunk - Left common carotid - Left subclavian arteries

32. Figure 19.9

33. The Aorta • Descending aorta - continues from the aortic arch and includes the: - Thoracic aorta: region of T5 – T12 and the - Abdominal aorta: ends a L4 • Divides into right and left common iliac arteries

34. Arteries of the Head and Neck Figure 19.10a

35. Major Arteries Serving the Brain Figure 19.10c

36. Arteries of the Upper Limb and Thorax Figure 19.11

37. Arteries of the Abdominal Aorta Figure 19.12

38. The Celiac Trunk and Main Branches Figure 19.13

39. Distribution of Superior & Inferior Mesenteric Arteries Figure 19.14

40. Internal Iliac Artery Figure 19.15b

41. Arteries of the Pelvis and Lower Limbs Figure 19.16a

42. Arteries of the Pelvis and Lower Limbs Figure 19.16b

43. Fig 19.17

44. Fenestrated Capillary Figure 19.5b

45. Abdominal Cavity Vasculature Figure 19.19

46. Major Veins of the Systemic Circulation Figure 19.18

47. Dural Sinuses • Superior and inferior sagittal sinuses • Straight sinus • Transverse sinuses • Sigmoid sinus Figure 19.20b

48. Venous drainage • Internal jugular veins • External jugular veins • Vertebral veins Figure 19.20a

49. Veins of the Right Limb Figure 19.21a

50. Antecubital Fossa • Form anastomese frequently • Median cubital vein is used to obtain blood or administer IV fluids Figure 19.22