The document summarizes the structure and function of blood vessels. It describes the three layers (tunics) that make up arteries and veins, as well as the single-layered endothelium of capillaries. It then compares different types of blood vessels, including elastic and muscular arteries, arterioles, venules and veins. It discusses the roles of each in conducting blood away from the heart in arteries and returning it to the heart in veins. It also describes the structure of capillaries and their role in exchanging materials with tissues.

1. Blood Vessels SMS 1053 Dr. Mohanad R. Alwan

2. Blood vessels Learning Objectives: Compare and contrast the structure and function of Arteries Veins Capillaries

3. Blood Vessels Closed circulatory system Arteries Arterioles Capillaries Venules Veins 3 tunics Lumen

4. The Vessels Functions: Distribution of blood Exchange of materials with tissues Return of blood to the heart Structure: Most have the same basic structure: – 3 layers surrounding a hollow lumen

5. General Structure of Blood Vessels Arteries and veins are composed of three tunics: tunica interna tunica media tunica externa Capillaries are composed of endothelium.

6. Generalized Structure of Blood Vessels Arteries and veins are composed of three tunics – tunica interna, tunica media, and tunica externa Lumen – central blood-containing space surrounded by tunics Capillaries are composed of endothelium with sparse basal lamina

7. The Anatomy Of Blood Vessels Layers: Tunica interna (intima): Endothelial layer that lines the lumen of all vessels. In vessels larger than 1 mm, a subendothelial connective tissue basement membrane is present 2. Tunica media: Smooth muscle and elastic fiber layer, regulated by sympathetic nervous system Controls vasoconstriction/vasodilation of vessels 3. Tunica externa (adventitia): Collagen fibers that protect and reinforce vessels Larger vessels contain vasa vasorum

8. General Structure

9. The Vessels 1. Tunica Intima Innermost smooth layer Simple squamous epithelium Continuous with the endocardium Present in all vessels

10. The Vessels 2. Tunica Media layer of smooth muscle - circular arrangement – contains elastin Supplied by sympathetic division of the ANS Depending on body’s needs – lumen is narrowed (vasoconstriction) or widened (vasodilation)

11. The Vessels 3. Tunica Externa (Adventitia) Thin layer of CT Elastic & collagen fibres

12. The Vessels Types of Vessels: Arteries – carry blood away from the heart Veins – carry blood towards the heart Capillaries – the most important part of the vascular system; site of exchange of materials

13. The Blood Vessels and the Cardiovascular System Arteries: blood from heart Strong & Elastic Conduct blood to capillaries Sphincters Capillaries: exchange with cells Veins Return blood to heart Valves

14. Histological Structure of Blood Vessels

15. Make Up of Blood Vessels: Arteries and Arterioles Endothelium Elastic tissues Rebounds Evens flow Smooth muscles Fibrous tissue Tough Resists stretch Figure 15-2: Blood vessels

16. Artery with thick wall Connective tissue Muscle layer Endothelium

17. 1 2 3 ELASTIC ARTERY (AORTA) Stained with orsein 1 - tunica intima 2 - tunica media 3 - tunica externa

19. Types of Blood vessels: Arteries Elastic Arteries: Thick-walled arteries near the heart; the aorta and its major branches . Large lumen allows low-resistance conduction of blood. Contain lots of elastin in all three tunics. walls stretch and recoil to propel blood Withstand and regulate large blood pressure fluctuations . Allow blood to flow fairly continuously through the body

20. Muscular (Distributing) Arteries and Arterioles Muscular arteries – distal to elastic arteries; deliver blood to body organs Have thick tunica media with more smooth muscle and less elastic tissue Active in vasoconstriction Arterioles – smallest arteries; lead to capillary beds Control flow into capillary beds via vasodilation and constriction

21. Types of Blood vessels: Arteries Muscular (distributing) arteries medium sized vessels tunica media more smooth muscle; less elastin major area of vaso-constriction & dilation to regulate blood flow

22. The Vessels Arterioles (diameter of 0.3 mm or less) smallest arteries; lead to capillary beds. close to capillaries - single layer of muscle spiralling around the endothelial lining regulates blood flow to capillary

23. General Structure

24. Venous System: Venules Are formed when capillary beds unite Allow fluids and WBCs to pass from the bloodstream to tissues Postcapillary venules – smallest venules, composed of endothelium and a few pericytes Large venules have one or two layers of smooth muscle (tunica media)

25. Venous System: Veins Veins are: Formed when venules converge Composed of three tunics, with a thin tunica media and a thick tunica externa consisting of collagen fibers and elastic networks Capacitance vessels (blood reservoirs) that contain 65% of the blood supply

26. Venous System: Veins Veins have much lower blood pressure and thinner walls than arteries To return blood to the heart, veins have special adaptations Large-diameter lumens, which offer little resistance to flow Valves (resembling semilunar heart valves), which prevent backflow of blood Venous sinuses – specialized, flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain)

28. Make Up of Blood Vessels: Veins and Venules (Contrasted to Arteries) Thinner walls Larger diameter Closer to skin Less muscle Less elastic Figure 15-3: Metarterioles

29. Anatomy of Vessels

30. Comparison of Veins and Arteries Arteries: Veins:

36. They also have semi-lunar valves to stop the blood flowing backwards

37. This is a medium sized vein, recognizable as such by its scanty wall and the presence of a valve inside it. The valve flaps are marked by arrows.

38. Artery and a vein

39. Vascular Anastomoses Merging blood vessels, more common in veins than arteries Arterial anastomoses provide alternate pathways (collateral channels) for blood to reach a given body region If one branch is blocked, the collateral channel can supply the area with adequate blood supply Thoroughfare channels are examples of arteriovenous anastomoses

40. A capillary wall is very thin and composed of (endothelium only) single layer of cells as it does not have to withstand high internal pressure. A capillary wall is often highly permeable, partly because its very thin and partly because of holes in and between cells in some capillaries (particularly those with high demand of exchange e.g endocrine glands) Capi ll aries

41. Capillaries Capillaries are the smallest blood vessels Walls consisting of a thin tunica interna, one cell thick Allow only a single RBC to pass at a time Pericytes on the outer surface stabilize their walls There are three structural types of capillaries: continuous, fenestrated, and sinusoids

46. A capillary bed

48. Make Up of Blood Vessels: Capillaries Figure 15-16: Types of capillaries

49. Continuous Capillaries Continuous capillaries are abundant in the skin and muscles, and have: Endothelial cells that provide an uninterrupted lining Adjacent cells that are held together with tight junctions Intercellular clefts of unjoined membranes that allow the passage of fluids

50. Continuous Capillaries Continuous capillaries of the brain: Have tight junctions completely around the endothelium Constitute the blood-brain barrier

51. Continuous Capillaries Figure 19.3a

52. Fenestrated Capillaries Found wherever active capillary absorption or filtrate formation occurs (e.g., small intestines, endocrine glands, and kidneys) Characterized by: An endothelium riddled with pores (fenestrations) Greater permeability to solutes and fluids than other capillaries

53. Fenestrated Capillaries Figure 19.3b

54. Sinusoids Highly modified, leaky, fenestrated capillaries with large lumens Found in the liver, bone marrow, lymphoid tissue, and in some endocrine organs Allow large molecules (proteins and blood cells) to pass between the blood and surrounding tissues Blood flows sluggishly, allowing for modification in various ways

55. Sinusoids

56. Capillary Beds A microcirculation of interwoven networks of capillaries, consisting of: Vascular shunts – metarteriole–thoroughfare channel connecting an arteriole directly with a postcapillary venule True capillaries – 10 to 100 per capillary bed, capillaries branch off the metarteriole and return to the thoroughfare channel at the distal end of the bed

57. Capillary Beds Figure 19.4a

58. Capillary Beds Figure 19.4b

59. Blood Flow Through Capillary Beds Precapillary sphincter Cuff of smooth muscle that surrounds each true capillary Regulates blood flow into the capillary Blood flow is regulated by vasomotor nerves and local chemical conditions, so it can either bypass or flood the capillary bed

61. Capillary Exchange Diffusion: Filtration: Reabsorption:

62. Capillary Exchange At the arteriole end of a capillary, water moves out of the blood due to the force of blood pressure . At the venule end, water moves into the blood due to osmotic pressure of the blood. Substances that leave the blood contribute to tissue fluid , the fluid between the body’s cells.

63. In the midsection of the capillary, nutrients diffuse out and wastes diffuse into the blood. Since plasma proteins are too large to readily pass out of the capillary, tissue fluid tends to contain all components of plasma except it has lesser amounts of protein. Excess tissue fluid is returned to the blood stream as lymph in lymphatic vessels . Capillary Exchange

64. Capillary exchange

65. Capillary exchange At the arterial end of a capillary, the blood pressure is higher than the osmotic pressure; therefore, water tends to leave the bloodstream. In the midsection, oxygen and carbon dioxide follow their concentration gradients. At the venous end of a capillary, the osmotic pressure is higher than the blood pressure; therefore, water tends to enter the bloodstream.

66. Capillary Exchang e

67. Blood Flow in Capillaries Blood moves slowly in capillaries because there are more capillaries than arterioles. This allows time for substances to be exchanged between the blood and tissues.

72. Major arteries and veins of the systemic circuit

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