TOPIC 3 Circulatory System – Blood Vessels

1. Biology 221 Anatomy & Physiology II TOPIC 3Circulatory System – Blood Vessels Chapter 20 pp. 718-747 E. Lathrop-Davis / E. Gorski / S. Kabrhel

2. Blood Vessel Functions • Blood vessels act as conduits for blood. Humans have a closed circulatory system, which is more efficient for accomplishing the cardiovascular system’s important functions (see Topic 1: Blood). • Separate systemic and pulmonary systems provide more efficient delivery of oxygen and nutrients to and removal of wastes from the tissues.

3. Blood Vessels - Major Types • Arteries carry blood away from the heart regardless of whether it is high or low in oxygen. Arteries divide into smaller and smaller vessels until they reach the capillaries. • Capillaries are sites of exchange of materials between blood and tissues. • Veins return blood to heart, regardless of whether it is high or low in oxygen.

4. Blood Vessel Histology There are three layers of blood vessel wall: • The tunica interna (tunica intima) is the inner layer. The tunical interna consists of: • an inner endothelium, which is simple squamous epithelium; and a • subendothelial layer of areolar connective tissue, which connects the endothelium to overlying structures. http://www.usc.edu/hsc/dental/ghisto/cv/d_1.html Fig. 20.1, p. 719

5. Blood Vessel Histology • The tunica media is the middle layer. It consists of: • varying amounts of dense connective tissue, which give it strength; and • smooth muscle, which can contract and relax, thus affecting the diameter of the vessel. This is important to managing flow and maintaining blood pressure (see Topic 4: Blood Flow, Blood Pressure, and Capillary Dynamics). • Vasomotor tone is a state of partial contraction maintained by near continuous sympathetic nervous outflow called “tone”. Changes in this “tone” lead to changes in blood vessel diameter. (see next page) Fig. 20.1, p. 719 http://www.usc.edu/hsc/dental/ghisto/cv/d_1.html

6. Blood Vessel Histology • An increase in sympathetic outflow leads to vasoconstriction, which is a decrease in vessel size. A decrease in size makes it more difficult to move blood through the vessel. • A decrease in sympathetic outflow leads to vasodilation, which is an increase in vessel size. An increase in size makes it easier to move blood through the vessel. Fig. 20.1, p. 719 http://www.usc.edu/hsc/dental/ghisto/cv/d_1.html

7. Blood Vessel Histology The tunica externa is the outermost layer. • It consists of connective tissue, which attaches it to the surrounding tissues. • Nerve fibers and lymphatic vessels run through the tunica externa of most medium to large vessels. • The vasa varsorum is a blood vessel system in the tunica externa of larger blood vessels (for example, elastic arteries). http://www.usc.edu/hsc/dental/ghisto/cv/d_2.html http://www.usc.edu/hsc/dental/ghisto/cv/d_5.html

8. Elastic (Conducting) Arteries • The aorta and its major branches are elastic arteries. • Their functions include that they: • carry blood rapidly away from heart; • decrease blood pressure fluctuations in the arteries during the course of each heart beat by: • expanding during ventricular systole, which gives the blood more room into which to move and decreases systolic pressure; and • recoiling during ventricular diastole, which maintains pressure on blood as the heart relaxes.

9. Elastic (Conducting) Arteries The structure of elastic arteries includes: • a large external diameter with a large lumen, which allows blood to flow quickly through (be rapidly conducted away from the heart); • thick walls, which allow them to withstand the high pressures generated by the ventricles during systole; • lots of elastic fibers (elastin), which allow they to stretch when blood is added during systole and recoil during diastole; and • the vaso vasorum (see previous slide), which nourishes their thick walls. http://www.usc.edu/hsc/dental/ghisto/cv/d_2.html

10. Muscular (Distributing) Arteries • Most of the named arteries are muscular arteries. • The function of muscular arteries is to deliver blood to the organs and to control flow to organs, that is, to control blood distribution to the organs. • The structure of musclar arteries includes: • an internal diameter smaller than that of elastic arteries; and • a thick tunica media with lots of smooth muscle, which allows flow to organs to be changed according to the needs of the body. For example, after eating more blood flows to the digestive system; during exercise, less flows there. http://www.usc.edu/hsc/dental/ghisto/cv/d_12.html

11. Arterioles • The function of arterioles is to distribute blood to tissues within organs; that is they are the major controllers of blood flow into capillaries. • The structure of arterioles includes : • that they branch and become smaller as they go into the organs/tissues they serve; and • that the thickness of their walls decreases as they near capillaries. • Near capillaries, the wall may consist of only of endothelium and scattered smooth muscle cells. http://www.usc.edu/hsc/dental/ghisto/cv/d_12.html

12. Capillaries • The function of capillaries is to act as sites of exchange of materials (gases, nutrients, wastes, etc.) between blood and tissues. • Structurally, capillary walls consist of tunica interna only with some with scattered pericytes (smooth muscle cells) to help control their diameter. • Three structural types of capillaries are: • continuous capillaries, • fenestrated capillaries, and • sinusoids. http://www.usc.edu/hsc/dental/ghisto/cv/d_25.html Fig. 20.3, p. 724

13. Continuous Capillaries • Most capillaries are of this type. • In continuous capillaries the endothelial cells are continuous; that is, they abut one another. • Cells that comprise the walls may be held together by tight junctions. • Intercellular clefts are gaps between tight junctions that aid exchange between blood and tissues. • Tight junctions are continuous in brain where they contribute to the blood-brain barrier(see A&P I, Unit 6). Fig. 20.3, p. 724 http://www.usc.edu/hsc/dental/ghisto/cv/d_25.html

14. Fenestrated Capillaries • Fenestrated capillaries are ones in which some endothelial cells have pores. • Most of these pores covered with membrane. • The presence of pores makes the walls very permeable. • Fenestrated capillaries are found in the small intestine, some endocrine glands, and glomeruli of the kidney; places where exchange rates of large molecules would be expected to be high. Fig. 20.3, p. 724 http://www.usc.edu/hsc/dental/ghisto/cv/d_26.html

15. Sinusoids • Sinusoids have large irregular lumens, which slows blood flow through them. • The walls are fenestrated or incompletely lined with endothelial cells, which allows large molecules (e.g., proteins) to pass through. • In the liver, the endothelium is discontinuous where macrophages (called Kupffer cells) form part of vessel wall. • In the spleen, phagocytes are on the tissue side (outside) of the endothelial lining and extend processes into the lumen. • There are few tight junctions in the walls. • Sinusoids are located in liver, spleen, bone marrow, lymphoid tissue, and some endocrine glands. Fig. 20.3, p. 724 http://www.usc.edu/hsc/dental/ghisto/cv/d_34.html

16. Capillary Beds • Capillary beds are formed from the many capillary branches from arteriole. • The flow of blood from arteriole to venule through the capillary bed is referred to as microcirculation. • The metarteriole - thoroughfare channel allows a fast, direct connection between arteriole and venule. • Flow goes as follows: terminal arteriole  metarteriole  thoroughfare channel  venule • This channel allows blood to by-pass the capillary bed when the tissue is inactive, thereby conserving the nutrients and oxygen in the blood for more active tissues. Fig. 20.4, p. 725

17. Capillary Beds: True Capillaries • True capillaries are sites of exchange between blood and tissues. • They originate as branches of the metarteriole and rejoin into the thoroughfare channel. • Precapillary sphincters control blood movement into the capillary bed at the arteriolar end. • The amount of blood entering a capillary bed depends on the gross needs of the body (sympathetic vasomotor nervous control) and local needs of the tissue (i.e., local chemical cues). Fig. 20.4, p. 725

18. Post-capillary Venules • The function of venules is to collect blood from capillary beds. • Their structure includes: • that they are formed by the union of capillaries; and • that the endothelium is leaky and has few pericytes. • White blood cells (WBCs) are abundant in venules where they help prevent spread of disease from potentially infected tissues. http://www.usc.edu/hsc/dental/ghisto/cv/d_36.html

19. Veins Functions of veins include: • carrying blood back toward the heart under low pressure; and • acting as blood reservoirs, or capacitance vessels, in that ~ 65% of the body’s blood is in the veins at any given time. Venoconstriction (constriction of the veins) can move this blood more quickly toward the heart and is especially important for maintaining adequate flow during hemorrhage. http://www.usc.edu/hsc/dental/ghisto/cv/d_39.html

20. Veins • Structurally: • Veins gradually increase in size and thickness of their walls as they near the heart. • Veins have all 3 vascular tunics present, but they are thinner than in arteries of corresponding size (external diameter). • The walls have little smooth muscle or elastin; and have a relatively thicker tunica externa. • Valves in the walls prevent backflow (i.e., keep blood flowing in one direction). • Varicose veins occur because valves fail and blood pools causing the venous walls to expand. • Phlebitis is any inflammation of a vein. http://www.usc.edu/hsc/dental/ghisto/cv/d_39.html

21. Venous Sinuses • Venous sinuses function to collect blood under low pressure. • Structurally: • sinuses are flattened veins with walls of endothelium only; • they are supported by surrounding tissues. • Examples include the coronary sinus of the heart and dural sinuses of the cranium.

22. Vascular Anastomoses • Arterial anastomoses form collateral channels to maintain flow in case one gets blocked. • Examples include: • the circle of Willis serving the brain; • around joints; • circulation to several abdominal organs; and • arterial circulation to the heart. • Arteriovenous anastomosesallow blood to flow directly from the arteriole to the venule and is represented by the metarteriole - thoroughfare channel • Venous anastomoses connect vein to vein and are more common.

23. Circulatory Patterns • The general pattern is: Ventricles of heart to the elastic arteries to the muscular arteries to the arterioles to the capillaries to the venules to the veins to the atria of heart • The two circulatory patterns are the pulmonary circulation and the systemic circulation. See Table 20.1, p. 721

24. Circulation: Pulmonary Circuit • The pulmonary circulation serves the alveoli of the lungs where exchange of gases between blood and air occur. • It starts from the right ventricle, which pumps blood into the pulmonary trunk, which branches into the right(R) and left (L) pulmonary arteries, which send branches to the alveolar capillaries of the lungs. Blood returns through the R and L pulmonary veinsto the left atrium. • It takes deoxygenated blood relatively high in CO2 to the lungs for exchange and and returns oxygenated blood that is also lower in CO2 to the heart. See Fig. 20.2, p. 720

25. Circulation: Systemic Circulation • The systemic circulation serves tissues of the body other than the alveoli of the lungs. • It starts from left ventriclewhich pumps the blood into the aortawhich leads to other elastic arterieswhich lead to muscular arteries which leads to arterioles which bring blood to the capillaries of body tissues (other than the alveoli of the lungs) blood returns via venules which lead to veins which flow into superior and inferior venae cavaewhich return blood to the right atrium. • It takes oxygenated blood low low in CO2 to tissues and returns deoxygenated blood higher in wastes and CO2 from the tissues. See Fig. 20.2, p. 720

26. Special Circulatory Patterns: • The hepatic portal system carries venous blood from intestines, pancreas, stomach, spleen to the liver. Fig. 20.27, p. 771 (Vessels of the hepatic portal system will be covered with the digestive system.) • The hypophyseal portal systemcarries venous blood with regulatory hormones from the hypothalamus to the anterior pituitary. Fig. 17.5, p. 617 (See also A&P I Unit 11: Endocrine System)

27. Special Circulatory Patterns • Coronary Circulation, which was covered with the heart, carries oxygen-rich blood to heart and removes oxygen-poor blood. Fig. 19.7, p. 690 • The cerebral circulation starts from the muscular carotid and vertebral arteries, which bring oxygenated blood the brain, and ends with jugular veins, which drain oxygen-poor blood from brain (details in lab). Fig. 20.20. p. 755 Fig. 20.25, p. 767

28. Special Circulatory Patterns • The fetal circulation serves the developing fetus. Fig. 29.13, p. 1136 • Two major differences between adult and fetal circulation exist: • the lungs are not developed or working and are thus by-passed (minimal flow); and • gases are exchanged at the placenta. • By-passing (limiting flow to) the developing lungs is accomplished by: • the ductus arteriosus, which carries blood from pulmonary trunk to aorta; and • the foramen ovale, which allows blood to flow from the right atrium into the left atrium.

29. Special Circulatory Patterns Fetal Circulation (con’t) • Gas & material exchange at placenta is accomplished through: • theumbilical arteriesthat carry deoxygenated blood high in wastes and CO2 to the placenta; and • the umbilical vein that brings oxygenated blood high in nutrients and low in CO2 from placenta. • What do all these fetal structures become? • What happens if they fail to close?

30. Vessel Disorders • Atherosclerosis occurswhen blood vessel walls become abnormally thick, narrowed and less compliant due to loss of elasticity. The lumen becomes filled with lipid-rich (especially cholesterol) plaques. • Occlusive coronary atherosclerosis is atherosclerosis of the coronary arteries; this often leads to ischemia and infarct. • Arteriosclerosisis a late stage of atherosclerosis in which the arterial walls begin to degenerate.

31. Vessel Disorders • Aneurysm refers to localized dilation or out-pouching of a blood vessel or a cardiac chamber. These are prone to rupture, which often leads to severe bleeding.