BLOOD VESSELS © 2013 Pearson Education, Inc.

Delivery system of dynamic structures that begins and ends at heart Blood Vessels Delivery system of dynamic structures that begins and ends at heart Arteries: carry blood away from heart; oxygenated except for pulmonary circulation and umbilical vessels of fetus Capillaries: contact tissue cells; directly serve cellular needs Veins: carry blood toward heart © 2013 Pearson Education, Inc.

Artery Vein © 2013 Pearson Education, Inc.

Structure of Blood Vessel Walls Lumen Central blood-containing space Three wall layers in arteries and veins Tunica intima, Tunica media, and Tunica externa Capillaries Endothelium with sparse basal lamina © 2013 Pearson Education, Inc.

• Internal elastic membrane • External elastic membrane Tunica intima • Endothelium • Subendothelial layer • Internal elastic membrane Tunica media (smooth muscle and elastic fibers) Valve • External elastic membrane Tunica externa (collagen fibers) • Vasa vasorum Lumen Lumen Artery Capillary network Vein Basement membrane Endothelial cells Capillary © 2013 Pearson Education, Inc.

Tunics Tunica intima Endothelium lines lumen of all vessels Continuous with endocardium Slick surface reduces friction Subendothelial layer in vessels larger than 1 mm; connective tissue basement membrane © 2013 Pearson Education, Inc.

Tunics Tunica media Smooth muscle and sheets of elastin Sympathetic vasomotor nerve fibers control vasoconstriction and vasodilation of vessels Influence blood flow and blood pressure © 2013 Pearson Education, Inc.

Tunica externa (tunica adventitia) Tunics Tunica externa (tunica adventitia) Collagen fibers protect and reinforce; anchor to surrounding structures Contains nerve fibers, lymphatic vessels © 2013 Pearson Education, Inc.

Sinusoid Metarteriole Venous system Arterial system Large veins (capacitance vessels) Heart Elastic arteries (conducting arteries) Large lymphatic vessels Lymph node Muscular arteries (distributing arteries) Lymphatic system Small veins (capacitance vessels) Arteriovenous anastomosis Lymphatic capillaries Sinusoid Arterioles (resistance vessels) Terminal arteriole Postcapillary venule Metarteriole Thoroughfare channel Capillaries (exchange vessels) Precapillary sphincter © 2013 Pearson Education, Inc.

Arterial System: Elastic Arteries Large thick-walled arteries with elastin in all three tunics Aorta and its major branches Large lumen offers low-resistance Act as pressure reservoirs—expand and recoil as blood ejected from heart Smooth pressure downstream © 2013 Pearson Education, Inc.

Arterial System: Muscular Arteries Distal to elastic arteries Deliver blood to body organs Thick tunica media with more smooth muscle Active in vasoconstriction © 2013 Pearson Education, Inc.

Arterial System: Arterioles Smallest arteries Lead to capillary beds Control flow into capillary beds via vasodilation and vasoconstriction © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc.

Microscopic blood vessels Walls of thin tunica intima Capillaries Microscopic blood vessels Walls of thin tunica intima In smallest one cell forms entire circumference Pericytes help stabilize their walls and control permeability Diameter allows only single RBC to pass at a time © 2013 Pearson Education, Inc.

In all tissues except for cartilage, epithelia, cornea and lens of eye Capillaries In all tissues except for cartilage, epithelia, cornea and lens of eye Provide direct access to almost every cell Functions Exchange of gases, nutrients, wastes, hormones, etc., between blood and interstitial fluid © 2013 Pearson Education, Inc.

Three structural types Capillaries Three structural types Continuous capillaries Fenestrated capillaries Sinusoid capillaries (sinusoids) © 2013 Pearson Education, Inc.

Continuous Capillaries Abundant in skin and muscles Intercellular clefts allow passage of fluids and small solutes © 2013 Pearson Education, Inc.

Pericyte Tight junction Red blood cell in lumen Intercellular cleft Endothelial cell Basement membrane Tight junction Pinocytotic vesicles Endothelial nucleus Continuous capillary. Least permeable, and most common (e.g., skin, muscle). © 2013 Pearson Education, Inc.

Fenestrated Capillaries Some endothelial cells contain pores (fenestrations) Permeable Function in absorption or filtrate formation (small intestines, endocrine glands, and kidneys) © 2013 Pearson Education, Inc.

Basement membrane Tight junction Pinocytotic vesicles Red blood cell in lumen Fenestrations (pores) Endothelial nucleus Intercellular cleft Basement membrane Tight junction Endothelial cell Fenestrated capillary. Large fenestrations (pores) increase permeability. Occurs in areas of active absorption or filtration (e.g., kidney, small intestine). © 2013 Pearson Education, Inc.

Blood flow sluggish – allows modification Sinusoid Capillaries Fewer tight junctions; usually fenestrated; larger intercellular clefts; large lumens Blood flow sluggish – allows modification Large molecules and blood cells pass between blood and surrounding tissues Found only in the liver, bone marrow, spleen, adrenal medulla © 2013 Pearson Education, Inc.

Sinusoid capillary. Most permeable. Occurs in special Endothelial cell Red blood cell in lumen Large intercellular cleft Tight junction Incomplete basement membrane Nucleus of endothelial cell Sinusoid capillary. Most permeable. Occurs in special locations (e.g., liver, bone marrow, spleen). © 2013 Pearson Education, Inc.

Blood Flow Through Capillary Beds Precapillary sphincters regulate blood flow into true capillaries Blood may go into true capillaries or to shunt © 2013 Pearson Education, Inc.

Precapillary sphincters Vascular shunt Precapillary sphincters Metarteriole Thoroughfare channel True capillaries Terminal arteriole Postcapillary venule Sphincters open—blood flows through true capillaries. Terminal arteriole Postcapillary venule Sphincters closed—blood flows through metarteriole – thoroughfare channel and bypasses true capillaries. © 2013 Pearson Education, Inc.

Venous System: Venules Formed when capillary beds unite Smallest postcapillary venules Very porous; allow fluids and WBCs into tissues Consist of endothelium and a few pericytes Larger venules have one or two layers of smooth muscle cells © 2013 Pearson Education, Inc.

Formed when venules converge Veins Formed when venules converge Have thinner walls, larger lumens compared with corresponding arteries Blood pressure lower than in arteries Thin tunica media; thick tunica externa of collagen fibers and elastic networks Called capacitance vessels (blood reservoirs); contain up to 65% of blood supply © 2013 Pearson Education, Inc.

Pulmonary blood vessels 12% Systemic arteries and arterioles 15% Heart 8% Capillaries 5% Systemic veins and venules 60% © 2013 Pearson Education, Inc.

Adaptations ensure return of blood to heart despite low pressure Veins Adaptations ensure return of blood to heart despite low pressure Venous valves prevent backflow of blood Most abundant in veins of limbs Venous sinuses: flattened veins with extremely thin walls (e.g., coronary sinus of the heart and dural sinuses of the brain) © 2013 Pearson Education, Inc.

© 2013 Pearson Education, Inc.

Artery Vein © 2013 Pearson Education, Inc.

Systemic Blood Pressure Pumping action of heart generates blood flow Pressure results when flow is opposed by resistance Systemic pressure Highest in aorta Declines throughout pathway 0 mm Hg in right atrium Steepest drop occurs in arterioles © 2013 Pearson Education, Inc.

120 Systolic pressure 100 Mean pressure 80 Blood pressure (mm Hg) 60 Diastolic pressure 40 20 Aorta Veins Arteries Venules Arterioles Capillaries Venae cavae © 2013 Pearson Education, Inc.