Blood Vessels Blood is carried in a closed system of vessels that begins and ends at the heart The three major types of vessels are arteries, capillaries, and veins Arteries carry blood away from the heart, veins carry blood toward the heart Capillaries contact tissue cells and directly serve cellular needs

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

Generalized Structure of Blood Vessels Figure 19.1b

Blood Vessel Anatomy Table 19.1

Elastic (Conducting) Arteries Thick-walled arteries near the heart; the aorta and its major branches Large lumen allow low-resistance conduction of blood Contain elastin in all three tunics Withstand and smooth out large blood pressure fluctuations Serve as pressure reservoirs

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

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

Vascular Components Figure 19.2a, b

Continuous Capillaries Continuous capillaries are abundant in the skin and muscles Endothelial cells provide an uninterrupted lining Adjacent cells are connected with tight junctions Intercellular clefts allow the passage of fluids

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

Continuous Capillaries Figure 19.3a

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 than other capillaries

Fenestrated Capillaries Figure 19.3b

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

Sinusoids Figure 19.3c

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

Capillary Beds Figure 19.4a

Capillary Beds Figure 19.4b

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

Venous System: Venules 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)

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

Veins have much lower blood pressure and thinner walls than arteries 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)

Merging blood vessels, more common in veins than arteries 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

Blood Flow Actual volume of blood flowing through a vessel, an organ, or the entire circulation in a given period: Is measured in ml per min. Is equivalent to cardiac output (CO), considering the entire vascular system Is relatively constant when at rest Varies widely through individual organs

Blood Pressure (BP) Force per unit area exerted on the wall of a blood vessel by its contained blood Expressed in millimeters of mercury (mm Hg) Measured in reference to systemic arterial BP in large arteries near the heart The differences in BP within the vascular system provide the driving force that keeps blood moving from higher to lower pressure areas

Resistance – opposition to flow Measure of the amount of friction blood encounters Generally encountered in the systemic circulation Referred to as peripheral resistance (PR) The three important sources of resistance are blood viscosity, total blood vessel length, and blood vessel diameter

Factors Aiding Venous Return Venous BP alone is too low to promote adequate blood return and is aided by the: Respiratory “pump” – pressure changes created during breathing suck blood toward the heart by squeezing local veins Muscular “pump” – contraction of skeletal muscles “milk” blood toward the heart Valves prevent backflow during venous return PLAY InterActive Physiology ®: Anatomy Review: Blood Vessel Structure and Function, pages 3–27

Factors Aiding Venous Return Figure 19.6

Maintaining Blood Pressure Maintaining blood pressure requires: Cooperation of the heart, blood vessels, and kidneys Supervision of the brain

Maintaining Blood Pressure The main factors influencing blood pressure are: Cardiac output (CO) Peripheral resistance (PR) Blood volume Blood pressure = CO x PR Blood pressure varies directly with CO, PR, and blood volume

Temperature Regulation As temperature rises (e.g., heat exposure, fever, vigorous exercise): Hypothalamic signals reduce vasomotor stimulation of the skin vessels Heat radiates from the skin Sweat also causes vasodilation via bradykinin in perspiration Bradykinin stimulates the release of NO As temperature decreases, blood is shunted to deeper, more vital organs

The vascular system has two distinct circulations Circulatory Pathways The vascular system has two distinct circulations Pulmonary circulation – short loop that runs from the heart to the lungs and back to the heart Systemic circulation – routes blood through a long loop to all parts of the body and returns to the heart

Differences Between Arteries and Veins Delivery Blood pumped into single systemic artery – the aorta Blood returns via superior and interior venae cavae and the coronary sinus Location Deep, and protected by tissue Both deep and superficial Pathways Fair, clear, and defined Convergent interconnections Supply/drainage Predictable supply Dural sinuses and hepatic portal circulation

Pulmonary Circulation Figure 19.18b

Systemic Circulation Figure 19.19

(b) Common carotid arteries Subclavian artery Aortic arch Coronary artery Thoracic aorta Branches of celiac trunk: Renal artery Superficial palmar arch Radial artery Ulnar artery Internal iliac artery Deep palmar arch • Left gastric artery Splenic artery Common hepatic artery Internal carotid artery Vertebral artery Brachiocephalic trunk Axillary artery Brachial artery Abdominal aorta Superior mesenteric artery Gonadal artery Common iliac artery External iliac artery Digital arteries Femoral artery Popliteal artery Inferior mesenteric artery Ascending aorta External carotid artery Anterior tibial artery Posterior tibial artery Arcuate artery Figure 19.20b

Ophthalmic artery Superficial temporal artery Basilar artery Maxillary artery Occipital artery Facial artery Vertebral artery Internal carotid artery Lingual artery External carotid artery Superior thyroid artery Common carotid artery Larynx Thyrocervical trunk Thyroid gland (overlying trachea) Costocervical trunk Clavicle (cut) Subclavian artery Brachiocephalic trunk Axillary artery Internal thoracic artery (b) Figure 19.21b

Arteries of the Brain Anterior Cerebral arterial circle (circle of Willis) Frontal lobe • Anterior communicating artery Optic chiasma Middle cerebral artery • Anterior cerebral artery Internal carotid artery • Posterior communicating artery Pituitary gland • Posterior cerebral artery Temporal lobe Basilar artery Pons Occipital lobe Vertebral artery Cerebellum Posterior (c) (d) Figure 19.21c,d

Common carotid arteries Vertebral artery Thyrocervical trunk Right subclavian artery Costocervical trunk Suprascapular artery Left subclavian artery Thoracoacromial artery Axillary artery Left axillary artery Subscapular artery Brachiocephalic trunk Posterior circumflex humeral artery Posterior intercostal arteries Anterior circumflex humeral artery Brachial artery Anterior intercostal artery Deep artery of arm Internal thoracic artery Common interosseous artery Descending aorta Radial artery Lateral thoracic artery Ulnar artery Deep palmar arch Superficial palmar arch Digitals (b) Figure 19.22b

Arteries of the Abdomen Liver (cut) Diaphragm Inferior vena cava Esophagus Celiac trunk Left gastric artery Hepatic artery proper Left gastroepiploic artery Common hepatic artery Splenic artery Right gastric artery Spleen Gallbladder Stomach Gastroduodenal artery Pancreas (major portion lies posterior to stomach) Right gastroepiploic artery Superior mesenteric artery Duodenum Abdominal aorta (b) Figure 19.23b

Arteries of the Abdomen Opening for inferior vena cava Diaphragm Inferior phrenic artery Hiatus (opening) for esophagus Middle suprarenal artery Celiac trunk Renal artery Kidney Superior mesenteric artery Lumbar arteries Gonadal (testicular or ovarian) artery Abdominal aorta Inferior mesenteric artery Median sacral artery Common iliac artery Ureter (c) Figure 19.23c

Arteries of the Abdomen Transverse colon Celiac trunk Superior mesenteric artery Middle colic artery Intestinal arteries Left colic artery Right colic artery Inferior mesenteric artery Ileocolic artery Aorta Ascending colon Sigmoidal arteries Descending colon Ileum Left common iliac artery Superior rectal artery Sigmoid colon Cecum Rectum Appendix (d) Figure 19.23d

Arteries of the Lower Limbs Common iliac artery Internal iliac artery Superior gluteal artery External iliac artery Deep artery of thigh Popliteal artery Lateral circumflex femoral artery Medial circumflex femoral artery Anterior tibial artery Obturator artery Femoral artery Fibular artery Posterior tibial artery Adductor hiatus Popliteal artery Dorsalis pedis artery (from top of foot) Lateral plantar artery Anterior tibial artery Medial plantar artery Plantar arch Posterior tibial artery Fibular artery (c) Dorsalis pedis artery Arcuate artery Metatarsal arteries (b) Figure 19.24b, c

brachiocephalic veins Vertebral vein Internal jugular vein Dural sinuses Subclavian vein External jugular vein Right and left brachiocephalic veins Vertebral vein Internal jugular vein Cephalic vein Superior vena cava Brachial vein Axillary vein Basilic vein Great cardiac vein Splenic vein Hepatic veins Median cubital vein Hepatic portal vein Renal vein Superior mesenteric vein Inferior mesenteric vein Inferior vena cava Ulnar vein Radial vein Digital veins Internal iliac vein Common iliac vein External iliac vein Femoral vein Great saphenous vein Popliteal vein Posterior tibial vein Anterior tibial vein Fibular vein Dorsal venous arch Dorsal digital veins (b) Figure 19.25b

Veins of the Head and Neck Ophthalmic vein Superficial temporal vein Facial vein Occipital vein Posterior auricular vein External jugular vein Vertebral vein Internal jugular vein Superior and middle thyroid veins Brachiocephalic vein Subclavian vein Superior vena cava (b) Figure 19.26b

Veins of the Brain Superior sagittal sinus Falx cerebri Inferior sagittal sinus Straight sinus Cavernous sinus Junction of sinuses Transverse sinuses Sigmoid sinus Jugular foramen Right internal jugular vein (c) Figure 19.26c

Veins of the Upper Limbs and Thorax Internal jugular vein External jugular vein Brachiocephalic veins Left subclavian vein Right subclavian vein Superior vena cava Axillary vein Azygos vein Accessory hemiazygos vein Brachial vein Cephalic vein Hemiazygos vein Basilic vein Posterior intercostals Inferior Median cubital vein vena cava Ascending lumbar vein Median antebrachial vein Basilic vein Ulnar vein Cephalic vein Deep palmar venous arch Radial vein Superficial palmar venous arch Digital veins (b) Figure 19.27b

Veins of the Abdomen Inferior phrenic vein Hepatic veins Inferior vena cava Left suprarenal vein Right suprarenal vein Renal veins Left ascending lumbar vein Right gonadal vein Lumbar veins Left gonadal vein Common iliac vein External iliac vein Internal iliac vein (b) Figure 19.28b

Veins of the Abdomen Hepatic veins Gastric veins Liver Spleen Inferior vena cava Hepatic portal vein Splenic vein Right gastroepiploic vein Inferior mesenteric vein Superior mesenteric vein Small intestine Large intestine Rectum (c) Figure 19.28c

Fibular (peroneal) vein Common iliac vein Internal iliac vein External iliac vein Inguinal ligament Femoral vein Great saphenous vein (superficial) Great saphenous vein Popliteal vein Popliteal vein Anterior tibial vein Fibular (peroneal) vein Fibular (peroneal) vein Small saphenous vein (superficial) Anterior tibial vein Posterior tibial vein Dorsalis pedis vein Plantar veins Dorsal venous arch Plantar arch Metatarsal veins Digital veins (c) (b) Figure 19.29b, c