The circulatory system III Blood vessels Arteries Capillaries Veins Chapter 21 The circulatory system III Blood vessels Arteries Capillaries Veins

Blood vessels Defined: hollow tubes of smooth muscle, elastic and dense connective tissue. Serve as conduits through which blood is circulated to every cell in the body. Tunica interna (intima) – -inner most layer of vessel wall. Lined with endothelial cells overlying a sparse layer of loose CT. Tunica media – -middle layer and thickest in arteries. Contains smooth muscle, collagen and in larger vessels ( e.g.-aorta) elastin fibers. Tunica externa (adventitia) -outer most layer covering vessels, consisting of loose CT. It merges with neighboring blood vessels, nerves and/or other organs. It contains the vasa vasorum which supply blood to outer ½ of vessel wall in larger vessels. Lumen: central canal or opening in the center of the vessel.

Neurovascular bundle

Conducting (elastic) arteries

Muscular arteries

Classes of Blood Vessels Arteries –-are the resistance vessels of the CV system. Designed to withstand large fluctuations in blood pressure. Conducting (elastic or large) arteries: 1.0–2.5 cm diam. Biggest arteries; Aorta, carotids, pulmonary trunk, common iliacs and subclavians are all examples. Contain a thin elastic layer between intima → media and the media → externa. Have up to 70 layers of elastic tissue. Distributing (muscular) arteries: 0.3 mm to1 cm diam. smaller branches that distribute blood to specific organs - have up to 40 layers of elastic tissue

Resistance (small arteries) and arterioles Small arteries are too variable in number and location to be given specific individual names. 10 -300 μ diam. Contain up to 25 layers of smooth muscle and little elastic tissue. Arterioles are the smallest of arteries and have only 3 layers of smooth muscle and little tunica externa. Metarterioles – terminal aretrioles before capillary bed; merge into through fare channels from which true capillaries branch off. Precapillary sphincters are at merge point and regulate blood flow based on tissue needs

Tissue blood flow: Terminal arterioles → metarterioles which → through fare channels. Precap. Sphincters- Regulate flow direction into true capillaries or through channels.

Capillaries For blood to serve any purpose, materials (eg. O2, CO2, nutrients, wastes and hormones) must pass through the walls of blood vessels. There are only 2 sites in the vascular system for this to occur: Capillaries and Venules. Capillaries are composed of only an endothelium and a thin basement membrane. They average ~ 5 μ to 9μ in size. Capillaries far out number venules in the cardiovascular system.

Capillaries Molecules pass into/out of capillary walls by 4 routes: 1. Direct diffusion through the endothelial cell membrane - down the concentration gradient O2 and CO2 2. Through intercellular clefts - small molecules 3. Pinocytotic vesicles and caveolae (in pocketing of vessel wall) - large molecules and proteins 4. Through fenestrations (pores) in the capillary wall CO2 and O2 easily diffuse through the endothelial cells

Capillaries a) Continuous b) Fenestrated c) Sinusoidal

Venules Venules and veins contain valves to maintain a one way flow of blood back to the heart. Venules are the smallest of veins (~15-20 μm). Possess a tunica interna with a few fibroblasts wrapped around them and tend to be more porous than capillaries. Most leukocytes migrate from blood stream through postcapillary venules.

Venous valves -muscles pump blood by contracting and compressing veins.

Veins Are sometimes called capacitance vessels At rest veins contain 54% of the blood volume Medium veins range up to 10 mm in diameter (ex. ulnar, great saphenous, radial). Medium veins have a tunica interna, an endothelium, a basement membrane and loose CT with a thin interna elastic lamina

Large veins Diameters greater than 10 mm. Contain longitudinal bundles of smooth muscle and a thick tunica externa. Ex: vena cava, pulmonary veins, internal jugular and renal veins

Pulmonary Circulation Blood flows through the lungs to get rid of CO2 and pick up O2. Begins in right ventricle → pulmonary trunk → pulmonary arteries → lobar arteries → arterioles → lung capillary beds → venules → pulmonary veins → left atrium

Pulmonary Circulation Pulmonary trunk → pulmonary arteries → lobar arteries

Systemic Circulation Blood flow to the entire body except for the pulmonary circuit Begins in left atrium → left ventricle → aorta → every cell in the entire body → arteries → arterioles → capillary beds → venules → veins → vena cava → right atrium → pulmonary circulation

Major arteries off of aortic arch

Major arteries off of aorta

Specialized circulations Bronchial circulation goes to lung tissues proper to provide nutritive blood to the lungs since the pulmonary circuit does not provide blood to the lung tissues Circle of Willis provides blood flow to the brain. It arrives there via two separate pathways. Hepatic Portal circulation provides blood flow to the liver via a portal pathway. A portal circulation passes through two capillary beds in series whereas most circulations pass through a single capillary bed.

Blood flow to the head and brain

Circle of Willis Provides blood flow to the brain by either the internal carotid artery or the basilar artery. The internal carotid enters the cranial cavity through the carotid foramen. The basilar artery is formed by the convergence of the two vertebral arteries that come up through the transverse foramen of the cervical vertebra.

Circle of Willis

Hepatic Portal Circulation The hepatic portal circulation is unique in that blood passes from the intestines into a primary capillary bed outside of the liver and then into the hepatic portal vein into a second capillary bed in the liver before it passes out of the liver via the hepatic vein into the inferior vena cava.

Hepatic Portal Circulation

Hepatic portal veins

Fetal Circulation The fetal circulation provides blood to the fetus from the mother via the umbilical vessels. Keep in mind that the fetal lungs are nonfunctional until after the child is born. Fetal circulation is provided by an interatrial shunt the foramen ovale and a pulmonary → aorta shunt the ductus arteriosis. Blood from the placenta comes to the fetus via the umbilical vein → right atrium → foramen ovale → LA → LV → body or → RV → pulmonary trunk → ductus arteriosis → aorta. Theses become the fossa ovale and ligamentum arterosis after birth of the child.

Umbilical vein →RA → LA →LV → aorta → body →umbilical artery → placenta. RV →Pulmonary trunk → ductus areteriosis → aorta → body → umbilical artery →

Arteries to know Aorta: ascending, aortic arch, thoracic and abdominal Coronary arteries: left ant. descending and circumflex Brachiocephalic Left and right Common Carotid Internal and external Carotid arteries Left and right Subclavian Axillary, Brachial, Radial, Ulnar Vertebral, Intercostal, Celiac trunk, Superior and Inferior mesenteric, Common iliac, External iliac, Suprarenal, Renal, Gonadal, Femoral, Popliteal, Ant. and Post. Tibial, Fibular

Major arteries Aorta Brachiocephalic Rt/Lt common carotids Int/ext carotids Left subclavian

Arteries of the upper limb Axillary Brachial Ulnar Radial

Abdominal blood flow - mesenteric arteries

Arteries in the abdominal cavity Suprarenal Renal Lumbar Common iliacs

Arteries to lower limb Femoral Popliteal Ant/post tibial

Veins to know Superior and inferior vena cava Brachiocephalic, Subclavian, Cephalic Internal and external jugular Axillary, Brachial, Basilic, Median cubital Hepatic, Suprarenal, Renal, Gonadal, Lumbar, Common iliac, External iliac, Internal iliac, Femoral Great saphenous, Popliteal, Ant and Post Tibial, Peroneal Superior sagittal and Transverse sinuses, Cavernous sinus

Major veins Superior vena cava Inferior vena cava Ext jugular vein Int jugular vein

Venous return from the head

Venous return from the upper limb Axillary Med. cubital Brachial Basilic

Venous return from abdomen Hepatic Suprerenal Renal Int/Ext iliac Gonadal Lumbar

Hepatic portal circulation

Venous return from the lower limb Femoral vein Gt saphenous Popliteal Ant/post tibial