Chapter 23 Blood Vessels

Blood Vessel Tunics Walls of blood vessels have three layers, or tunics 1.Tunica externa (adventitia) – anchor BV to an organ Larger blood vessels require own blood supply – vasa vasorum in tunica externa 2.Tunica media – smooth muscle Sympathetic input  vasoconstriction Parasympathetic input  vasodilation 3.Tunica intima (interna) – endothelium (simple squamous ET lining) Continuous with endocardium

Blood Vessel Tunics

Microscopic Comparison of Arteries and Veins

Types of Arteries Three types of arteries: 1.Elastic arteries Largest arteries, close to heart Thick tunica media w/ elastic fibers 2.Muscular arteries Medium diameter Proportionally thicker tunica media 3.Arterioles –smallest arteries Thin tunica media (< 6 layers) Connect to capillaries

Capillaries – Diameter slightly larger than erythrocyte Tunica intima only – Allows for rapid diffusion Form capillary beds – Blood flow regulated by precapillary sphincters – Thoroughfare channel bypasses bed Site of metabolic exchange

Types of Capillaries 1.Continuous – most common – continuous and complete endothelium (no physical holes) 2.Fenestrated – endothelial cells possess small “holes” – allow fluid exchange between blood and interstitial fluid eg. kidneys 3.Sinusoid – large gaps between endothelial cells – promotes transport of large molecules and cells to and from blood eg. Liver and spleen

Veins Drain capillaries – return blood to heart Pressure much lower than in arteries – Walls much thinner – Very little muscle in tunica media At rest, veins hold about 60% of body’s blood – function as blood reservoirs

Venules Smallest veins Postcapillary venules - smallest – Diapedesis occurs here Venules merge to form veins

Veins Skeletal muscle pump moves blood toward heart – Contraction of muscles Blood pressure too low to overcome gravity – valves prevent backflow and pooling of blood in limbs – formed from tunica intima

Varicose Veins

Venous Return from the Abdomen Special system of circulation - hepatic portal system Drains blood from GI organs and shunts blood to liver – Allows for filtering of ingested substances

Fetal-Placental Circulation Fetal circulation bypasses the developing lungs, kidneys, and digestive tract ttp:// All nutritional, respiratory, and excretory needs are met by the placenta The exchange occurs via capillaries- mom’s and baby’s blood don’t mix.

Fetal system has structures that are modified or cease to exist after birth Fetal circulatory pathway: – Oxygenated blood from placenta  umbilical vein  ductus venosus (bypasses liver)  inferior vena cava  right atrium  foramen ovale (shunt to LA)  left ventricle  aorta – Some blood from RA  RV  pulmonary trunk  ductus arteriosus (shunt to aorta) – Aorta  body  umbilical arteries (now deoxygenated)  placenta  nutrient and gas exchange Fetal-Placental Circulation

Postnatal changes – Umbilical vessels constrict and cease function – Ductus venosus becomes ligamentum venosum – Foramen ovale becomes fossa ovalis Failure to close at birth = patent foramen ovale – Ductus arteriosus becomes ligamentum arteriosum Failure to close at birth = patent ductus arteriosus Fetal-Placental Circulation