Blood vessels Chapter 23

• Systemic circulation


  Extends to all body regions
• Pulmonary circulation


  Consists of vessels to and from lungs
• Arteries


  Carry blood away from the heart
• Arteries


  Become smaller as they branch and lead to capillaries
• Veins


  Return blood to the heart
• Veins


  Become progressively larger as they merge and approach heart
• Anastomosis


  Convergence of two or more vessels
• Veins anastomose with each other more than arteries do
• End arteries do not form anastomoses
• Companion vessels


  Arteries and veins that lie next to each other
• Tunics of artery and vein walls

  • Tunica intima
  • Tunic media
  • Tunica externa
• Tunica intima


  Composed of endothelium (simple squamous lining) and subendothelial layer of areola connective tissue
• Tunic media

  Comprised of circularly arranged smooth muscle
• Tunica externa


  Outer connective tissue that helps anchor blood vessel to surroundings
• Large (thick) blood vessels require vaso vasorum - their own network of blood small vessels on their outer layer
• Capillary walls contain only tunica intima
• Types of arteries


  • Elastic arteries
  • Muscular arteries
  • Arterioles
• Elastic arteries


  • Largest arteries
  • Walls contain many elastic fibers
  • Elastic fibers especially abundant in tunica media
  • Most examples are near heart
• Muscular arteries


  • Medium-sized arteries
  • Possess elastic fibers in two concentric rings
  • Muscular arteries have a proportionately thicker tunica media with multiple layers of smooth muscle cells
  • Most named arteries are muscular (e.g., brachial artery)
• Arterioles


  • Smallest arteries
  • Number of tunics and thickness of walls varies between larger and smaller arterioles
• Capillaries


  • Smallest blood vessels
  • Diameter only slightly larger than an erythrocyte
  • For most, their wall consists solely of tunica intima
  • Single layer of endothelial cells and basement membrane
  • Allow metabolic exchange between blood and tissues
• Capillary beds


  • Groups of capillaries
  • Each bed is fed by a metarteriole
  • Several true capillaries branch from the metarteriole to form the bulk of the capillary bed
  • The origin of each true capillary has a precapillary sphincter of smooth muscle around it that controls blood flow
  • Centrally, metarteriole continues distally to thoroughfare channel which then connects to postcapillary venule
• Types of capillaries


  • Continuous
  • Fenestrated
  • Sinusoids
• Continuous capillaries


  Endothelial cells form a complete lining aided by tight junctions
• Fenestrated capillaries


  Endothelial cells contain pores that allow fluid exchange between blood and interstitial fluid
• Sinusoid capillaries


  Have large gaps between endothelial cells and a discontinuous (or absent) basement membrane; allows transport of large molecules and cells to and from blood
• Veins


  • Drain capillaries and return blood to the heart
  • Pressure in veins is much lower than in arteries
  • At rest, veins hold about 60% of the body's blood
  • Veins function as blood reservoirs
• Venules


  • The smallest veins
  • Companion vessels with arterioles
  • Smallest are the postcapillary venules
• Diapedesis


  Leukocytes migrating from bloodstream to interstitial fluid occurs through walls of postcapillary venules
• Smaller and medium-sized veins

  Travel with muscular arteries
• Large veins


  Travel with elastic arteries
• Valves in veins


  • Formed from tunica intima
  • Prevent pooling of blood
• Skeletal muscle pump


  Muscles bulge as they contract and push on veins, thereby helping move blood toward the heart
• Respiratory pump


  Assists venous return through changes in intrathoracic and intra-abdominal pressure
• Blood pressure


  The force per unit area that blood places on the inside of a blood vessel
• Systolic blood pressure


  Pressure during ventricular contraction
• Diastolic blood pressure


  Pressure during ventricular relaxation
• General arterial flow out of the heart

  1. Oxygenated blood is pumped out of left ventricle into ascending aorta

  2. Two branches emerge from aorta: left coronary artery and right coronary artery

  3. Three arterial branches emerge from the aortic arch: brachiocephalic trunk, left common carotid artery, left subclavian artery
• General arterial flow out of the heart (continued)

  1. Descending thoracic aorta follows aortic arch and several arteries emerge to thoracic wall

  2. Changes name to descending abdominal aorta after it passes through diaphragm

  3. At fourth lumbar vertebra, the aorta bifurcates into left and right common iliac arteries

  4. Common iliac arteries further divide into internal and external iliac arteries
• General venous return to the heart

  1. Superior vena cava is formed from a fusion of the right and left brachiocephalic veins and drains into the right atrium

  2. Inferior vena cava returns blood to the right atrium from lower limbs, pelvis and perineum, and abdominal structures