10.1 Blood Vessels

• The cornea of the eye is nearly free of capillaries
• How the cornea obtains nutrients

  By diffusion from tears on outside and fluids on inside
• Importance of capillaries in homeostasis

  • Regulate body systems by exchanging water, wastes, and nutrients across membranes between blood and tissues
• Capillary bed opening and closing


  1. Only certain beds open at a time

  2. Open after eating to serve digestive system, close to muscles

  3. Sphincter muscles relax to open, contract to close

  4. Closed beds bypass through arteriovenous shunts
• Why capillary beds need to open and close

  To divert blood to areas needing more oxygen, nutrients, and waste removal
• Recent evidence suggests hormones from digestive tract, not blood flow, may cause postprandial somnolence (sleepiness after eating)
• Artery


  Carry blood away from the heart
• Capillary


  • Exchange material with tissues
  • Walls are one cell thick
• Vein

  • Carry blood toward the heart
  • Contain valves
  • Contain approximately 70% of blood
  • Act as a blood reservoir
• Arteries

  • Thickest walls
  • Lead to arterioles
  • Blood flow kept moving by the pumping of the heart
• Capillaries

  • Extremely narrow
  • Contain shunts
  • Drain blood to venules
• Veins

  • Largest holding capacity
  • Carry oxygen-poor blood to the lungs
  • Carry oxygen-rich blood from the lungs
  • Deliver oxygen-poor blood to the heart
• Aorta


  Largest artery
• Vena cava

  Largest vein
• The circulatory system has three types of blood vessels: Arteries, Capillaries, and Veins
• All three blood vessel types have an inner endothelium, a simple squamous epithelium attached to a connective tissue basement membrane that has elastic fibres
• Aorta


  The largest artery in the body, carries O2-rich blood from the heart to other parts of the body
• Arterial wall

  • Inner layer: endothelium
  • Middle layer: smooth muscle that contracts and relaxes to regulate blood flow and pressure
  • Outer layer: fibrous connective tissue
• Arterioles

  Small arteries that branch off from an artery
• Arteriole wall

  • Inner layer: endothelium
  • Middle layer: some elastic tissue but mostly smooth muscle
  • Outer layer: fibrous connective tissue
• Smooth muscle contracts


  Blood vessel constricts, resulting in higher blood pressure
• Smooth muscle relaxes


  Blood vessel relaxes, resulting in lower blood pressure
• Capillaries


  Narrow blood vessels that join arterioles to venules
• Capillary composition

  • Composed of a single layer of epithelium with a basement membrane
  • Form vast networks (capillary beds) throughout the body
• Exchange of substances across capillary walls

  1. Oxygen and nutrients diffuse out of the capillary and into the tissue fluid that surrounds cells

  2. Wastes (carbon dioxide) diffuse into the capillary

  3. Some water leaves the capillaries, and excess is picked up by lymphatic vessels
• Veins and venules

  Take blood from the capillary beds to the heart
• Vein and venule walls

  • Have the same three layers as arteries, but there is less smooth muscle and connective tissue
  • Veins have valves, which allow blood to flow toward the heart when open and prevent blood from flowing backward when closed
  • Veins act as a blood reservoir, their walls can expand to a greater extent, about 70% of blood is in the veins
• Venae cavae


  The largest veins in the body, deliver O2-poor blood to the heart