Structure & function of blood vessels

• blood vessels must be

  • resilient
  • flexible
  • always remain open
• structure of blood vessel


  label them

  A) lumen

  B) tunica intima

  C) tunica media

  D) tunica adventitia

  
• tunica intima

  • endothelium - simple squamous epithelium
  • basal lamina of the epithelial cells
  • subendothelial connective tissue
• tunica media

  • smooth muscle fibres in loose connective tissue
  • may contain elastic fibres
• tunica externa/adventia

  • connective tissue
  • merges with surrounding connective tissue
  • contain vaso vasorum (vessels within the vena)
• characteristics of arteries

  • thicker wall
  • smaller lumen
  • maintains its shape
  • more resilient
  • no valves
• types of arteries


  elastic (conducting)

  muscular (distributing)

  arterioles (resistance vessels)
• features of elastic arteries

  • e.g aorta, brachiocephalic, common carotid.
  • 2.5 cm diameter
  • withstand changes in pressure during cardiac cycle
  • ensures continuous blood flow
• structural adaptations of elastic arteries

  • thick tunica media
  • many elastic fibres
  • few smooth muscle cells

  
• muscular (distributing) arteries

  • brachial & femoral
  • 0.5 mm - 0.4 cm diameter
  • distribute blood to muscles and organs
  • capable of vasodilation & vasoconstriction (to control the rate of blood flow to suit the needs of the organ)
• structural adaptations of muscular arteries

  • smooth muscle cells in tunica media
  • distinct internal IEL & external EEL elastic laminae
  • thick tunica externa

  
• arterioles vessels

  • capable of vasoconstriction & vasodilation
  • control blood flow to organs
  • involved in blood control
  • 30 um diameter
• structural adaptation of arterioles

  • one to two layers of smooth muscle cells in tunica media
  • poorly defined tunica externa

  
• capillaries

  • connect arterioles and venules (microcirculation)
  • site of gaseous exchange
  • thin walls facilitate diffusion
  • blood flow through capillaries is slow
  • structure permits 2-way exchange
  • 8 micrometers diameter
  • found near all cells
• types of capillaries

  • continuous
  • fenestrated
  • sinusoidal
• continuous capillaries

  • majority are continuous
  • skeletal and smooth muscle, CT and lungs.

  
• fenestrated capillaries

  • pores penetrate the endothelial lining
  • rapid exchange of water or large solutes
  • absorption (kidney, choroid plexus and endocrine glands)

  
• sinusodial capillaries

  • spaces between endothelial cells
  • incomplete or absent BM
  • exchange of large solutes i.e. plasma proteins
  • specialised lining cells (liver, phagocytic cells engulf damaged RBCs)
  • blood moves slowly through sinusoids

  
• what are capillary beds


  capillaries organised into groups (a network).

  
• what metarterioles

  • small blood vessels that serve as transitional channels between arterioles (the smallest arteries) and the capillary beds.
  • continues as a throughfare channel -> reduces flow to a whole capillary bed.

  
• precapillary sphincter


  guard the entrance to each capillary

  • contraction narrows the entrance therefore the flow decreases.
  • relaxation dilates the entrance therefore the flow increases.

  
• arteriovenuous anastomoses

  form direct communication between arteriole and venule

  when diluted blood bypasses the capillary bed and flows directly to venuous circulation

  
• what are venules?

  • collect blood from capillary beds and deliver to small veins.
  • 20 micrometer diameter
• structural adaptations of venules

  • small - endothelium on a basement membrane
  • larger - increasing number of smooth muscle cells located outside endothelium

  
• veins (capacitance vessels)


  classified according to size:

  • small <2mm
  • medium 2-9 mm
  • large > 9mm

  low pressure system

  easily distensible
• structural adaptation of veins


  thin walled

  tunica externa is predominant

  valves to aid blood flow (no flow back)