blood vessels

• Structure of blood vessels

  • Tissues have and arterial supply and venous drainage
  • The aorta is the main artery
  • Aorta → artery → arterioles → capillaries → venules → veins → inferior venacava
  • Inferior vena cava is the main vein
  • All vessels need to be:
  • Resilien
  • Flexible
  • Remain open
• 1. Lumen: where blood is situated
  2. Tunica intima

  • inner layer
  • lined with simple squamous epithelium
  • stuck of basement membrane called basal lamina of epithelial cells
  • under layer of tissue called subendothelial connective tissue

  1. Tunica media

  • smooth muscle fibres in loose connective tissue
  • in some vessels it can have elastic tissues

  1. Tunica externa/adventitia

  • merges with surrounding connective tissue
  • May have vaso vasorum
  • Comprised mostly of collagen
  • when large can have vessels within (vessels of the vessels)
• The artery

  • Smaller lumen
  • Maintains shape
  • Blood under high pressure
  • No valves
  • thick walls
• • elastic/conducting arteries
  • Largest arteries (aorta, brachiocephalic and common carotid)
  • Diameter: up to 2.5cm
  • Can withstand pressure changes during cardiac cycle to ensurecontinuous flow of blood
  • Structural adaptations:
  • ○ Thick tunica media with elastic fibres and few smooth muscle cells. Elastic fibres coil and recoil during contraction and relax
• muscular/distributing arteries

  • Distribute blood to organs and muscles
  • Most names arteries in the body (brachial and femoral)
  • Diameter: 0.5mm - 0.4cm
  • Capable of vasodilation and vasoconstriction in order to control rate ofblood flow to organs and muscles
  •  Structural adaptations:
  •  Smooth muscle cells in tunica media
  • distinct internal and external elastic laminae
  • thick tunica externa
• arterioles/resistance vessels

  •  Capable of vasodilation and vasoconstriction
  • Control blood flow to organs
  • Involved in blood pressure control
  • Diameter: 30um or less
  • Structural adaptations:
  • One or two layers of smooth muscle cells in tunica media
  • Poorly defines tunica externa
• The capillaries

  • connect arterioles and venules (microcirculation)
  • Site of gaseous exchange
  • Thin walls to facilitate diffusion (endothelium and basement membrane)
  • Diameter: 8um
  • Slow blood flow
• Continuous capillaries

  *Make the majority

  *Found in skeletal and smooth muscle, connective tissue and lungs
• Fenestrated capillaries

  • Pores penetrate endothelium
  • Rapid exchange of water or larger solutes such as small peptides
  • Found in kidney, choroid plexus and endocrine glands
  • More permeable
• Sinusoids capillaries

  • Spaces between endothelial cells
  • Incomplete or absent basement membrane
  • Exchange of large solutes (plasma proteins)
  • Slow flow
  • Found in the live
• Capillary beds

  • Capillaries are organised into groups called capillary beds
  • Flow through beds is controlled by: metarterioles, pre capillary sphincter, arteriovenous anastomoses
• 1. metarterioles (capillary bed)

  • Supply whole capillary beds
  • Continues as a thoroughfare channel which leads directly into a veinand has numerous capillaries leading off it
  • Constriction can reduce flow to a whole capillary bed
• 2. Precapillary sphincter (capillary bed)

  • Guard entrance to each capillary
  • Contraction narrows entrance and reduces flow
  • Relaxation increases flow
• 3. Arteriovenous anastomoses (capillary bed)

  • Direct communication between arteriole and venule
  • When dilated blood bypasses capillary bed and flows directly to venouscirculation
• Venules

  • Collect blood from capillary beds and deliver it to small veins
  • Diameter: varies, average 20um
  • Structural adaptations:
  • Small: endothelium on basement membrane
  • Large: increasing numbers of smooth muscle cells located outside theendothelium
• Veins/capacitance vessels

  *Classified according to size:

  • Small: >2mm in diameter
  • Medium: 2-9mm
  • Large: <8mm (superior and inferior vena cava)

  *Low pressure system

  *Easily distensible (capacitance)

  *Structural adaptations:

  • Thin walls
  • Tunica externa is predominant
  • Valves to aid blood flow
• Blood circulatory systems

  • Systemic
  • Pulmonary
  • Specialised (portal, coronary and foetal)