L4.5.1: Microscopic Anatomy

• MICROSCOPIC ANATOMY

  ● Tunics / Layers of Blood Vessels (except the capillaries)

  ○ Tunica Intima

  ○ Tunica Media

  ○ Tunica Externa
• ○ Tunica Intima

  ■ Lines the lumen/interior of the blood vessels

  ■ Forms a friction-reducing lining

  ■ Thin layer of endothelium
• ○ Tunica Media

  ■ Bulky; made up of smooth muscle and elastic fibers

  ■ Controlled by sympathetic nervous system

  ■ Some larger arteries have elastic laminae in addition to the scattered elastic fibers
• ○ Tunica Externa

  ■ Forms protective outermost covering

  ■ Mostly fibrous connective tissue

  ■ Supports and protects the vessel
• ○ Arteries

  ■ Have a heavier, stronger, stretchier tunica media than veins to withstand changes in pressure

  ■ Internal elastic lamina — can be found between tunica intima and media

  ■ External elastic lamina — found between tunica media and externa

  ■ They subdivide into arterioles, and as they run out of oxygen, they run into venules through the capillary bed, Awhich then connects to the veins
• ○ Veins

  ■ Have a thinner tunica media than arteries and operate under low pressure

  ■ Have valves to prevent backflow of blood

  ■ Have larger lumens

  ■ Skeletal muscle “milks” blood in veins toward the heart since blood flows against gravity
• ○ Capillaries

  ■ Only one cell layer thick; tunica intima

  ■ Allow for exchanges between blood and tissue

  ■ Form networks called capillary beds

  ■ Blood flow through a capillary bed is known as microcirculation (terminal arteriole → exchange vessels of capillary bed → postcapillary venule)
• Special capillary beds of the mesentery have:

  • Precapillary sphincters
  • Vascular shunt
• Precapillary sphincters

  • Regulates blood flow through a capillary bed
  • If open, blood flows through, and exchanges with cells can occur
  • If closed, blood flows through the shunt and bypasses cells in that region
• Vascular shunt

  • Directly connects the arteriole and venule at opposite ends of the bend
• OPERATION OF THE MUSCULAR PUMP

  ● A vein receives blood from multiple branches and blood flows against gravity

  ● When a vein gets squeezed by nearby skeletal muscle, the valve below the muscle contraction closes, while the valve above the muscle contraction opens, thereby allowing blood to flow upward