Blood vessels

Cardiovascular system function

• A transport system of oxygenated and deoxygenated blood.

• Blood vessels contribute to perfusion and homeostasis according to structure, location and functional changes due to vasoactive molecules and the local microenvironment
  (e.g. pH, CO2, O2, Cytokines etc.)

Histology

The study of tissues and cells under a microscope

Blood vessel histology

Vascular differences: artery vs vein diagrams

Endothelium

• Single layer of polygonal, flattened epithelial cells. Elongated in the direction of blood flow

• Basement membrane

• Sub-endothelium contains loose connective tissue and fibroblasts

Collagen and Elastin fibers

• Longitudinal arrangement

• Autonomic innervation

Vasa vasorum ‘vessels of the vessel’

Provide O2 and nutrients to the cells in the vessel wall; more frequent in larger vessels than in arteries

Elastic arteries

• It contains alternating layers of smooth muscle cells and elastic laminae

• Left (H&E) shows SMCs and elastic fibers (E); right, elastin staining (black)

• Right out of the ventricles

• Function is to receive energy from when the heart contracts, stores it and releases the energy to pump the blood around the body.

Muscular arteries

• It contains layers of smooth muscles cells and some elastic fibers

• Prominent internal elastic lamina (IEL) and less accentuated external elastic lamina (EEL, absent in smaller muscular arteries)

• M, tunica media

• A, tunica adventitia

Arterioles

• Contains 1 - 2 layers of smooth muscle cells

• Elastic fibers limited to internal elastic lamina (IEL)

• No external elastic lamina (EEL)

• The transition from large to small arterioles is indicated by the loss of IEL

• Tunica adventitia is very thin

• E, endothelium

• M, tunica media

Venules

• Capillaries drain into postcapillary venules, similar in structure to capillaries: bearing endothelium and pericytes, larger diameter = 15 - 20 um.

• Postcapillary venules drain into collecting venules: larger diameter with an increase in pericyte density

• Collecting venules drain into muscular venules: media layer with 2 - 3 layers of smooth muscle cells.

Pericytes

cells present at intervals along the walls of capillaries (and post-capillary venules).

Small and medium veins

• Tunica media contains a few layers of smooth muscle cells

• Tunica adventitia contains some elastin and collagen fibers arranged, longitudinally

• No elastic laminae

• M, tunica media; Ad, tunica adventitia

Large veins

• Tunica media contains a few layers of smooth muscle cells

• Tunica adventitia contains some elastin and collagen fibers, arranged longitudinally

• No elastin laminae

• M, tunica media; A, tunica adventitia

Vein structure allows fluid capacitance

• Significance of thin wall and wide lumen

• Easily collapsed and expanded

• Facilitates function as capacitance (storage) vessels

Structural comparison among arteries and veins

• Arteries are characterised by:

  • A narrow lumen

  • A thicker wall and media layer

  • Greater wall-to-lumen and media-to-lumen ratios.

• Greater elastin content of the arterial media layer:

  • Especially in elastic arteries (windkessel effect)

  • Dampens pulse pressure

  • Veins possess valves (intimal projections) that prevent backflow, facilitating venous return

Capillaries

• A single layer of endothelial cells located upon a basement membrane

• Diameter 4 - 10 um

• Three forms of capillary classified according to the nature of the endothelium:

  • Continuous

  • Fenestrated

  • Discontinuous (sinusoids)

Continuous capillaries (electron micrograph)

• M, marginal fold

• P, pericyte (partly surrounds endothelial cell layer, endowed with limited contractile ability; contributes to basement membranes protein secretion)

Fenestrate capillaries

• Small circles are the transportation of lipids and others.

Sinusoids

• Large intercellular gaps

• Large fenestrations (no diaphragms)

• Discontinuous basement membrane

• Diameter = 30 - 40um: liver, bone marrow, spleen.

Summary: structure follows function