Blood Vessel Histology Quiz

Study Notes

Blood Vessel Histology

Blood vessels are classified into arteries, arterioles, capillaries, venules, and veins.

Learning Objectives

Students will be able to classify blood vessels.
Students will be able to list the layered architecture of blood vessel walls.
Students will be able to differentiate between types of blood capillaries.
Students will be able to compare an artery and vein of similar size.

General Structure of the Vascular Wall

The vascular wall is comprised of three layers:
- Tunica intima (inner layer):
  - Endothelium resting on basement membrane
  - Subendothelium (layer of loose connective tissue)
  - Internal elastic lamina (composed of elastic connective tissue).
- Tunica media (middle layer):
  - Smooth muscle tissue
  - External elastic lamina (composed of elastic connective tissue) (sometimes, absent).
- Tunica adventitia (outer layer):
  - Loose areolar connective tissue rich in elastic fibers.

Types of Blood Vessels

Large Elastic Arteries:
- Include the aorta and its large branches.
- Function: stabilize blood flow.
- Contain yellowish elastic tissue in the media (middle layer).
- Tunica intima (inner layer) is thick and contains an internal elastic lamina.
- Tunica media (middle layer) contains thick external elastic lamina.
- Tunica adventitia (outer layer) is relatively thin. The adventitia contains collagen fibers.
Medium-sized Muscular Arteries:
- Include renal, mesenteric, and femoral arteries.
- Function: control blood flow to organs.
- Tunica intima (inner layer) is thick with an internal elastic lamina.
- Tunica media (middle layer) contains up to 40 layers of smooth muscle cells and external elastic lamina.
- Tunica adventitia (outer layer) is well-developed, containing collagen fibers.
Arterioles and Small Arteries:
- Generally less than 0.5 mm in diameter.
- Function: form the microcirculation, branching out from arteries to capillaries.
- Tunica intima (inner layer) is thin; internal elastic lamina may be absent.
- Tunica media (middle layer) has one or two layers of smooth muscle; external elastic lamina is absent.
- Tunica adventitia (outer layer) is very thin.
Capillaries:
- Connect arteries and veins for metabolic exchange between blood and tissues.
- Originate from meta-arterioles (small branches of arterioles).
- Composed of a single layer of endothelial cells rolled up in a tube, resting on a basement membrane.
- Diameter ranges from 7 to 9 microns. Capillaries vary in structure. Continuous, fenestrated, and sinusoidal capillaries.
Types of Capillaries:
- Continuous: abundant in skin, muscle, connective tissue, and nervous tissue.
- Characterized by the absence of fenestrations (openings), tight junctions, and a continuous basement membrane.
- Fenestrated: present in the viscera (internal organs).
- Characterized by large fenestrations (openings) in the endothelium and a continuous basement membrane. These capillaries have pores that allow for quick passage of materials.
- Discontinuous (Sinusoidal): observed in the liver, spleen, and bone marrow.
  - Characterized by large fenestrations without diaphragms, large intercellular clefts, tight junctions, and an incomplete basement membrane. These capillaries allow for large molecule movement.
Post-capillary Venules: Pericytes are present.
- Also called pericytic venules due to pericytes.
- Tunica intima is composed of endothelium plus pericytes and subendothelial layer.
- Function similarly to capillaries, participating in inflammatory processes and exchange of cells/molecules between blood and tissues.
Collecting Venules:
- Similar to post-capillary venules but larger.
Muscular Venules:
- Contain additional smooth muscle layers.
Small and Medium-sized Veins:
- Diameter of 1-9 mm.
- Media (middle layer) contains bundles of smooth muscle cells and collagenous tissue.
- Adventitia (outer layer) is well-developed, with collagenous tissue. Thickest layer compared to arteries.
- Contain valves formed by folds of tunica intima that extend into the lumen, excluding those in head and neck veins. Valves direct venous blood toward the heart (against gravity).
Large Veins:
- Include major venous trunks (e.g., vena cava).
- Close to the heart.
- Tunica intima (inner layer) is distinct.
- Tunica media (middle layer) contains layers of smooth muscle cells.
- Tunica adventitia (outer layer) is thick, with elastic fibers mixed throughout.
- May contain valves in their lumens.