Blood Vessels Overview

Study Notes

Types: Arteries, veins, arterio-venous connections
Large Arteries:
- Example: Aorta
Large Veins:
- Example: Inferior vena cava
- Wide lumen with numerous valves
- Thin, flat wall
- Tunica intima: similar to medium-sized veins
- Tunica media: thin with only smooth muscle fibers
- Tunica adventitia: thick layer of loose connective tissue (CT) containing collagenous fibers, CT cells, vasa vasorum, and nerves; also contains longitudinally arranged bundles of smooth muscle fibers
Medium-Sized Arteries:
Medium-Sized Veins:
Arterioles:
Venules:
Blood Capillaries:
- Thin-walled, tubular blood channels connecting terminal arterioles to venules
- Diameter: 8-10 μm
- Network of capillaries called a capillary bed
- Histological Structure:
  - Single layer of endothelial cells resting on a basal lamina
  - Pericytes lie between the endothelium and basal lamina
  - Endothelial cell cytoplasm contains:
    - Usual organelles
    - Numerous microfilaments
    - Intermediate filaments
    - Pinocytotic vesicles
  - Cells held together by zona occludens and gap junctions
- After tissue injury: Pericytes proliferate and differentiate to form new blood vessels and CT cells

Continuous Capillaries:
- No fenestrae present
- Location: All kinds of muscle tissue, CT, exocrine glands, and nervous tissue
Fenestrated Capillaries:
- Large fenestrae present in the cytoplasm of endothelial cells
- Fenestrae closed by diaphragms that are thinner than a cell membrane
- Continuous basal lamina present
- Location: Kidney, intestine, and endocrine glands
Fenestrated Capillaries Without Diaphragm:
- Have a very thick basal lamina
- Location: Renal glomerulus capillary
Discontinuous Sinusoidal Capillaries:
- Discontinuous basal lamina and endothelial cells
- Multiple fenestrae without diaphragm found in the endothelial cells

Blood Sinusoids:
- Dilated, irregular blood spaces (5-30 μm in diameter)
- Location: Bone marrow, spleen, liver, and endocrine glands
- Histological Structure:
  - Lined by fenestrated endothelial cells (simple squamous epithelium)
  - Walls contain pores that are not covered by diaphragms
  - Endothelium rests on a thin layer of reticular CT
  - No basement membrane present
  - Macrophages present outside the sinusoidal wall, extending pseudopodia inside the wall to phagocytose any foreign body

Diameter:
- Capillaries: Uniform 8-10 μm
- Sinusoids: Irregular 5-30 μm
Lumen:
- Capillaries: Narrow
- Sinusoids: Wide
Sites:
- Capillaries: Present everywhere in the body
- Sinusoids: Present in certain sites: bone marrow, spleen, liver, and endocrine glands
Structure:
- Lining Endothelium:
  - Capillaries: Fenestrated or continuous
  - Sinusoids: Fenestrated, not covered with diaphragm
- Basement Membrane:
  - Capillaries: Well developed
  - Sinusoids: Absent
- Pericytes:
  - Capillaries: Absent
  - Sinusoids: Present
- Macrophages:
  - Capillaries: Absent
  - Sinusoids: Present

Function:
- Capillaries: Gas and nutrient exchange
- Sinusoids: Function according to their location (e.g., blood cell production, filtration, etc.)