BLOOD VESSEL WALLS
Arteries and veins:
Vessel walls consist of 3 layers.
1. Tunica Intimia :
- the innermost layer.
- Lined by endothelium (comprised of simple squamous epithelial cells,.
- Deep to the endothelium are a basement membrane, and a layer of subendothelial connective tissue.
- Has elastic membrane called internal elastic membrane at the boundary between the tunica intima and the tunica media.
Composed of elastic fibers, so it allows for elasticity facilitating the expansion and contraction of the cardiac cycle.
Tunica Media :
- The middle and thickest layer.
- Thicker in arteries than in veins.
- Consists of smooth muscle cells, collagen fibers and elastic fibers.
- External elastic membrane at the boundaries between tunica media and .
Tunica Externa (adventitia)
- The outermost layer.
- Composed of connective tissue arranged longitudinally also with collagen and elastic fibers.
The C.T in the wall of veins is accompanied by smooth muscle cells.
Capillaries :
- The innermost layer is formed by a single layer of endothelial cells.
- Thin Basement membrane.
- Lack of smooth muscle and Adventitia.
This absence allows capillaries to be highly permeable and facilitates efficient exchange of substances.
- Capillary types:
- Continuous Capillaries: The most common type, with tight junctions between endothelial cells, limiting permeability.
- Fenestrated Capillaries: Contain pores or fenestrations, found in tissues with high rates of nutrient exchange, such as the kidneys and intestines.
- Discontinuous (Sinusoidal) Capillaries: Have irregular gaps between endothelial cells and are found in the liver, spleen, and bone marrow.