Histology of Blood Vessels

Blood Vessel Structure

• The blood vessel wall consists of three layers: tunica intima, tunica media, and tunica adventitia.
• Tunica intima has a simple squamous epithelium, known as endothelium, which secretes its own basal lamina.
• Subendothelial tissue is a loose tissue formed by fine collagen and elastic fibers with occasional smooth muscle cells.
• Internal elastic lamina separates intima from media and consists of elastic fibers with fenestrae (gaps) that allow diffusion of substances for nourishment.

Tunica Media

• Tunica media consists of concentric layers of elastic and reticular fibers, proteoglycans, glycoproteins, and smooth muscle cells.
• Elastic fibers are arranged as fenestrated laminae, and the thickness and components vary depending on the type of blood vessel.
• Pericytes replace the media in capillaries and post-capillary venules.

Tunica Adventitia

• Tunica adventitia is a connective tissue layer that consists of collagen (type 1) and elastic fibers.
• Thickness varies in different vessels and contains small blood vessels and nerves.

Functions of Blood Vessel Layers

• Endothelium regulates transport of materials, secretes substances, and controls blood flow.
• Elastic fibers passively alter the size of the lumen to withstand and maintain blood pressure (elastic recoil).
• Smooth muscles regulate the amount of blood flow to an area by contracting and relaxing.

Vascular Tree

• The vascular tree consists of the heart, large elastic artery, large vein, muscular/medium artery, medium vein, arterioles, metarteriole, capillaries, venule, small vein, venule, post-capillary, and large/elastic/conducting artery.

Types of Arteries

Elastic Artery

• Tunica intima has all three layers and is thicker compared to other arteries.
• Subendothelial layer has fibroblasts, and the internal elastic lamina is not that distinguishable.
• Tunica media consists of 90% layers of fenestrated elastic lamina, and tunica adventitia is thin and relatively underdeveloped.

Muscular Artery

• Tunica intima has all three components, but it is thinner than the elastic artery.
• Tunica media is 50% thick with circularly arranged smooth muscle fibers, and the thickness decreases as the arteries become smaller.
• External elastic lamina is seen in large muscular arteries but absent in small muscular arteries.

Arteriole

• Less than 0.5mm in diameter, with a narrow lumen, and tunica intima is present.
• Internal elastic lamina is absent in small arterioles, and tunica media is made up of 1-2 smooth muscle cell layers.

Capillaries

• Endothelial tubes with a diameter of 7-9μm, resting on a basement membrane.
• Zonula occludens is present, and the vast surface area and low flow rate allow for efficient diffusion of substances.
• Single layer of endothelial cells resting on a basement membrane, with pericytes at various locations.

Types of Capillaries

• Continuous (somatic) capillaries have smooth, non-porous endothelial lining and continuous basal lamina, found in brain, muscle, nerves, connective tissue, and exocrine glands.
• Fenestrated capillaries have endothelial cells forming small openings called fenestrations, with a continuous basal lamina that acts as a filter, favoring rapid transport of substances.