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.

Test your knowledge on the histological structure of blood vessels, focusing on the layers such as Tunica Intima, Subendothelial tissue, and Tunica Media. Learn about the composition of each layer and their functions.