Histology of Blood and Lymphatic Vessels

Comprehensive vocabulary flashcards covering the histological structure, classification, and function of blood and lymphatic vessels, including arteries, veins, microcirculation, capillaries, and valves.

Intima

The innermost layer of a blood vessel wall, consisting of an endothelium (a single layer of cells resting on a basal membrane) and subendothelial connective tissue elements.

Media

The middle layer of a blood vessel wall, composed of smooth muscle fibres, elastic laminae or fibres, and collagen fibres arranged according to the vessel type.

Adventitia

The outermost layer of a blood vessel wall, composed of loose connective tissue, collagen fibres, elastic fibres, nerve fibres, and small nutrient vessels.

Vasa Vasorum

Small nutrient or feeding blood vessels located within the adventitia that supply the walls of larger blood vessels.

Elastic Arteries

Large-calibre juxta-cardial conduit arteries with an external diameter $> 7\,\text{mm}$ (such as the aorta and pulmonary artery) whose elastic wall enables them to absorb and release ventricular pressure waves.

Muscular Arteries

Medium-calibre ($2.5\text{ to }7\,\text{mm}$) and small-calibre ($< 2.5\,\text{mm}$) subcardial distribution arteries characterized by a thick media consisting of smooth muscle fibres arranged in several layers to form a compact sheath.

Endarterium

The subendothelial region of the arterial intima, which in elastic arteries comprises a connective tissue reticulum, a mucoid layer of amorphous ground substance, a striated layer of smooth muscle fibres, and an internal elastic lamina.

Internal Elastic Lamina

A layer of elastic fibres bounding the intima that is hardly distinguishable from media laminae in elastic arteries, formed by large elastic fibres packed into 2 or 3 strata in muscular arteries, and fragmentary in arterioles.

Outer Elastic Lamina

A boundary structure found in muscular arteries formed by one or more layers of elastic fibres that converge in the adventitia.

Arteriole

A small arterial vessel with a diameter of $100\text{--}50\,\mu\text{m}$, characterized by a thin flattened endothelium on a basal lamina, a fragmentary internal elastic boundary, a media reduced to 2 to 3 smooth muscle layers, and a dense collagenous adventitia.

Metarteriole

A terminal or pre-capillary arteriolar branch ($50\text{--}15\,\mu\text{m}$ in diameter) where the internal elastic lamina is absent, the media is reduced to 1 or rarely 2 layers of smooth muscle, and adventitial nerve terminals form myoneural junctions.

Myo-endothelial Junction

A specialized contact formed in terminal arterioles where endothelial cell protrusions extend through the basement membrane to meet smooth muscle cells of the media.

Myoneural Junction

A junction formed where unmyelinated nerve fiber terminals in the adventitia penetrate between the smooth muscle cells of the arteriolar media.

Pre-capillary Sphincter

A musculo-endothelial mechanism ($\varnothing = 10\,\mu\text{m}$) containing numerous myoneural and myo-endothelial junctions that regulates blood pressure and flow entering the capillaries.

Mesovene

The tunica media of a vein, composed of smooth muscle fibres interspersed with abundant connective tissue septa.

Perivenous Layer

The tunica adventitia of a vein, which is thicker than the media and contains strong longitudinal collagen bundles, a few elastic fibres, vasa vasorum, and nerve fibres.

Muscular Vein (Smooth Muscle Type)

The most common type of vein in the venous system, characterized by the presence of smooth muscle fibres within the tunica media.

Muscular Vein (Striated Muscle Type)

A venous vessel possessing striated muscle within its adventitia, found in the juxta-cardial segments of the venae cavae and pulmonary veins.

Fibrous Vein

A non-deformable vein whose intima is lined by a connective-fibrous layer, characteristic of intracranial veins.

Fibro-elastic Vein

A vein characterized by anastomosing elastic laminae in contact with the endothelium and concentric collagen bundles, found in supracardiac veins such as neck and axillary veins.

Venous Valves

Paired folds of the intima covering a dense connective-elastic core with smooth muscle fibres oriented perpendicularly to the vessel axis, functioning to prevent backward flow in large veins of the lower body.

Venule

A small venous vessel with a poorly defined wall consisting of an endothelium, a connective-elastic layer, and a few smooth muscle fibres.

Capillary Bed

The entire microvascular network formed by branching arterioles that serves as the principal site of nutrient exchange between the blood and tissue cells.

Portal Arterial Network

A specialized capillary network positioned between two arterioles, found in the kidney.

Portal Venous Network

A specialized capillary network positioned between two venules, found in the liver.

Junction Capillaries

Typical capillaries that form a direct connecting conduit between an arteriole and a venule.

True Capillaries

Typical capillaries that arise directly from an arteriole or a junction capillary to form the exchange bed.

Continuous Capillary

A typical capillary whose wall consists of a single layer of flattened endothelial cells with nuclei protruding into the lumen, a complete basal membrane (sometimes lined with reticulin fibres), and peripheral pericytes or mesenchymal cells.

Embryonic Capillary

An atypical capillary characterized histologically by possessing a syncytial wall.

Sinusoidal Capillary

An atypical capillary characterized by large endothelial pores, a discontinuous wall, and an absent or discontinuous basal lamina (e.g., in the liver).

Lymphatic Vascular System

A drainage network that collects lymph originating in connective tissue spaces and returns it through capillaries, venules, and trunks into the subclavian veins.

Lymphatic Trunk

A large-calibre collecting lymphatic vessel possessing internal valves and a wall formed by an endothelial intima, a thin media, and a musculo-elastic sheath.