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Circulatory System

This lecture on human circulatory system explains the structure of the arteries, veins and capillaries, along with other related aspects.

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Overview of Lecture

Circulatory System


Blood and lymphatic vascular systems

The blood vascular system is composed of heart, arteries, capillaries and veins

The lymphatic vascular system comprises lymphatic capillaries and ducts which drain into  blood stream

Macrovasculature > 0.1 mm in diameter


Vascular Wall

Three basic structural constituents: the endothelium, the muscular tissue, and the connective tissue

Mechanical factors

Metabolic factors

Present in different proportions in the vascular wall, except for capillaries and post-capillary venules, in which the only structural elements represented are the endothelium, its basal lamina, and pericytes


Endothelium is a special type of epithelium ,btw the blood plasma and the interstitial fluid


Exchange of materials between blood and surrounding tissues


Production of vasoactive factors


Anti-thrombogenic action

Vascular Smooth Muscle

Present in all vessels except capillaries and post-capillary venules

Smooth muscle cells arranged in helical layers in the tunica media

Vascular smooth muscle cells, mainly of arterioles and small arteries, are connected by communicating (gap) junctions

Vascular Connective Tissue

Collagen fibers are found between muscle cells, in adventitia, and in some sub-endothelial layers. Collagen types I, III ,IV is present in adventitia, tunica media and basement membranes respectively

Elastic fibers predominate in large elastic arteries

Ground substance forms a heterogeneous gel in the extracellular spaces of the vessel wall. It affects the diffusion and permeability across the wall.

Structure of Blood Vessels

Composed of three layers (tunics)

Tunica intima

one layer of endothelial cells (simple squamous epithelium) supported by a sub-endothelial layer of loose connective tissue

In arteries, the intima is separated from the media by an internal elastic lamina. This lamina, composed of elastin, has gaps (fenestrae) that allow the diffusion of substances to nourish cells deep in the vessel wall

Tunica media

Consists of smooth muscle which are helically arranged  in concentric layers

Interposed among these cells are variable amounts of elastic fibers and lamellae, reticular fibers (collagen type III), proteoglycans, and glycoproteins. Smooth muscle cells produce this extracellular matrix

In arteries, the media has a thinner external elastic lamina, which separates it from the tunica adventitia

Tunica adventitia (tunica externa)

Dense fibrous connective tissue which thins out to loose fibrous connective tissue

Protects the blood vessel (strong)

Gives vessel strength for shape

Anchors vessel to surrounding tissue; loosens with age

The adventitia consists principally of collagen (type1)and elastic fibers

The adventitial layer gradually becomes continuous with the connective tissue of the organ through which the vessel runs

Vasa Vasorum

Large vessels usually have vasa vasorum (“vessels of the vessel”), which are arterioles, capillaries, and venules that branch profusely in the adventitia and the outer part of the media

The vasa vasorum provide metabolites to the adventitia and the media

Frequent in veins than in arteries


Unmyelinated sympathetic nerve fibers (vasomotor nerves), neurotransmitter is norepinephrine

Arteries vs Veins

Vasodilation in arteries of skeletal muscles.. ACH-nirtic oxide (endothelium)-cyclic GMP mechanism in smooth muscles-relaxation

Structure of Arteries, Veins, and Capillaries

Large Elastic Arteries

Aorta and its large branches

Yellowish color

Intima is thicker

Internal elastic lamina

Media consists of elastic fibers and a series of concentrically arranged, perforated elastic laminae

The tunica adventitia is relatively underdeveloped

The several elastic laminae contribute to the important function of making the blood flux more uniform.

During ventricular contraction (systole)

During ventricular relaxation (diastole)

Elastic artery (aorta)

Medium (Muscular) Arteries

May control the flow of blood to the organs

Intima is somewhat thicker than that of the arterioles

internal elastic lamina is prominent

tunica media may contain up to 40 layers of smooth muscle cells

An external elastic lamina, the last component of the media, is present only in the larger muscular arteries

The adventitia

Muscular artery (transverse section)


< 0.5 mm in diameter

The subendothelial layer is very thin

In the very small arterioles, the internal elastic lamina is absent, and the media is generally composed of one or two circularly arranged layers of smooth muscle cells

It shows no external elastic lamina

The tunica adventitia is very thin


Single layer of endothelial cells

The average diameter 7-9 µm, and their length is usually not more than 50 µm

When cut transversely, their walls are observed to consist of one to three cells .The external surfaces of these cells usually rest on a basal lamina

Polygonal cells, elongated in the direction of blood flow. The nucleus causes the cell to bulge into the capillary lumen

Its cytoplasm contains few organelles

Zonula occluda junctions type are present between endothelial cells

Pericytes, contractile function and tissue repair

Capillaries are grouped into three types, depending on the continuity of both the endothelial sheet and the basal lamina

Continuous Capillary

Found in all types of muscle tissue, connective tissue, exocrine glands, and nervous tissue

There is a complete layer of cytoplasm throughout each endothelial cell

In some places, but not in the nervous system, numerous pinocytic vesicles are present on both surfaces of endothelial cells

Fenestrated Capillary

Present in intestinal villi, ciliary process of eye, choroid plexus, glomeruli of kidney

Circular transcellular openings in the endothelium membrane called fenestrae (70-100 nm)

The basal lamina of the fenestrated capillaries is continuous

Each fenestra is obliterated by a diaphragm

The cytoplasm of endothelial cells differs from continuous capillaries, it lacks any alkaline  phosphatase activity


Discontinuous Sinusoidal Capillaries

Discontinuous Sinusoidal Capillaries

Present in liver, bone marrow and spleen

The capillaries have a tortuous path and greatly enlarged diameter (30-40µm), which slows the circulation of blood

The basal lamina is discontinuous

The endothelial cells form a discontinuous layer and are separated from one another by wide spaces

The cytoplasm of the endothelial cells has multiple fenestrations without diaphragms

Macrophages are located either among or outside the cells of the endothelium

Sinusoidal capillaries


The arterioles branch into small vessels surrounded by a discontinuous layer of smooth muscle, the metarterioles

Constriction of metarterioles helps to regulate the circulation in capillaries

In some tissues, there are arteriovenous anastomoses that enables the arterioles to empty directly into venules

This is an additional mechanism that contributes to regulation of the capillary circulation (skeletal muscle and in the skin of the hands and feet)

Post-capillary Venules

Range in diameter from 0.1-0.5 mm and in length from 0.5-70 mm

Characterized by the presence of pericytes

The tunica intima of these vessels is composed of endothelium and a very thin sub endothelial layer

Post-capillary venules have several features in common with capillaries

Muscular Vein

The majority of veins are small or medium-sized ,with a diameter of 1-9 mm

The intima usually has a thin subendothelial layer, which may be absent at times

The media consists of small bundles of smooth muscle cells intermixed with reticular fibers and a delicate network of elastic fibers

The adventitia is well developed


The valves consist of 2 semilunar folds of the tunica intima that project into the lumen

They are composed of connective tissue rich in elastic fibers and are lined on both sides by endothelium

Designed to prevent backflow

Comparison of a Typical Artery and a Typical Vein

Lymphatic Vascular System

Returns the extracellular liquid to the bloodstream

System of endothelium-lined thin-walled channels that collect fluid from the tissue spaces and returns it to the blood. This fluid is called lymph; unlike blood, it circulates in only one direction, toward the heart

Lymph Capillaries

The lymphatic capillaries originate in the various tissues as thin, closed-ended vessels that consist of a single layer of endothelium and an incomplete basal lamina

Lymphatic capillaries are held open by numerous microfibrils of the elastic fiber system, which also bind them firmly to the surrounding connective tissue

Lymphatic Vessel

The lymphatic vessels have a structure similar to that of veins except that they have thinner walls and lack a clear-cut separation between layers (intima, media, adventitia)

They also have more numerous internal valves

The lymphatic vessels are dilated and assume a beaded appearance between the valves

Like arteries and veins, large lymphatic ducts contain vasa vasorum and a rich neural network

Blood and Lymphatic vessel in connective tissue




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