This lecture on human circulatory system explains the structure of the arteries, veins and capillaries, along with other related aspects.
Overview of Lecture
Circulatory System
Introduction
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
Microvasculature
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
Endothelium is a special type of epithelium ,btw the blood plasma and the interstitial fluid
Functions
Exchange of materials between blood and surrounding tissues
Conversion
Production of vasoactive factors
Lipolysis
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
Innervation
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)
Arterioles
< 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
Capillaries
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
MICROCIRCULATION
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
Valves
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
