The blood vessels form a network of tubes that carry blood away from the heart, transport it to the tissues of the body, and then return it to the heart. The blood vessels that the blood away from the heart are called carry arteries, while those, which return the blood to the heart, are called veins.
General Structural Plan of the Blood Vessels
All arteries and veins have a common basic design with regard to the composition of their walls. Generally, the wall of a blood vessel is composed of three concentric coats (tunics):
- Tunica intima
- Tunica adventitia
This is the innermost coat and consists further of two layers: (I) endothelium which lines the lumen of the blood vessel, and (ii) a thin layer of subendothelial connective tissue.
It is the middle layer and is composed chiefly of circularly arranged smooth muscle fibers. Depending on the type of blood vessel, variable amounts of collagenous fibers and elastic fibers are found between the smooth muscle fibers.
This is the outermost coat and is composed of collagenous and elastic fibers running mainly in a longitudinal direction.
CLASSIFICATION OF BLOOD VESSELS
А. Anatomical Classification
Taking into account the flow of blood, first away and then toward the heart, the blood vessels are classified into the following 5 categories:
The arteries are efferent vessels that carry blood away from the heart to the organs and tissues of the body. On their way to different parts of body the arteries branch in a tree-like manner; the branches progressively decrease in diameter. The arteries are classified into two main types: (1) elastic arteries and (2) muscular arteries.
Elastic arteries are also those in which the tunica media is mainly compos of elastic variety of connective tissue fibers and the amount of smooth muscle is comparatively very low. These arteries possess large caliber and, hence are also known as large arteries. The main arteries issuing from the heart, i.e., the aorta and pulmonary trunk are elastic arteries. In addition, main branches of the aorta (the brachiocephalic, left common carotid, left subclavian and common iliac), and those of the pulmonary trunk also belong to the elastic variety of arteries.
The muscular arteries are so named because their tunica media chiefly
composed of smooth muscle cells. Most of the named arteries of the body belong to this group, e.g., the axillary, brachial, radial and ulnar arteries of the upper limb, and the femoral, popliteal, anterior tibial and posterior tibial arteries of the lower Limb.
The arterioles also called small arteries, constitute a very important segment of the circulatory system because they form the principal component of the peripheral resistance to flow that regulates the blood pressure, Arterioles range 0.2 to 0.4 mm in diameter. The tunica intima of these vessels consists of endothelium and a thin subendothelial layer of connective tissue.
The tunica media of the larger arterioles consists of two layers of circularly-arranged smooth muscle fibers. The smaller arterioles also called terminal arterioles, have only one layer of smooth muscle in their wall and each individual smooth muscle cell completely encircles the endothelium. The arterioles branch into smaller vessels, called metarterioles, which are surrounded by a discontinuous layer of smooth muscle. The metarterioles branch into capillaries which form networks.
This smooth muscle ring is known as a precapillary sphincter. Constriction of this sphincter can completely stop the blood flow within a capillary.
A narrow lumen with relatively thick muscular walls enables the arterioles to dilate or constrict to a considerable extent. Consequently, the arterioles play a very important role in controlling the flow of blood from arteries into capillaries and, later, into organs according to their functional requirements. If needed, an arteriole can dilate to increase the blood flow to capillaries by as much as four times the normal supply.
Capillaries occur as networks of microscopic vessels which connect the arterial and venous systems. Networks (plexuses) of capillaries exist in almost every part of the body. Microscopically the capillaries are delicate endothelial tubes. Outer to the endothelial lining, a very thin layer of collagenous and reticular connective tissue fibers is present. Through the thin walls of the capillaries substances also exchanged between the blood and tissues of the body. Arrangement of capillaries in the form of networks serves to increase the surface area for the exchange of materials. Capillaries generally measure 0.75 mm in length and 7 to 9 µm in diameter; however, the sinusoidal capillaries have much larger diameter.
Types of Capillaries
The capillaries are classified into three different types; each type is related to specific functions. These three types are:
- Continuous capillaries
- Fenestrated capillaries
- Sinusoidal capillaries
These capillaries measure 7-9 um in diameter and also do not have any pores, gaps or discontinuities in their walls. Most of the capillaries of the body belong to this variety. Examples of continuous capillaries are capillaries in muscles, lungs and brain.
These capillaries have the same diameter as that of the continuous capillaries but the walls of fenestrated capillaries are characterized by the presence of circular pores in the lining endothelial cells (in Latin, fenestra=window). These pores range from 60 to 80 nm in diameter. Fenestrated capillaries are found in those locations in the body where rapid exchange of materials between the blood and tissues is required, e.g., in intestines, endocrine glands and kidneys.
These capillaries, or sinusoids, exhibit the following characteristics:
1. Their luminal diameter (30–40 um) is much larger than the continuous or fenestrated capillaries.
2. Their walls are irregular and tortuous.
3. Intercellular gaps exist between the endothelial cells (due to which
the blood can diffuse out of the circulation with only a minimal hindrance).
4. Lining endothelial cells show pores.
5. Phagocytic cells may found to be located in the walls of sinusoids.
Major locations in the body where sinusoidal capillaries are include liver, spleen, bone marrow and medulla of suprarenal gland.
The blood from capillaries drains into venules. Two or more capillaries converge and join together to form a postcapillary venule. These venules
are about 30 micro meter in diameter and consist mainly of endothelium and a thin tunica adventitia. Postcapillary venules play important role in the exchanges between the blood and intercellular fluid. In response to inflammation, they allow water, solutes and leukocytes to move out into the intercellular space. The postcapillary venules also join to form muscular venules, which contain smooth muscle in their tunica media. The muscular venules converge and join to form collecting venules, which drain blood into veins.
In some regions of the body the metarterioles, after giving rise to capillaries, continue as low resistance channels which open into venules; these channels are preferential channels or thoroughfare channels.
Preferential channels open when constriction of precapillary sphincters reduces blood flow through the local capillary network. The thoroughfare channels serve to by-pass the capillary bed and sustain blood flow through the region when the capillaries are not being used. Preferential channels are abundant in the mesentery of the intestine, and in those body sites where thermoregulation is important, e.g., skin and external ear.
Venules join together to form veins. Smaller veins joining to form a larger vein are tributaries of that vein. The veins generally have larger lumen but thinner walls than those of accompanying arteries. The veins have considerably less smooth muscle cells and elastic fibers in their wall but they contain more collagenous fibers. Consequently, the veins can distend considerably to adapt to variations in the volume and pressure of blood passing through them. The veins are generally further classified into three categories:
1. Small veins which include the un-named veins of the body.
2. Medium-sized veins which include most of the named veins.
3. Large veins which include the superior vena cava, inferior vena cava and the portal vein.
Many veins, such as those in the limbs, contain valves that permit blood to flow only in one direction preventing any back flow. There are no valves small veins and, in those veins, which are present in the regions of great muscular pressure, i.e., the thoracic and abdominal cavities. Valves strength by a small amount of collagen and elastic fibers. The valve cusps are semilunar in shape. A valve is usually consist of two cusps.
Venae comitantes. Generally, each named artery is accompanied by a corresponding vein, which receives blood from the area supplied by the artery. However, in the limbs the veins that accompany major deep arteries usually occur in pairs which are known as venae comitantes (i.e., one artery is accompanied by two veins which are given the same name as that of the artery which they accompany). The two venae comitantes flank the artery on either side and connected to each other by numerous cross-connections.
В. Functional Classification of Blood Vessels
Functionally the blood vessels can be classified into the following five groups:
1. Conducting vessels. This group consists of the large (i.e.., elastic) arteries. They include the big arteries arising from the heart (aorta and pulmonary trunk) and the main branches of these arteries.
2. Distributing vessels. This variety of vessels consists of the muscular arteries, which reach the individual organ and divide into branches within these organs.
3. Resistance vessels. This group includes the arterioles. As already mentioned, these vessels have a small lumen but relatively thick muscular walls and, hence, are the principal source of the peripheral resistance to blood flow.
4. Exchange vessels. Walls of these vessels allow exchange of substances and defensive cells between blood and intercellular fluid. This group of vessels also includes capillaries, sinusoids and postcapillary venules.
5. Reservoir vessels. These vessels, also called capacitance vessels, include larger venules and all types of veins. Due to their distensibility, these vessels accommodate large volume of blood and, thus, serve as reservoirs of blood. As a matter of fact, more than 70% of the total blood volume is in this part of the CVS at any one time.