Blood vessels are dynamic structures which form a system of pipes. There are 3 main types of blood vessels: arteries, veins, and capillaries, and the structure of each vessel varies with its function. The system begins at the heart. Arteries carry blood away from the heart, and veins bring the blood back. There are two circulatory systems; systemic and pulmonary:
Blood vessels supply the body. Arteries carry oxygenated blood to where it is required, and the veins bring back the deoxygenated blood.
Arteries carry deoxygenated blood from the heart, into the lungs. Once gas exchange has occurred, the veins bring the oxygenated blood back to the heart, where it is pumped out into systemic circulation.
Blood vessels are made up of a number of different cell types: smooth muscle cells and endothelial cells. In addition to this there are two types of connective tissue fibres; elastin and collagen. How much of each of these components is present, varies with the function of the vessel.
These are spindle shaped cells with a centrally located nucleus, and lack the striations of skeletal and cardiac muscle cells. These cells are tightly arranged to form sheets, and contact to produce a squeezing motion, therefore propelling blood.
These are simple squamous epithelial cells, and form the inner (“endo”) lining of blood vessels. These cells are flattened, with a disc shape nucleus, and provide a slick surface for blood flow. These cells form a thin, permeable layer, which is perfect for the exchange of nutrients and waste.
These are long, thin fibres which form a branching network. They are made from the elastin protein, which allows them to stretch like elastic under tension. When the tension is released, they recoil back to their original shape.
These fibres consist of collagen proteins cross-linked to form a fibril, which bundle to form thick collagen fibres. These fibres are very strong, and are able to withstand large amounts of pressure.
There are three distinctive layers, or tunics, to arteries and veins. These are the tunica intima, tunica media, and tunica externa. In arteries, between these tunics there are also layers of elastin fibres.
The innermost layer, containing the endothelium. The squamous epithelial cells reduce friction as blood flows through the vessel. In vessels larger than 1mm wide, there is also a subendothelial layer to support the endothelium.
The middle layer, containing smooth muscle cells and elastin fibres. This layer contracts and relaxes to regulate blood flow and is vital in regulating blood pressure. This layer is supplied by sympathetic nerves.
The outer layer, containing collagen fibres. This layer’s function is to reinforce the vessel, and the collagen fibres help anchor the vessel in place. This layer also contains nerve fibres, lymphatic vessels, microvasculature, and in larger veins, elastin fibres.
Arteries can be further divided into 3 groups based on their size and function:
The major vessels leaving the heart and their branches e.g. the aorta.
Elastic arteries branch into smaller muscular arteries e.g. radial artery.
Capillaries are the smallest blood vessels, and supply most tissues. They are so small (8-10µm) that red blood cells can only fit through in single file. Capillaries only consist of a layer of endothelium (sometimes only one cell thick) and the surrounding basement membrane.
The basement membrane consists of connective tissues and sometimes smooth-muscle like cells known as pericytes, which anchor the capillaries in place. The thin structure of capillaries makes them perfectly suited to exchange of gas, nutrients, and waste, between the blood and tissues.
There are three different types of capillary; continuous, fenestrated, and sinusoidal. The function of each type of capillary varies, and therefore they each have different structures.
Capillaries work in groups called capillary beds to achieve microcirculation. Arterioles bring blood to the capillaries, and if precapillary sphincters are open, blood will flow across the capillary bed and into the venous system. If the sphincters are closed, the blood will flow directly from the arterioles into ametarteriole and into a throughfare channel, therefore bypassing the capillary bed and going straight back into the venous system.
After leaving the capillary beds, the blood is carried back to the heart via a series of venous vessels. The venous system comprises of venules and veins. These vessels increase in size as they progress towards the heart.
Boundless. Arteries, Veins, and Capillaries. [accessed 18/02/16] https://www.boundless.com/biology/textbooks/boundless-biology-textbook/the-circulatory-system-40/mammalian-heart-and-blood-vessels-226/arteries-veins-and-capillaries-853-12098/
Ivy Rose Holistic. Blood vessels. [accessed 18/02/16] http://www.ivyroses.com/HumanBody/Blood/Blood_Vessels.php
Marieb EN & Hoehn K. 2010. Human Anatomy and Physiology, Chapter 19. 8th Edition, London: Pearson International Edition.
Fastbleep © 2019.