The cardiovascular (CV) system consists of all those structures which have the role of transporting the blood to all parts of the body. In this way the CV system enables the blood to perform its roles of protection, providing nutrients, and removal of toxins for all the bodily tissues.
The CV system consists of two main parts which you will consider in turn in this lesson, the blood vessels and the heart. The vessels are the channels through which the blood flows.
The three main types of blood vessels are called arteries, veins and capillaries. The heart, of course, is the pump which provides the power to create the flow of blood through the vessels.
Sometimes the lymphatic system, which you studied in Lesson 2.1d, is also included as part of this system. However, in this course, for the sake of clarity, the lymphatic system will be treated as a separate system.
The blood (haematological system) is not conventionally considered to be a part of the CV system.
Conventionally the cardiovascular system is seen simply as a pump and a series of pipes.
It is not equated with any of the deep emotional qualities to which it is linked in Chinese medicine.
The vascular system is the term used to describe the vessels alone. It consists of a network of tubes, lined by endothelium, which are at their widest as they leave the heart, and which branch, like a tree, to produce thousands of tiny capillaries within the tissues. Thecapillaries then meet up again to form wider and wider tubes which return the blood back to the heart.
Blood vessels which lead blood away from the heart are called arteries and those which lead blood back again to the heart from the tissues are called veins. Arterioles and venules are names given to the smallest types of arteries and veins, but you need not be concerned about the differences between these and the larger arteries and veins.
The major characteristic of arteries is that they contain muscle and an elastic material called elastin in their wall, which means that they have the capacity to contract and expand.
This means that arteries can alter the flow of blood to an area of the body by altering in width.
The blood in arteries is under high pressure as it has just been pumped from the heart. A cut artery will spurt blood in pulses. It is at arteries and not veins where the acupuncturist can sense the state of Blood and Qi when "taking the pulse".
One major characteristic of veins is that they contain valves. This is because the blood within them is under much lower pressure, and the valves ensure that the flow of blood back to the heart remains one-way. Veins rely on the pumping action of the bodily muscles to aid in the movement of blood back to the heart, which is partly why sedentary people can start accumulating fluid in their legs.
Varicose veins is a common condition which results from damage to the structure of the valves in the veins.
Capillaries are minute vessels, the walls of which are only one cell thick. The major characteristics of capillaries are that they enable the rapid transfer of nutrients, gases and waste products to and from the blood across their very thin walls.
In inflammation the gaps between the cells in the capillary wall widen, making the capillaries leaky.