The heart and the blood vessels all have the same basic plan. They are all tubes. Each tube is specialised to perform functions additional to simply providing a channel through which blood will flow. The heart develops from a tube during embryogenesis. The walls of the tube have 3 layers.
The innermost layer consists primarily of an epithelium. This lines the tubes and provides the barrier which separates the lumen of the tube from the exterior environment. Its luminal surface prevents the blood clotting but if the epithelial lining is breached then clotting of the blood will occur. The epithelial layer is specialised to allow the passage across it of gasses, proteins and cells in a selective manner dependent on the location within the body.
The middle layer is primarily muscle. In the heart it is cardiac muscle. The co-ordinated contractions of the cardiac muscle in the heart wall generate a pumping mechanism for driving the blood around the blood vessels. In the blood vessels there is smooth muscle. Contraction of this muscle will change the diameter of the blood vessel and control the distribution of blood to the different regions of the body. Muscle contraction is under the control of the autonomic nervous system.
The outer layer is connective tissue. Generally, this is responsible for binding the tubes to surrounding structures.
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