Arteries and veins do not actively take part in the delivery of blood since they have no peristalsis. However, blood vessels can partly adjust their diameter. Vasodilation is the widening of the blood vessels, while vasoconstriction is a narrowing of the vessels by contracting the vascular smooth muscle. Vasodilation and vasoconstric- tionare not under conscious control, which are controlled by the autonomic nervous system. They are regulated by vasoconstrictors. These include a number of hormones, paracrine factors, and neurotransmitters from the nervous system. In addition, the most prominent vasodilator is NO. Vasodilation and vasoconstriction are also used antagonistically. Oxygen, which is the most vital nutrient carried by the blood, is bound to hemoglobin in red blood cells. In the arterial system, this is usually around 120 mmHg systolic and 80 mmHg diastolic. However, pressures in the venous system are constant and rarely exceed 10 mmHg.
ECs line the entire vascular system, from the heart to the smallest capillary. They form a one-cell-thick layer called the endothelium. Moreover, the ECs act as a semiselective barrier between the surrounding tissue and vessel lumen to transport white blood cells out of and into the bloodstream. Molecules can pass through the endothelium by diffusion and enter the surrounding tissues. In addition, waste products diffuse back into the blood.
The endothelium also plays a vital role in modulating blood flow. ECs provide an anticoagulant barrier between the vessel wall and blood. Perturbations disrupt these activities and promote ECs to create an antifibrinolytic and prothrombotic microenvironment. Secretion and uptake of vasoactive substances by the endothelium may also partly regulated blood flow .
ECs also have several other specialized functions. Firstly, ECs can produce nitric oxide (NO) and NO can bring down inflammation and prevent platelets from sticking to vessel walls as well as dilate blood vessels to increase blood flow. Secondly, the endothelium allows white blood cells to the infected tissue and increases blood flow. Thirdly, ECs prevent blood clotting from happening inside the blood vessel. Fourthly, ECs release clotting proteins like factor VII to help in blood clotting.
Vascular smooth muscle cells (VSMCs) are the main constitutive cells of the vascular, which are subjected to biochemical components and mechanical stress. They participate in all the pathological changes and the physiological functions. VSMCs withstand the high pressure of the circulating blood compartment and generate the ECM during development. Moreover, VSMCs are also took part in arterial repair after injury in adult life and participate in the regulation of blood pressure due to their contractile tonus. VSMCs respond to hypertension-induced increases by an increase in cell protein synthesis and ECM secretion .