The arteries

VASCULATURE
Large blood vessels and those of the microvasculature branch frequently and undergo gradual transitions into structures with different histologic features and functions.
Elastic Arteries
Elastic arteries are the aorta, the pulmonary artery, and their largest branches; these large vessels are also called conducting arteries because their major role is to carry blood to smaller arteries. The most prominent feature of elastic arteries is the thick media in which elastic lamellae, each about 10 µm thick, alternate with layers of smooth muscle fibers. The adult aorta has about 50 elastic lamellae (more if the individual is hypertensive).
The intima: is well developed, with many smooth muscle cells in the subendothelial connective tissue, and often shows folds in cross section as a result of the loss of blood pressure and contraction of the vessel at death . The internal elastic lamina is not easily discerned because it is similar to the elastic laminae of the next layer . The adventitia is much thinner than the media. The numerous elastic laminae of these arteries contribute to their important function of making blood flow more uniform. During ventricular contraction (systole), blood is moved through the arteries forcefully and the elastin is stretched, distending the wall within the limit set by the wall´s collagen. When the ventricles relax (diastole), ventricular pressure drops to a low level, but the elastin rebounds passively, helping to maintain arterial pressure. The aortic and pulmonary valves prevent backflow of blood into the heart, so the rebound continues the blood flow away from the heart. Arterial blood pressure and blood velocity decrease and become less variable as the distance from the heart increases.
Arterial Sensory Structures:
Carotid sinuses are slight dilations of the bilateral internal carotid arteries where they branch from the (elastic) common carotid arteries; they act as important baroreceptors monitoring arterial blood pressure. At these sinuses the media is thinner, allowing greater distension when blood pressure rises, and the adventitia contains many sensory nerve endings from cranial nerve IX, the glossopharyngeal nerve. The brain’s vasomotor centers process these afferent impulses and adjust vasoconstriction, maintaining normal blood pressure. Functionally
similar baroreceptors are also present in the aortic arch.
Histologically more complex chemoreceptors that monitor blood levels of CO2 and O2, as well as its hydrogen ion concentration (pH), are found in the carotid bodies and aortic bodies, located in the walls of the carotid sinuses and aortic arch, respectively. These structures are parts of
the autonomic nervous system called paraganglia with rich capillary networks. The capillaries are closely surrounded by numerous, large, neural crest-derived glomus (type I) cells filled with dense-core vesicles containing dopamine, acetylcholine, and other neurotransmitters, which are supported by smaller satellite (type II) cells. Appropriate ion channels in the glomus cell membranes respond to stimuli in the arterial blood, primarily hypoxia (low O2), hypercapnia (excess CO2), or acidosis, by activating release of neurotransmitters. Sensory fibers branching from the glossopharyngeal nerve form synapses with the glomus cells and signal brain centers to initiate cardiovascular and respiratory adjustments that correct the condition.