MEDICAL BIOLOGY, HUMAN ANATOMY, PHYSIOLOGY AND PATHOLOGY - Y.I. Fedoniuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 5. BLOOD AND LYMPH CIRCULATION AND ITS PATHOLOGY

PHYSIOLOGY OF BLOOD VESSELS

5.REGULATION OF BLOOD CIRCULATION

5.3.REFLEX REGULATION OF VASCULAR TONE

ARTERIAL Blood PRESSURE in a healthy Organism is maintained at a constant level. If, for some reason (such as psycho-emotional stress, physical exertion, etc.), blood pressure rises, it is rather quickly normalized through reflex action based on THE PRINCIPLE OF self-regulation.

Let us examine how this process occurs. The Vascular System contains reflexogenic zones—clusters of baroreceptors that respond to changes in arterial blood pressure levels. Particularly important reflexogenic zones are located in the aortic arch, the bifurcation area of the common carotid artery (the carotid sinus), and the junction where the venae cavae enter the right atrium. The reflex originating from the baroreceptors of the aortic arch is known as the Cyon-Ludwig reflex. When blood pressure in the aorta increases, its walls stretch, stimulating the baroreceptors. This generates an impulse that is transmitted from the baroreceptors via the depressor nerve to the Medulla Oblongata. Here, the tone of the Vagus nerve nuclei increases reflexively, while the tone of the vasoconstrictor center decreases. Subsequently, impulses traveling along the vagus nerve fibers reach The Heart, slowing its activity, while simultaneously causing vasodilation. As a result, arterial blood pressure drops.

Such reflex self-Regulation of Blood pressure occurs continuously and is of great physiological significance. It protects the body from sharp fluctuations in arterial pressure. The Cyon-Ludwig reflex is the primary mechanism regulating arterial blood pressure levels.

The reflexogenic zone of the carotid sinus holds the same physiological significance as the aortic arch zone. Because the carotid sinus baroreceptors are located at the entrance to the Brain, they mediate the reflex regulation of cerebral blood flow. The latter is characterized by remarkable stability despite significant fluctuations in central hemodynamics.

Vascular Reflexes can be triggered by stimulating not only the baroreceptors of reflexogenic zones but also chemoreceptors, which are highly sensitive to Changes in the blood gas composition. Such chemoreceptors are located in the aortic and carotid bodies. They are particularly sensitive to Hypoxia, elevated carbon dioxide concentrations, nicotine, and cyanides. Excitation from these receptors is transmitted via afferent nerves to the vasomotor center, increasing its tone. Consequently, Blood Vessels constrict and arterial blood pressure rises.

Excitation originating from the chemoreceptors of the aorta and carotid artery elicits pressor reflexes, whereas stimulation of the baroreceptors in these reflexogenic zones triggers depressor reflexes.