Neurological and humoral control of blood pressure

Blood must be maintained under pressure to overcome the resistance offered by blood vessels, and thus ensure an adequate rate of flow to metabolizing tissues. If pressure is too low, the flow of blood cannot deliver sufficient oxygen; if it is too high, damage occurs to the blood vessels and organs. Hence, blood pressure is regulated around a ‘set point’. Pressure in the arterial system is regulated on a minute-to-minute basis by the autonomic nervous system and in the long term by a number of hormones that act on the kidney. High-pressure sensors (baroreceptors) are located in the carotid sinus and aortic arch, which monitor pressure generated by the beating heart. Afferent fibres of the ninth and tenth cranial nerves (glossopharyngeal and vagus, respectively) project into the cardiovascular control centre in the medulla oblongata. Parasympathetic vagal tone acts to slow heart rate and thus cardiac output, whereas sympathetic tone increases both force and rate of contraction, as well as stimulating vasoconstriction of blood vessels to increase resistance. Long-term regulation of blood pressure depends on the maintenance of blood volume. This is achieved by the combined actions of the renin–angiotensin system, aldosterone and vasopressin (antidiuretic hormone), which act on the kidney to promote retention of sodium and water. Blood volume is reduced by atrial natriuretic peptide, which causes diuresis and natriuresis. Together, the nervous and endocrine systems act to correct fluctuations in blood pressure and ensure that it is maintained at an appropriate level.