Certain pressure is always exerted on the walls of the arteries as the blood flows through them during systolic and diastolic phases of cardiac function. The pressure is highest during contraction of the heart and lowest when the heart is relaxing. Hence it is known as systolic and diastolic blood pressure respectively. An average blood pressure for a normal healthy adult is 120 mmHg (systolic) and 80 mmHg (diastolic). Age, sex, time of the day, posture, exercise, emotions, lack of oxygen and changes in the metabolism are some of the factors, which can influence the heart rate as well as the blood pressure.
CONTROL OF BLOOD PRESSURE
The carotid arteries in the upper neck region are divided into external and internal carotid arteries. At this bifurcation, wall of the artery is thinner, containing large number of nerve endings. This small portion of the artery is called the carotid sinus. These nerve endings are highly sensitive to stretch. Stretching of the wall of the artery is related to the pressure within the artery and hence the carotid sinus serve as a pressure receptor (baroreceptor). These nerve endings are connected with the cardiac center in the medulla oblongata by glossopharyngeal nerve. Aortic arch also possesses similar kind of nerve endings, which constitutes second important arterial baroreceptor. The afferent fibres from these receptors travel through the vagues nerve. An increase in the arterial pressure stimulates the carotid sinus and baroreceptors at the aortic arch. These impulses reach to the medulla oblongata through afferent nerves. The cardiac centers inhibit sympathetic activity and increase parasympathetic activity, which causes slowing down of the heart and dilation of the arteriols. As a result, the cardiac output is reduced and blood pressure returns to the normal level.