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Blood Pressure



Blood Pressure - Anatomy and Physiology


Anatomy and Physiology
Blood pressure is highest in thick-walled arteries nearest the heart and lowest in veins which are thinner and away from the heart. A pressure of 100 mmHg in the brachial artery, close to the heart, means a force sufficient to push a column of Mercury up 100 millimeters in a manometer tube.

Regulation of Blood Pressure
Sympathetic fibers (efferents) from the vasomotor center in the medulla innervate arterioles. Arterioles regulate blood pressure. Normally they are in a state of partial constriction, or arteriolar tone. Several types of adrenergic receptors exist in the autonomic nervous system. Alpha, beta-1 and beta-2 are found on many organs. Alpha receptors are predominant in smooth muscle of arterioles. Stimulation of the sympathetic portion of the vasomotor center causes vasoconstriction and a rise in blood pressure; nonstimulation causes vasodilation and a fall in blood pressure. Factors affecting the vasomotor center and blood pressure are:

1. Cardiac status. In general, factors causing an increase and decrease in heart rate cause an increase and decrease in blood pressure. Tachycardia elevates blood pressure. Bradycardia or severe tachycardia lowers blood pressure because of decreased cardiac output.
2. Baroreceptors are nerve endings sensitive to pressure, or stretch. The more important are located near chemoreceptors at the arch of the aorta and at the bifurcation of carotid arteries. Afferents run to the vasomotor center via the glossopharygeal and vagus nerves. An increase in blood pressure stimulates the vagal portion of the vasomotor center in the brainstem, the blood pressure decreases and the heart rate slows. Baroreceptor stimulation by massaging the carotid artery area on one side (carotid massage) in the emergence department may convert supraventricular tachycardia to a normal sinus rhythm. In hypotension, baroreceptors are not stretched and sympathetic output is increased, causing alpha and beta-1 stimulation, vasoconstriction and tachycardia.
3. Other factors may act on the vasomotor center to increase or decrease blood pressure. Emotions play a role: nerve fibers from the cortex synapse in centers in the hypothalamus, and stimulation may cause an increase or decrease in blood pressure. Stimulation of the lateral spinothalamic tract (pain and temperature pathway to the brain) causes a rise in blood pressure. When oxygen is low and carbon dioxide is high, as in hypotension, chemoreceptors are activated. Chemoreceptor afferents pass with baroreceptor fibers to the vasomotor center, assisting in elevating the blood pressure. An increase in carbon dioxide causes vasodilation of the vessels, particularly in the brain and skin. A decrease causes vasoconstriction.

Normal Blood Pressure

In the average adult, a pressure greater than 160/90 mmHg is considered high (hypertension) and a pressure less than 90/60 mmHg low (hypotension). Blood pressure may be measured directly by arterial cannulation, or indirectly using a sphygmomanometer. Direct monitoring is performed on critically ill patients: an angiocatheter is inserted into the radial artery and connected to a pressure transducer/monitor. With the exception of severe hypotension, indirect monitoring is usually within 10 mmHg of direct monitoring.

The mean pressure, or mean arterial pressure (MAP) is the average of pressures recorded over a period of time (not an average of systolic and diastolic pressures). One-third of the pulse pressure is added to the diastolicpressure.

Example: what is the mean pressure of 140/80 mmHg?
The pulse pressure is 40 mmHg. 1/3 of 40 = 13, and 13 + 80 = 93.
Answer: the mean pressure is 93 mmHg.


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