Arterial blood pressure is the lateral pressure exerted by a column of blood against the arterial wall. It is the result of cardiac output and peripheral vascular resistance. Blood pressure depends on the volume of blood ejected, its velocity, the distensibility of the arterial wall, the viscosity of the blood, and the pressure within the vessel after the last ejection.
Systolic blood pressure is the peak pressure in the arteries. It is regulated by the stroke volume and the compliance of the blood vessels. Diastolic blood pressure is the lowest pressure in the arteries and depends on peripheral resistance. The difference in the systolic and diastolic pressures is the pulse pressure. Systolic blood pressure in the legs is 15 to 20 mm Hg greater than in the arms, even while the individual is lying flat. This is in part related to Poiseuille's law, according to which the total resistance of vessels connected in parallel is greater than the resistance of a single large vessel. The blood pressure in the aorta is less than the blood pressure in the branched arteries of the lower extremities.
Blood pressure varies greatly, according to the patient's degree of excitement, degree of activity, smoking habits, pain, bladder distention, and dietary pattern. There is normally an inspiratory decline of up to 10 mm Hg in systolic blood pressure during quiet respiration.
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