Arterial Physiology

Overview

Arteries serve two major functions within the circulatory context. First, arteries must conduct blood to downstream vasculature without significant loss of blood pressure so that a sufficient blood pressure gradient exists to actuate blood through the remaining circulation. Secondly, arteries must modify the enormous fluctuations in blood pressure generated during the systolic and diastolic phases of cardiac contraction to a fairly uniform pressure, thus allowing continuous rather than pulsatile flow of blood through the remaining circulation. Because the volume of blood within the arteries is under large amounts of pressure, the arterial blood volume is also known as the "Stressed Volume" of blood.

Early Arterial Pressures

Early pressures in the arterial tree, especially in the aorta and its large branches are heavily influenced by the pumping of the heart. This is a product of two factors: 1) The large stroke volume of blood which is rapidly pushed into these vessels during cardiac systole, and 2) The rather low vascular compliance of arteries. Consequently, the large changes of blood volume in these low compliance vessels causes large swings in their intravascular pressure. This is manifested by the wide difference in intra-arterial pressure during the systolic and diastolic phases of cardiac contraction. The difference between these two values is known as the pulse pressure which can be clinically measured in certain accessible arteries.

Early arteries are especially suited for downstream transmission of blood pressure with little loss. This is both a product of their wide diameter, which thus offers little [rResistance]] to blood flow, and the large amount of elastin in their tunica media. The elastic nature of early arteries is critical because the volume of blood which can physically flow out of the arteries during the systolic phase is generally less than the stroke volume that is pushed into them. If arteries were not elastic, much of the energy of cardiac ejection would be lost as freshly ejected blood would simply hit a wall of previously-ejected blood and thus lose energy. Instead, the elastic nature of early arteries means that the kinetic energy that the heart exerts in rapidly pushing blood into the arteries is saved in the form of elastic arterial stretch. As the elastic arteries recoil during diastole, their potential energy is re-converted back into downstream kinetic blood flow.

Late Arterial Pressures

As blood flows through the arterial tree, the fluctuations of the pulse pressure begin to dampen and largely dissipate by the time capillaries are reached. This is a product of the large resistance offered by arterioles and their greater vascular compliance. When low resistance vessels ramify into high resistance vessels, clean fluctuations of pressure tend to degrade during the transition. Sadly, the explanation for this phenomenon of Fluid Mechanics is beyond the scope of this work. Additionally, the larger vascular compliances mean that similar changes in blood volume within the vascular segment result in lower amounts of pressure fluctuation.

Mean Arterial Pressure

The Mean Arterial Pressure is a theoretical average of the arterial pressure measured over the entire course of fluctuation between systolic and diastolic pressures in large arteries. Because more time is spent in diastole than in systole, the mean arterial pressure is closer to the diastolic pressure than the systolic pressure, rather than being a simple mathematical average of the two individual values

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