Clinical Note

Narrow Pulse Pressure

Patients with a very narrow pulse pressure are a cause for concern. Equation 20 indicates that a narrow pulse pressure could result from either a reduced stroke volume or increased compliance of the aorta. Reduced stroke volume can result from inadequate blood volume as might occur after hemorrhage or from a failing heart.

A common cause of increased aortic compliance is the presence of a distended region of the aorta, called an aneurysm. Aneurisms tend to be unstable because, once distended, the wall tension further increases because of the law of LaPlace, putting the patient at risk of a catastrophic rupture of the aorta.

velocity is only approximately 0.017 cm/sec in the capillaries. This low velocity allows time for exchange of oxygen, carbon dioxide, nutrients, and waste to take place between the tissues and the blood in the capillaries. If the kinetic energy component for the flow in the root of the aorta is calculated from the above figures, we arrive at only 0.028 mm Hg. The calculation assumed that the cardiac output was delivered over the entire cardiac cycle. Actually peak velocity during the rapid ejection period exceeds 50 cm/sec, which would yield a peak kinetic component of 0.54 mm Hg.

The heart and blood vessels contain approximately 5000 mL of blood, approximately 4000 mL in the systemic vascular system and 1000 mL in the pulmonary system at any instant. The distribution of blood among the various vessel types in both systems depends on the side pressures existing in the vessels and the vessel capacitances at any instant. As it turns out, the large capacitance and unstressed volume of the veins causes most of the blood volume to be found in the veins (Fig. 22).

Because of their large blood volume, the veins serve an important reservoir function in the body. Smooth muscle in the walls of the veins allows them to change their compliance as needed. As this compliance changes, so does venous pressure, which is the filling pressure for the heart. Modulation of venous compliance is an important mechanism for controlling cardiac output. Stimulating the sympathetic nerves to the veins causes smooth muscle in their walls to contract, which raises the venous pressure. As we will see in Chapter 13, filling of the heart is determined by the venous pressure. The more the heart fills, the more it will eject with every beat. Thus, the body can easily increase the cardiac output by simply constricting the veins.

FIGURE 22 At any instant, the volume of blood in the systemic circulation is distributed as indicated. Owing to their large unstressed volume and capacitance, the veins contain most of the blood, even though venous pressures are low.
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