Biomedical Engineering Reference
In-Depth Information
8.2.3 The Cardiac Output
The cardiac output is normally about 5 L/min but can triple during strenuous exercise.
This is the product of the stroke volume (the volume of a single output) and the heart rate.
These are nominally 70 cm
3
per stroke at 70 beats per minute (BPM).
As the heart rate increases, the total output increases proportionally until it reaches
about 200 BPM, when it does not have time to fill properly, so the maximum rate is limited
to about 15 L/min.
8.2.4 Pressure Regulation
The blood pressure (BP) in the aorta alternates between a high pressure (systole) of about
120 mm Hg and low pressure (diastole) of about 80 mm Hg. This lower limit is determined
by the elasticity of the ventricular walls. Blood pressure is reported as the systolic pressure
over the diastolic pressure, typically 120/80.
Pressure will obviously be a function of the flow rate and the resistance to flow, and
the body changes both the cardiac output (CO) and the resistance to maintain the required
level, as shown in Figure 8-5. The baroreceptor reflex makes short-term adjustments to
the blood pressure (BP) with a time constant of less than 1 minute. An example of this
is the increase in heart rate (HR) and stroke volume (SV) that occurs when you stand up
suddenly.
Pressure is measured by sensors in the arch of the aorta and the carotid sinus where
nerves convey the information to the nucleus tractus solitarius of the medulla oblongata
in the brain. Regulation is provided via the Vagus nerve, which slows the heart, and the
accelerator nerve, which speeds it up (Orme, 2002).
FIGURE 8-5
Short-term pressure
regulation. [Adapted
from (Orme 2002).]
