Nanomedicine, Volume I: Basic Capabilities
© 1999 Robert A. Freitas Jr. All Rights Reserved.
Robert A. Freitas Jr., Nanomedicine, Volume I: Basic Capabilities, Landes Bioscience, Georgetown, TX, 1999
18.104.22.168 Arteriovenous Macrocirculation
Topologically, the arteriovenous circulatory system resembles a "figure-eight" with a four-chambered heart positioned at the central junction. In the top half of the "figure-eight," oxygen-depleted blood is pumped from the heart to the lungs via the pulmonary artery. Oxygen-rich blood leaves the lungs and returns to the heart via the left and right pulmonary veins. In the bottom half of the "figure-eight," oxygen-rich blood received from the lungs is pumped into the aorta for distribution to the tissues, as shown in Figure 8.1. Oxygen-depleted blood is collected by the venous network (Fig. 8.2) and returns to the heart via the vena cava. (In both Figures, superficial vessels are drawn as solid lines and deep vessels as broken lines.) Most veins 1 mm in diameter or larger are fitted with a series of one-way valves to prevent backflow (Fig. 8.3). Such valves are most numerous in the lower extremities: the vena cava and the mesenteric, pulmonary and portal veins. Arteries have no valves. The smaller arteries are heavily anastomosed except across the midline of the body.
Table 8.1 provides a summary of the ~19,000 kilometers of human arteriovenous vasculature (mostly capillaries); additional data for a few selected major vessels is in Table 8.2. Singhal et al828 have carefully surveyed the complete branching structure of the pulmonary arterial network, spanning the entire range from principal artery to final capillary (Table 8.3). These data may provide a useful model for estimating capillary branching systems in other tissues, bearing in mind that pulmonary capillaries are generally wider and shorter than capillaries found elsewhere in the body, and that capillary bed geometries are unique to each organ (Section 22.214.171.124). The measured thickness of the glycocalyx of the endothelium of the systemic arteries and vena cava may also be utilized for navigational purposes. For example, the thickness of rabbit glycocalyx ranged from 45 ± 1 nm in the coronary artery to 81 ± 2 nm in the carotid; the glycocalyx was 20 ± 1.5 nm thicker on the downstream side of intercostal ostia than on the upstream side.3164
How much time does a nanorobot flowing with the blood take to transit individual organs and the entire vascular circuit? It is generally accepted that the roundtrip circulation time through the entire "figure-eight" averages ~60 seconds under resting conditions,398 a time that appears constant for nearly all mammals.362 During heavy exercise, the minimum circulation time may fall to 11-15 sec along most pathways primarily due to vasodilation.
Tissue transit times have been widely studied.839 For example, bloodborne particles in the lung normally take only 1-2 seconds to pass the alveolar sheet but 5-10 seconds to transit the entire pulmonary vasculature.361,780 Lung transit time is not significantly reduced if heart rate is simply increased;829 however, during strenuous exercise the alveolar transit time falls to 0.3 sec and the maximum lung transit time falls from 10 sec to 2 sec due to vasodilation. Organ transit times are dominated by capillary flow speeds which range from 0.20-1.50 mm/sec (Table 8.2). Given that capillaries average ~1 mm in length, minimum organ transit times are 0.7-5.0 seconds.
Last updated on 19 February 2003