Simple circulatory routes are those where a single capillary bed, a source of resistance, is connected in series with (between) an artery and a vein (see Figure 7.02). The head, limbs, coronary circulation, one hepatic circulation, and most skeletal muscle circuits are of this type. Two capillary beds may be in series with one another; in the kidney blood first passes through the glomerular capillaries, then flows to the tubules where it passes through a second capillary bed. Total resistance in this case is the sum of the two individual resistances.
There are also more complicated series and parallel arrangements; e.g. in the portal system, the spleen and mesentery are in parallel and both in turn are in series with the liver. All of the above are left (heart) to right (heart) circuits. In contrast, the pulmonary circulation is a right to left circuit.
Some normal shunts also exist: In the Thebesian circulation blood percolates directly into the right and left ventricle lumina, bypassing vena caval-right atrial entry. In the bronchial circulation, blood serving a nutritive function perfuses non-alveolar regions of the lung, then returns to the heart via the pulmonary veins. Again the right heart is bypassed in this shunt.
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