Box Extension 25.2

An Incompletely Divided Central Circulation Can Be an Advantage for Intermittent Breathers

Intermittent breathing is common in lungfish, amphibians, lizards, snakes, and turtles—groups in which the heart is incompletely divided. It is also common in crocodilians, in which the central circulation outside the heart is incompletely divided. An intermittent breather, having ventilated its lungs with air, holds its breath for a substantial time before ventilating again (see page 607). Intermittent breathing presents both opportunities and challenges because the lungs vary from time to time in how effectively they are able to oxygenate the blood. Immediately after an animal has taken a series of breaths, the air in the lungs is rich in O2, and blood flowing through the lungs can become well oxygenated. However, after a long interval of apnea (cessation of breathing), the air in the lungs may be depleted of O2, and little opportunity may exist for blood flowing through the lungs to gain O2.

A great deal of research has convincingly demonstrated that most or all air-breathing vertebrates with incompletely divided central circulations in fact modulate blood flow to their lungs independently of flow to the rest of the body—at least to some degree and under certain circumstances. They increase blood flow to the lungs immediately after each period of breathing, whereas they decrease it toward the end of each period of apnea. In doing so, they achieve ventilation–perfusion matching to some degree. That is, they achieve functionally efficient matching of air flow and blood flow to the lungs. These topics are discussed further in Box Extension 25.2.

Mammals and birds cannot vary their lung blood flow without varying blood flow throughout their bodies: Because their pulmonary and systemic circuits are anatomically connected in series, the rate of blood flow to their lungs must always be identical to the rate of flow to their systemic tissues. However, animals with incompletely divided hearts—or with central circulations that are incompletely divided in other ways—have flexibility in this regard. They can potentially redistribute their total blood flow along alternative paths through the central circulation so that the amount of blood pumped to the lungs can be increased or decreased relatively independently of flow to the systemic tissues. Blood flow to the lungs can be increased when the lung air is rich in O2 and decreased when the lung air is O2-depleted even as—all the while—blood is circulated steadily, round and round, in the systemic circuit. Redistribution of total blood flow in this way could have advantages from the viewpoint of ventilation–perfusion matching, an important tenet in the study of circulatory and respiratory physiology.

According to the tenet of ventilation–perfusion matching, the ability of the circulatory system to perfuse the breathing organ—and thus carry O2 away—should ideally be closely matched to the ability of the breathing system to ventilate the breathing organ with O2 from the environment. In that way, excess effort is not invested in perfusion relative to the O2 made available by ventilation; and conversely, excess effort is not invested in ventilation relative to the ability of perfusion to transport O2 from the lungs to the rest of the body. The study of ventilation–perfusion matching is the study of whether and how the breathing and circulatory systems function together to approximate (or not approximate) this ideal.

Circulation in Protopterus lungfish exemplifies these concepts. The circulatory physiology of these lungfish changes markedly in synchrony with their breathing cycle. Immediately after a lungfish has come to the water’s surface and ventilated its lungs with air, circulatory adjustments take place that favor rapid uptake of O2 from the lungs and efficient distribution of the O2 to the body. One key adjustment is a fourfold increase in lung blood flow. Although this increase in blood flow is achieved in part by an increase in cardiac output, it also results from the redistribution of the cardiac output by changes in the pattern of flow through the heart and the rest of the central circulation. The percentage of the heart’s total outflow directed to the pulmonary arteries, although as low as 20% just before ventilation, may rise to as high as 70% just after ventilation. Then, during the ensuing apnea, it falls back to 20% again. In this way, the lungfish achieve a significant degree of ventilation–perfusion matching.

Copyright 2016 Sinauer Associates
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