The Controller: Directing the Orchestra

Learning Objectives

At this moment you are, hopefully, sitting in a comfortable chair, reading this book. Every 5 seconds or so, you take a breath. Why? Most people respond to that question by answering, "Because we all need oxygen to live." Although that statement is true, consider a second question before deciding if a need for oxygen really explains why we breathe. How low does your PaO2 decrease in the 5 seconds between breaths? The answer to that is, "Not very much," perhaps not even a measurable amount. So, if we do not breathe in response to hypoxemia, you might think it is because we are becoming hypercapnic; our PaCO2 must be increasing. Again, we ask you to consider another question, "How high does our carbon dioxide increase in the 5 seconds between breaths?" As you probably have guessed by now, the answer to that question is also, "Not very much." So why do we breathe?

The physiological control of the respiratory system is unique among organ systems. Because breathing is essential to life and must occur 24 hours a day, 365 days a year, whether you are awake or asleep, conscious or unconscious, there must be an automatic mechanism to determine the rate and depth of breathing on a minute-by-minute basis. Most enzymes and chemical reactions in the body operate best within a very narrow range of oxygen level and pH. Even short periods of absent breathing or breathing in excess of the metabolic needs of the body can lead to life-threatening derangements in the internal milieu. In this sense, the respiratory system is similar to the cardiovascular, endocrine, and gastrointestinal systems, which are all regulated without the need for conscious instructions.

At the same time, humans (and other mammals, for that matter) need to be able to temporarily interrupt the normal pattern of breathing to perform other functions such as vocalizing, eating, and lifting heavy objects. Imagine, for example, that you are just about to swallow some food and your inspiratory muscles are activated, thereby creating a negative intrathoracic pressure. Material in the posterior portion of the pharynx could be sucked down into the trachea and lungs, leading to obstruction of the airways and pneumonia. To prevent such mishaps and to use ventilatory muscles for other functions at times, we possess the ability to temporarily change our breathing patterns. To call for someone down the block, you can instantly take a deep breath and forcefully exhale to shout a loud “stop!” Alternatively, to swim under water, you can hold your breath, at least for a while, to avoid aspiration. This voluntary control of the respiratory system is unique—you certainly cannot make your heart double its rate or stop completely by willing it to happen, nor can you control peristalsis or consciously instruct your adrenal gland to secrete epinephrine.

In this chapter we explore the role of the cerebral cortex (the volitional component to the controller) and the brainstem (the automatic component) in breathing. In addition, we describe many receptors that, when stimulated, provide information to the brain and modify the output from both the cortex and the brainstem (Fig. 6-1). The chapter contains a moderate amount of detail about the receptors in the upper airways, lungs, and chest wall, not only because of their relevance for the physiology of ventilatory control but also because an understanding of these receptors is essential for our exploration of respiratory sensations and the symptom of "shortness of breath" that are pursued in Chapter 8.

The control of breathing is a very complex process, especially when the heart and lungs become diseased. Nevertheless, there are doctors you will encounter in the next few years who will tell you it is all about low oxygen or high carbon dioxide. We are confident that by the time you finish this chapter, you will understand and appreciate the beauty and complexity of the ventilatory controller.