Principles of Pulmonary Medicine E-Book

Steven E. Weinberger, Barbara A. Cockrill, Jess Mandel

• Éditeur: Elsevier
• Année: 2017
• ISBN:
  9780323523721
• Édition: 7
• Sujet: Medicine
• Sous-sujet: Pulmonary & Thoracic Medicine

With an emphasis on the pathophysiologic basis of pulmonary disease, Principles of Pulmonary Medicine, 7th Edition, by Drs. Steven E Weinberger, Barbara A Cockrill, and Jess Mandel, provides a superbly illustrated introduction to this fast-changing field. This essential text employs a concise and understandable approach, integrating clinical topics with underlying physiologic, pathophysiologic, and basic science concepts critical for medical students, trainees, and those looking for a practical update on both acute and chronic pulmonary diseases

1

Pulmonary Anatomy and Physiology

The Basics

Abstract

Chapter 1 provides the fundamentals of pulmonary anatomy and physiology that are critical for understanding both the normal functioning of the lung and the mechanisms for its disruption in the various disease states discussed throughout the book. After a presentation of basic lung anatomy, the relationships between pressure and volume within various compartments of the thorax are explored. An extensive discussion of how ventilation and circulation interact in oxygen uptake and carbon dioxide elimination by the lungs is followed by an explanation of how the relationship between ventilation and perfusion affects gas exchange. The chapter concludes with an explanation of the mechanisms behind abnormal gas exchange resulting in hypoxemia or hypercapnia.

Keywords

Pulmonary anatomy; Lung mechanics; Ventilation-perfusion relationships; Oxygen transport; Carbon dioxide transport; Hypoxemia; Hypercapnia

ANATOMY, 1

PHYSIOLOGY, 3

Mechanical Aspects of the Lungs and Chest Wall, 3

Ventilation, 5

Circulation, 7

Diffusion, 8

Oxygen Transport, 8

Carbon Dioxide Transport, 11

Ventilation-Perfusion Relationships, 12

ABNORMALITIES IN GAS EXCHANGE, 15

Hypoxemia, 15

Hypercapnia, 17

To be effective at gas exchange, the lungs cannot act in isolation. They must interact with the central nervous system (which provides the rhythmic drive to breathe), the diaphragm and muscular apparatus of the chest wall (which respond to signals from the central nervous system and act as a bellows for movement of air), and the circulatory system (which provides blood flow and thus gas transport between the tissues and lungs). The processes of oxygen uptake and carbon dioxide elimination by the lungs depend on proper functioning of all these systems, and a disturbance in any of them can result in clinically important abnormalities in gas exchange. This chapter begins with an initial overview of pulmonary anatomy, followed by a discussion of the mechanical properties of the lungs and chest wall and a consideration of some aspects of the contribution of the lungs and the circulatory system to gas exchange. Additional discussion of pulmonary and circulatory physiology is presented in Chapters 4, 8, and 12; neural, muscular, and chest wall interactions with the lungs are discussed further in Chapter 17.

Anatomy

It is appropriate when discussing the anatomy of the respiratory system to include the entire pathway for airflow from the mouth or nose down to the alveolar sacs. En route to the alveoli, gas flows through the oropharynx or nasopharynx, larynx, trachea, and finally a progressively arborizing system of bronchi and bronchioles of diminishing diameter (Fig. 1.1). The trachea divides at the carina into right and left mainstem bronchi, which branch into lobar bronchi (three on the right, two on the left), segmental bronchi, and an extensive system of subsegmental and smaller bronchi. These conducting airways divide approximately 15 to 20 times down to the level of terminal bronchioles, which are the smallest units that do not actually participate in gas exchange.

Figure 1.1 Schematic diagram of airway branching. LLL, Left lower lobe bronchus; LM, left mainstem bronchus; LUL, left upper lobe bronchus; RLL, right lower lobe bronchus; RM, right mainstem bronchus; RML, right middle lobe bronchus; RUL, right upper lobe bronchus; Tr, trachea.

Conducting airways include all airways down to the level of the terminal bronchioles.

Beyond the terminal bronchioles, further divisions include the respiratory bronchioles, alveolar ducts, and alveoli. From the respiratory bronchioles on, these divisions form the portion of the lung involved in gas exchange and constitute the terminal respiratory unit or acinus. At this level, inhaled gas comes into contact with alveolar walls (septa), and pulmonary capillary blood loads O₂ and unloads CO₂ as it courses through the septa.

The acinus includes structures distal to a terminal bronchiole: respiratory bronchioles, alveolar ducts, and alveoli (alveolar sacs).

The surface area for gas exchange provided by the alveoli is enormous. It is estimated that the adult human lung has on the order of 300 million alveoli, with a total surface area approximately the size of a tennis court. This vast surface area of gas in contact with alveolar walls is a highly efficient mechanism for O₂ and CO₂ transfer between alveolar spaces and pulmonary capillary blood.

The pulmonary circulation and blood within provide the other crucial requirement for gas exchange: a transportation system for O₂ and CO₂ to and from other body tissues and organs. After blood arrives at the lungs via the pulmonary artery, it courses through a widely branching system of smaller pulmonary arteries and arterioles to the major locale for gas exchange, the pulmonary capillary network. The capillaries generally allow red blood cells to flow through in single file only, so that gas exchange between each cell and alveolar gas is facilitated. Upon completion of gas exchange and travel through the pulmonary capillary bed, the oxygenated blood flows through pulmonary venules and veins and arrives at the left side of the heart for pumping to the systemic circulation and distribution to the tissues.

Further details about the anatomy of airways, alveoli, and the pulmonary vasculature, particularly with regard to structure-function relationships and cellular anatomy, are given in Chapters 4, 8, and 12.

Physiology

Mechanical Aspects of the Lungs and Chest Wall

The discussion of pulmonary physiology begins with an introduction to a few concepts about the mechanical properties of the respiratory system, which have important implications for assessment of pulmonary function and its derangement in disease states.

Both the lungs and chest wall have elastic properties. Each has a particular resting size (or volume) it would assume if no internal or external pressure were exerted on it, and any deviation from this volume requires some additional influencing force. If the lungs were removed from the chest and no longer had the external influences of the chest wall and pleural space acting on them, they would collapse to the point of being almost airless; they would have a much lower volume than they have within the thoracic cage. To expand these lungs, positive pressure would have to be exerted on the air spaces, as could be done by putting positive pressure through the airway. (Similarly, a balloon is essentially airless unless positive pressure is exerted on the opening to distend the elastic wall and fill it with air.)

Alternatively, instead of po...