Organization of the Respiratory System

There are two lungs, the right and left, each divided into several lobes. Pulmonary is the adjective referring to "lungs." The lungs consist mainly of tiny air-containing sacs called alveoli (singular, alveolus), which number approximately 300 million in the adult. The alveoli are the sites of gas exchange with the blood. The airways are all the tubes through which air flows between the external environment and the alveoli.

Inspiration (inhalation) is the movement of air from the external environment through the airways into the alveoli during breathing. Expiration (exhalation) is movement in the opposite direction. An inspiration and an expiration constitute a respiratory cycle. During the entire respiratory cycle, the right ventricle of the heart pumps blood through the capillaries surrounding each alveolus. At rest, in a normal adult, approximately 4 L of fresh air enters and leaves the alveoli per minute, while 5 L of blood, the entire cardiac output, flows through the pulmonary capillaries. During heavy exercise, the air flow can increase twenty-fold, and the blood flow five- to sixfold.

The Airways and Blood Vessels

During inspiration air passes through either the nose (the most common site) or mouth into the pharynx (throat), a passage common to both air and food (Figure 15-1). The pharynx branches into two tubes, the esophagus through which food passes to the stomach, and the larynx, which is part of the airways. The larynx houses the vocal cords, two folds of elastic tissue stretched horizontally across its lumen. The flow of air past the vocal cords causes them to vibrate, producing sounds. The nose, mouth, pharynx, and larynx are termed the upper airways.

The larynx opens into a long tube, the trachea, which in turn branches into two bronchi (singular, bronchus), one of which enters each lung. Within the lungs, there are more than 20 generations of branchings, each resulting in narrower, shorter, and more numerous tubes, the names of which are summarized in Figure 15-2. The walls of the trachea and bronchi contain cartilage, which gives them their cylindrical shape and supports them. The first airway branches that no longer contain cartilage are termed bronchioles. Alveoli first begin to appear in respiratory bronchioles, attached to their walls. The number of alveoli increases

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

Pharynx

Right main bronchus

Right lung

Pharynx

Right main bronchus

Functions The Respiratory System

Nasal cavity

Nostril

Mouth Larynx

Left lung

Diaphragm

FIGURE 15-1

Organization of the respiratory system. The ribs have been removed in front, and the left lung is shown in a way that makes visible the airways within it. flf

Nasal cavity

Nostril

Mouth Larynx

Trachea

Left main broncus

Left lung

Diaphragm

FIGURE 15-1

Organization of the respiratory system. The ribs have been removed in front, and the left lung is shown in a way that makes visible the airways within it. flf

Branching Airways

FIGURE 15-2

Airway branching. %

in the alveolar ducts (Figure 15-2), and the airways then end in grapelike clusters consisting entirely of alveoli (Figure 15-3). The airways, like blood vessels, are surrounded by smooth muscle, the contraction or relaxation of which can alter airway radius.

The airways beyond the larynx can be divided into two zones: (1) The conducting zone extends from the top of the trachea to the beginning of the respiratory bronchioles; it contains no alveoli and there is no gas exchange with the blood (Table 15-2). (2) The respiratory zone, which extends from the respiratory bronchioles on down, contains alveoli and is the region where gases exchange with the blood.

The epithelial surfaces of the airways, to the end of the respiratory bronchioles, contain cilia that constantly beat toward the pharynx. They also contain glands and individual epithelial cells that secrete mucus. Particulate matter, such as dust contained in the inspired air, sticks to the mucus, which is continuously and slowly moved by the cilia to the pharynx and then swallowed. This mucus escalator is important in keeping the lungs clear of particulate matter and the many bacteria that enter the body on dust particles. Ciliary activity can be inhibited by many noxious agents. For example, smoking a single cigarette can immobilize the cilia for several hours.

The airway epithelium also secretes a watery fluid upon which the mucus can ride freely. The production of this fluid is impaired in the disease cystic fibrosis, the most common lethal genetic disease of Caucasians, and the mucous layer becomes thick and dehydrated, obstructing the airways. The impaired secretion is due to a defect in the chloride channels involved in the secretory process (Chapter 6).

A second protective mechanism against infection is provided by cells that are present in the airways and alveoli and are termed macrophages. These cells, described in detail in Chapter 20, engulf inhaled particles and bacteria, rendering them harmless. Macrophages, like cilia, are injured by cigarette smoke and air pollutants.

The pulmonary blood vessels generally accompany the airways and also undergo numerous branchings. The smallest of these vessels branch into networks of capillaries that richly supply the alveoli (Figure 15-3). As noted in Chapter 14, the pulmonary circulation has a very low resistance compared to the systemic circulation, and for this reason the pressures within all pulmonary blood vessels are low. Therefore, the diameters of the vessels are determined largely by gravitational forces and passive physical forces within

FIGURE 15-2

Airway branching. %

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

III. Coordinated Body Functions

15. Respiration

© The McGraw-Hill Companies, 2001

PART THREE Coordinated Body Functions

Trachea

Left pulmonary artery Left main bronchus Pulmonary veins Bronchiole

Heart

Alveoli Capillaries Relationship

Smooth muscle

Heart

Terminal bronchiole

Branch of pulmonary artery

Branch of pulmonary vein

Smooth muscle

Respiratory bronchiole

Alveoli

Capillary

FIGURE 15-3

Relationships between blood vessels and airways. (a) The lung appears transparent so that the relationships can be seen. The airways beyond the bronchiole are too small to be seen. (b) An enlargement of a small section of Figure 15-3a to show the continuation of the airways and the clusters of alveoli at their ends. Virtually the entire lung, not just the surface, consists of such clusters. %

TABLE 15-2 Functions of the Conducting Zone of the Airways

1. Provides a low-resistance pathway for air flow; resistance is physiologically regulated by changes in contraction of airway smooth muscle and by physical forces acting upon the airways.

2. Defends against microbes, toxic chemicals, and other foreign matter; cilia, mucus, and phagocytes perform this function.

3. Warms and moistens the air.

4. Phonates (vocal cords).

the lungs. The net result is that there are marked regional differences in blood flow. The significance of this phenomenon will be described in the section on ventilation-perfusion inequality.

Site of Gas Exchange: The Alveoli

The alveoli are tiny hollow sacs whose open ends are continuous with the lumens of the airways (Figure 15-4a). Typically, the air in two adjacent alveoli is separated by a single alveolar wall. Most of the air-facing surface(s) of the wall are lined by a continuous layer, one cell thick, of flat epithelial cells termed type I alveolar cells. Interspersed between these cells are thicker specialized cells termed type II alveolar cells (Figure 15-4b) that produce a detergent-like substance, surfactant, to be discussed below.

The alveolar walls contain capillaries and a very small interstitial space, which consists of interstitial fluid and a loose meshwork of connective tissue (Figure 15-4b). In many places, the interstitial space is absent altogether, and the basement membranes of the alveolar-surface epithelium and the capillary-wall en-dothelium fuse. Thus the blood within an alveolar-wall capillary is separated from the air within the alveolus by an extremely thin barrier (0.2 ^m, compared with

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

Respiration CHAPTER FIFTEEN

Respiration CHAPTER FIFTEEN

Capillaries

Respiratory bronchiole

Alveolus

Alveolar duct Alveolus

Alveolus pore

Alveolus

Alveolus

Capillary Alveolar air Type II cell Basement endothelium membrane

Erythrocyte

Interstitium

Plasma in capillary

Erythrocyte

Relation of the Lungs to the Thoracic (Chest) Wall

The lungs, like the heart, are situated in the thorax, the compartment of the body between the neck and abdomen. "Thorax" and "chest" are synonyms. The thorax is a closed compartment that is bounded at the neck by muscles and connective tissue and completely separated from the abdomen by a large, dome-shaped sheet of skeletal muscle, the diaphragm. The wall of the thorax is formed by the spinal column, the ribs, the breastbone (sternum), and several groups of muscles that run between the ribs (collectively termed the intercostal muscles). The thoracic wall also contains large amounts of elastic connective tissue.

Each lung is surrounded by a completely closed sac, the pleural sac, consisting of a thin sheet of cells called pleura. The two pleural sacs, one on each side of the midline, are completely separate from each other. The relationship between a lung and its pleural sac can be visualized by imagining what happens when you push a fist into a balloon (Figure 15-5): The arm represents the major bronchus leading to the lung, the fist is the lung, and the balloon is the pleural sac. The fist becomes coated by one surface of the balloon. In addition, the balloon is pushed back upon itself so that its opposite surfaces lie close together. Unlike the hand and balloon, however, the pleural surface coating the lung (the visceral pleura) is firmly attached to

Type I cell

Alveolar air

FIGURE 15-4

(a) Cross section through an area of the respiratory zone. There are 18 alveoli in this figure, only four of which are labelled. Two frequently share a common wall.

From R.O. Greep and L. Weiss, "Histology," 3d ed., McGraw-Hill New York, 1973.

(b) Schematic enlargement of a portion of an alveolar wall.

Adapted from Gong and Drage. ^

the 7-^m diameter of an average red blood cell). The total surface area of alveoli in contact with capillaries is roughly the size of a tennis court. This extensive area and the thinness of the barrier permit the rapid exchange of large quantities of oxygen and carbon dioxide by diffusion.

In some of the alveolar walls, there are pores that permit the flow of air between alveoli. This route can be very important when the airway leading to an alveolus is occluded by disease, since some air can still enter the alveolus by way of the pores between it and adjacent alveoli.

Intrapleural Sac Alveoli

Lung Heart

Intrapleural fluid Parietal pleura Visceral pleura

FIGURE 15-5

Relationship of lungs, pleura, and thoracic wall, shown as analogous to pushing a fist into a fluid-filled balloon. Note that there is no communication between the right and left intrapleural fluids. For purposes of illustration in this figure, the volume of intrapleural fluid is greatly exaggerated. It normally consists of an extremely thin layer of fluid between the pleura membrane lining the inner surface of the thoracic wall (the parietal pleura) and that lining the outer surface of the lungs (the visceral pleura).

Lung Heart

Intrapleural fluid Parietal pleura Visceral pleura

FIGURE 15-5

Relationship of lungs, pleura, and thoracic wall, shown as analogous to pushing a fist into a fluid-filled balloon. Note that there is no communication between the right and left intrapleural fluids. For purposes of illustration in this figure, the volume of intrapleural fluid is greatly exaggerated. It normally consists of an extremely thin layer of fluid between the pleura membrane lining the inner surface of the thoracic wall (the parietal pleura) and that lining the outer surface of the lungs (the visceral pleura).

PART THREE Coordinated Body Functions

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

PART THREE Coordinated Body Functions the lung by connective tissue. Similarly, the outer layer (the parietal pleura) is attached to and lines the interior thoracic wall and diaphragm. The two layers of pleura in each sac are so close to each other that normally they are always in virtual contact, but they are not attached to each other. Rather, they are separated by an extremely thin layer of intrapleural fluid, the total volume of which is only a few milliliters. The in-trapleural fluid totally surrounds the lungs and lubricates the pleural surfaces so that they can slide over each other during breathing. More important, as we shall see in the next section, changes in the hydrostatic pressure of the intrapleural fluid—the intrapleural pressure (Pip) (also termed intrathoracic pressure)— cause the lungs and thoracic wall to move in and out together during normal breathing.

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  • fosco
    What is the function of the left bronchus in the respiratory system?
    7 years ago
  • louise thompson
    What constitute a respiratory cycle?
    7 years ago
  • Sabrina
    Is constantly mooved by cilia to the pharynx?
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  • seredic t
    What substance produced by type 2 alveolar cells is detergent like?
    7 years ago
  • Speranza
    What fluid surrounds the lungs and its function?
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  • johanna
    What is part of the airway that has two folds?
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    How is the respiratory system organized in the body?
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  • Mike Eberhardt
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  • JOSEPH
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    Where is the outer pleura in respiratory system?
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    What is the function of the red blood cell with the alveolus?
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    What is the organisation of the respiratory system?
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