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Figure

Oxygen diffuses from the air within the alveolus into the capillary, whereas carbon dioxide diffuses from the blood within the capillary into the alveolus.

Blood vessel Capillary Alveolus

Blood vessel Capillary Alveolus

Figure

Scanning electron micrograph of casts of alveoli and associated capillary networks. These casts were prepared by filling the alveoli and blood vessels with resin and later removing the soft tissues by digestion, leaving only the resin casts (420x).

Tissues and Organs: A Text-Atlas of Scanning Electron Microscopy, by Richard D. Kessel and Randy Kardon. © 1979 W. H. Freeman and Company.

Alveolus

Alveolus

Bronchiole

Figure

Scanning electron micrograph of lung alveoli and a bronchiole (70x).

300 million alveoli in the human lung were spread out, they would cover an area of between 70 and 80 square meters—nearly half the size of a tennis court.

During gas exchange, oxygen diffuses through the alveolar walls and enters the blood in nearby capillaries. Carbon dioxide diffuses from the blood through these walls and enters the alveoli (figs. 19.17 and 19.18).

Lungs

The lungs are soft, spongy, cone-shaped organs located in the thoracic cavity. The right and left lungs are separated medially by the heart and the mediastinum, and they are enclosed by the diaphragm and the thoracic cage (see figs. 1.7, 19.19 and reference plates 56, 57, and 71).

In the inherited illness cystic fibrosis, airways become clogged with thick, sticky mucus, which attracts bacteria. As damaged white blood cells accumulate at the infection site, their DNA may leak out and further clog the area. A treatment that moderately eases breathing is deoxyribonuclease (DNase), an enzyme normally found in the body that degrades the accumulating extracellular DNA.

What is the function of the cartilaginous rings in the tracheal wall?

How do the right and left bronchi differ in structure?

List the branches of the bronchial tree.

Describe the changes in structure that occur in the respiratory tubes as their diameters decrease.

How are gases exchanged in the alveoli?

Several techniques enable a person who has stopped breathing to survive. In artificial respiration, a person blows into the mouth of a person who has stopped breathing. The oxygen in the rescuer's exhaled breath can keep the victim alive.

In extracorporeal membrane oxygenation, blood is pumped out of the body and across a gas-permeable membrane that adds oxygen and removes carbon dioxide, simulating lung function. Such a device can keep a person alive until he or she recovers from other problems, but is too costly and cumbersome to maintain life indefinitely.

A lung assist device, called an intravascular oxygena-tor, consists of hundreds of tiny porous hair-thin fibers surgically implanted in the inferior vena cava. Here, de-oxygenated blood returning to the heart receives oxygen and is rid of carbon dioxide — but only at about 30% the capacity of a healthy respiratory system.

Thyroid cartilage Cricoid cartilage

Clavicle

Scapula

Superior lobe of right lung

Middle lobe of right lung

Inferior lobe of right lung

Rib cartilage

Thyroid cartilage Cricoid cartilage

Clavicle

Scapula

Superior lobe of right lung

Middle lobe of right lung

Inferior lobe of right lung

Rib cartilage

Trachea

Superior lobe of left lung

— Inferior lobe of left lung

Figure 19.19

Locations of the lungs within the thoracic cavity.

Trachea

Superior lobe of left lung

— Inferior lobe of left lung

Figure 19.19

Locations of the lungs within the thoracic cavity.

Each lung occupies most of the thoracic space on its side and is suspended in the cavity by a bronchus and some large blood vessels. These tubular structures enter the lung on its medial surface through a region called the hilus. A layer of serous membrane, the visceral pleura, is firmly attached to the surface of each lung, and this membrane folds back at the hilus to become the parietal pleura. The parietal pleura, in turn, forms part of the mediastinum and lines the inner wall of the thoracic cavity (fig. 19.20).

There is no significant space between the visceral and parietal pleurae, since they are essentially in contact with each other. The potential space between them, the pleural cavity (ploo'ral kav'i-te), contains only a thin film of serous fluid that lubricates the adjacent pleural surfaces, reducing friction as they move against one another during breathing. This fluid also helps hold the pleural membranes together.

The right lung is larger than the left lung, and it is divided by fissures into three parts, called the superior, middle, and inferior lobes. The left lung is similarly di vided and consists of two parts, a superior and an inferior lobe.

A lobar bronchus of the bronchial tree supplies each lobe. A lobe also has connections to blood and lymphatic vessels and is enclosed by connective tissues. Connective tissue further subdivides a lobe into lobules, each of which contains terminal bronchioles together with their alveolar ducts, alveolar sacs, alveoli, nerves, and associated blood and lymphatic vessels.

Table 19.1 summarizes the characteristics of the major parts of the respiratory system. Clinical Application 19.2 considers substances that irritate the lungs.

H Where are the lungs located?

^9 What is the function of the serous fluid within the pleural cavity?

^9 How does the structure of the right lung differ from that of the left lung?

□ What kinds of structures make up a lung?

Shier-Butler-Lewis: I V. Absorption and I 19. Respiratory System I I © The McGraw-Hill

Human Anatomy and Excretion Companies, 2001

Physiology, Ninth Edition

Right lung

Pericardium Pleura

Left lung

Visceral pleura

Parietal pleura

Figure 19.20

The potential spaces between the pleural membranes called the left and right pleural cavities are shown here as actual spaces.

Visceral pleura

Parietal pleura

Right pleural Pericardial Heart cavity cavity

Left pleural cavity

Figure 19.20

The potential spaces between the pleural membranes called the left and right pleural cavities are shown here as actual spaces.

Part

Description

Function

Nose

Part of face centered above the mouth and inferior to the space between the eyes

Nostrils provide entrance to nasal cavity; internal hairs begin to filter incoming air

Nasal cavity

Hollow space behind nose

Conducts air to pharynx; mucous lining filters, warms, and moistens incoming air

Sinuses

Hollow spaces in various bones of the skull

Reduce weight of the skull; serve as resonant chambers

Pharynx

Chamber posterior to the oral cavity and between the nasal cavity and larynx

Passageway for air moving from nasal cavity to larynx and for food moving from oral cavity to esophagus

Larynx

Enlargement at the top of the trachea

Passageway for air; prevents foreign objects from entering trachea; houses vocal cords

Trachea

Flexible tube that connects larynx with bronchial tree

Passageway for air; mucous lining continues to filter air

Bronchial tree

Branched tubes that lead from the trachea to the alveoli

Conducts air to the alveoli; mucous lining continues to filter incoming air

Lungs

Soft, cone-shaped organs that occupy a large portion of the thoracic cavity

Contain the air passages, alveoli, blood vessels, connective tissues, lymphatic vessels, and nerves of the lower respiratory tract

^ Breathing Mechanism

Breathing, which is also called ventilation, is the movement of air from outside the body into the bronchial tree and alveoli, followed by a reversal of this air movement. The actions responsible for these air movements are termed inspiration (in"spi-ra'shun) or inhalation and expiration (ek"spi-ra'shun) or exhalation.

Inspiration

Atmospheric pressure due to the weight of the air is the force that moves air into the lungs. At sea level, this pressure is sufficient to support a column of mercury about 760 millimeters high in a tube. Thus, normal air pressure equals 760 millimeters (mm) of mercury (Hg). (Other units are in common usage: 760 mm Hg = 760 Torr = 1 atmosphere.)

Lung Irritants

The lungs are exquisitely sensitive to the presence of inhaled particles. Such exposures can cause a variety of symptoms, both acute and chronic, that range from a persistent cough to cancer.

Asbestos

Asbestos, a naturally occurring mineral, was once widely used in buildings and on various products because it resists burning and chemical damage. Asbestos easily crumbles into fibers, which, when airborne, can enter human respiratory passages. Asbestos-related problems include

• asbestosis (shortness of breath resulting from scars in lungs)

• mesothelioma (a rare cancer of the pleural membrane)

Asbestos fibers that are longer than 5 micrometers (0.0002 inch) and thinner than 2 micrometers (0.00008 inch) can cause illness when inhaled. Table 19A indicates how risk of becoming ill rises with duration of exposure.

Although asbestos clearly causes respiratory illness, it does so only if it is disturbed so that fibers break free and become airborne. Experts must determine whether it is safer to encapsulate asbestos in a building and leave it in place or remove it. Often, removing asbestos is actually more dangerous because this releases fibers. Today, synthetic fiberglass or plastics stand in for asbestos.

Berylliosis

Beryllium is an element used in fluorescent powders, metal alloys, and in the nuclear power industry. A small percentage of workers exposed to beryllium dust or vapor develop an immune response, which damages the lungs. Symptoms include cough, shortness of breath, fatigue, loss of appetite, and weight loss. Fevers and night sweats indicate the role of the immune system. Radiographs show granuloma scars in the lungs. Pulmonary function tests and simply listening to breath sounds with a stethoscope reveal impaired breathing.

Symptoms of berylliosis typically begin about a decade after the first exposure, but this response time can range from several months to as long as forty years. Many people who worked with the element in the Rocky Flats plant in Colorado have developed berylliosis, and they are being monitored at the National Jewish Medical and Research Center in Denver. Affected individuals include those who directly contacted the element frequently, as well as support staff such as secretaries who probably inhaled beryllium.

Berylliosis can be distinguished from other lung ailments with a blood test that detects antibodies to beryllium. Affected individuals and those who do not have symptoms but know that they were exposed to beryllium are advised to have periodic blood tests and chest radiographs to detect the condition early. The steroid drug pred-nisone is used to control symptoms.

A Disorder with Many Names

Repeatedly inhaling dust of organic origin can cause a lung irritation called extrinsic allergic alveolitis. An acute form of this reaction impairs breathing and causes a fever a few hours after encountering dust. In the chronic form, lung changes occur gradually over several years. The condition is associated with several occupations and has a variety of colorful names:

Bathtub refinisher's lung Bird breeder disease Cheese worker's lung Enzyme detergent sensitivity Farmer lung

Laboratory technician's lung Maltworker lung Maple bark stripper disease Mushroom picker disease Snuff taker's lung Plastic worker's lung Poultry raiser disease Wheat weevil disease ■

Situation

Level of Exposure (fibers/cubic centimeter)

Cancer Cases per Million Exposed People

Asbestos workers with

10 fibers/cc

200,000

20 years' exposure

Essentials of Human Physiology

Essentials of Human Physiology

This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.

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