CHAPTER 1

Anatomy and physiology of the humanrespiratory system

R.L. Johnson, Jr., & C.C.W. HsiaDepartment of Internal Medicine, University of Texas SouthwesternMedical Center, Dallas, Texas, USA.

Abstract

A brief review of the anatomy and fundamental physiologic concepts of the humanrespiratory system is presented, including postnatal lung growth and development,mechanical function of the airway and lung parenchyma, alveolar gas exchange,ventilation-perfusion and diffusion-perfusion relationships, alveolar microvascularrecruitment, pulmonary circulatory function and respiratory muscle energetics. Theemphasis is placed on the close interactions among different respiratory componentsand between the heart and the lung, which are essential for optimizing oxygentransport in health and disease.

1 Postnatal growth and development

1.1 Anatomy of the adult lung

In the right lung there are 3 lobes (upper, middle and lower), which are subdividedinto 9 sublobular segments. In the left lung there are 2 lobes (upper and lower), whichare subdivided into 8 sublobular segments. There are approximately 23 generationsof airways (Z) that distribute respired air to and from the smallest gas-exchangeunits in the lung (alveoli) [1], Fig. 1.

The path length and number of generations from trachea to different acini in thelung are not uniform owing to nonspherical asymmetry. Path lengths from tracheato acini can vary widely. Alveoli are small pockets that bud off of acinar airways,shaped like the cells of a beehive with a hexagonal mouth and a diameter of about150 m at birth.

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doi:10.2495/978-1-85312-944-5/01

2 Human Respiration

Figure 1: Diagram of the branching airways. Z designates the order of generationswith the trachea as zero, followed by bronchi (BR) and bronchioles (BL).On average the first 16 generations are conducting airways that do notparticipate in gas exchange. The last conducting airway is a terminalbronchiole (TBL), which empties into an acinus. In the acinus there are3 generations of partially alveolated respiratory bronchioles (RBL) fol-lowed by alveolar ducts (AD) that are completely alveolated in adults andempty into an alveolar sac (AS). From Weibel [1].

1.2 Postnatal development

According to Hislop et al [2] conducting airways at birth are miniature versions ofthose in the adult. No new generations are added after birth; only size and lengthchange during postnatal growth. This is not true of the acinus. Alveoli multiplyby adding more alveolar ducts as well as by lengthening of existing ducts andincreasing alveolization [3]. The number of alveoli continues to increase after birth(Fig. 2) [46], most rapidly in the first 2 years but continues up to age 5 to 8 years.

Based on these measurements about 2/3 of the increase in acinar volume duringpostnatal growth is from an increase in number of alveoli within the first 5 to 8 yearsand the remaining 1/3 from a steady increase in alveolar size that continues untilthe lung reaches adult size. Rates of increase in size of the thorax and of the lung

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Anatomy and Physiology of the Human Respiratory System 3

0

100

200

300

400

Num

ber

of A

lveo

li (

mill

ions

)

0 5 10 15 20 25

Years since birth

Adult

Figure 2: Number of alveoli increase about 15-fold after birth. Alveolar diameteralso increases linearly with time after birth until growth is complete [4];alveolar diameters approximately double, causing an approximate 8-foldincrease in volume.

must remain exactly matched throughout postnatal growth and there probably existmechanisms of feedback control between the lungs and thorax controlling this rate.

2 Conducting airways structure and function

2.1 Functional anatomy

2.1.1 Distribution of inspired airConducting air passages distribute respired air to and from lung acini where gasexchange occurs; they constitute what is called anatomical dead space that must bedisplaced during inspiration before fresh air reaches the acinus where gas exchangeoccurs. Upper airways consist of the nasopharynx, oropharynx and glottis. Belowthe glottis, Fig. 1, there is the trachea followed by an average of 16 generations ofdichotomously branching conducting airways down to terminal bronchioles, whichaverage approximately 0.06 cm in diameter. Terminal bronchioles empty into acinarairways where gas exchange occurs in alveolated airways. In each acinus there areapproximately 3 generations of respiratory bronchioles that are partially alveolated,and additional generations of alveolar ducts that are completely alveolated, finallyending in alveolar sacs, Fig. 2. Based on Weibels model of regular dichotomy[1] there are 65,000 terminal bronchioles in a 3/4 inflated lung to a volume of4800 ml minus the volume of conducting airways (150 ml); hence, the average sizeof individual acini would be about 0.072 cm3. These structures are made up of veryfine alveolar septa where circulating red blood cells are separated from alveolar airby a distance of less than a m [1].

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4 Human Respiration

2.1.2 Air conditioning and filtering inspired airAnother major function of conducting air passages is to humidify the inspiredair [7] and filter out small particulate matter to protect this thin gas-exchangesurface from dehydration and accumulation of inspired atmospheric debris [8, 9].Particulates with an aerodynamic diameter larger than 10 m are filtered out byinertial impact at airway branches and seldom reach an acinus; they are removedby retrograde ciliary transport in the conducting airways to be coughed out or swal-lowed. A fraction of smaller particulates may sediment out in the central portionof the acinus and be removed by scavenging macrophages or breathed out againwithout being deposited. There appears to be little difference between the adult andchilds lung in the efficiency of humidification and filtration of particulate matter.If anything, however, the childs lung may be a little more efficient than the adultlung [2].

2.2 Viscous resistance to flow in upper airways, lungs and thorax

In 1915 Fritz Rohrer [10] published a classic paper describing the quantitativeanatomy of the irregular dichotomous branching of airways down to subsegmen-tal airways 1 mm in diameter. These emptied into what he designated as a lobule,which, based on Weibels model [10], would be about 5 generations above the aci-nus. Between the carina of the trachea and lobules in different regions of the lunghe measured the numbers of generations, airway diameters and lengths, angles ofbranching and changes in cross-sectional area at each branch point.Airways dimen-sions in the lobules were generated by an equation for regular dichotomy derived byvon Recklinghausen [11]. Based on the anatomy of the branching airways systemand physics of flow in tubes, Rohrer derived the pressureflow relationships anddistribution of ventilation in human airways. Distribution was not uniform becausethe dichotomous branching was not uniform. Path length from the carina to thelobules varied from 8 cm in the central regions of the lung to 14 cm in the moreperipheral regions causing uneven distribution of resistance. The overall pressuredrop from nasopharynx to alveoli required to generate increasing flow could bereduced to a single comprehensive equation of the following form:

P = K1V + K2(V )2, (1)

where P is the pressure drop in cmH2O across the lung; V is gas flow in l/s; K1is a constant derived from the summation of laminar-flow resistances in branchingairways in cmH2O/(l/s) and K2 is a constant derived from changing resistancefrom turbulence and from inertance at airway branch points and at changes incross-sectional area of the airways; units are in cmH2O/(l/s)2. Results reported byRohrer for resistances across the upper airways, lower airways and total are givenin Table 1 [10].

Fifty years later K1 and K2 in eqn. (1) derived by Rohrer have been determinedexperimentally to describe viscous properties of lung airways, lung tissues andthorax, Table 2.

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Anatomy and Physiology of the Human Respiratory System 5

Table 1: Pressureflow relationships derived by Rohrer.

Upper airways P = 0.426 V + 0.714 (V )2Lower airways

Bronchial P = 0.106 V + 0.080 (V )2Lobular P = 0.258 V + 0.006 (V )2

Total P = 0.790 V + 0.800 (V )2

Table 2: Resistance to airflow in the respiratory system of an average normal personbreathing through the mouth or nose [12].

Mouth breathing Nasal breathing

Flow Laminar Turbulent Laminar Turbulent

Resistance K1 K2 K1 K2Units cmH2O/(l/s) cmH2O/(l/s)2 cmH2O/(l/s) cmH2O/(l/s)2

Air flow 1.2 0.3 1.8 3.0Lung tissue 0.2 0 0.2 0Chest wall 1.0 0 1.0 0Resp. system 2.4 0.3 3.0 3.0

Thus, to describe the pressure in cmH2O to generate a volume flow of V l/sthrough the mouth would be P = 2.4V + 0.3 [V ]2 and through the nose wouldbe P = 3.0V + 3.0[V ]2. Results are close to those originally described by Rohrer,particularly when resistances imposed by viscoelastic properties of the lung andthoracic tissues are removed, since these are not a part of Rohrers estimates [10].

2.3 Alveolar ventilation is not uniform even in the normal lung

Th