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Unlike some animals, human females can have sex any time of the month, and they do not have to orgasm to ovulate or get pregnant.

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Anatomical differences

John Gray used this provocative title for his book to describe the fundamental psychological differences between the sexes. Many other controlled studies and brain scans demonstrate that men and women are physically and mentally different. The purpose of this physiology masterclass is to illustrate how sex-related differences are present in respiratory function and their possible clinical implications.

From the 26th to 36th weeks of gestation, female fetuses show a more mature phospholipid profile that reflects the production of surfactant. After birth, female neonates seem to be characterised by higher ratio of large to small airways.

They tend to have higher flow rates and specific airway conductance than males. This has been attributed to the role of surfactant in maintaining patency of the smaller airway [ 1 ]. Men are characterised by larger nasal cavities, and longer, narrower and higher nasal floors than females of the same body size. Such sexual dimorphism in the human skull influences the morphology of the upper airways [ 2 ].

Male average skeletal cranial airways are larger, with taller piriform apertures and, more consistently, taller internal nasal cavities and choanae than females [ 3 ]. Absolute retropalatal cross-sectional area is larger in males during both wakefulness and sleep, but when it is corrected for body surface area there is no sex difference.

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Men are characterised by larger neck circumference both as an absolute value and after correction for body mass index. Neck circumference is considered a surrogate index of pharyngeal soft tissue volume and fat distribution. Men have higher fat deposition at the level of the palate. These non-neuromuscular properties of the upper airways are important determinants of retropalatal compliance [ 4 ].

There are also sex-related differences in the pharynx in terms of size and resistance. The cross-sectional area is higher in men than in women [ 56 ]. Although age and percentage of ideal body weight are contributors, the strongest independent factor impacting on pharyngeal area is sex [ 6 ]. Differences also emerge dynamically with lung volume variations. The consequent changes in laryngeal area are larger in males both as absolute value and normalised for laryngeal size or expiratory reserve volume [ 6 ]. Males also demonstrate stronger volume dependence than females.

The percentage change in pharyngeal area between total lung capacity TLC and residual volume RVand also between functional residual capacity FRC and RV are ificantly higher in males than in females [ 56 ]. Both the pharyngeal resistance, in the segment of upper airway between the choanae and epiglottis, and the supraglottic resistance are higher in men than women [ 7 ]. As pharyngeal resistance is higher in men, we can speculate that they also presumably have lower patency.

The development of pharyngeal collapse and obstructive apnoea may be increased, explaining, at least in part, the male predominance in this syndrome [ 7 ]. By contrast, there is no sex effect on glottic cross-sectional area and epiglottal shape [ 89 ]. Glottic area depends on and changes with lung volume.

At any given lung volume, there is no difference in glottic area between men and women, and its reduction between TLC and RV is similar. However, this reduced glottic area among females occurs predominantly at low lung volumes, whereas it is more uniform throughout the vital capacity range in males [ 8 ]. Glottal closure in men is more complete, but briefer than in women [ 9 ].

The structure of the larynx shows ificant sex-related differences in all absolute dimensions. They are particularly pronounced in the sagittal diameters and in the thyroid angles, and only to a lesser extent in the transverse diameters. However, the relative proportions are much more constant and are not sex-specific [ 10 ]. These differences are related to growth. In fact, while no differences between the sexes are present during infancy, in the phase from puberty to maturity differentiation between males and females takes place both in terms of morphology and size, due to the different curves of the body which are shaped during maturation [ 11 ].

Tracheal area shows a good correlation with flows in women, but much less so in men [ 13 ]. However, the differences in tracheal and mainstem bronchi size between men and women disappear when airways measurements are standardised for lung size [ 14 ].

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Sex differences in lung growth and development start in the prenatal period. Lung maturation is more advanced in the female fetus. Between the 16th and 26th weeks of gestation, mouth movement starts, reflecting fetal breathing, and is considered a critical determinant for the development of the lung [ 1 ]. Other fundamental regulators of lung maturation are sex hormones, with androgens having mainly inhibitory effects and oestrogens stimulatory. Oestradiol is produced by the placenta, while testosterone is secreted also by fetal testes.

While androgens delay the surge of surfactant lipid production, oestrogens have positive effects on both the production of fetal surfactant and on the alveologenesis during neonatal and pubertal periods [ 1617 ]. The different impacts of androgens and oestrogens on the production of surfactant may be one of the reasons why premature female neonates are at lower risk They also have a better response than male neonates do to hormone accelerators of surfactant production.

As a result, premature males with respiratory distress syndrome show the highest incidence of morbidity [ 16 ]. At birth, females have smaller lungs than males with fewer respiratory bronchioles [ 1 ]. The sex-related differences in lung growth persist from childhood to adulthood.

The fact that men have bigger lungs than women have been shown using different approaches: standard morphometric methods [ 21 ], chest radiographs [ 22 ] and three-dimensional geometric morphometric methods on computed tomography scans [ 23 ]. However, the of alveoli per unit area, the of alveoli per unit area volume, individual lung units and alveolar dimensions do not differ between males and females.

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Because boys have bigger lungs per unit of stature, they have a larger total of alveoli and a larger alveolar surface area for a given age and stature [ 21 ]. The intrinsic elasticity of lung parenchyma is similar between sexes, whereas the recoil pressure differs because of the differences in lung size and in maximum distending forces [ 2425 ]. The shape of the lung differs between males and females, being more pyramidal in the former and more prismatic in the latter [ 23 ].

G reen et al.

In other words, large lungs are not necessarily associated with a larger airways size than in a person with smaller lungs. For the first time, they introduced the term dysanapsis from the Greek: dys meaning unequal and anaptixy meaning growth to indicate the disproportionately growing pattern between the constituent parts of an organ that allows normal physiological function of the whole [ 26 ]. The former is sensitive to airway size, the latter to lung size. He found that lung size and airway length are not associated [ 27 ].

As airways and lung dimensions are ificantly different between males and females, what about the relationship between their sizes? M ead [ 27 ] showed that females are characterised by smaller ratios at a given size than adult males of comparable age.

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Women, therefore, have smaller airways relative to lung size than men. He also showed that these sex differences develop late on in growth [ 27 ]. Similar were found based on a direct measurement of tracheal area using an acoustic reflection technique or chest radiograph [ 1314 ]. A different sex-related dysanaptic pattern emerged.

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It seems that in males the airways—parenchymal dysanapsis starts in childhood and persists into adulthood. By contrast, tracheal and lung volume grow proportionally during childhood in females, but then the airways start to grow faster than the lung and women show dysanapsis [ 13 ]. More recently, the dysanapsis ratio was found to be similar between the sexes [ 2528 ], but after correction for vital capacity, the were smaller in woman [ 25 ].

Dysanapsistherefore, strongly depend on the methods used to quantify airways and lung sizes and whether the data have been normalised for some parameter height, lung volume, lung recoil pressure etc. There is also the need to understand if there is a cut-off between physiological and pathological conditions [ 30 ], but this is not the purpose of the present masterclass.

Important sex differences are present in both volume and configuration of the ribcage.

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Women are characterised by a disproportionately smaller rib cage size than males [ 2231 ]. Specifically, the cross-sectional area, the internal anterior—posterior and the lateral diameters are lower at different lung volumes. The thoracic index i. At TLC, women show a rounder rib cage than men.

The different thoracic configuration in females is also evidenced by a different relationship between rib cage cross-sectional area and the height of the diaphragm dome [ 22 ]. These were obtained from chest radiographs [ 22 ], but higher ribcage dimensions in men were also found using other techniques. Opto-electronic systems for motion analysis, based on infrared TV cameras, reconstruct the geometry of the ribcage using the three-dimensional coordinates of external, passive, reflecting markers placed on anatomical points.

from this technique demonstrate the male ribcage is not only characterised by higher antero—posterior diameter, but also by larger perimeters, cross-sectional area and volume [ 32 ]. More recently, the use of semi-landmark methods on computed tomography reconstructions has allowed a more accurate morphometric quantification of the three-dimensional structure of the ribcage.

This method adds more details on the sex-related differences in ribcage size. Rib cages are wider in men, particularly at the caudal part, whereas the sternum is in a higher position in females [ 2 ]. This emerges both from quantification of the angle formed by the lower border of the sixth rib and the vertical on lateral films of chest radiographs and from the three-dimensional rib cage morphology using a semi-landmark approach for computed tomography reconstruction [ 2223334 ].

This difference may be a consequence of the different orientation of the spinous processes, which are more horizontal in females and more caudal in males.

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Such greater dorsal orientation of the transverse processes of men may reorient the ribs leading to greater radial ribcage diameters [ 35 ]. T orres- T amayo et al. The rib cage has the dual role of accommodating both lung and abdominal volume displacements, in particular when the abdomen is distended. The higher volume capacity of the rib cage of females in relationship to the size of their lungs, therefore, is suggested to be well suited to accommodate the increased abdominal distension caused by pregnancy.

In this way, the effects on lung function and abdominal pressure of the enlarging uterus may be minimised [ 2236 — 38 ]. The aforementioned sex-related differences in the ribcage ultimately also affect the chest wall, as shown by R omei et al. In general, all the dimensions of the chest wall are greater in males than in females.

Only the antero—posterior diameter and the volume of the abdomen are higher in men. For this reason, the chest wall differences can be only ascribed to the ribcage.