3D analysis of sexual dimorphism in size, shape and breathing kinematics of human lungs

Abstract

Sexual dimorphism in the human respiratory system has been previously reported at the skeletal (cranial and thoracic) level, but also at the pulmonary level. Regarding lungs, foregoing studies have yielded sex-related differences in pulmonary size as well as lung shape details, but different methodological approaches have led to discrepant results on differences in respiratory patterns between males and females. The purpose of this study is to analyse sexual dimorphism in human lungs during forced respiration using 3D geometric morphometrics. Eighty computed tomographies (19 males and 21 females) were taken in maximal forced inspiration (FI) and expiration (FE), and 415 (semi)landmarks were digitized on 80 virtual lung models for the 3D quantification of pulmonary size, shape and kinematic differences. We found that males showed larger lungs than females (P < 0.05), and significantly greater size and shape differences between FI and FE. Morphologically, males have pyramidal lung geometry, with greater lower lung width when comparing with the apices, in contrast to the prismatic lung shape and similar widths at upper and lower lungs of females. Multivariate regression analyses confirmed the effect of sex on lung size (36.26%; P < 0.05) and on lung shape (7.23%; P < 0.05), and yielded two kinematic vectors with a small but statistically significant angle between them (13.22°; P < 0.05) that confirms sex-related differences in the respiratory patterns. Our 3D approach shows sexual dimorphism in human lungs likely due to a greater diaphragmatic action in males and a predominant intercostal muscle action in females during breathing. These size and shape differences would lead to different respiratory patterns between sexes, whose physiological implications need to be studied in future research.

Keywords: expiration; inspiration; landmark; lungs; semilandmark; shape; size.

Figure 1

Template of digitization. Landmarks (red), sliding curve semilandmarks (green) and surface semilandmarks (other colors) used to describe lung morphology (see Table 1 for the anatomical position); front view (A), right lateral view (B), axial view (C) and inferior view (D).

Figure 2

Lung mean forms of: females in forced inspiration (FI; A–C), females in forced expiration (FE; A′–C′), males in forced inspiration (FI; D–F) and males in forced expiration (FE;D′–E′). Frontal view (row 1), right lateral view (row 2) and inferior view (row 3).

Figure 3

Scatterplot showing the relationship of the different subjects studied at principal component (PC)1–PC2. Females in forced inspiration (FI; pink filled squares), females in forced expiration (FE; pink unfilled squares), males in FI (blue filled circles) and males in FE (light unfilled circles). Larger squares and circles represent the mean of each group.

Figure 4

Surface warps associated to principal component (PC)1. Grids were warped from overall lung mean shape to the lung shape at forced inspiration (FI; left: negative values of PC1), and to the lung shape at forced expiration (FE; right: positive values of PC1). Frontal view (row 1), right lateral view (row 2) and inferior view (row 3).

Figure 5

Surface warps associated to principal component (PC)2. Grids were warped from overall lung mean shape to the more pyramidal lung shape of males (left: negative values of PC2), and to the more prismatic lung shape of females (right: positive values of PC2). Frontal view (row 1), right lateral view (row 2) and inferior view (row 3).