Anurans of the genus Brachycephalus are among the smallest vertebrates in the world, due to an extreme process of miniaturization. As an example of this process, Brachycephalus species show loss of fingers, loss of the eardrum and middle ear, bone fusions, and the presence of paravertebral plates and parotic plaque. Thus, this study aimed to investigate if overall body small size has consequences to the cardiorespiratory system. We investigated, via dissections, individuals of four Brachycephaloidea species: Brachycephalus rotenbergae, B. pitanga, Eleutherodactylus johnstonei, and Ischnocnema parva. We observed that B. rotenbergae and B. pitanga present a reduction of the atrial septum and the absence of the carotid body. In Ischnocnema, despite being a member of the sister genus to Brachycephalus (both genera are Brachycephalidae), individuals present a heart with a complete septum and the presence of the carotid body; this is also observed in E. johnstonei (Eleutherodactylidae). We also observed that B. rotenbergae and B. pitanga have a thin skin, in which a 1-2 cells thick germ layer could be observed, and the lungs present little blood supply compared to individuals of the species E. johnstonei and I. parva. Based on the observed structures, we suggest that in species of Brachycephalus, respiration is performed mainly through the skin, and their lungs may have a reduced respiratory function.