The homeostatic control of sodium and water levels and of the blood pressure involves a complex humoral and neural mechanism. In the last 30 years several studies analyze the participation of the heart in this mechanism. The heart cells, mainly those that compose the atria complex produces a hormone denominated Atrial Natriuretic Factor (ANF), responsible for the increase of the diuresis with excretion of sodium and decrease of the arterial pressure. The understanding of the mechanisms involved in the control of the heart secretion has as one of the fundamental premises the meticulous knowledge of the morphology and function of the structures of the atrium-auricular complex. The aim of the present work is analyze the morphoquantitative aspects of the atria complex in young and adult guinea pigs of both sexes using mesoscopic, transmission and scanning electron microscopy, immuno and topocitochemistry and morphometric analysis. The myocardial and vascular architecture, endocardial surface miocytes ultra-structure, intercellular space permeability, subepicardial nervous plexus and the presence and location of ANF were studied.  Finally, the endocardial and epicardial surfaces were morphometrically compared. It was observed that the muscular arrangement in the atrium is formed by muscular bundles in a uniform organization, while in the auricles the muscular bundles are complex in organization. The arterial vascular arrangement in the atrium shows an elongated, uniform and dense network. The auricles present a dense and irregular network, with tortuous vessels with narrow spaces. The endocardial atria complex surface shows projections concerning to the nuclei of the endothelial cells. The intercellular limits are evident. It was also observed projections similar to microvili. The analysis of the eletronmicrographies demonstrated the presence of eletrondense granules located close to the perinuclear area, mitochondrias, Golgi complex and miofilaments. The intercellular spaces are permeable.
The subepicardial nervous plexus have a medium number of 846±201 neurons, with medium size of 322±10µm², distributed in ganglia with varied forms in specific areas of the posterior wall of the atrium. The relationship between external and internal surfaces is 1:1 in the atria and 1:2,6 in the auricles. In conclusion, the muscular and vascular structure, as well as the endocardial surface of the atria complex resembles the architecture of the glands with a dense vascular network associated to a complex internal surface of the auricles. Thus, the internal surface would increase the contact surface and consequently, its secretion capacity. We believed that these data are important, because it has been observed in researches related to the heart surgeries, an increasing interest concerning to the acute and chronic effects of the decrease in the liberation of ANF and consequent unbalance in the renal functions associated to the control of the corporal fluid, common in maze procedure and appendectomy.