1. Introduction
The application of the Second Law of Thermodynamics to living beings has its initial applications from the middle of the past century with two symbolical works proposed by Schrödinger [
1] and Prigogine and Wiane [
2]. These works open a new field of application of the Second Law of Thermodynamics, nowadays usually applied by means of the Exergy Analysis. As proposed by Oliveira [
3], one of the most interesting applications of the Second Law is understanding the functioning of living beings and use the physical property entropy generation (or destroyed exergy) as an indicator or diagnosis of pathology. This assessment tool was applied to a cancerous cell [
4], neuron under hypoxia [
5], human lungs [
6,
7], human heart [
8,
9,
10], athletes under different training level [
11,
12], hypothermia as a therapy [
13], orientated healing of burned patients [
14] and, the most common application, the analysis of thermal comfort conditions [
15,
16,
17] (comparison of the comfort indexes with destroyed exergy and exergy efficiency) systematized in [
18]. Furthermore, Ozilgen [
19] carried out an extensive review of this area, pointing out that this field has promising applications in several areas of interaction of Thermodynamics with living beings’ behavior.
Regarding the pollutant carbon monoxide (CO), it is considered to be a harmful gas since it bounds to hemoglobin, decreasing the blood capacity to deliver oxygen to tissues [
20]. Pregnant women can be poisoned by work-related sources like some industrial process (hot furnaces, gas refineries), or household sources like defective water heaters, or even passing through a tunnel in an urban environment. Smoking or being a passive smoker can also cause damage: a smoke-filled room has a carbon monoxide concentration as high as 100 ppm. This concentration is enough to cause a chronic poisoning, characterized by carboxyhemoglobin (HbCO) in the 5–25% range.
The bind between O
and hemoglobin (
) forms oxyhemoglobin (
). Each hemoglobin can carry four molecules of O
. The easiness of the reaction increases when the first oxygenation occurs. The gas CO is toxic because it has an affinity with the hemoglobin 200–300 times greater than O
. Hence, even relatively small concentrations of CO inspired can reduce the delivered oxygen to the tissues, causing hypoxia. Therefore, the toxicity of CO is related to the increase of carboxyhemoglobin (reaction of carbon monoxide with hemoglobin). The usual treatment of CO poisoning consists of reducing the inspired CO to zero and increasing the oxygen inspired, thus generating an increment in the gradients of these gases in the alveolar ventilation and increasing O
partial pressure in the blood [
7,
21].
The intoxication symptoms usually are headaches, sensations of weakness, dizziness, sleepiness, impaired physical performance, visual difficulties, palpitations, nausea, and vomiting [
22]. Effects in fetus depend on the pregnancy stage in which the poisoning occurs. During the embryonic stage, it can cause skeletal and neurological effects [
22]. Later in pregnancy, it is responsible for anoxic encephalopathy, which occurs when there is not enough O
delivered to the fetus’ brain. During gestation, oxygen and nutrients are provided to embryo through the placenta, which uses several transport mechanisms to accomplish this task: simple or facilitated diffusion, active transport, pinocytosis and phagocytosis. Transport of O
and CO
occurs mainly by simple diffusion [
23].
Herein, O
, CO
and CO transport are modeled to occur as a result of simple diffusion of the number of gases dissolved in maternal blood. However, some authors suggest that, for carbon monoxide, this mechanism could happen due to facilitated diffusion [
22]. Therefore, this work aims at modeling and expanding the phenomenological behavior of a healthy male human lung subjected to the inhalation of O
, CO
, and CO, first proposed by Albuquerque Neto et al. [
20]. Moreover, based on the exergy method proposed by Henriques et al. [
6] and Cenzi et al. [
7], the objective is to expand the phenomenological and exergy model to a pregnant woman subjected to conditions usually found in urban ecosystems and during stress periods such as passive smoking or prolonged exposure to a tunnel environment. The distinguishing feature of this article is the application of these conditions to a pregnant woman, with a phenomenological of the mother [
20] and fetus (proposed in the present article).
4. Conclusions
In the past decade, the exergy analysis of biological systems has been a focus of attention, considering that the destroyed exergy and exergy efficiency are quality indexes in an energy conversion process. These are used to estimate indexes of thermal comfort [
18], health care [
13,
14], and even for performance in sports [
9,
11,
12]. In this study, the effect of carbon monoxide intoxication in a pregnant woman was analyzed.
The phenomenological model of the mother [
20] and exergy models [
6,
7] were taken from literature. This article proposes a new phenomenological model of the placenta. Moreover, it has a distinguishing point, because it is the first attempt to use the respiratory model applied to the mother and fetus in energy and exergy basis.
In the analyses, the destroyed exergy rate for three models was calculated. The first model took into account the gases dissolved in the blood only as ideals. The second method associated the Gibbs free energy variation of the hemoglobin reaction with CO and O. Finally, the third model considered only irreversibilities related to gas transportation.
Regarding the mother’s exergy behavior of the lungs, for the first model, a decrease in the destroyed exergy as a function of the intoxication of CO was obtained. The second and third models had similar trends, where the destroyed exergy increases as a function of the CO poisoning. Moreover, the third model indicates the irreversibilities of the mass transfer, which corroborates the results of the second model.
It was also possible to conclude that the main effects in the fetus are caused by hypoxia due to the low oxygen pressures in the fetus’s blood. In this analysis, the HbCO concentration was found to be smaller in the fetus’s blood than in the mothers. The exergy efficiency of the maternal respiratory system, calculated from Model 1, is higher than the average male. This can be related to increased ventilation of the lungs during pregnancy. Moreover, Models 2 and 3 indicated a similar trend, which suggests that the main modifications in the lung are associated with the increase of irreversibilities of mass transfer.
The placenta destroyed exergy rate is significantly higher than the maternal, due to its high metabolism. The third analysis is useful in this case because it shows that the exergy destroyed rate increases, although to a small amount. This is the reason that makes the other two analyses appear without a slope.
Eventually, the destroyed exergy rate was correlated with an indicator often used in the medical area, which is the saturation of oxygen in the hemoglobin, showing that the exergy analysis may be a proper index to evaluate the human body in healthy and unhealthy scenarios. This type of study may be incorporated in the medical area first because the exergy analysis may give a complementary diagnosis of diseases. Second, with the phenomenological model, it is possible to suggest different scenarios of intoxication without the necessity of experiments in living beings. A limitation of this study is regarding the saturation curve of the mother subjected to some poisoning, which suffers a change; moreover, the two phenomenological models could be integrated to obtain a more extensive range of results of a pregnant woman.