Introduction
The body is on an ongoing and continuous quest to maintain an environment characterized with equilibrium. Scientifically, this state is called homeostasis. It is the body’s internal mechanism that is meant to make the person survive, despite the numerous internal and external changes it may encounter. The body of a person who was acutely exposed to a colder than usual environment would, for example, react by preserving the heat that is currently stored in and generated by the body—something which can be done in a lot of ways such as, but may not be limited to vasoconstriction, shivering, and sweat prevention.
The body’s internal systems allow it to adapt to the changes to its external environment and sometimes, the body’s own internal system malfunctions. Without these homeostatic maintenance mechanisms, the body would simply succumb to the perils of the positive feedback mechanism where it would fail to counteract or balance all of the potentially harmful things and processes that the body may undertake or be subjected to, leading to the demise of the entire organism and in a larger scale, extinction. Needless to say, the body’s organs and organ systems work together in maintaining this homeostatic environment. In terms of function, the different structures in the body can be divided into two: sensory and effector organ. This functional division is based on the theoretical assumption that suggests that the body’s homeostatic maintenance ability behaves like a reflex arc.
A reflex is anything that can be considered as the body’s response to a perturbing stimulus which can either be internal or external. This happens involuntarily. A reflex happens because of the following components; receptor organ and the corresponding sensory neuron, integration center (often the spinal cord), and the effector organ with the corresponding motor neuron. Essentially, the body’s own homeostatic maintenance mechanism can be considered a reflex because it is the body’s own automatic response towards perturbations and it also happens involuntarily. So far, it satisfies all the fundamental definitions of a reflex. In this case, however, it would be important to focus more on the afferent and efferent components because these two hold the most relevance in this paper.
The objective of this paper is to discuss an internal body structure called carotid body in general, and discuss its functions in (including the effects of its stimulation on) the cardiovascular, endocrine and renal, and respiratory systems and also in patients with hypotension. Within the realm of anatomy, the carotid body may well be considered as a part of the cardiovascular system, which would make it logical to think that its functions are related to the cardiovascular system. This theoretical assumption about the carotid body is only true partially. This is because apart from being a component of the body’s cardiovascular system, it has been discovered that its function is much more than just being a component of the said system. For the purposes of this paper, however, the author only focused on its functions in the respiratory, endocrine and renal, and cardiovascular systems.
Carotid Body’s Stimulation’s Effects on the Respiratory System
The carotid body plays a role in detecting and therefore regulating the respiratory system. The body has the internal ability to detect hypoxia (a state where oxygen is lacking). This is done through chemoreceptors that have the ability to detect the level of certain hypoxia-relevant chemicals in the blood and tell the brain (through synapses) whether adjustments are needed. The two chemicals or elements that are most important for the regulation of homeostasis as far as the respiratory system is concerned are oxygen and carbon dioxide. Carbon dioxide is a byproduct of the body’s cell’s processes and so an ineffective respiratory system would often manifest with excessively high levels of it in the blood. Excessively high levels of carbon dioxide in the blood are considered toxic. Most chemoreceptors depend on their ability to detect carbon dioxide levels in the blood to prevent hypoxia. In the case of the carotid body, however, it depends on oxygen detection.
The carotid body’s chemoreception of hypoxia starts with the identification of the structure that there is a decrease in the partial pressure of oxygen in the blood (below normal and tolerable levels). This stimulus leads to the blockage of potassium channels in the Type I Glomus Cell or the chief glomus cells. This leads to the depolarization of the cells which then allows the entry of the calcium ions located outside the chief cells. The entry is accompanied with the delivery of the vesicles that contain the necessary neurotransmitters into the inside of the cell.
The neurotransmitters are then released into a synaptic type cleft between the glomus cells and the nerve ending where the neurotransmitters are then interpreted as a signal to start an action potential. Eventually, the signals that the carotid body delivers would be interpreted by the brain as a form of instruction that would tell it whether there is a need to increase or decrease oxygen levels in the blood. If the goal is to prevent hypoxia, the instruction would most likely be to improve or normalize respiratory function in a negative feedback scheme. Once the chemoreception signals that the brain receives from the carotid body normalizes (through its continuous sending of neurotransmitters to the postsynaptic receptors), the brain would stop sending signals to the efferent neurons to stimulate the effector organs (e.g. lungs).
In a study published in the Proceedings of the National Academic of Sciences of the United States of America, the researchers examined the relationship between defective carotid body functions and the presence of impairments in ventilation responses to chronic hypoxia. The researchers used mice that were diagnosed to have partial deficiencies in hypoxia inducible factor 1 alpha. Basically, what they wanted to know is whether the hypoxia inducible factor 1 alpha and therefore the carotid body are necessary to prevent and regulate hypoxia.
Typically, when the carotid bodies are exposed to or detect hypoxia (by the chemoreceptive mechanism that was explained earlier), an increase in the carotid body’s sinus activity can be experienced. Using histological methods and analyses, the researchers in the said study was able to discover that “no abnormalities of carotid body morphology in HIF1A mice; wild type mice exposed to hypoxia for 3 days manifested an augmented ventilation response to a subsequent acute hypoxic challenge; in contrast, prior chronic hypoxia resulted in a diminished ventilation response to acute hypoxia in HIF1A mice; thus, partial HIF1A deficient has a dramatic effect on carotid body neural activity and ventilation adaptation to chronic hypoxia” .
This study only confirms, but in a more specific manner, how the carotid body’s stimulation contributes to the regulation of normal respiratory system processes, particularly in the prevention of hypoxia. Other similar studies that have come up with essentially the same findings were also noted and reviewed. So far, the consensus is that carotid body function as a peripheral chemoreceptor is an important component of the body’s own hypoxia-prevention mechanism .
Carotid Body’s Stimulation’s Effects on the Endocrine and Renal System
The carotid body is also responsible for assisting the nervous system in the regulation of the body’s endocrine functions, at least based on theories. Specifically, it is proposed that it aids in sending signals to the brain on whether certain endocrine hormones and neurotransmitters are needed by the body in order for the cardiovascular and respiratory systems to fulfill their respective functions. The carotid body has been highly associated with the excretion of the neurotransmitters acetylcholine, dopamine, norepinephrine, and other homeostasis-relevant components such as adenosine triphosphate (the functional unity of energy that the cells use in various metabolic processes), peptides, and gamma amino butyric acid (GABA), among others.
In terms of actual endocrine functions, however, there is no concrete scientific and academic evidence that shows that the carotid body has an actual endocrine role; this is according to Heath’s (2016) study about the human carotid body . According to Heath (2016) much of the study about the carotid body has been focused on rats. Although humans share a lot of anatomic and physiologic qualities with rats, to say that the human carotid body would behave the same way as that of rats without having observed the reactions of the different rat-tested experiments on the human carotid body would not only be academically unverifiable but also unethical—one has to be able to test a hypothesis or a theory first before publishing it as a result. .
In terms of the renal functions, numerous studies converge on the notion that suggests that the chemoreceptive function of the carotid body is essential in maintain optimal renal function, particularly in renal hemodynamics , which again may be correlated with its main although indirect function which falls under that of the cardiovascular system. The renal system is responsible for the excretion of wastes and solutes that are not needed by the body and may be categorized as wastes. The carotid body, through its, cardiovascular function, helps maintain the integrity of the kidneys, the primary renal system organ, by ensuring a stable flow of blood. The kidney continuously filters the blood; it would not be able to do so without a continuous flow of blood and this is where the indirect cardiovascular function of the carotid body plays a major role on .
Carotid Body’s Stimulation’s Effects on the Cardiovascular System and Hypotension
The carotid body is a small cluster of cells that is located in the area adjacent to the carotid arteries. It is medially located in the area where the two carotid arteries’ fork or the location where they bifurcates into the left and right carotid arteries. Its cellular components are mainly chemoreceptors and supporting cells. It is composed of the glomus cells. Now, there are two types of glomus cells present in the carotid body and each has its own function. They are the Glomus Type I and Glomus Type II cells. The former is also known as the chief glomus cells while the latter as the sustentacular cells. Judging the organ based on its location, it would be logical to assume that one of the organ’s main functions is related to the adjacent structures such as the carotid arteries—therefore giving the carotid body a primarily cardiovascular function. The carotid area is the most vascularized area in the human body. The carotid arteries are among the biggest blood vessels in the body (although it is slightly smaller than the aorta); and the carotid bodies were placed in the common carotid artery bifurcation area for a reason.
The carotid body is a receptor, a sensor if one will. Its cardiovascular function is an indirect one. This means that it does not deal directly with other cardiovascular organs such as the heart and the blood vessels. Rather, it deals directly with the central nervous system, particularly the brain by performing a cardiovascular system-related sensory function. The blood vessels, as shown in the example earlier, constrict and dilate, depending on what the body needs in a certain situation.
Because it is the cardiovascular system being talked about primarily in this section, one metric or outcome that would come to mind would be blood pressure. When the blood vessels dilate, the body tends to experience a lower blood pressure because of the increase in the diameter of the vessels allowing for more blood to pass through each vessel, lessening the need to increase the flow of blood to sustain the oxygen and nutrient needs of the target organs. On the contrary, the blood pressure increases when the blood vessels constrict because the total volume of blood that each vessel can potentially deliver over the same period of time decreases, creating the need to increase blood pressure to meet the target organs’ needs.
The stimulation of the carotid body plays a major role in the maintenance of blood pressure. For starters, hypotension is a condition wherein the person’s blood pressure drops below the standard levels; hypertension is an abnormally high blood pressure based on the same standards. The carotid body, based on its location and its indirect cardiovascular function, can therefore be considered as a peripheral chemoreceptor organ. Unlike other receptor organs that have cardiovascular functions that rely on carbon dioxide concentrations to detect abnormalities, the carotid body is more dependent on oxygen levels. It is important to note, however, that it also has the ability to detect levels of carbon dioxide in the blood vessels but to a much lesser degree compared to oxygen.
The consensus among studies about the effects of the stimulation of the carotid body on cardiovascular system, particularly in hypotension, suggests that more active peripheral chemoreceptor organs tend to contribute to hypertension; and therefore less active peripheral chemoreceptor organs tend to lead to hypotension . In a study published in the Journal of Physiology, the researchers used this theory to formulate their working hypothesis.
Specifically, they suggested that heightened peripheral chemoreceptor sensitivity and activity contributes to the development and chronic appearance of hypertension—the confirmation of which reversely confirms the theory about hypotension as well. They focused on spontaneously hypertensive rats; specifically, in the intervention group, the researchers surgically resected the innervation of the carotid body so that it would stop its sensory and regulatory function. Therefore, if the spontaneously hypertensive rats would start to manifest normal blood pressure readings or hypotension signs and symptoms, their working hypothesis would have been confirmed. However, the researchers concluded that “carotid sinus nerve inputs from the carotid body are, in part, responsible for elevated sympathetic tone and critical for the genesis of hypertension in the developing SHR and its maintenance in later life”. In general, however, the results were mixed and so further research may be needed. Basing solely on the results of this and other similar studies reviewed, however, it can be inferred that an abnormally under-stimulated and hyposensitive set of chemoreceptor organs (a category which the carotid body falls under) would indeed lead to hypotension .
Apart from the regulation of blood pressure, the carotid body also has a direct relevance in the development and maintenance of chronic heart failure (CHF). Studies have shown that a hyperactive carotid body (which means that it is overly stimulated) is common among CHF patients. This theory was used as the hypothesis in a study published in the Current Hypertension Reports. The researchers hypothesized that a sensitive and hyperactive carotid body can be identified as precursor of cardiovascular diseases such as CHF, mainly as a result of the failure of the body to correctly regulate blood pressure and oxygen and carbon dioxide (among other important chemicals) levels .
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