Blood pressure, or the force that drives blood through the system of organs, is one of the most significant indicators of the state of the cardiovascular system. In fact, blood pressure is a variable value, which is influenced by both external and internal factors. Nature gave a person specific mechanisms that allow maintaining equilibrium in the body. The mechanism responsible for regulation of blood pressure is highly complicated. It is necessary to indicate such components as the parasympathetic and sympathetic braches of the autonomic nervous system, the central nervous system, the endocrine system, receptors located at the branching of the carotid arteries, and receptors based in the aortic arch. Such hormones as norepinephrine, aldosterone, and atrial natriuretic peptide play an important role in the mechanism of regulation of blood pressure and, thus, it is necessary to study them.
Norepinephrine is the hormone produced by the adrenal medulla, the neurotransmitter of sympathetic nervous system. It is the second hormone after adrenaline in terms of quantitative indices of secretion but not less important for the body. Its role as a neurotransmitter is extremely imperative (Russell, Hertz, & McMillan, 2016). According to its chemical structure, noradrenaline differs from adrenaline by the absence of a methyl group in the nitrogen atom of the side chain amino group (Russell et al., 2016). Norepinephrine is a precursor of epinephrine during the synthesis in chromaffin tissue. Receptors through which noradrenaline realizes its effect on the body cells are called adrenoreceptors. There are two types of adrenoreceptors, such as alpha (?) and beta (?) (Kunos & Ciriello, 2012). When norepinephrine is combined with ?-adrenergic receptors, vessels narrow. When norepinephrine is combined with ?-adrenoreceptors, vessels expand. Norepinephrine has a vasoconstrictive effect on most arterial vessels through ?-adrenoreceptors (Kunos & Ciriello, 2012). Besides, it also has a vasodilating effect on the blood vessels of the heart, kidneys, brain, and skeletal muscles when it is combined with ?-adrenergic receptors (Kunos & Ciriello, 2012). In the heart, there are only ?-adrenergic receptors, while ?-adrenergic receptors are absent. Norepinephrine is connected by ?-adrenergic receptors (Kunos & Ciriello, 2012). Therefore, the metabolic processes in the heart muscle increase. This fact leads to an increase in the strength of the heartbeat and blood pressure. At the same time, oxygen consumption by cardiac muscle cells rises. The lack of oxygen in cardiomyocytes increases the splitting of ATP in them with the formation of adenosine (Kunos & Ciriello, 2012). The latter is released from the cardiomyocyte and has a vasodilating effect on cardiac arteries. It upsurges blood flow and oxygen consumption of the cardiomyocytes preventing their oxygen starvation and increasing heart rate.
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Norepinephrine participates actively in the regulation of blood pressure and peripheral vascular resistance. For example, when moving from a reclining to a standing or sitting position, a normal level of norepinephrine in blood plasma rises several times within a minute (Russell et al., 2016). The level of norepinephrine in the blood rises with stressful conditions, trauma, shock, blood loss, fear, anxiety, and nervous tension (Russell et al., 2016). A cardiotropic action of noradrenaline is connected with a stimulating effect of ?-adrenoreceptors on the heart. Nonetheless, ?-adrenostimulating action is masked by reflex bradycardia and increased vagal tonus caused by amplified blood pressure (Kunos & Ciriello, 2012). Norepinephrine causes an increase in cardiac output (Russell et al., 2016). Due to increased blood pressure, perfusion pressure in the coronary and cerebral arteries increases. At the same time, peripheral vascular resistance and central venous pressure also significantly increase. In addition, noradrenaline is the main neurotransmitter in adrenergic CNS systems (Russell et al., 2016). In fact, it is different from adrenaline, which plays a less significant role in the transmission of nerve impulses in adrenergic brain systems and mainly performs the role of the hormonal regulator than the neurotransmitter (Russell et al., 2016). Therefore, the role of norepinephrine in regulation of blood pressure is difficult to overestimate.
Another significant component in the regulation of blood pressure is aldosterone. In the book Essentials of Pathophysiology, it is written that В“the rennin-angiotensin-aldosterone system plays a central role in blood pressure regulationВ” (Porth, 2011, p. 426). An important role in the multistage control of the volume of circulating fluid and arterial pressure belongs to aldosterone, which is synthesized by the cells of the glomerular zone of the adrenal cortex. Under the influence of aldosterone, the transport of potassium and sodium through the epithelial membrane is enhanced (Porth, 2011). The aldosterone is applied as the distal segment of the tubular apparatus, in which the active reabsorption of sodium and the secretion of potassium increase (Porth, 2011). It is very likely that the action of aldosterone also extends unto the proximal segment of the tubule.
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Aldosterone regulates the state of water-salt metabolism in the body. In the book Pharmacology for Nursing Care, it is stated В“after being released from the adrenal cortex, aldosterone acts on distal tubules of the kidneys to cause retention of sodium and excretion of potassium and hydrogenВ” (Lehne & Rosenthal, 2014, p. 502). A volume of circulating blood directly depends on its activity, what represents the third component of blood pressure. Aldosterone is necessary for the regulation of renal diuresis, which is the balance of consumed and discharged fluid by the body. This hormone is able to retain sodium ions present in the urine. If its level in blood is not too high, sodium is released in normal amounts (Lehne & Rosenthal, 2014). According to the law of the gradient, ions of water move behind the sodium (Lehne & Rosenthal, 2014). Every person has experienced a similar manifestation when after an excess consumption of salty foods he/she gets an urgent desire to drink. If, for a number of reasons, the aldosterone content in the blood rises, sodium ions stay in the kidneys, making the water to detain. Liquid that could not leave the body with urine passes into the tissues. This fact is characterized by the appearance of edema both noticeable and localized directly in the vascular system. The result is a narrowing of the lumen of the vessels. The excess of sodium ions increases the sensitivity of endothelial cells to vasoconstrictor factors, such as adrenaline and norepinephrine (Lehne & Rosenthal, 2014). Fluid that lingers in the body not only results in swelling but also increases the volume of the liquid part of the blood (plasma). Therefore, the effects of high level of aldosterone in the blood are connected with hypertension.
Atrial Natriuretic Peptide
The heart is known as the main circulatory organ and as the so-called pumping station, which circulates blood. Without it, a person cannot continue living, because a stop of this organ for only a several minutes will lead to death. The heart is considered one of the most powerful muscles in the body, which is constantly working (White, 2015). However, in addition to its main function, the heart also releases hormones. In such a way, it regulates the work of many systems and organs (White, 2015). It is important to understand the role these substances play in the body to correctly diagnose and prescribe treatment.
For a long time, it was believed that the heart only provides blood flow and maintains the necessary level of blood pressure. Nevertheless, numerous researches showed that some cells synthesize substances of the protein nature, such as peptides. In the experiment, they were administered to experimental rats and it was revealed that diuresis increased in animals (Samson & Levin, 2012). Besides, after the body lost a large amount of liquid and sodium, pressure decreased. It has become clear that the heart has another extremely important function, which is the formation of biologically important substances. One of such substances is the atrial natriuretic peptide (Samson & Levin, 2012). The hormone is synthesized in the cells of the atria and in individual cells of the ventricles. It accumulates in the cardio myocyte, provoking substances being released into the blood. First, it is a prohormone, or an inactive substance that becomes a full-fledged hormone directly in the channel (Samson & Levin, 2012). A small amount can be released by the brain, endothelium of the vessels, lungs, and kidneys. Nonetheless, the functional significance has atrial natriuretic peptide (Samson & Levin, 2012). A great number of tissues have receptors. In such a way, it is necessary to study the role of this hormone in the regulation of blood pressure.
The effects of this substance in the body are highly diverse. In its essence, it is the main factor of the renin-angiotensin system, performing the opposite functions. The hormone has a strong influence on kidneys. It upsurges the blood flow, increasing the lumen of the vessels. Besides, atrial natriuretic peptide stimulates filtration and inhibits the reabsorption of sodium and water (Dun, Machado, & Pilowsky, 2011). Thus, a large amount of sodium is released from the body followed by water, while the amount of circulating fluid decreases. It is especially necessary for swelling and increased intracranial or intraocular pressure. Atrial natriuretic peptide dilates blood vessels, improves blood flow in the organs, and helps reduce blood pressure (Dun et al., 2011). More blood is deposited in the dilated vessels, leading to lower resistance. Under the influence of this hormone, the permeability of the vascular wall increases. Excess fluid from the vascular bed goes into the tissue that also reduces the total amount of circulating fluid. With arterial hypertension, the level of atrial natriuretic peptide significantly rises (Samson & Levin, 2012). An increase of its level in the blood plasma in patients with arterial hypertension testifies to the level of its secretion, which is obviously associated with a change in hemodynamics due to increased blood pressure. In the early stages of the disease, the compensatory role of atrial natriuretic peptide is significantly weakened (Samson & Levin, 2012). It is confirmed by the predominance of pressor factors, which lead to vasoconstriction and water and sodium retention (Samson & Levin, 2012). Therefore, atrial natriuretic peptide plays a highly significant role in the regulation of blood pressure.
A permanent condition for the continuous translational movement of blood in the vessels is a stable pressure in the arterial system gradually decreasing to the periphery and changing within narrow limits around a medium-level constant. A single system of regulation of blood pressure consists of at least several subordinate components. In such a way, the regulation of pressure goes along two pathways, including nervous with inclusion of all levels of the central nervous system and humorous, a central link of which includes kidneys as an endocrine and excretory organ. Such hormones as norepinephrine, aldosterone, and atrial natriuretic peptide play an active role in the regulation of blood pressure in the human body. The activity of all regulatory mechanisms of blood pressure is aimed at maintaining the optimal minute volume as well as tonus of the vessels responsible for certain physiological state of the organism. Derivatives of this activity are sufficient circulation of blood and a stable level of blood pressure.