Clase 28 Fisiología Circulatoria - Regulación nerviosa y control rápido de la PA (IG:@doctor.paiva)

28ª Clase de Fisiología: Regulación Nerviosa y Control Rápido de la Presión Arterial

Introducción a la Regulación Nerviosa

• La clase se centra en la regulación nerviosa de la circulación y el control rápido de la presión arterial. Se mencionan los mecanismos reflejos para normalizar la presión arterial y la respuesta isquémica del sistema nervioso central.

Sistema Nervioso Autónomo

• El sistema nervioso autónomo, compuesto por el simpático y parasimpático, es el principal regulador nervioso de la presión arterial. El simpático tiene mayor relevancia en la regulación circulatoria, mientras que el parasimpático se enfoca más en la función cardíaca.

Inervación Simpática

• La inervación simpática afecta arterias, arteriolas y esfínteres precapilares; sin embargo, no hay inervación capilar debido a su falta de músculo liso. Un estímulo simpático provoca vasoconstricción en pequeñas arterias, aumentando así la resistencia vascular.

Efectos del Estímulo Simpático

• El estímulo simpático incrementa tanto la frecuencia cardíaca como el gasto cardíaco, elevando así la presión arterial. En contraste, el sistema parasimpático disminuye ambos parámetros.

Centro Vasomotor

• El bulbo raquídeo alberga centros vasomotores que regulan tanto vasoconstricción como vasodilatación mediante neuronas preganglionares del sistema simpático. La zona sensitiva envía señales reflejas sobre funciones circulatorias y cardíacas a través de los nervios vagos y glosofaríngeo.

Tono Vasoconstrictor Simpático

• Los vasos sanguíneos mantienen un estado continuo de contracción leve conocido como tono vasoconstrictor simpático; un aumento en este tono resulta en una mayor presión arterial. Un experimento mostró que al inhibir el centro vasomotor se produjo una caída significativa en la presión arterial debido a pérdida del tono constrictor.

Noradrenalina y Control Cardíaco

Cardiovascular Regulation Mechanisms

Overview of Cardiac Output and Blood Pressure Regulation

• The increase in cardiac output leads to a rise in blood pressure, stimulated by sympathetic nervous system activity, which enhances heart rate and contraction strength.

• Blood pressure can rapidly double or halve within seconds due to the nervous system's quick response mechanisms. This makes it the fastest regulatory mechanism for blood pressure.

Reflex Mechanisms and Baroreceptors

• Reflex mechanisms operate continuously to maintain normal blood pressure levels, primarily through negative feedback loops involving baroreceptors located in the carotid arteries and aortic arch.

• Baroreceptors detect changes in vessel wall stretch caused by blood pressure exertion, also known as "pressure receptors." Their location is crucial for effective monitoring of arterial pressure.

Functionality of Baroreceptors

• These receptors send signals via the glossopharyngeal nerve to the medulla oblongata when stretched by high pressures, leading to vasodilation and decreased heart rate, thus lowering blood pressure.

• Conversely, low blood pressure triggers an increase in sympathetic activity to restore normal levels through reflex actions initiated by baroreceptor stimulation.

Importance of Baroreceptors in Postural Changes

• When transitioning from lying down to standing up, gravity causes a drop in head-level blood pressure; baroreceptors quickly activate sympathetic responses to counteract this drop and prevent fainting.

• A comparison between normal dogs and those lacking baroreceptors illustrates significant differences in blood pressure regulation under varying conditions, highlighting their critical role.

Long-term vs Short-term Regulation

• While baroreceptor regulation is highly effective for short-term adjustments, its long-term impact on blood pressure is minimal; renal mechanisms play a more significant role over extended periods.

Chemoreceptor Functions

Sensitivity of Chemoreceptors

• Unlike baroreceptors that respond to mechanical stretch, chemoreceptors are sensitive to chemical changes such as oxygen depletion and carbon dioxide accumulation within the bloodstream.

• Located similarly near the carotid bifurcation but distinct from baroreceptors, these chemoreceptors have abundant arterial supply ensuring they can promptly relay information about chemical status back to the central nervous system.

Impact on Cellular Oxygen Levels

Understanding the Role of Chemoreceptors and Cardiac Reflexes in Blood Pressure Regulation

The Impact of Lactic Acid on Blood Pressure

• The presence of lactic acid leads to an increase in hydrogen ions, stimulating chemoreceptors that send signals to the vasomotor center, raising blood pressure back to normal levels.

• Chemoreceptor reflexes are not significant controllers of blood pressure unless it drops below 80 mmHg. Their primary role is in respiratory control through carbon dioxide and hydrogen ion receptors.

Atrial Reflexes and Blood Pressure Control

• Atrial walls contain receptors similar to baroreceptors that respond to increased blood volume by stretching, initiating various reflexes. This includes dilation of renal arteries for enhanced fluid filtration.

• Stretching of atria sends signals to the hypothalamus, inhibiting antidiuretic hormone (ADH) secretion, leading to reduced water reabsorption in kidneys and increased fluid elimination.

Atrial Natriuretic Peptide (ANP) Release

• Stretched atria release atrial natriuretic peptide (ANP), which decreases sodium and water reabsorption in kidneys, regulating blood volume effectively.

• Increased heart rate results from atrial stretching; this occurs via direct stimulation of the sinoatrial node or through Bainbridge reflex mechanisms that enhance cardiac output significantly.

Bainbridge Reflex Mechanism

• The Bainbridge reflex increases heart rate by 40% to 60% when atria stretch, sending signals via vagus nerve to the medulla oblongata which adjusts heart rate accordingly. This helps pump accumulated blood more efficiently from veins and atria.

• All discussed atrial reflexes involve low-pressure receptors detecting stretch within the atria, crucial for maintaining cardiovascular stability during fluctuations in blood volume.

Ischemic Response in Central Nervous System Regulation

• Most neural control over blood pressure arises from peripheral circulation through baroreceptors and chemoreceptors; however, ischemic conditions can disrupt this balance if cerebral blood flow diminishes significantly.