Heart Regulation by Autonomic Nervous System, Cardiac Output and Cardiac Cycle : Pharmaguideline -->

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Heart Regulation by Autonomic Nervous System, Cardiac Output and Cardiac Cycle

A nerve impulse or hormone can influence the speed with which the SAN produces an electrical impulse by affecting the cells in the SAN.
Heart regulation by the autonomic nervous system
Sinoatrial Node (SAN) - the pulse generator of the heart muscle - is responsible for establishing the heart rate. Without influences, it can be assumed that the SAN pacing rate is 100 bpm; however, the heart rate and cardiac output will vary based on individual requirements. A nerve impulse or hormone can influence the speed with which the SAN produces an electrical impulse by affecting the cells in the SAN. It affects the frequency and magnitude of heartbeats, which, in turn, affect the output of the heart.

The autonomic nervous system
The autonomic nervous system (ANS) is responsible for maintaining homeostasis and a wide range of bodily functions. As part of this system, the heart contracts, and its rate increases. The peripheral resistance of the blood vessels is also controlled by this hormone. A functioning nervous system relies on the coordination of sympathetic and parasympathetic activity.

Through the vagus nerve (CN X), parasympathetic messages are sent to the heart. Synaptic connections between the vagus nerve and the postganglionic cells of the SAN and AVN occur within the atrioventricular node. As acetylcholine is stimulated, it binds to M2 receptors which cause the pacemaker potential to slope downward. As a consequence, the heart rate will decrease (a negative chronotropic effect).

From the sympathetic trunk, the postganglionic fibers innervate the SAN and AVN, which are the main pathways for sending sympathetic signals to the heart. As a result of the release of noradrenaline by the postganglionic fibers, B1 adrenoreceptors begin to increase the pacemaker potential slope steepness. Consequently, your heart rate and contraction force will be greater (positive chronotropic effect) and positively inotropic (positive inotropic effect). In the parasympathetic pathway, the heart rate is in the range of 60bpm at rest since the parasympathetic pathway dominates the SAN. When parasympathetic outflow is reduced, there is an increase in a heartbeat that usually reaches over 100bpm. Initially, heart rate increases due to an increase in sympathetic activity.

Baroreceptor reflex
Located in both the aortic arch and the carotid sinus, baroreceptors are mechanoreceptors. An arterial stretch and tension change is their primary function. The second function of these zones is to detect changes in arterial pressure and transmit these changes to the medulla oblongata. A medullary center in the brain is responsible for controlling the symptoms of the autonomic nervous system and coordinating responses according to data displayed by baroreceptors:

A heart rate reduction is caused by activation of the parasympathetic pathway when arterial pressure increases. Additionally, this has the effect of reducing arterial pressure along with increasing vessel vasodilation.

In response to a decrease in arterial blood pressure, the sympathetic pathway is activated and the heartbeat and contractility are increased. In combination with the increased vasoconstriction of vessels, this contributes to an increase in arterial pressure.

Cardiac cycle
Heartbeats are marked by contractions and then relaxations, which are known as cardiac cycles. When a muscle contracts, it is defined as systole, while when it relaxes, it is described as diastole.

Stages of the cardiac cycle
In the case of 74 bpm, another example, each cycle lasts 0.8 of a second. Atrial systole occurs when the atria are contracted. Cardiac diastole occurs when the ventricles and atria are relaxed. No matter when a description starts, it does not matter at what stage of the cardiac cycle it begins. This period has been selected to accommodate the filling of the atria. A heartbeat causes oxygen-rich blood to flow into the right atrium through the superior vena cava and inferior vena cava, while oxygen-poor blood goes into the left atrium through the pulmonary veins. Blood flows passively through the atrioventricular valves and into the ventricles through these valves.

A wave of contraction is triggered by the SA node, which spans both atria, causing the atria to empty and the ventricular filling process to complete (atrial systole 0.1 s). An electrical impulse is slowed Down by the AV node during atrioventricular transmission, thereby delaying it. Due to this delay, the mechanical activity (atrial contraction) of atrial stimulation lags behind the electrical activity for a fraction of a second. After the atrium has finished emptying into the ventricles before a contraction begins, the ventricles can begin contracting. Upon passing the brief delay, the AV node sends an electrical impulse to the ventricular muscle, which is carried to the ventricular muscle by the AV bundle, bundle branches, and Purkinje cells. Within 0.3 seconds, there is an upward wave of contraction sweeping both ventricles and pumping blood into the pulmonary arteries and aorta.

In contracting heart muscle, the ejection of blood from the ventricles creates high pressures which close the atrioventricular valves and prevent blood from returning to the atria. There is a long period of relaxation in the atria and ventricles after ventricle contractions: cardiac diastole lasts 0.4 seconds. This is the time when the myocardium gets ready for the next heartbeat, as well as when the atria recharge so they are ready for the next cycle. Valve openings and closings of the heart and the great vessels are affected by the pressure within our heart chambers. Atrial filling and systole are characterized by the AV valves remaining open while ventricular muscle relaxes.

When the ventricles contract, the pressure in these chambers increases rapidly, and the atrial and ventricular valves close when they rise above the atrial pressure. In response to a rise in ventricular pressure, blood flows into the pulmonary and aortic valves, which make these vessels function. In the reverse process, the pressure within the ventricles falls when the ventricles relax. The atrioventricular valves open following the closure of the pulmonary and aortic valves, creating a cycle within the heart. Blood can only flow one way through a sequence of valves that open and close.

Cardiac output
A person's cardiac output can be calculated by calculating how much blood is ejected from their ventricles each minute. An individual's stroke volume refers to how much each ventricle extrudes during every contraction. A healthy adult at rest has a stroke volume of approximately 70 ml, and his cardiac output is approximately 5 l/min. Multiply the stroke volume by the cardiac output (in liters per minute): It can be increased to 25 l/minute or even 35 l/minute for athletes to meet the demands of physical exercise. An increase in cardiovascular reserve occurs when the heart is exercising. Heart rate and stroke volume may require an increase when more oxygen and nutrients are required by tissues.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of pharmaguideline.com, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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