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The Physiology of the Human Heart

The constant beating of the heart is controlled by the conducting system of the heart, which is a series of specialized nerve tissues that fire through the heart and coordinate the actions of the heart beat:

  • Sinoatrial (SA) node: This pacemaker initiates the impulse. It’s located anterolaterally just under the epicardium where the superior vena cava enters the right atrium. The impulse from the sinoatrial node spreads through the myocardium of the right and left atria, and it’s also quickly transmitted to the atrioventricular node.

  • Atrioventricular (AV) node: This node is located in the posterior and inferior portion of the interatrial septum, close to the opening of the coronary sinus in the right atrium. From there the signal is transmitted to the ventricles by a bundle of nerves called the atrioventricular bundle.

  • Atrioventricular bundle: This bundle of nerves runs from the atrioventricular node to the ventricles along the interventricular septum. It divides into left and right bundle branches that run deep to the endocardium to become the subendocardial branches (also called the Purkinje fibers):

    • Subendocardial branches of the right bundle stimulate the interventricular septum, the papillary muscle, and the wall of the right ventricle.

    • Subendocardial branches of the left bundle stimulate the interventricular septum, the papillary muscle, and wall of the left ventricle.

The heart is innervated by the autonomic nerves from superficial and deep cardiac plexuses. The deep cardiac plexus is located on the bifurcation of the trachea, and the superficial cardiac plexus is located on the base of the heart below the arch of the aorta.

The autonomic nervous system is made up of a two-neuron chain (using the presynaptic neuron and the postsynaptic neuron) from the central nervous system to the heart. The presynaptic sympathetic fibers branch off the first five or six thoracic segments of the spinal cord. They enter the sympathetic trunks and synapse with postsynaptic neurons located in the cervical and upper thoracic ganglia. Fibers of the postsynaptic neurons join the cardiac plexus and terminate on the SA node, AV node, cardiac muscle fibers, and coronary arteries.

Sympathetic stimulation increases heart rate, force of contraction, and dilation of coronary arteries. Parasympathetic innervation to the heart is provided by the vagus nerve (CN X). The presynaptic parasympathetic fibers of the vagus nerve join the postsynaptic sympathetic fibers in the cardiac plexus. The postsynaptic parasympathetic neurons are located in intrinsic ganglia (within the wall of the heart) and terminate on the SA node, AV node, and coronary arteries. Parasympathetic stimulation has the opposite effect of sympathetic stimulation.

The cardiac cycle is the sequence of events of each heart beat:

  • Diastole: During this process, the ventricles fill with blood from the atria. The atrioventricular valves are open, and the pulmonary and aortic valves are closed.

  • Systole: In this process, the ventricles empty into the aorta and pulmonary arteries. The atrioventricular valves are closed, and the pulmonary and aortic valves are open.

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The major blood vessels of the thorax include arteries that branch off the aorta and veins that drain into the vena cava. Following are the parts of the aorta and its branches:

  • Ascending aorta: This part of the aorta leaves the left ventricle and ascends up to the sternal angle. It has spaces between the walls of the vessel and the cusps of the aortic valve called aortic sinuses.

  • Arch of the aorta: Continuing from the ascending aorta, this part arches posteriorly to the left of the trachea and esophagus, above the left primary bronchus

  • Thoracic aorta: The thoracic aorta continues from the arch and descends in the posterior mediastinum and left of the vertebral column.

  • Posterior intercostal arteries: These arteries branch off the posterior part of the thoracic aorta and run laterally and anteriorly in the intercostal spaces.

  • Bronchial arteries: These arteries branch off the anterior part of the aorta or a posterior intercostal artery.

  • Esophageal arteries: Starting at the anterior part of the thoracic aorta, these arteries run to the esophagus.

  • Superior phrenic arteries: These arteries start at the anterior part of the thoracic aorta and run to the diaphragm.

Following are the parts of the vena cava and its tributaries:

  • Right and left brachiocephalic veins: These veins unite to form the superior vena cava near the brachiocephalic trunk (at the level of the 1st costal cartilage).

  • Superior vena cava: This large vein runs inferiorly to enter the right atrium.

  • Inferior vena cava: This vein is formed by the union of the iliac. It enters the heart at the lowest part of the right atrium.

  • Azygos vein: This vein arises from the right ascending lumbar vein and passes through the posterior mediastinum to drain into the superior vena cava.

  • Hemiazygos vein: This vein starts at the left ascending lumbar vein and crosses the vertebral column around the level of the 8th thoracic vertebra to join the azygos vein.

  • Accessory hemiazygos vein: This vein is formed by the union of the left fourth to the eighth posterior intercostal veins and joins the azygos vein at the level of the 7th thoracic vertebra.

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