!Heart - Anatomical Overview
The heart as the centre of the ((Circulatory - System Overview|circulatory system)) is located anterior to the vertebral column and posterior to the sternum.

It is enclosed in a double-walled sac called the ((Circulatory - Pericardium and Epicardium|pericardium)). The pericardium's outer wall is called the parietal pericardium and the inner one the visceral pericardium. Between them there is some pericardial fluid which functions to permit the inner and outer walls to slide easily over one another with the heart movements. Outside the parietal pericardium is a fibrous layer called the fibrous pericardium which is attached to the mediastinal fascia. This sac protects the heart and anchors it to the surrounding structures.

The outer wall of the human heart is composed of __three layers__; the outer layer is called the ((Circulatory - Pericardium and Epicardium|epicardium)), or visceral pericardium since it is also the inner wall of the pericardium. The middle layer is called the ((Circulatory - Myocardium - Ventricles|myocardium)) and is composed of contractile cardiac muscle. The inner layer is called the ((Circulatory - Endocardium and Heart Valves|endocardium)) and is in contact with the ((Circulatory - Blood|blood)) that the heart pumps. Also, it merges with the inner lining (endothelium) of blood vessels and covers heart valves.

{img fileId="225" thumb="box"}

The human heart has __four chambers__, two superior ((Circulatory - Myocard - Atria|atria)) and two inferior ((Circulatory - Myocardium - Ventricles|ventricles)). The ((Circulatory - Myocard - Atria|atria)) are the receiving chambers and the ((Circulatory - Myocardium - Ventricles|ventricles)) are the discharging chambers. During each cardiac cycle, the atria contract first, forcing blood that has entered them into their respective ventricles, then the ventricles contract, forcing blood out of the heart. The pathway of the blood consists of a __pulmonary circuit__ and a __systemic circuit__ which function simultaneously. Deoxygenated blood from the body flows via the vena cava into the right atrium, which pumps it through the __tricuspid ((Circulatory - Endocardium and Heart Valves|valve))__ into the right ventricle, whose subsequent contraction forces it out through the __pulmonary valve__ into the pulmonary arteries leading to the lungs. Meanwhile, oxygenated blood returns from the lungs through the pulmonary veins into the left atrium, which pumps it through the __mitral valve__ into the left ventricle, whose subsequent strong contraction forces it out through the __aortic ((Circulatory - Endocardium and Heart Valves|valve))_ to the aorta leading to the systemic circulation.

{img fileId="226" thumb="box"} graphic: wikipedia

The myocardium contracts after stimulation. Under normal conditions, electrical activity is spontaneously generated by the __SA node__, the physiological __pacemaker__, and is closely tied to sympathetic discharge. This electrical impulse is propagated throughout the right atrium, and through Bachmann's bundle to the left atrium, stimulating the myocardium of the atria to contract. As the electrical activity is spreading throughout the atria, it travels via specialized pathways, known as internodal tracts, from the SA node to the AV node. The __AV node functions as a critical delay__ in the conduction system. Without this delay, the atria and ventricles would contract at the same time, and blood wouldn't __flow effectively__ from the atria to the ventricles. Second order electrical control of the myocardium is closely tied to parasympathetic influence from the spinal vertebral ganglia and vagus nerves.

{img fileId="230" thumb="box"} conduction: 1.Sinus node 2. AV node

graphic: Wikipedia, Patrick J Lynch

From an evolutionary point of view, the heart has made a turn so that curiously, in the Plexus Cardiacus (autonomous nerve system or XIII brain nerve), fibres are running together which relate both to the brain stem relay crossing over, and from cerebral cortex relays not crossing over. So, the relay for the ((Circulatory - Myocard - Atria|sinus node)) that stimulates heart rhythm via the right atrium, lies in the left brain stem.

!!Brain
{img fileId="356" thumb="box"}heart innervation

graphic: [http://philschatz.com/anatomy-book/resources/2032_Automatic_Innervation.jpg|Phil Schatz]

{img fileId="357" thumb="box"}cardiac plexus

graphic: McGraw-Hill Companies, 2002

!!!Relays
The Plexus Cardiacus contains fibres related to following relays:
__Cerebral Medulla__ (-/+) medial of "Throacic region" left & right, for ((Circulatory - Endocardium and Heart Valves|Endocardium & Valves)) and ((Circulatory - Myocardium - Ventricles|Ventricular Myocardium))
{img fileId="175" thumb="box"} brain-organ relation uncrossed!

__Midbrain__ (+/-) fronto-lateral left & right, for ((Circulatory - Myocard - Atria|Atrial Myocardium)) and stimulation via sinoatrial node (heart rhythm)
compare __Brain Stem__ {img fileId="190" thumb="box"} brain-organ relation crossed!

__Cerebral Cortex__ (-/+) insular region medio-lateral left & right, for ((Circulatory - Coronary Arteries (Intima)|Coronary Arteries)) and ((Circulatory - Coronary Veins (intima)|Veins)) and AV Node (pulse regulation, bradycardia)
{img fileId="125" thumb="box"}

__Cerebellum__ (+/-) medial for ((Circulatory - Pericardium and Epicardium|Pericardium & Epicardium))
{img fileId="174" thumb="box"}
