The heart is divided into two parts.
Each part consists of an atrium and a ventricle.
The right side of the heart (right atrium and ventricle) drives the pulmonary circulation.
The left side of the heart (left atrium and ventricle) drives the systemic circulation.
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Approximately 5-10% of the stroke volume serves to nourish the cardiac muscle alone.
The right and left coronary arteries enter the cardiac muscle from outside.
Scheme of the organs of the circulatory system
Vessels carrying blood to the heart are referred to as veins.
Vessels carrying blood away from the heart are referred to as arteries.
Arteries in the pulmonary circulation transfer poorly oxygenated blood, whereas in systemic circulation, they transfer oxygen-rich blood.
The pulmonary veins transfer oxygen-rich blood from the lung to the left cardiac atrium.
This blood then flows into the left ventricle and is subsequently forced into the aorta.
Arteries and arterioles transfer blood to the capillaries, where oxygen is given off and carbon dioxide is taken up.
The poorly oxygenated blood flows via veins into the right atrium.
Poorly oxygenated blood enters the right ventricle from the right atrium from where it is forwarded to the lung capillaries via the pulmonary veins. Here, the gas exchange takes place. The oxygen-rich blood is transferred to the left atrium via the pulmonary veins.
The excitations needed for the contractions of the myocardium originate in the heart itself.
Pulse frequency and contraction strength are influenced by the sympathetic and the parasympathic parts of the autonomic nervous system.
The heart rate is usually between 60 and 100 per minute. A failure of this system can have fatal consequences and lead to cardiac arrest. Therefore, nature has built in several precautionary measures. Should the sinus node fail, other mechanisms of impulse propagation will come into action.
Several structures within the heart are part of the cardiac conduction system:
- The sinus node
The sinus node lies in the right atrium at the opening of the superior vena cava. Here, the electrical impulses are generated and then conducted. At rest, the sinus node independently produces 60 - 80 impulses/minute and conducts them to the AV node.
- The AV node (atrio-ventricular node)
The AV node is in the right atrial wall at the border between atrium and ventricle. It transfers the impulses produced by the sinus node to the His bundle. If the sinus node fails, the AV node can still produce a rhythm of 40-60 impulses/minute.
- The His bundle
The His bundle is a bundle of fibres running in the ventricular septum dividing into the right and left bundle branches. The impulses are conducted from here to the Purkinje fibres. If sinus and AV nodes fail, here a rhythm of 20 impulses/minute can still be produced.
- The Purkinje fibres Anaesthetic
The Purkinje fibres are terminal branches of the His bundle and conduct the impulses to the ventricular muscles.