Knotted Fabric

Nodular tissue: heart rate regulator

Nodal tissue is specialized tissue that regulates heart contractions. It is found only in the heart and has the properties of both muscle and nervous tissue. In lower vertebrates there is a venous sinus, which is a separate chamber of the heart, but in higher forms the venous sinus has disappeared and only nodular tissue remains.

Nodular tissue consists of two main nodes: the sinus node and the atrioventricular (atrioventricular) node. The sinus node is located at the junction of the superior vena cava into the right atrium, and the atrioventricular node lies between the atria, just above the ventricles. Scientists have proven that the atrioventricular node gives the first impulse for heart contractions and regulates their frequency.

When the contraction wave reaches the atrioventricular node, the impulse is transmitted through a bundle of nodal tissue to the ventricles. There is no muscular connection between the atria and ventricles; their contractions are coordinated only by specialized nodular tissue. Conduction of the impulse by nodular tissue ensures simultaneous contraction of all parts of the ventricle.

If conduction in the ventricles were carried out by ordinary muscle tissue, then the muscles at the base of the ventricles would contract first, causing stretching of the still relaxed apex of the heart, which could lead to its damage. That the sinus node regulates the heart rate is demonstrated by the fact that exposing this node to heat causes the heart to beat more frequently, while cooling causes the heart to beat faster.

An increased heart rate during fever is caused by warmer blood stimulating the sinus node. In the event of destruction of the sinus node due to injury or illness, the role of the leading node takes over the atrioventricular node.

Each heartbeat consists of a contraction, or systole, of the heart muscle and its subsequent relaxation, or diastole. At normal frequency (70 beats per minute), each complete contraction cycle takes about 0.85 seconds. The atria and ventricles do not contract simultaneously: atrial systole occurs first, which takes about 0.15 seconds, followed by ventricular systole, which lasts about 0.30 seconds. For the remaining 0.40 seconds, all chambers rest, or are in diastole.

Nodular tissue plays an important role in regulating heart rhythm and ensuring the coordination of contractions of the atria and ventricles. It may also be susceptible to medical conditions such as junctional bradycardia, where the heart rate slows due to a faulty sinus node, or atrioventricular node block, where impulses cannot travel from the atria to the ventricles. These diseases can lead to serious cardiac problems and require medical intervention.