Heart: Tissue Function

Cardiac Conduction System

The human heart has a need for a power source and also uses electricity. It is able to create its own electrical impulses and control the route that the impulses will course through via a specialized conduction pathway.

This pathway is made up of five main elements:

• 1.
  The sino-atrial (SA) node
• 2.
  The atrio-ventricular (AV) node
• 3.
  The bundle of His
• 4.
  The right and left bundle branches
• 5.
  The Purkinje fibers

The SA node is the natural pacemaker of the heart. It releases electrical stimuli at regular rates, which are dictated by the needs of the body. Each stimulus passes through the myocardial cells of the atria, creating a wave of contraction that spreads rapidly through both atria.

The electrical stimuli from the SA node eventually reaches the AV node, upon which there is a brief delay so that the contracting atria have sufficient time to pump all the blood into the ventricles. Once the atria are empty of blood, the valves between the atria and ventricles close. At this point the atria begin to refill and the electrical stimuli pass through the AV node and Bundle of His, into the bundle branches and Purkinje fibers.

After complete ventricular contraction, the atria are full and the valves between atria and their respective ventricles are closed, the SA node will release another electrical stimulus and the conduction cycle will repeat itself. However, the SA node and AV node contain only one stimulus. The nodes must recharge after every stimulus fired. The SA node recharges during atrial refill, and the AV node recharges during the ventricular refill, hence causing no lag in the pumping motion of the organ.

Pressure Generation

The left ventricle fills with blood returning from the lungs during diastole. After it has been filled up completely, it contracts to eject the blood into the arteries with a certain degree of pressure. The blood pressure in the arteries during contraction of the ventricle is higher because blood is being actively ejected into the vessels; this is known as the systolic pressure. Likewise, diastolic pressure is the low pressure during relaxation of the ventricle when no blood is being ejected. The pulse that is felt when a finger is placed over an artery is caused by the contraction of the left ventricle and the ejection of blood.

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