Biology » Introducing Animal Systems » Circulatory Systems In Animals

The Cardiac Cycle

The Cardiac Cycle

A cardiac cycle refers to the sequence of events that happens in the heart from the start of one heartbeat to the start of the subsequent heartbeat. During a cardiac cycle the atria and the ventricles work separately. The sinoatrial node (pacemaker) is located in the right atrium and regulates the contraction and relaxing of the atria.

  • At rest, each heartbeat takes approximately \(\text{0.8}\) seconds.
  • The normal heart rate at rest is approximately \(\text{72}\) beats per minute.
  • During systole the heart muscle contracts.
  • During diastole the heart muscle relaxes.

The phases of the cardiac cycle will be broken down and explained in the following section:

Phase 1: Atrial systole (Atrium contracts)

  • Blood from the superior and inferior vena cava flows into the right atrium.
  • Blood from the pulmonary veins flows into the left atrium.
  • The atria contract at the same time.
  • This contraction lasts for about \(\text{0.1}\) seconds.
  • Blood is forced through the tricuspid and bicuspid valves into the ventricles.

Phase 2: Ventricular systole (Ventricle contracts)

  • Ventricles relax and fill with blood.
  • The ventricles contract for \(\text{0.3}\) seconds.
  • Blood is forced upwards, closing the bicuspid and tricuspid valves (lubb sound).
  • The blood travels up into the pulmonary artery (on the right) and the aorta (on the left).
  • The atria are relaxed during ventricular systole.

Phase 3: General diastole: (General relaxation of the heart)

  • The ventricles relax, thus decreasing the flow from the ventricles.
  • Once there is no pressure the blood flow closes the semi-lunar valves in the aorta and the pulmonary artery (dubb sound).
  • General diastole lasts for about \(\text{0.4}\) seconds.

The sound the heart makes

The heart makes two beating sounds. One is loud and one is soft. We call this the lubb dubb sound. The lubb sound is caused by the pressure of the ventricles contracting, forcing the atrioventricular valves shut. The dubb sound is caused by the lack of pressure in the ventricles which causes the blood to flow back and close the semi-lunar valves in the pulmonary artery and aorta.

A doctor uses a stethoscope to listen to the heartbeats. Alternatively, a person’s pulse can be measured by pressing a finger (other than the thumb which already has a pulse) against the brachial artery in the wrist or the carotid artery next to the trachea. The pulse of the heart allows us to measure the heart rate which is the number of heartbeats per unit time.

Mechanisms for controlling cardiac cycle and heart rate (pulse)

The cardiac cycle is controlled by nerve fibres extending from nodes of nerve bundles through the heart muscle. There are two nodes, namely the sinoatrial node (SA node) and the atrioventricular node (AV node). The SA node is located within the wall of the right atrium while the AV node is located between the atria and the ventricles. Electrical impulses generated in the SA node cause the right and left atria to contract first, initiating the cardiac cycle.

The electrical signal reaches the AV node, where the signal pauses, before spreading through conductive tissues called the bundles of His and Purkinje fibres. These fibres branch into pathways which supply the right and left ventricles, causing the ventricles to contract. The SA node is the pacemaker of the heart since electrical signals are normally generated there – without any stimulation from the nervous system (automaticity). However, although the heart rate is automatic, it changes during exercise or when experiencing intense emotions like fear, anger and excitement. This is as a result of added stimulation from the nervous system and hormones, such as adrenaline.

Simple simulation of how electrical activity spreads over the heart

The Cardiac Cycle

Principle of ECG formation. Credit: Wikimedia Commons, CC BY-SA 3.0

Electrical activity

The electrical activity in the heart is so strong that it can be measured from the surface of the body as an electrocardiogram (ECG). A normal heart has a very regular rhythm. Arrhythmia is a condition where the heart has an abnormal rhythm, as shown in the figures. Tachycardia is when the resting heart rate is too fast (more than \(\text{100}\) beats per minute), and bradycardia is when the heart rate is too slow (less than \(\text{60}\) beats per minute).

The Cardiac Cycle

Electrocardiogram depicting different heart rhythms.

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