CARDIAC CYCLE Renee Anderson

OBJECTIVES Define the cardiac cycle Describe and explain the three stages involved in a complete cardiac cycle Describe the myogenic stimulation of the heart rate. Describe the importance of the sino-atrial node (pacemaker) in the initiation of the heart beat.

What is the cardiac cycle? The cardiac cycle is the sequence of events that occur when the heart beats. One cardiac cycle is defined as the contraction of the two atria followed by contraction of the two ventricles. This is pumping of the blood from the heart when the heart is filled with blood.

The events of the cardiac cycle described below trace the path of the blood as it enters the heart, is pumped to the lungs, travels back to the heart and is pumped out to the rest of the body. These events will be further explained in the following slides.

Three Stages involved in a complete cardiac cycle Diastole: is the stage of relaxation in the heart after it contracts (systole). Blood returns to the heart under low pressure in the vein enter the two artia. Oxygenated blood enters the left atrium and deoxygenated blood enters the right atrium.

The atrioventricular (AV) valves (tricuspid and mitral) are open, allowing blood to flow from the atria to the ventricles. The semi-lunar valves (pulmonary and aorta) are closed to prevent the back flow of blood into the heart. This stage lasts for about 0.4 second.

Atrial Systole: is the period of time the atria are contracting, pushing blood into the ventricles. The means that the AV valves are open (tricuspid and mitral) and the semi-lunar valves are closed. This takes about 0.1 second. Ventricular systole: is the period of time when the ventricles are contracting pushing blood to the lungs and to the body. The semi-lunar valves are open and the AV valves are closed. This takes about 0.3 second. The total cycle takes about 0.8 second while the subject is resting. This gives a heart rate of about 75 beats per minute.

Atrial Systole Ventricular Systole Ventricular Diastole The walls of the atria contract. This reduces the volume of the atria, increasing the pressure. More blood is forced through the atrio-ventricular valves into the ventricles. Ventricular Systole The walls of the ventricles now contract, reducing the volume in the ventricles. The pressure increases and blood is forced into the arteries. Ventricular Diastole The ventricle walls relax and the pressure in the ventricles falls. Blood starts to flow from the atria into the ventricles again.

Myogenic Stimulation of the Heart rate Heart muscle does not need to be stimulated by a nerve before it will contract. The heart beat originates in the muscle itself and, for this reason, it is described as being myogenic. If the heart was removed form a mammal and placed in a well oxygenated salt solution at 37 degrees Celsius, it will continue to beat rhythmically for a considerable time without stimuli from the nervous system.

The SA node sends out an excitation wave of electrical activity over the atrial walls. The cardiac muscle responds to this wave by contracting at the same speed as the SAN. This results in both atria contracting simultaneously. However, there is a delay between atrial contraction and ventricular contraction. Fibres between the two chambers that do not conduct the excitation phase cause this delay. Therefore, the wave is conducted through a patch of fibres in the septum known as the atrio-ventricular node or AVN.

The cardiac cycle is initiated by a small patch of muscle called the Sinoatrial node (SAN)or pacemaker located in the right atrium. This node sets the rhythm for all the other cardiac muscle. The cells of the have an inbuilt rhythm that is faster than the other cells in the heart.

The artial muscle fibres between the two chambers that do not conduct the excitation phase cause this delay. Therefore, the wave is conducted through a patch of fibres in the septum known as the atrio- ventricular node or AVN. After about a delay of 0.1 seconds electrical impulse passes to ventricles via AV node and the Bundle of His. The bundle of His is a group of fibres that conduct impulses to Purkinje fibres which carries the impulses to the centre of the septum to left & right ventricles.

The wave of ventricular contraction begins at the bottom of the heart and spreads upwards, squeezing blood out of the ventricles towards the arteries which pass vertically upwards out of the heart . The electrical activity that spreads through the heart during the cardiac cycle can be detected using electrodes placed on the skin and an instrument called electrocardiogram.

Diagram of Electrocardiogram Diagram of Electrodes