Cardiac cycle

Question 1

How does the cardiac cycle work

Answer 1

Refers to a complete heartbeat- one contraction and a relaxation of each chamber of the heart.
Under normal conditions a cardiac cycle takes about 0.8s.
Divided into two basic phases: systole and diastole.
Systole is the period of contraction and emptying of the heart.

Diastole is when the relaxed atria fill with blood. This blood puts pressure on the AV valves which open and the blood fills the relaxed ventricles. When the ventricle is fill it contracts and this begins systole.
The pressure on the ventricle shuts the AV valve and forces the blood through the semilunar valves into the arteries.

Question 2

How does the heartbeat work

Answer 2

1. The heart beat begins when the heart muscles relax and blood flows into the atria.
2. The atria then contract and
   the valves open to allow blood into the ventricles.
3. The valves close to stop blood
   flowing backwards.

The ventricles contract forcing
the blood to leave the heart.

At the same time, the atria are relaxing and once again filling with
blood.

Question 3

Heart rhythm

Answer 3

When a heart is removed from a live animal, it continues to beat for a short time.
Heart muscle has the unusual ability to contract and relax on its own without stimulation from an external source.
This type of muscle is known as myogenic muscle. It is a safety mechanism to ensure the heart will beat even if the nervous system is damaged.
The heartbeat is initiated by a cluster of cells in the right atrium sinoatrial (SA) node.

Question 4

Sinoatrial (S.A.) Node

Answer 4

Pacemaker of the heart
Beats are regulated at about 72 beats/minute
The electrical signal first passes over the atria in a wave causing the muscles to contract.

Question 5

AV Node & Purkinje Fibres

Answer 5

The signals then reach a second mass of cells called the atrioventricular (AV) node. It is located in the wall of the heart between the right atrium and right ventricle.
Special conducting fibres , Purkinje fibres, then run down the septum and throughout the muscles of the ventricles.
The heart rate can be adjusted by the sympathetic and parasympathetic nervous system. This is done in response to stress and other activities.