The cardiac cycle

We all know that the main purpose of the heart is to pump blood around the body so that our vital organs and our muscles can receive the oxygen that they require!!

In this part I will be looking at the cardiac cycle of the heart and how this process works!

The heart is a cardiac muscle it has four chambers these are left ventricle, left atrium, right ventricle and right atrium.(The atrium's are at the top of the heart and the ventricles are at the bottom of the heart)
Oxygenated blood passes through the pulmonary vein in the left atrium, when the atrium is full the atrioventricular valves open allowing the blood to be pushed into the ventricles. As the blood enters the ventricles the atrioventricular valves stop the blood from flowing back into the atrium. The ventricles contract pushing the blood through the semi-lunar valves into the aorta. This is where the blood leaves the heart and travels around the body via the arteries, arterioles and capillaries. Deoxygenated blood enters the heart through the superior vena cava and the inferior vena cava into the right atrium, as the pressure builds up in the atrium the semi-lunar valves are pushed open allowing the blood to enter the right ventricles. When the heart contracts the deoxygenated blood is forced into the pulmonary artery so that the blood can enter the lungs and become oxygenated.

Did you know that your heart beats 70 times a minute! and that each heartbeat lasts around 0.8seconds!

Within each heart beat the heart relaxes and contracts this is known as systole(contract) and diastole (relax). The heart is relaxed longer then it is contracted. This is because the nervous system which is in a part of the brain called the medulla oblongata sends electrical impulses to the heart in order to stimulate the sinoatrial node (SAN), This is know as the hearts natural pacemaker. These electrical impulses allow the cardiac muscle to contract the right and left atrium allowing the blood to pass through the atrioventricular valves into the relaxed ventricles. There is a slight delay here so that the atria can empty fully. The electrical impulses spread into tissues called Bundle of His. These tissues connect the atrioventricular node to the septum. The bundle of his has branches leading into the ventricle walls known as purkinje fibres. The purkinje fibres allow the impulse to travel through them so that they can get to the ventricle walls. as this happens it the impulses cause the ventricles to contract (also known as ventricular systole). The impulses are stronger at the ventricles because the blood has further to travel than the atria.
This process is repeated every time the heart beats and it is known as THE CARDIAC CYCLE.

Cardiac output is the volume of blood pumped by each ventricle in one minute.
It is calculated by a simple sum Q = SV x HR (this means Q(cardiac output) = SV(stroke volume) x HR(heart rate)
Measuring Cardiac output is extremely important as it allows us to know how healthy a heart is. This allows us to know if the heart is under strain i.e. heart failure. In a healthy human being the average cardiac output is around 5 litres of blood per minute.

The heart has its own system called a pacemaker which is connected to the SA node located at the top of the heart. It contains sympathetic and parasympathetic nerve fibres. These fibres release hormones which can either slow the heart rate down or speed the heart rate up. These hormones are delivered under conditions of rest(parasympathetic nerve fibres) or emotional and physical activity (sympathetic nerve fibres).