The Cardiac Cycle (Edexcel International A Level Biology): Revision Note

The Cardiac Cycle

• Circulatory systems are systems which transport fluids containing materials needed by the organism, as well as waste materials that need to be removed

• Circulatory systems are described as being either open or closed

  • In a closed circulatory system blood is pumped around the body and is always contained within a network of blood vessels

  • In an open circulatory system the blood, or blood equivalent, is inside the body cavity and bathes the organs

• Circulatory systems can also be either double or single

  • Double circulatory systems have two loops; one to the lungs and one to the body

  • Single circulatory systems have one loop that includes the lungs and the body

• Humans have a closed, double circulatory system; in one complete circuit of the body blood passes through the heart twice

  • The right side of the heart pumps deoxygenated blood to the lungs for gas exchange; this is the pulmonary circulatory system

  • Blood then returns to the left side of the heart, so that oxygenated blood can be pumped at high pressure around the body; this is the systemic circulatory system

Heart structure

• The human heart has a mass of around 300 g and is roughly the size of a closed fist

• The heart is a hollow, muscular organ located in the chest cavity

• It is protected in the chest cavity by the pericardium, a tough and fibrous sac

• The heart is divided into four chambers

  • The two top chambers are atria

  • The bottom two chambers are ventricles

• The left and right sides of the heart are separated by a wall of muscular tissue called the septum

  • The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart

The human heart is adapted for pumping blood around the body

The heart is a muscular organ; the muscle itself is supplied with blood by the coronary arteries, enabling it to pump blood via a series of major blood vessels

Valves in the heart

• Valves are important for keeping blood flowing forward in the right direction and for maintaining the correct pressure in the chambers of the heart

  • The right atrium and right ventricle are separated by an atrioventricular (AV) valve known as the tricuspid valve

    • Remember; the right side of the heart has a tricuspid valve

  • The left atrium and left ventricle are separated by another AV known as the bicuspid valve

  • The right ventricle and the pulmonary artery are separated by a semilunar (SL) valve known as the pulmonary valve

  • The left ventricle and aorta are separated by another SL valve known as the aortic valve

• Valves in the heart

  • Open when the pressure of blood behind them is greater than the pressure in front of them

  • Close when the pressure of blood in front of them is greater than the pressure behind them

• The valves are attached to the heart walls by valve tendons, or cords; these prevent the valves from flipping inside out under high pressure

Adaptations of the Structures of the Heart Table

Blood vessels and the heart

• There are two blood vessels bringing blood into the heart; the vena cava and pulmonary vein

  • The vena cava brings blood from the body

  • The pulmonary vein brings blood from the lungs

• There are two blood vessels taking blood away from the heart; the pulmonary artery and aorta

  • The pulmonary artery takes blood to the lungs

  • The aorta takes blood to the body

• The muscle of the heart itself is supplied with blood by a series of blood vessels known as the coronary arteries

  • The coronary arteries can be seen running across the surface of the heart

The cardiac cycle

• The cardiac cycle is the series of events that take place in one heartbeat, including muscle contraction and relaxation

  • The contraction of the heart is called systole, while the relaxation of the heart is called diastole

• One cardiac cycle is followed by another in a continuous process

  • There is no gap between cycles where blood stops flowing

Volume and pressure changes

• Contraction of the heart muscle causes a decrease in volume in the corresponding chamber of the heart, which then increases again when the muscle relaxes

• Volume changes lead to corresponding pressure changes

  • When volume decreases, pressure increases

  • When volume increases, pressure decreases

• Throughout the cardiac cycle, heart valves open and close as a result of pressure changes in different regions of the heart

  • Valves open when the pressure of blood behind them is greater than the pressure in front of them

  • They close when the pressure of blood in front of them is greater than the pressure behind them

• Valves are an important mechanism to stop blood flowing backwards

Atrial systole

• The walls of the atria contract

  • Atrial volume decreases

  • Atrial pressure increases 

• The pressure in the atria rises above that in the ventricles, forcing the atrioventricular (AV) valves open

• Blood is forced into the ventricles

  • There is a slight increase in ventricular pressure and chamber volume as the ventricles receive the blood from the atria

• The ventricles are relaxed at this point; ventricular diastole coincides with atrial systole

Ventricular systole

• The walls of the ventricles contract

  • Ventricular volume decreases

  • Ventricular pressure increases

• The pressure in the ventricles rises above that in the atria

  • This forces the AV valves to close, preventing back flow of blood

• The pressure in the ventricles rises above that in the aorta and pulmonary artery

  • This forces the semilunar (SL) valves open so blood is forced into the arteries and out of the heart

• During this period the atria are relaxing; atrial diastole coincides with ventricular systole

  • The blood flow to the heart continues, so the relaxed atria begin to fill with blood again

Diastole

• The ventricles and atria are both relaxed

• Pressure in the ventricles drops below that in the aorta and pulmonary artery, forcing the SL valves to close

• The atria continue to fill with blood  

  • Blood returns to the heart via the vena cava and pulmonary vein

• Pressure in the atria rises above that in the ventricles, forcing the AV valves open

• Blood flows passively into the ventricles without need of atrial systole

• The cycle then begins again with atrial systole

The events of the cardiac cycle allow the heart to pump blood to the lungs and body

Valves during the cardiac cycle table

The pressure changes in the cardiac cycle can be analysed

Analysing the cardiac cycle

• The lines on the graph represent the pressure of the left atrium, aorta, and the left ventricle

• The points at which the lines cross each other are important because they indicate when valves open and close

Point A - the end of diastole

• The atrium has filled with blood during the preceding diastole

• Pressure is higher in the atrium than in the ventricle, so the AV valve is open

Between points A and B - atrial systole

• Left atrium contracts, causing an increase in atrial pressure and forcing blood into the left ventricle

• Ventricular pressure increases slightly as it fills with blood

• Pressure is higher in the atrium than in the ventricle, so the AV valve is open

Point B - beginning of ventricular systole

• Left ventricle contracts causing the ventricular pressure to increase

• Pressure in the left atrium drops as the muscle relaxes

• Pressure in the ventricle exceeds pressure in the atrium, so the AV valve shuts

Point C - ventricular systole

• The ventricle continues to contract

• Pressure in the left ventricle exceeds that in the aorta

• Aortic valve opens and blood is forced into the aorta

Point D - beginning of diastole

• Left ventricle has been emptied of blood

• Muscles in the walls of the left ventricle relax and pressure falls below that in the newly filled aorta

• Aortic valve closes

Between points D and E - early diastole

• The ventricle remains relaxed and ventricular pressure continues to decrease

• In the meantime, blood is flowing into the relaxed atrium from the pulmonary vein, causing an increase in pressure

Point E - diastole

• The relaxed left atrium fills with blood, causing the pressure in the atrium to exceed that in the newly emptied ventricle

• AV valve opens

After point E - late diastole

• There is a short period of time during which the left ventricle expands due to relaxing muscles

• This increases the internal volume of the left ventricle and decreases the ventricular pressure

• At the same time, blood is flowing slowly through the newly opened AV valve into the left ventricle, causing a brief decrease in pressure in the left atrium

• The pressure in both the atrium and ventricle then increases slowly as they continue to fill with blood