IB Biology SL

Revision Notes

6.2.4 The Blood System: Cardiac Cycle

Introduction to the Heart

Heart structure

  • The human heart has a mass of around 300g 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 Human Heart, downloadable AS & A Level Biology revision notes

The human heart has four chambers and is separated into two halves by the septum

  • The heart is divided into four chambers. The two top chambers are atria and 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 portion of the septum which separates the left and right atria is called the interatrial septum, while the portion of the septum which separates the left and right ventricles is called the interventricular septum
  • The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart

Valves in the heart

  • 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
  • Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards. They are also important for maintaining the correct pressure in the chambers of the heart
  • The right atrium and right ventricle are separated by the atrioventricular valve, which is otherwise known as the tricuspid valve
  • The right ventricle and the pulmonary artery are separated by the pulmonary valve
  • The left atrium and left ventricle are separated by the mitral valve, which is otherwise known as the bicuspid valve
  • The left ventricle and aorta are separated by the aortic valve
  • There are two blood vessels bringing blood to the heart; the vena cava and pulmonary vein
  • There are two blood vessels taking blood away from the heart; the pulmonary artery and aorta

Coronary arteries

  • The heart is a muscle and so requires its own blood supply for aerobic respiration
  • The heart receives blood through arteries on its surface, called coronary arteries
  • It’s important that these arteries remain clear of plaques, as this could lead to angina or a heart attack (myocardial infarction)

The Outside of the Heart, downloadable AS & A Level Biology revision notes

The coronary arteries cover the outside of the heart, supplying it with oxygenated blood

Sinoatrial Node

  • The contraction of the heart is called systole, while the relaxation of the heart is called diastole
  • Atrial systole is the period when the atria are contracting and ventricular systole is when the ventricles are contracting
  • Atrial systole happens around 0.13 seconds after ventricular systole
  • During ventricular systole, blood is forced out of the pulmonary artery (to the lungs) and aorta (to the rest of the body)
  • One systole and diastole makes a heartbeat and lasts around 0.8 seconds in humans.
  • This is the cardiac cycle

Initiation of the heartbeat by the sinoatrial node

  • Control of the basic heartbeat is myogenic, which means the heart will beat without any external stimulus
    • This intrinsic rhythm means the heart beats at around 60 times per minute
  • The sinoatrial node is a group of cells in the wall of the right atrium which have extensive membranes specialized to promote rapid depolarization
  • A wave of electrical activity leads to contraction of the atria in the following way
    1. Firstly, the membranes of cells in the sinoatrial become depolarized and this initiates the start of the cardiac cycle as the cells contract
    2. The electrical impulse moves to adjacent cells so they also contract
    3. The electrical activity spreads across the atria causing contraction of the cardiac muscle cells, this is called atrial systole
  • The sinoatrial node is considered the pacemaker of the heart because it initiates the heart beat and so controls the speed at which the heart beats
  • Artificial pacemakers are electronic devices implanted just underneath the skin. They can be used to replace or regulate the sinoatrial node if it becomes defective

Cardiac Cycle

  • Once the wave of depolarization has passed over the atria, it reaches a region of non-conducting tissue which prevents the signal spreading straight to the ventricles
    • Instead, the electrical impulse is carried to the atrioventricular node
    • This is a region of conducting tissue between atria and ventricles
  • After a slight delay, the atrioventricular node is stimulated and passes the stimulation along conductive fibres (bundles of His) in the septum (middle) of the heart
    • This delay means that the atria can complete their contraction before the ventricles contract
  • The electrical signal is then carried through Purkyne fibres which spread around the ventricles, causing contraction of the ventricles from the apex (bottom) of the heart
  • This is called ventricular systole
  • Blood is forced out of the heart into the pulmonary artery and aorta

Stages in the cardiac cycle table

Stages in the Cardiac Cycle Table, downloadable AS & A Level Biology revision notes

Electrical Activity of the Heart_1, downloadable AS & A Level Biology revision notes

The wave of depolarisation spreads across the heart in a coordinated manner

Worked Example

Explain the roles of the sinoatrial node, the atrioventricular node and the conductive fibres in a heartbeat.

The sinoatrial node sends out a wave of excitation and this spreads across both atria, causing atrial systole. Non-conducting tissue between the atria and ventricles prevents the excitation from spreading to the ventricles and so this ensures that atria and ventricles don’t contract at the same time. The Atrioventricular node then sends the wave of excitation to the ventricles after a short delay of around 0.1 – 0.2 seconds, ensuring that the atria have time to empty their blood into the ventricles. The conductive (Purkyne) fibres conduct the excitation down the septum of the heart and to the apex, before the excitation is carried upwards in the walls of the ventricles. This means that during ventricular systole, the blood contracts from its base and blood is pushed upwards and outwards.

Cardiac Cycle Pressure Changes

  • Throughout the cardiac cycle, heart valves open and close as a result of pressure changes in different regions of the heart
  • Valves are an important mechanism to stop blood flowing backwards
  • During diastole, the heart is relaxing
    • The atrioventricular valves open
    • The semilunar valves are closed
  • During systole, the heart contracts and pushes blood out of the heart
    • The atrioventricular valves are closed
    • The semilunar valves are open
  • The contraction of the muscles in the wall of the heart reduces the volume of the heart chambers and increases the pressure of the blood within that chamber
  • When the pressure within a chamber/vessel exceeds that in the next chamber/vessel the valves are forced open and the blood moves through
  • When the muscles in the wall of the heart relax they recoil which increases the volume of the chamber/vessel and decreases the pressure so that the valves close

Pressure Changes in the Heart Table

Pressure Changes in the Heart Table, downloadable AS & A Level Biology revision notes

Pressure Changes in the Cardiac Cycle, downloadable AS & A Level Biology revision notes

Cardiac Cycle Labelled, downloadable AS & A Level Biology revision notes

Image showing the pressure changes within the aorta, left atrium and left ventricle during the cardiac cycle.

Analysing the cardiac cycle

  • There are several key points to keep in mind when analysing the cardiac cycle
  • The curves on the graph represent the pressure of the left atria, aorta and the left ventricle
  • The points at which the curves cross each other are important because they indicate when valves open and close

Point A – both left atrium and left ventricle are relaxed

  • Pressure sits at roughly 0 kPa

Between points A and B – atrial systole

  • Left atria contracts and empties blood into the left ventricle

Point B – beginning of the ventricular systole

  • Left ventricular pressure increases
  • AV valve shuts
  • Pressure in the left atria drops as the left atrium expands

Point C – pressure in the left ventricle exceeds that in the aorta

  • Aortic valve opens
  • Blood enters the aorta

Point D – diastole

  • Left ventricle has been emptied of blood
  • Muscles in the walls of the left ventricle relax and pressure falls below that in aorta
  • Aortic valve closes
  • AV valve opens

Point E – expansion of the left ventricle

  • There is a short period of time during which the left ventricle expands
  • This increases the internal volume of the left ventricle which decreases the pressure

Exam Tip

When looking at the heart, remember the right side of the heart will appear on the page as being on the left. This is because the heart is labelled as if it were in your body and flipped around.

Remember that the heart is myogenic, which means that the heart will generate a heartbeat by itself and without any other stimulation. Instead, the electrical activity of the heart regulates the heart rate.

The maximum pressure in the ventricles is substantially higher than in the atria. This is because there is much more muscle in the thick walls of the ventricles which can exert more force when they contract.



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