L5 - cardiac cycle

• Contraction is systole
• Relaxation is diastole
• The walls of the heart:

  -muscular walls of the atria are thinner than the ventricles

  -when atria contract, the thin wall do not generate much force but enough to force blood down into the ventricles

  -walls of the ventricles are thicker and more muscular

  -ventricles contract and squeeze blood inwards, increasing the pressure, and out of the heart
• The cardiac cycle:

  -the series of events that take place in one heartbeat

  -continuous process so no gap between cycles where blood stops flowing
• Valves:

  -cusp shaped, flexible, fibrous tissue

  -cusp shape has a concave and convex side

  -string links tendons attached to papillary muscles stop valves inverting
• Higher blood pressure above a valve forces it open
• Higher blood blood pressure below a valve forces it shut
• 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
• Volume and pressure changes:

  -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 (contraction):

  -atria contract causing an increase in pressure in the atria

  -atrioventricular valves open

  -semi lunar valves (pulmonary and aortic) closed

  -blood forces from the atria into the ventricle
• Ventricular systole (contraction):

  -ventricle contracts so pressure increases

  -atrioventricular valves forced shut so pressure increases more

  -semi lunar valves open when ventricular pressure becalmed greater than the aortic and pulmonary artery pressures

  -when the aortic and pulmonary artery pressures get bigger than the ventricular pressure the semi lunar valves are forced shut
• Diastole (relaxation):

  -heart relaxes and atria fill a with blood from the veins

  -then the pressure in the atria gets larger than the ventricular pressure forcing the atrioventricular valves open

  -blood then passively starts to fill the ventricles

  -semi lunar valves are closed
• Heart muscle cells are myogenic
• Myogenic means that a muscle is able to contract with no external stimulus, like a nerve cell
• Cardiac muscle structure:

  -consist of fibres that branch, producing cross bridges that help to spread the wave of excitation across the heart

  -they have lots of mitochondria to release energy for contraction and are separated by intercalated discs which helps to synchronise the contraction
• Intercalated discs are unique structural formations between the myocardial cells of the heart. They play vital roles in bonding cardiac muscle cells together and in transmitting signals between cells
• Wave of excitation pattern:

  -sino atrial node found in the right atrium acts as a pacemaker sending regular waves of electrical activity over atrial walls

  -wave of excitation passed to the atrio ventricular node where a band of non conducting tissue prevents this wave from passing to the ventricles

  -delay allows atria to fully empty

  -avn causes slight delay before passing wave down to the bundle of His

  -purkyne fibres make ventricle walls simultaneously contract from the bottom up for efficiency
• Bundle of His: a bundle of conducting tissue containing purkyne fibres
• Purkyne fibres: conductive muscle fibres which branch upwards
• Electrocardiogram:

  -also called an ECG

  -used to record the spread of electrical excitation in the heart

  -used to measure tiny differences in electrical activity on the skin, which have resulted from the electrical activity of the heart

  -electrodes stuck to skin using a gel to help detect the electrical activity, signal fed to machine that produces a trace

  -used to help doctors diagnose heart problems or show heart attacks
• Depolarisation: discharge of electrical energy
• Repolarisation: decrease of voltage after depolarisation
• Healthy ECG trace:

  
• P wave:

  -caused by the depolarisation of the atria, which results in atrial contraction (systole)
• QRS complex:

  -caused by the depolarisation of the ventricles, which results in ventricular contraction (systole)

  -the largest wave because the ventricles have the largest muscle mass
• T wave:

  -caused by the repolarisation of the ventricles, which results in ventricular relaxation (diastole)
• PQ segment:

  -flat line following the P wave

  -signal pauses at the non conductive tissue in the AV node

  -sometimes called PA segment
• ST section:

  -represents the interval between ventricular depolarisation and repolarisation
• QT section:

  -shows the complete time for the depolarisation and repolarisation of the ventricles
• Tachycardia:

  -when the heart beats too fast it is tachycardic

  -an individual with a resting heart rate of over 100bpm is said to have tachycardia
• Bradycardia:

  -when the heart beats too slow it is bradycardic

  -an individual with a resting heart rate below 60bpm is said to have bradycardia

  -a lot of fit individuals or athletes tend to have lower heart rates and it is usually not dangerous
• Ectopic heartbeat:

  -this condition is caused by an early heartbeat followed by a pause

  -it is common in the population and usually requires no treatment unless very severe
• Fibrillation:

  -an irregular heartbeat will disrupt the rhythm of the heart

  -severe cases of fibrillation can be very dangerous, even fatal

  -arrhythmia is an abnormal heart rhythm
• Ectopic heartbeats are when either the atria or ventricle has an extra beat. we all usually have at least one a day
• Cardiac output:

  -the term used to describe the volume of blood that is pumped by the heart (ventricles) per unit of time
• Cardiac output = heart rate x stroke volume
• Heart rate: number of beats per minute
• Stroke volume: volume of blood ($cm^3$) pumped by heart in 1 beat
• Blood pressure (key points):

  -measured in blood vessels (artery)

  -determined by cardiac output and resistance to flow of blood in vessels

  -resistance to flow affected by diameter of blood vessels

  -narrower vessels (vasoconstriction)

  -wider vessels (vasodilation)
• Systolic blood pressure:

  -maximum blood pressure

  -occurs when ventricles are contracting (at the end of the cardiac cycle)