3.1.2f/h Cardiac Cycle & Control Flashcards
What is the cardiac cycle
An ongoing sequence of contraction & relaxation of the atria & ventricles that keeps blood continuously circulating around the body.
How does the cardiac cycle work
- The volumes of the atria & ventricles change as they contract & relax, altering the pressure in each chamber.
- This causes valves to open & close, which directs the blood flow through the heart.
The cardiac cycle can be simplified into 3 stages
3 stages of the cardiac cycle
1. Ventricles relax, atria contract
2. Ventricles contract, atria relax
3. Ventricles relax, atria relax
3 stages of the cardiac cycle: 1. Ventricles relax, atria contract
- The ventricles are relaxed. The atria contract, which decreases their volume & increases their pressure.
- This pushes the blood into the ventricles through the atrioventricular valves.
- Theres a slight increase in ventricular pressure & volume as the ventricles receive the ejected blood from the contracting atria
3 stages of the cardiac cycle: 2. Ventricles contract, atria relax
- The atria relax. The ventricles contract, decreasing their volume, increasing their pressure.
- The pressure becomes higher in the ventricles than the atria, which forces the atrioventricular valves shut to prevent back-flow.
- The high pressure in the ventricles opens the semi-lunar valves - blood is forced out into the pulmonary artery & aorta
3 stages of the cardiac cycle: 3. Ventricles relax, atria relax
- The ventricles & the atria both relax. The higher pressure in the pulmonary artery & aorta causes the semi-lunar valves to close, preventing back-flow.
- The atria fill with blood (increasing their pressure) due to the higher pressure in the vena cava & pulmonary vein.
- As the ventricles continue to relax, their pressure falls below the pressure in the atria.
- This causes the atrioventricular valves to open & blood flows passively (w/o being pushed by atrial contraction) into the ventricles from the atria.
- The atria contract, & the whole process begins again.
What is cardiac output
The volume of blood pumped by the heart per minute
measured in cm^3 min^-1
Formula to calculate cardiac output
cardiac output = heart rate x stroke volume
What is heart rate
The number of beats per minute
(bpm)
What is stroke volume
The volume of blood pumped during each heartbeat
(cm^3)
see pg83 for dia of cardiac cycle stages
What is the cardiac (heart) muscle described as
Cardiac (heart) muscle is ‘myogenic’ - it can contract & relax w/o receiving signals from nerves. This pattern of contractions controls the regular heartbeat
How is the heart beat initiated & coordinated
- The sino-atrial node (SAN) (pacemaker) is located in the wall of the right atrium, & is responsible for initiating heartbeat by sending out regular waves of electrical excitations to the atrial walls.
- SAN sends a wave of electrical excitation across the atria. This causes the right & left atria to contract at the same time.
- A band of non-conducting collagen tissue prevents the waves from being passed directly from the atria to the ventricles.
- Instead, the wave of electrical excitation is transferred from the SAN to the atrioventricular node (AVN).
- The AVN is responsible for passing the wave of excitation on to the bundle of His. But, theres a slight delay (0.1secs) before the AVN reacts, to make sure the ventricles contract after the atria have emptied
- The bundle of His is a group of muscle fibres responsible for conducting the wave of excitation to the finer muscle fibres in the right & left ventricle walls, called the Purkyne tissue.
- The Purkyne tissue. carries the wave of excitation into the muscular walls of the right & left ventricles, causing them to contract simultaneously, from the apex up.
What is an Electrocardiograph
A doctor can check someone’s heart function using an electrocardiograph - a machine that records the electrical activity of the heart.
The heart muscle depolarises (loses electrical charge) when it contracts, & repolarises (regains energy) when it relaxes.
An electrocardiograph records these changes in electrical charge using electrodes placed on the chest
What is an electrocardiogram
The trace produced by an electrocardiograph is called an electrocardiogram, or ECG
What does a normal ECG look like
- The P wave is caused by contraction (depolarisation) of the atria
- The main peak of the heartbeat, tg with the dips at either side, is called the QRS complex - its caused by contraction (depolarisation) of the ventricles
- The T wave is due to relaxation (depolarisation) of the ventricles
- The height of the wave indicates how much electrical charge is passing through the heart - a bigger wave means more electrical charge, so for the P & R waves, a bigger wave means a stronger contraction
(graph on goodnotes images)
What do doctors use ECGs for
Doctors compare their patient’s ECGs with a normal trace.
This helps them diagnose any heart problems
What does an ECG of tachycardia look like
- This heartbeat is too fast - around 120bpm. Its called tachycardia
- This may be acceptable during exercise, but at rest it shows that the heart isnt pumping blood efficiently
- A heartbeat can also be too slow - below 60bpm at rest. This is called bradycardia
(graph on goodnotes images)
What does an ECG of ectopic heartbeat look like
- The 5th heartbeat on this ECG is an ectopic heartbeat - an ‘extra’ heartbeat
- Here its caused by an earlier contraction of the atria than in the previous heartbeats (you can see P wave is different & comes earlier than it should)
- However, it can be caused by early contraction of the ventricles too.
- Occasional ectopic heartbeats in a healthy person dont cause a problem (most ppl have 1 a day) - only linked to serious conditions when frequent
What does an ECG of fibrillation look like
- This is fibrillation - a really irregular heartbeat
- The atria or ventricles completely lose their rhythm & stop contracting properly
- It can result in anything from chest pain & fainting to lack of pulse & death
Who usually have bradycardia
- Many ppl have bradycardia bc they are fit - training makes the heart beat more slowly & efficiently.
- Severe bradycardia can be serious & may need an artificial pacemaker to keep heart beating steadily
