The Heart Flashcards
What type of muscle is the heart muscle?
Myogenic
Explain what myogenic means
- Contracts without nervous stimulation
- Even if removed from nervous system it still beats
What causes heart muscle contraction? 4
- Electrical charge changes in cardiac muscle
- As polarity changes spread from cell to cell
- Stimulate contraction of cardiac muscle
- Pattern of contractions co-ordinate regular heartbeat
How is a cell polarised?
If it is slightly positive on the outside
How is a cell depolarised?
If it is slightly negative on the outside
What is the sinoatrial node [SAN]? 2
- Small group of specialised muscle fibres known as the pacemaker
- Set rhythm of the heartbeat by sending regular waves of electrical activity to atrial walls
Explain the electrical activity of the heart - 6
- SAN initiates depolarisation
- passes through wall of atria
- causes atrial systole
- AVN conducts to ventricles
- conducts to Purkyne fibres - bundle of His
- ventricular systole follows from apex
Where is the AVN located?
Right and left ventricle walls
What is the AVN?
Group of large specialised muscle fibres conducing impulses rapidly to apex of ventricles
Where is the SAN located?
Right atrium wall under the superior vena cava opening
What is an ECG?
Graphic record of electrical activity in the cardiac cycle
How is an ECG formed?
- The heart depolarises - loses electrical charge - when it contracts
- The heart polarises - regains electrical charge - when it relaxes
- Changes in polarisation cause small electrical currents to be detected on skin’s surface by chest electrodes
What are the 4 parts of an ECG from order of left to right? 4
- P wave
- PR interval
- QRS complex
- T wave
What does the P wave indicate?
Contraction and depolarisation of atria in atrial systole
What does the PR interval indicate?
Time for conduction of impules from SAN across atria to ventricles through AVN
Why is there a time delay between the conduction of impulses from the atria to the ventricles?
Ensures ventricles contract after atria have emptied
What does the QRS complex indicate?
Contraction/depolarisation of ventricles in ventricular systole
What does the T wave indicate?
The repolarisation/relaxation of the ventricles in cardiac diastole
What is bradycardia?
Heart rate of less than 60bpm
What condition is indicated by a heart rate of less than 60bpm?
Bradycardia
What is tachycardia?
Heart rate of more than 100bpm
What condition is indicated by a heart rate of more than 100pm?
Tachycardia
What can cause bradycardia? 4
- Athlete at rest
- Hypothermia
- Heart disease
- Drugs
What can cause tachycardia? 4
- Anxiety
- Heart disease
- Exercise
- Anemia
What does a low QRS complex indicate? 2
- Muscle damage
- AVN not conducting impulses to ventricles properly
What does an early/odd P wave indicate? 2
- Ecoptic heartbeat
- Early contraction of atria
What is fibrillation?
Ventricles and atria completely lose rhythm and eventually stop contracting completely - could lead to death
Label an ECG diagram
SEE NOTES
Define cardiac output
Total volume of blood pumped out of ventricle every minute
Give the formula for calculating cardiac output
cardiac output : heart rate x stroke volume
What are the units of cardiac output?
cm3min-1
What are the units of stroke volume?
cm3
Define stroke volume
The volume of blood pumped out by a ventricle each time it contracts
How does increased exercise increase stroke volume and cardiac output? 4
- During exercise there is a greater venous return: more blood returning to heart because more respiration
- Muscle stretched to greater extent
- Comes into contact with greater force
- More blood is expelled
What is residual blood?
When body is at rest ventricles don’t completely empty with each beat
How does exercise affect residual blood?
Stronger contractions eject more residual blood from the heart
What 2 things influence heart rate?
- Heart size
- Amount of training
How does heart size lower heart rate? 2
- Increased stroke volume with each beat
- So need to beat less frequently to circulate same volume of blood around the body
The larger the heart, the x the resting heart rate
Lower
What 2 things affect heart size?
- Genetics
- Body size
How does training reduce heart rate? 4
- Increase in heart size
- Due to thickening of heart muscle walls
- Increased stroke volume with each beat
- So need to beat less frequently to circulate same volume of blood around the body
How is heart rate increased? 4
- increased impulses to SAN
- via sympathetic nervous system
- stimulates more frequent depolarisation in SAN
- increases heart rate
