13. Phases of the Cardiac Cycle

• Relaxation Period .4 seconds

- heart is in diastole for .4 seconds
- the atria are filling
- cuspid valves are open, semilunar valves are closed
- rapid ventricular filling
- SA Node fires and AP spreads over atria
- P wave occurs

• Atrial Systole .1 seconds

- atrial contraction for .1 seconds sends 25 mls of blood into ventricles
- slow ventricular filling = end-diastolic volume of 130 mls

• Ventricular Systole .3 seconds

- action potential passes over AV Node to bundle of His and purkinje fibers
- ventricles depolarize, QRS Complex
- turbulence as ventricles begin to contract closes A-V valves "LUBB"
- semilunar valves open as pressure in ventricles rises above pressure in
   pulmonary arteries and aorta
- 80 mmHg to 120 mmHg
- ventricles complete contraction in .25 seconds
- ventricles repolarize, T wave
- 70 mls of blood is ejected, stroke volume
- blood remaining in ventricles = end-systolic volume of 60 mls
- AV valves open inferiorly, semilunar valves close inferiorly "dupp"

• reasons for a long refractory period

- allows cadiac muscle to repolarize
- allows chambers to fill
- allows circulation through coronary system

14. Cardiac Output

• Stroke Volume = amount of blood ejected at each contraction
• CO = SV X HR
• End Diastolic Volume = amount of blood in ventricles before contraction
• End Systolic Volume = amount of blood in ventricles after contraction
• SV = EDV - ESV
• Frank-Starling’s LAW OF THE HEART;  the greater the heart muscle is stretched, the more forceful the contraction

15. Arrhythmias

• AV block
  - conduction through AV Node slowed

- causes:

1. vagus nerve stimulation
2. destruction of AV Node by an MI
3. atherosclerosis
4. depression caused by digitalis
5. inflammation of myocardium

- ECG misses a QRS complex
- corrected with an artificial pacemaker

• Atrial Fibrillation
  - atria flutter rapidly rather than contract
  - causes:

1. MI
2. rheumatic heart disease
3. mitral valve stenosis

- ECG has quivering P wave
- digitalis to increase contraction force and slow heart rate

• Ventricular Fibrillation

- a cardiac emergency as ventricles contract haphazardly
- ECG no pattern
- defibrillation is used

• Bradycardia
  - heart rate below 60 BPM
  - causes:

1. heart block
2. low body temperature
3. digitalis and beta blockers
4. parasympathetic stimulation

- ECG waves normal

• Tachycardia

- heart rate above 100 BPM
- causes:

1. fever
2. ectopic foci
3. thyroid disease
4. infections
5. anxiety

            - ECG waves normal

16. Factors that regulate Heart Rate

• Nervous system: Cardiovascular center in medulla oblongata

Cardioacceleratory center
1. increases heart rate
2. over sympathetic nervous system
3. norepinephrine speeds rate of depolarization of SA node
     and increases influx of Ca⁺⁺ through slow channels

Cardioinhibitory center
  1. slows heart rate
  2. over parasympathetic nervous system X
  3. acetylcholine is the neurotransmitter

• Carotid and aortic bodies monitor blood pressure and adjust heart rate as needed
• Chemicals

- thyroxine increases heart rate
- decreased heart rate due to

1. acidosis/alkalosis
2. low pO₂
3. hyperkalemia
    - heart rate weak and irregular with little repolarization
    - kidneys excrete excess K⁺ but when pH falls H⁺ are
       excreted rather than K⁺
4. hypokalemia
    - cell membrane hyperpolarizes
    - K⁺ stays in cell muscle weakness
5. hypocalemia
    - weak contractions