Topic 7B (SAN & AVN, ECG, heartrate and breathing)

Explain how the coordinated contraction of heart muscle is brought about/heart rate is controlled
(5marks)

• Cardiac muscle is myogenic (in nature)
  
  

• SAN found in the right atrium initiates electrical excitation (or depolarization)
  
  

• this causes atria to contract (or atrial systole)
  
  

• the electrical excitation is transferred from SAN to AVN, where there is a small delay

(to ensure atrial systole is complete and that ventricles are filled with blood)

• AVN then passes electrical excitation down the Bundle of His which splits into purkyne fibres
  
  

• Purkyne fibres pass E.E into the ventricles, causing ventricles to contract starting from apex of heart (bottom)

NOTE:
SAN → atrial depolarisation(electrical excitations) → atrial systole → AVN delay → Bundle of His → Purkyne fibres → ventricular depolarisation → ventricular systole from apex upwards

The cardiovascular centre in the medulla oblongata can alter the rate of excitation of the SAN via sympathetic and parasympathetic nerves.

Describe the role of the sinoatrial node (SAN) in controlling heart rate

Explain the role of chemoreceptors

Explain the role of barroreceptors