control of cardiac output

• Cardiac output

  • CO = heart rate x stroke volume
  • CO = volume of blood ejected by each ventricle per minute
  • HR = number of heart beats per minute
  • SV = volume of blood ejected by each ventricle per beat
  • To increase CO, increase HR and/or SV
  • Athletes generally have lower HR and higher SV
  • Physiological ventricular hypertrophy = thickening of the wall of the heart'smain pumping chamber
• Autonomic innervation and hormonal regulation of heart rate

  • Factors affect HR = chronotropic effects under control of autonomicinnervation and hormones
  • Cardiovascular system and CNS have receptors which detect changes in bloodand cerebrospinal fluid to increase of decrease HR from CO
  • Chemoreceptors = in medulla and carotid bodies
  • Baroreceptors = internal carotid artery and aorta
  • Receptors detect changes needed to change CO and impulses are sent along sensory nerves via a process call cardiac reflex
  • Impulses reach medulla at the cardioregulatory centre
• Cardioinhibitory centre

  • Connected to parasympathetic nervous system
  • Sends impulses along vagus nerve (cranial nerve 10) usingneurotransmitter acetylcholine
  • Impulses arrive at SA node which tells pacemaker cells to slowHR
• Cardioacceleratory centre

  • Connected to sympathetic nervous system
  • Sends impulses along sympathetic ganglia (T1-T4 in spinal cord)using noradrenaline
  • Acts on SA node and stimulates adrenal medulla to secretehormones such as adrenaline and NA which are secreted into the circulation and reach the heart
• Autonomic effects on ionic control at the SA node

  • Parasympathetic: K+ channels activated by Acetylcholine. Allows for K+ to exit cells and membrane potential is hyperpolarized so it takes longer for depolarisation to reach threshold
  • Sympathetic: allows for Na+ to enter which means reduced repolarization so less of a +ve increase to reach threshold potential
• Vagal tone

  • Vagal tone is activity of the vagus nerve, the 10th cranial nerve and afundamental component of the parasympathetic branch
  • At rest vagal tone reduces HR to 60-100 bpm where there is little to nosympathetic activity
  • Athletes have higher vagal tone = lower HR
• Effects of venous return and atrial reflex on HR

  • With increased venous return the right atrium has stretch receptors whichsenses more blood returning and sends impulses to the medulla to activate thesympathetic NS and increase HR
  • This is the atrial/bainbridge reflex
  • When pacemaker cells are stretched = increased HR
• Stroke volume and CO

  • Increase ESV = increase SV
  • decreased ESV = increased SV
• • Bradycardia = slow heart rate
  • Tachycardia = high heart rate