Cardiovascular Lecture 7 Flashcards
What is cardiac output?
The quantity of blood pumped by the heart per minute
What may vary cardiac output?
Heart rate or stroke volume
CO=HRxSV
What is HR determined by?
SA node (pacemaker) activity (diastolic depolarisation)
What is SV determined by?
Myocardial performance (preload, afterload, contractility)
Describe heart system regulation in terms of ANS activity.
Autonomic nervous system controls HR. Both division influence SA node activity. Parasympathetic tone usually predominates over the sympathetic.
Describe the parasympathetic pathways
• Fibres originate in the medulla oblongata, cells in the dorsal motor nucleus of the
vagus, or in the nucleus ambiguous
• Right vagus predominantly affects SA node, slowing heart rate
• Left vagus mainly inhibits AV conduction, resulting in various degrees of AV block
• Stimulation has very short latency, as acetylcholine (ACh) rapidly activates special
ACh-regulated K+ channels (KACh)
• KACh channels open quickly as muscarinic receptor coupled directly to channel by a
guanine nucleotide-binding protein (Gi
)
• Response decays rapidly, SA and AV nodes rich in
cholinesterase, enzyme that rapidly hydrolyses ACh
• Brief latency and rapid delay allow beat-by-beat
control of SA and AV node function
Describe the sympathetic pathways
• Fibres originate in the intermediolateral columns of thoracic and cervical
segments of the spinal cord
• Innervate the heart chambers as an extensive epicardial plexus of mixed efferents
• Response to stimulation begins slowly:
- Norepinephrine released slowly
- Effects mediated by relatively slow second messenger system involving cAMP
• Response decays gradually, as nerve terminals take
up only ~70% of released norepinephrine, with
remainder carried away by blood
Describe baroreceptor reflex
• Sudden changes in arterial blood pressure initiate reflex that evokes an inverse
change in heart rate
• Most pronounced over an intermediate range of blood pressure
Describe bainbridge reflex
• Increase in heart rate due to distension of the atria by increased volume
• Magnitude and direction of response depends on prevailing heart rate:
- With slow heart rate, increased volume increases heart rate
- At more rapid heart rates, increased volume decreases heart rate
• Due to competing activity of Bainbridge reflex and baroreceptor reflex
Describe atrial/ventricular receptors & natriuretic peptides
• Both atrial chambers have receptors (cardiopulmonary baroreceptors) that
respond to increased blood volume by decreasing heart rate
• Send afferents to brain-stem in the vagi and efferent impulses return to the SA node
via fibres from both autonomic divisions
• Stimulation by stretch also:
- Increases urine volume by a neurally mediated reduction in vasopressin
(antidiuretic hormone) secretion by posterior pituitary gland
- Releases atrial natriuretic peptide from atria (and brain natriuretic peptide from
ventricles) with potent diuretic, natriuretic, and vasodilator effects
Describe respiratory sinus arrhythmia
• Rhythmic variations in heart rate at the frequency of respiration:
- Heart rate increases with inspiration, while sympathetic activity increases
- Heart rate decreases with expiration, while parasympathetic activity increases
Describe chemoreceptor reflex
• Effect on heart rate result of complex interaction of primary and secondary effects
• Stimulation in response to increased CO2
results in increased ventilatory rate / depth
• Magnitude of ventilatory response determines if heart rate increases or decreases
• Mild stimulation results in decreased heart rate, more pronounced stimulation results
in increased heart rate
What is stroke volume regulation determined by?
- Preload - ventricular filling=end-diastolic volume
- Afterload - resistance to blood flow=total peripheral resistance
- Contractility (force/speed of contraction after a given preload or afterload
How do preload and afterload affect SV?
increase preload = increase SV
increase afterload = decrease SV
Describe sympathetic stimulation
Enhances contractility, by increasing magnitude and rate of force generation, as well as rate of relaxation. Due to the effects on intracellular Ca2+ dynamics.
