Cardio Vascular System Heart Rate Youtube Lecture Handout Medical Science

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Heart Rate

Heart Rate

  • Normal heart rate (Sinus Rhythm)
    • 70 to 80 beats/min. range is 50 to 100
  • Sinus Bradycardia
  • Sinus Tachycardia
  • Intrinsic Heart Rate when all nerves are cut 100/min
  • Heart rate is low in trained athelets

Sinus Arrhythmia

  • Sinus Arrhythmia HR varies with the phases of respiration
    • Increases during inspiration
    • Decreases during expiration

Explanations:

1. Central irradiation … Impulses irradiates from Res. to Cardiac centre

2. Bainbridge Reflex

3. Afferent impulses arise from the lungs during inspiration & reflexly inhibit the vagal tone

Factors Affecting

  • Size of animal-
    • Elephant … 25 beats/min
    • Dog
    • Canary bird-1000/min.
  • Age
    • Foetus … 140 to 150/min
    • At birth … 130 to 140 ″
    • 12 yrs … 100 to 110/ ″
    • Adult … 70 to 80/ ″
    • Old age … up to 100/ ″
  • Sex-HR more in females as they have high sympathetic tone
  • Body temp-HR ↑ by 10 beats for every 1°F increase in temp.
  • Emotional stimuli
    • Fear, anger, excitement … ↑ HR.
    • Grief, shock … ↓ HR.
  • Exercise … ↑HR
  • Pain
    • Superficial pain … ↑HR
    • Deep pain … ↓HR
  • 8. Respiration … Sinus arrhythmia

Pathological Cond

Diseases

  • Thyrotoxicosis ↑ HR
  • Hypothyroidism ↓ HR

Drugs

  • Ephinephrine ↑HR
  • Acetylcholine ↓HR

Heart Rate Regulation

Adjusted according to need of body

  • Increases during exercise
  • Falls during sleep

Two mechanisms

1. Local Mech. — any change in SAN function or junction tissue affect HR.

2. Nervous mechanism

Includes:

  • Cardio excitatory center (Vasomotor center) acts through sympathetic nerves
  • Cardio inhibitory center acts through Vagus
  • They are in reciprocal relation
  • Vagus exerts a Vagal tone

Sympathetic Innervation

  • From T3 & T4
  • Nerves of right side have more influence on SAN
  • Of left side have more on AVN
  • Neurotransmitter … Norepinephrine
  • Receptor β1 adrenergic receptor
  • Mechanism of action – acts by ↑ concentration of Cyclic AMP which leads to↑ influx of Na + & Ca ++
  • Blocker … Propranolol, atenolol

Actions

Sympathetic (Stimulation Causes)

  • + Chronotropic effect (↑rate)
  • + Inotropic effect (↑force)
  • + Bethmotropic effect (↑excitability)
  • + Dromotropic effect (↑conductivity)

Sympathetic Vasodilator Fibers

Mainly supply skeletal muscles. They

Cause vasodilatation only during exercise & this leads to anticipatory rise in blood flow

Is controlled by Anterior hypothalamus

Parasympathetic Innervation

  • Parasympathetic Rt. & Lt. Vagus
    • Right supplies to SAN
    • Lt. to AVN
  • Ventricles have no or very little parasympathetic innervation
  • Neurotransmitter-Acetylcholine
  • Receptors- Muscarinic M1
    • Mechanism of Action - ↓ Ca ++ entry in nodal tissue
  • Blocker- Atropine

Parasympathetic Stimulation

Causes

  • Chronotropic effect (↓rate)
  • Inotropic effect
  • Bethmotropic effect
  • Dromotropic effect

Vagal Tone

  • Vagal tone is reflexly produced through sinoaortic nerves & exerts a stronger inhibitory action
  • Action is suppressed by Atropine & it can increase HR up to 150/min.
  • Normal physiological variations in heart rate are by change in vagal tone

Vasovagal Syncope

  • Fainting associated with intense emotional disturbances
  • Strong inhibitory signals come to heart through vagus - intense stimulation of vasodilator fibers to skeletal muscles
  • Fall in BP. & ↓ Blood supply to brain
  • Person loses consciousness
Sympathetic Nerve and Parasympathetic Nerve

Reflexes Influencing HR

  • Sino-aortic reflex/Cardio inhibitory/Marey՚s Reflex
  • ↑BP. Results in ↓ HR. through Vagus. Baroreceptors are involved

Marey՚s Law: Heart rate & BP have an inverse relation

  • 2. Bainbridge reflex Cardio acceleratory reflexes
  • 3. Other reflexes
  • Sensory stimuli can alter H. R.
  • Stimulation of splanchnic nerves ↓ H. R.
  • Sudden blow on abdomen can stop heart

Other Factors

  • Body temp. Affect HR. by
    • Direct action on SAN
    • Stimulate Cardio acceleratory centre
  • Impulses from higher centers — Frontal lobe of cerebral cortex & hypothalamus
    • Excitement increases H. R.
    • Sudden shock may even stop heart

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