Lec. 6 & 7. Heart rate.

Factors affecting HR.

Heart rate is regulated by many factors that include Nervous, Physical, and Chemical factors.

Nervous factors.

---

Impulses from higher centers in the brain sending signals to the heart to regulate HR.

1. The respiratory center.
   • ==Respiratory sinus arrythmia:== HR increases with inspiration and decreases with expiration. Due to the proximity of the respiratory and cardiac centers allowing irradiating signals from RC to reach CC.
2. Due to afferent cardiac nerves.
   • ==Bainbridge==: HR increases with increased cardiac filling / atrial pressure. Some say it is due to direct SAN stretching causing excitability.
   • ==Marey’s law==: HR decreases with increased arterial blood pressure. Increased ABP causes stimulation of arterial baroreceptors which send signals to higher centers causing inhibition of SA node. Stimulation of arterial baroreceptors also causes: (Anything that reduces BP.)
     1. Inhibition of vasomotor center causing VD.
     2. Inhibition of adrenaline secretion.
     3. Inhibition of respiratory center.
     4. inhibition of ADH secretion causing urination, decreased BV and decreased BP.
   • ==Carotid and aortic chemoreceptor reflex==: HR and rate of breathing (hyperpnea) increase with increased CO₂ levels, decreased O₂ levels, or blood acidity. This is to restore blood concentrations to normal.

(Bainbridge is concerned with AT-RIAL BP. Marey’s is concerned with ART-ERIAL BP.)

Physical factors.

---

Temperature is the major physical factor affecting HR. HR increases with increased temperature. This is due to an effect on the hypothalamus and SA node excitability. However, in some cases increased temperature is not followed by increased HR, such as: Diphtheria, Meningitis, Tetanus. This is due to toxins that are released by these organisms that slow down the heart.

Chemical factors.

---

Blood gases.

Moderate changes in blood gasses usually cause increased HR, while severe changes cause decreased HR. (moderate hypoxia, hypercapnia, acidosis = increased HR.) (severe hypoxia, hypercapnia, acidosis = decreased HR.)

Hormones.

1. Adrenaline:
   • Small doses activate cardiac ${\beta }_1$ receptors -⇒ Increased HR.
   • Large doses activate vascular $\alpha$ receptors -⇒ VC -⇒ Increased ABP -⇒ Decreased HR. (Marey’s law.)
2. Noradrenaline:
   • Activates vascular $\alpha$ receptors -⇒ VC -⇒ Increased ABP -⇒ Decreased HR. (Marey’s law.)
3. Thyroxine, atropine, histamine, and sympathetic drugs -⇒ Increased HR.
4. Parasympathetic drugs -⇒ Decreased HR.