CARDIAC REGULATION

I. Nervous Control

A.  Autonomic Nervous System (ANS)

1. Parasympathetic Relaxation

• controlled by the cardio-inhibitory center of the medulla oblongata
• via the Vagus Nerve à acetylcholine à DECREASES heart rate

2. Sympathetic Stimulation

• controlled by the cardio-acceleratory center of the medulla oblongata
• via the Accelerator Nerve à nor-epinephrine à INCREASES heart rate

B.  Cardiac Reflexes

1. Chemoreceptors

• located in the aortic arch & carotid sinus
• monitors pO₂, pCO₂, & pH
• low pO2, high pCO2, & low pH stimulate cardio-acceleratory center
• increased heart rate à increases cardiac output à increases pulmonary blood flow

2. Baroreceptors (= Pressoreceptors)

a.  Marey's Law of the Heart (= Aortic Reflex + Carotid Sinus Reflex) [ARTERIAL BP]

• increased arterial BP à stimulates stretch receptors in aortic arch and/or carotid sinus
• sensory neurons travel to cardio-inhibitory center à Vagus Nerve à acetylcholine released
• heart rate DECREASES (decreases "pushing") à decreases cardiac output à decreases arterial blood volume à decreases arterial BP

b.  Bainbridge Reflex (= Right Atrial Reflex) [VENOUS BP]

• increased venous BP à stimulates stretch receptors in right atrium (= Vena Cavae)
• sensory neurons travel to cardio-acceleratory center à Accelerator Nerve à nor-epinephrine released
• heart rate INCREASES (increases "pulling") decreases venous blood volume à decreases venous BP

II. Endocrine Control

A.   Adrenal Medulla

• sympathetic response to fight, fight, and/or flight
• epinephrine released à INCREASES heart rate

B.   Thyroid

• hyperthyroidism à hypersecretion of thyroxin à increases metabolic rate à INCREASES heart rate
• hypothyroidism à hyposecretion of thyroxin à decreases metabolic rate à DECREASES heart rate

III.  Serum Control

A.  Sodium (Na⁺)

• low sodium (= hyponatremia) à interferes with depolarization à decreases HR (could stop HR)
• high sodium (= hypernatremia) à P-wave flattens, QRS-complex widens, T-wave peaks à increases BP

B.  Potassium (K⁺)

• low potassium (= hypokalemia) à interferes with repolarization à decreases HR à prolonged P-R interval, occassional T-wave inversions
• high potassium (= hyperkalemia) à interferes with depolarization à decreases HR (stops in diastole) à elevated T-waves, prolonged QRS-complex, atrial paralysis

C.  Calcium (Ca⁺⁺)

• low calcium (= hypocalcemia) à lengthens Q-T interval
• high calcium (= hypercalcemia) à causes fibrillation; eventually stops in systole (= calcium rigor)

D.  Magnesium (Mg⁺⁺)

• low magnesium à lengthens heart beat; slows heart rate
• high magnesium à causes cardiac arrest