The pressure that must be overcome before a semilunar valve can open is the afterload ◦ Ejection of blood from heart begins when pressure in right ventricle exceeds pressure in pulmonary trunk and when pressure in left ventricle exceeds pressure in aorta ◦ At that point, the higher pressure in ventricles causes blood to push semilunar valves open
NS regulation of heart originates in cardiovascular center in medulla oblongata ◦ This region receives input from sensory receptors and from higher brain centers (cerebral cortex) ◦ Cardiovascular center then directs appropriate output by increasing or decreasing frequency of nerve impulses in autonomic nervous system
Hormones ◦ Epinephrine and norepinephrine (from adrenal gland) enhance heart’s pumping effectiveness Increase both heart rate and contractility ◦ Exercise, stress, and excitement cause adrenal gland to release more hormones ◦ Thyroid hormones – increases heart rate and contractility One sign of hyperthyroidism (excess thyroid hormone) is tachycardia ( elevated resting heart rate)
Cations ◦ Sodium and potassium are crucial for production of action potentials ◦ Elevated blood levels of potassium or sodium decreases heart rate and contractility ◦ Excess sodium blocks calcium inflow during cardiac action potentials, thereby decreasing force of contraction ◦ Excess potassium blocks generation of action potentials (can cause bradycardia, slowing down of heartbeat) ◦ Moderate increase of calcium speeds heart rate and strengthens heartbeat
Newborn baby has resting heart rate over 120 beats/min, rate then gradually declines throughout life Adult females often have slightly higher resting heart rates than males Regular exercise tends to bring resting heart rate down in both genders bradycardia – resting heart rate under 50 beats/min ◦ Beneficial in endurance type training because slower heart rate is more energy efficient than one that beats more rapidly
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- Fall '16
- Lambrini Nicopoulos
- right ventricle