• Increased contractility – ESV ↓ because of increased volume of ejected blood – Because both EDV and ESV decrease, SV can remain unchanged Parasympathetic NS opposes sympathetic effects • ACh hyperpolarizes pacemaker cells by opening K + channels, which slows HR • Has little to no effect on contractility Autonomic Nervous System regulation of HR Regulation of Heart Rate
Autonomic Nervous System regulation of HR cont. Regulation of Heart Rate – Heart at rest exhibits vagal tone • Parasympathetic is dominant influence on heart rate • Decreases rate about 25 beats/min • Cutting vagal nerve leads to HR of 100 beats/min – When SNS is activated, PNS is inhibited, and vice-versa Atrial ( Bainbridge ) reflex : sympathetic reflex initiated by increased venous return , hence increased atrial filling • Atrial walls are stretched with increased volume • Stimulates SA node, which increases HR
Chemical regulation of heart rate • Hormones • Epinephrine from adrenal medulla ↑ HR & contractility • Thyroxine ↑ HR; enhances effects of NE & E • Ions • Intra- and extracellular ion concentrations (e.g., Ca 2+ and K + ) must be maintained for normal heart function – Imbalances are very dangerous to heart – Hypocalcemia : depresses heart – Hypercalcemia : increases HR and contractility – Hyperkalemia : alters electrical activity, which can lead to heart block and cardiac arrest – Hypokalemia : results in feeble heartbeat; arrhythmias Regulation of Heart Rate
Regulation of Heart Rate Other factors that influence heart rate – Age • Fetus has fastest HR; declines with age – Gender • Females have faster HR than males – Exercise • Increases HR • Trained athletes can have slow HR – Body temperature • HR increases with increased body temperature
Fill in the blanks: The contraction of the ventricles is referred to as and the period of ventricular relaxation is called . The first heart sound is a result of closure of the valves and closure of the valves causes the second heart sound. The heart chambers that have just been filled when you hear the first heart sound are the . The chambers that have just emptied when you hear the first heart sound are the . Immediately after the second heart sound, the are filling and the __________________ have emptied. Abnormal sounds, or usually indicate valve problems. 1 . During atrial systole: a) the atrial pressure exceeds ventricular pressure b) 70% of ventricular filling occurs c) the AV valves are open d) valves prevent backflow into the great veins e) a and c 2. Threshold in pacemaker cells is marked by: a) opening of sodium gates b) opening of calcium slow channels c) opening of calcium fast channels d) opening of potassium gates CHECK YOUR UNDERSTANDING
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- Winter '16
- Josef Buttigieg
- right ventricle