Question

8. Describe how the heartbeat originates and is regulated. O2, COs, nutrients, and wastes into and out of the capillaries.

Answer 1

8). SA node (Sino Auricular node) is present in the right auricle, which is also called as the pace maker of the heart. The electric signals for cardiac contraction are generated in the SA node.

Ectopic pacemaker is a group of excitable cells that are present outside the SA node, which causes premature heartbeat. Ectopic pacemaker results in cardiac arrhythmias. For example, atrial fibrillation (AF) is an abnormal rhythm of the heart (a type of cardiac arrhythmia) characterized by palpitations and discomfort in the chest region. It is caused due to the generation of atrial ectopic pacemaker firings that are dominated by the firings of SA node.

The rhythmic contractions of heart (produces lubb-dupp sounds) supply the blood to all the body parts. In a healthy adult, the normal heart rate is 72 beats per minute and this may increase or decreases depend upon the physiological changes. The time taken for one cardiac cycle is 0.08sec. During the cardiac cycle, the contraction (systole) and relaxation (diastole) of auricles and ventricles take place simultaneously.

This rhythmic contraction of the heart is regulated by the pacemaker cells and neurotransmitters. Overactivity of the sympathetic nervous system increases heart rate, and parasympathetic system decreases heart rate. The cardiac center is located in the medulla oblongata.

For example, if the auricles are overfilled with the blood, these receptors trigger a protective reflex known as “atrial reflex” or “Bainbridge reflex.” In response to the atrial reflex, the cardiac center responds and adjust the sympathetic and parasympathetic innervation of the heart (i.e. either by increasing the heart rate) to avoid overstretching of the heart valves.