- A fish has only two-chambered heart (single circuit).
- In amphibians and most of the reptiles (except crocodilia), heart is three-chambered (incomplete circuit).
- In crocodiles, birds and mammals the heart is divided into four chambers (double circuit).
- Single circuit means blood flows only once through the heart while double circuit means blood flows twice through the heart as shown below in human heart. A new born baby's heart weight about 20 g.
- The chambers which receive the blood from body tissues are called auricles and the chambers of heart which pump blood to body tissues are called ventricles.
- Human heart beat is myogenic in nature, i.e. initiated by a patch of modified heart muscles itself without requiring an external stimulation. This patch is called SA node (sino-auricular node) or pacemaker.
- When SA-node becomes defective, i.e., it does not generate cardiac impules, it can be cured by surgical grafting of an artificial pacemaker (an electric device), in the chest of the patient. It stimulate the heart electrically at regular intervals.
- The normal rate of the heart beat at rest is about 70-72 per minute. In a newly born baby, heart rate is about 140 per minute.
- During heavy exercises it may be high as 170-200 per minute. Heart is the only organ which can function for some time outside our body.
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