What is meant by stimulus and reaction?
Stimulus is the change in the environment or environmental factors to to which the organism can respond or react. Theresponse shown by organisms towards or awaya stimulus is called reaction.
Describe the structure of human heart in brief.
The human heart lies near the middle of the thoric cavity between two lungs. It is the hollow, conical organ made up of cardiac muscles. It is about the size of a fist of the owner. It measures about 12cm in length and 9 cm in breadth. The average weight of the heart is about 300 gm in adults.
The human heart contains four complete chambers. It E is divided into two halves (right and left) by septum. Each half consists of two chamber the upper , small sized atrium or auricle and lower larger sized ventricle. The human heart remains enclosed in a double layered membranous sac called pericardium. The space between two pericardium layers is filled with slippery fluid called pericardial fluid that prevents heart from mechanical injury and helps to beat effectively. Various blood vessels entering and leaving the heart can be seen externally.