(i) The endocrine system is a control system of ductless glands that secrete hormones directly through blood. Thus, a smaller size helps in easy diffusion through the blood. Moreover, the endocrine molecules are basically responsible for chemical communication in an organism. These glands secrete special small molecules which needs to be diffused with cells of target organs to detect them with special molecules. Thus, to make the process of diffusion faster in the target organ cells. Hence these hormones consists of small molecules.
(ii) The three endocrine glands and the chemicals they produce are:
1. Pineal gland - It produces melatonin. It is the hormone that affects the sleeping patterns. 2. Thyroid – This gland produces hormones which regulate the heart rate, the rate of metabolism and affect the growth.
3. Pituitary gland – The most important gland, that produces more than 9 hormones – example - thyroid stimulating hormone (TSH).
The organs of endocrine glands are shown here:
