The Human Endocrine System

In the previous chapter (Coordination), we have seen one way in which information is sent from a receptor to an effector (through nerves!). We will be learning the other way – hormones. 

In this section, we will be studying about the next way of transmitting information- through hormones.

  • Chemicals that transmit information from one part of the body to other are called as hormones
  • Hormones are made in endocrine glands
  • Endocrine glands release hormones directly into the blood and so they are called as ductless glands (as they aren’t carried in ducts)
  • Hormones only have effect on target organs (specific organs)

Here is a list of some endocrine glands:

  • Pituitary gland
  • Thyroid gland
  • Adrenal gland
  • Testes (in males)
  • Ovaries (in females)
  • Pancreas


  • In our body, there are two adrenal glands, one above each kidney.
  • They secrete a hormone called adrenaline.
  • Adrenaline is secreted when you are frightened, exited, or keyed up (anxious)
  • Importance of adrenaline: Needed for fight and flight response.
What adrenaline does How it helps the body
Makes your heart beat faster (supplying more oxygen to brain and muscles) Gives more energy for fighting or running away
Increases your breathing rate More oxygen enters the lungs; improved blood circulation
Gives you butterflies in your stomach (by contracting blood vessels in the skin and digestive system) It ensures that maximum volume of blood is transported to your muscles and brain, and you can think faster
Causes the pupils in the eye to widen Helps you to see danger more clearly
Causes the liver to release glucose into the blood It again gives the muscles extra energy to fight or run

To summarise, adrenaline prepares body for vigorous action

Important endocrine glands and their functions:

Gland Hormone secreted Function of hormone
Adrenal Adrenaline E prepares body for vigorous action
Pancreas Insulin E Reduces the concentration of blood glucose
Testis Testosterone E Causes the development of male secondary sexual characteristics
Ovary Oestrogen E Causes the development of female secondary sexual characteristics

E Helps in the control of the menstrual cycle

The difference between the nervous and endocrine system:

Nervous system Endocrine system
Made up of neurones Made up of secretory cells (glands)
Information transmitted in form of electrical impulses Information transmitted in form of hormones
Impulse transmitted along nerve fibres Hormones transmitted along blood plasma (in dissolved form)
Impulses travel very quickly Hormones travel more slowly
Effect of impulse only lasts for a shorter period Effect of hormone usually lasts for longer duration

Plant hormones

Plants respond to light and gravity with the help of plant hormones.

Plants need to use hormones as they don’t have sense organs.

One kind of plant organ is Auxin

Auxin is being made all the time on the tip of the shoot.

The Auxin then spreads to the rest of the plant by diffusing across cells.

Auxin is responsible for the rate of growth for plants; the more Auxin present, the greater will its growth be.

How plants respond to phototropism?

Hormones - Auxins

Normally, Auxin is equally distributed at the tip of the shoot; however, when light is shined at the shoot’s tip in one direction, the Auxin gets concentrated at the shady part of the shoot; hence the growth at the shady part increases and the plant begins to grow in the direction of light.

How plants respond to gravitropism?

If a potted Coleus plant is placed horizontally in the absence of light, Auxin will collect at the lower part of the shoot and increase the rate of growth in that specific part. This will cause the plant’s shoot to change direction and give a negative response to gravity.

Similarly, a bean seedling’s root will always show positive response to gravitropism by growing downwards, irrespective of the position in which it is planted.


Seedlings grown in the dark tend to be pale and weak. But the reason why they are tall and thin is Auxin. Auxin gets evenly distributed at the tip of the shoot and grows steadily using the energy from the food stored in cotyledons of the seed. But once that gets exhausted, it gets ethiolated and will soon die if it is not given light to photosynthesise.


Weedkillers are chemicals that are needed to kill unwanted plants that compete with other plants for minerals, light and water.

Weedkillers are usually selective and kill only specific plants.

They contain a synthetic form of Auxin called 2,4D which causes rapid growth in weeds, so that they grow faster and die quickly.

This leaves ample space for the other plants, leading to an increase in their growth.


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