1. Hormones

Chemical messengers
Circulate in the blood stream
Acts as effectors on specific target cells or tissues
- control and regulate processes like water/ion homeostasis, glucose levels and growth
Produced and secreted in specific organs
- mostly secreted from Endocrine Glands but also from the Hypothalamus, Placenta, Testes and other tissues

Properties of Hormone

Hormones can act global because they are transported in the blood stream
They are highly reactive and their concentration in the blood needs to be tightly regulated
- All hormones require a specific receptor at the target cell
  - The same hormone can have different effects in different tissues
  - Achieved by Hormone-specific Receptors expressed by the target cells
- Different tissues or even different cells in the same tissue can express different receptors the number of which may be regulated
  - The receptor determines the effect a hormone has on a target cell
  - Effects depends on processes the receptor in the target cell is linked to
Hormones can be produced and stored or produced de-novo on release
Hormonal effects may kick in on different time scales, from seconds to days
Often work far away from where they have been produced and may have global and/or multiple effects