Hormone

Hormones are the body's chemical messengers which travel through the bloodstream to tissues and organs, and control most of our body's major systems
The endocrine messengers are called hormones.
Chemical messengers can travel from a signaling cell to nearby target cells by diffusion in a process called paracrine communication. These messengers can even affect the signaling cell, in a process called autocrine communication.
For long-distance cell-to-cell communication, animals use the endocrine system.
 In the endocrine system, the chemical messenger travels from the signaling cell to the target cell carried by the circulatory system
Animals can even send chemical messengers between individu-als, a system termed exocrine communication
In exocrine communication, a chemical termed a  pheromone is released by one individual and travels through the external environment (e.g., air or water) to exert its effects on a different individual.
Autocrine and paracrine communication are very rapid, because chemical signals need only diffuse across very small distances. Diffusion is a rapid process at these scales, so autocrine and paracrine communication occurs on a time scale of milliseconds to seconds. Nervous communication is similarly rapid. Propagation of electrical signals within a neuron occurs on a millisecond scale, and diffusion of a neurotransmitter across the synapse is also rapid. In contrast, endocrine communication is usually slower, because it relies on transport of hormones in the circulatory system
The secretory cells of the exocrine and endocrine tissues are often grouped into structures called  glands

Type of hormones -

The chemical structure of the hormones is the critical property that affects the way in which indirect signaling is accomplished. Hydrophobic hormones use different mechanisms for signaling than do hydrophilic hormones, because hydrophobic hormones can diffuse freely across cell membranes whereas hydrophilic hormones can not.

There are six main classes of chemicals which are as follows -     

         1- Peptide hornhorm

         2- Steroids hormones

         3-Amines

        4- lipid hormones

         5-purines hormones

        6- Gasious hormones

    1 - Peptide hormones

Peptide and protein messengers consist of two or more amino acids linked in series, and range in size from 2 to 200 amino acids in length. Chains of fewer than 50 amino acids are usually called peptides, while the word  protein is used for longer chains. Peptide and protein hormones are hydrophilic chemicals that cannot diffuse across the membranes, they require receptor for that peptide hormones present on the outer surface of the cell

   Like - insulin

Peptide hormones are often synthesized as large, inactive polypeptides called  preprohormones and by the proteolytic activity the prohormones converted into active form of hormones

   2 - Steroids hormones

   Steroids are derived from the molecule cholesterol, and are important hormones in both vertebrates and invertebrates. Steroids can also act as paracrine and autocrine signals in some tissues, and are important pheromones involved in communication among animals

Mineralocorticoids are involved in regulating sodium uptake by the kidney, and are important for fluid and electrolyte balance in the body. 

 Aldosterone is the primary mineralocorticoid in mammals. The glucocorticoids (cortisol, cortisone, and corticosterone), also called the stress hormones, have widespread actions including increasing glucose production, increasing the breakdown of proteins into amino acids, increasing the release of fatty acids from adipose tissue, and regulating the immune system and inflammatory responses.

 The reproductive hormones (estrogens, progesterone, testosterone), regulate sex-specific characteristics and reproduction.

     3- Amines

   Amines are chemicals that possess an amine(–NH2) group attached to a carbon atom.

 Amines that function in cellular signaling are termed biogenic amines. 

Many amines are synthesized from amino acids. The catecholamines(dopamine,norepinephrine, and epinephrine) are synthesizedfrom the amino acid tyrosine. 

Dopamine, which is found in all animal taxa, acts as a neurotransmitter. Norepinephrine and epinephrine are known only from vertebrates, and can act as neurotransmitters, paracrines, and hormones. 

 The thyroid hormonesare synthesized from a polypeptide containing the aminotyrosine. These messengers are found only in the vertebrates, and act as hormones.

 Serotonin, which is synthesized from the amino acid tryptophan, is a neurotransmitter found in all animal taxa. 

Melatonin, which is also synthesized from the amino acid tryptophan, is found in almost all organisms and acts as a neurotransmitter and a hormone

  4  - Lipid hormones

   A class of lipids known as the eicosanoids can act as neurotransmitters and paracrine chemical messengers

Most eicosanoids are derivatives of arachidonic acid, a 20-carbon fatty acid common in plasma membrane phospholipids.

Prostaglandins are one of the most studied groups of eicosanoids because they are involved in pain perception. Many common painkillers (including aspirin and ibuprofen) work by blocking prostaglandin synthesis.

5- Purine hormones

A variety of purines including adenosine, adenosine monophosphate (AMP), adenosine triphosphate (ATP), and the guanine nucleotides are known to act as neurotransmitters or paracrines.

Adenosine acts on the immune system to promote wound healing, can change the rhythm of the heartbeat in vertebrates, and is a potent calming neurotransmitter in the brain.

  6- Gasious hormones

   Only three gases are known to act as chemical messengers in animals: nitric oxide, carbon monoxide, and hydrogen sulfide.  Nitric oxide (NO) was the first gas identified as a chemical messenger, and a great deal is now known about its mechanisms of action. Nitric oxide is produced by the enzyme  nitric oxide synthase (NOS), which catalyzes the reaction of the amino acid arginine with oxygen to produce nitric oxide and citrulline (another amino acid).