Hormones that are dissolved in blood typically exhibit a half-life on the order of ________, whereas hormones that are bound to carrier proteins can remain undegraded for ________.

Chemical messengers are molecules that transmit messages between neurons, or from neurons to muscles.  Some types of chemical messengers include: Neurotransmitters: These are molecules used by the nervous system to transmit messages. Hormones: These are chemicals that carry messages through your blood to your organs, skin, muscles, and other tissues. Neuropeptides Pheromones  Chemical messengers can also be local or paracrine. Autocrine agents: These are local chemical messengers that are released into the extracellular fluid and act upon the same cells that secreted them. Paracrine... Show more

Chemical messengers are molecules that transmit messages between neurons, or from neurons to muscles. 

Some types of chemical messengers include:
Neurotransmitters: These are molecules used by the nervous system to transmit messages.
Hormones: These are chemicals that carry messages through your blood to your organs, skin, muscles, and other tissues.
Neuropeptides
Pheromones 

Chemical messengers can also be local or paracrine.

Autocrine agents: These are local chemical messengers that are released into the extracellular fluid and act upon the same cells that secreted them.
Paracrine chemical messengers: These are local messengers that are secreted by one cell type but affect neighboring cells of a different type.
Paracrine signaling: This is a mechanism in which one cell secretes a molecule that acts on a second cell in close proximity. 
 

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