Hormone Transport In Blood

Hormone Transport In Blood

When hormones are secreted into the blood, many are immediately bound to plasma proteins.

The proteins may recognize the hormone specifically and bind it with high affinity and specificity, for example the binding of sex hormones by sex hormone - binding globulin (SHBG). Other proteins, such as albumin, also bind many hormones, including thyroid hormone and the sex hormones, with much lower affinity. Equilibrium is set up between the free and bound hormone, so that a fixed proportion of the hormone travels free and unbound, while most is carried bound. It is currently believed that only the free fraction of the hormone is physiologically active and available to the tissues and for metabolism. When a hormone is bound to plasma proteins it is physiologically inactive and is also protected from metabolic enzymes in organs such as the liver. Some drugs, such as aspirin, can displace other substances such as anticoagulants from their binding sites, which in the case of anticoagulants may cause haemorrhage.

Classification of Endocrine Hormones

Classification of Endocrine Hormones

Hormones are chemical messengers. They may be classified several ways (see Figure):

1. Autocrine: acting on the cells that synthesized them; for example insulin - like growth factor (IGF - 1) which stimulates cell division in the cell which produced it.

2. Paracrine: acting on neighbouring cells. An example is insulin, secreted by pancreatic b cells and affecting secretion of glucagon by pancreatic a cells.

3. Endocrine: acting on cells or organs to which they are carried in the bloodstream or through another aqueous ducting system, such as lymph. Examples include insulin, estradiol and cortisol.

4. Neuroendocrine: this is really paracrine or endocrine, except that the hormones are synthesized in a nerve cell (neurone) which releases the hormone adjacent to the target cell (paracrine), or releases it into the bloodstream, which carries it to the target cell, for example from the hypothalamus to the anterior pituitary gland through the portal system.

5. Neural: this is neurotransmission, when a chemical is released by one neurone and acts on an adjacent neurone. These chemicals are termed neurotransmitters. Neurotransmitters produce virtually instantaneous effects, for example acetylcholine, whereas some chemicals have a slower onset but longer lasting effect on the target organ, and are termed neuromodulators, for example certain opioids.

6. Pheromonal transmission is the release of volatile hormones, called pheromones, into the atmosphere, where they are transmitted to another individual and are recognized as an olfactory signal.