H.1.1 State that hormones are chemical messengers secreted by endocrine glands in the blood and transported to specific target cells
H.1.2 State that hormones can be steroids, proteins and tyrosine derivatives, with one example of each
H.1.3 Distinguish between the mode of action of steroid hormones and protein hormones
H.1.4 Outline the relationship between the hypothalamus and the pituitary gland
H.1.5 Explain the control of ADH (vasopressin) secretion by negative feedback
As we saw earlier, hormones are secreted by the Endocrine System through a series of endocrine glands. The endocrine glands are ductless glands. They simply secrete hormones into the blood, which transports it around the body. As the hormones pass cells, only the cells with special receptors will react to the presence of hormones. These cells are called target cells.
Hormones have many affects in the body. They can be categorized into seven areas:
- Regulate the chemical composition and volume of the internal environment (extracellular fluid)
- Help regulate metabolism and energy balance.
- Help regulate contraction of smooth and cardiac muscle fibers and secretion by glands.
- Help maintain homeostasis despite emergency environmental disruptions, such as infections, trauma, emotional stress, dehydration, starvation, hemorrhage, and temperature extremes.
- Regulate certain activities if the immune system.
- Play a role in the smooth, sequential integration of growth and development.
- Contribute to the basic processes of reproduction, including gamete production (sperm and egg), fertilization, nourishment of the embryo and fetus, delivery and nourishment of the newborn.
Hormones can be steroids (testosterone), peptides (insulin) or tyrosine derivatives (thyroxine).
Hormones can be produced in the adrenal cortex, kidneys, testes, ovaries, thyroid, adrenal medulla, Mast cells in the connective tissues, hypothalamus, pituitary gland, pancreas, stomach and small intestine. (see handout)
Mode of Action of Hormones
Once the hormones have reached the target cells, they need to effect a change inside the cell. Hormones can act on cells in two different ways in the body.
- Steroid hormones and thyroxine, lipid soluable hormones, will bind to receptors on the cell surface membrane and they will be transported into the cell. There they will bind to receptors in the cytoplasm or nucleus. The hormone-receptor will then switch specific genes on or off. (This was discussed in the lac operon model.)
- Polypeptide hormones (hydrophilic) will bind to receptors on the cell membrane surface. The binding of the polypeptide hormone on the outside will cause a change on the inside of the cell membrane surface. This will change the concentration of a substance known as the ‘second messenger’, which will activate, for example, a specific enzyme. An example of a second messenger is cyclic AMP. Cyclic AMP is synthesized from ATP by an enzyme called adenylate cyclase.
cAMP synthesis allows certain enzymes to exert their physiological responses. The hormones are antidiuretic hormone (ADH), oxytocin, follicle stimulating hormone (FSH), luteinizing hormone, thyroid stimulating hormone (TSH), adrenocorticotropic hormone (ACTH), calcitonin, parathyroid hormone (PTH), glucagons, epinephrine, norepinephrine,
and hypothalymic releasing hormone. For growth inhibiting hormone, (somatostatin) the decrease in cAMP allows it to bind.
The Hypothalamus and Pituitary Gland
Most hormones are released in short bursts, with little or not secretion between bursts. Regulation of the secretion normally maintains homeostasis and prevents the overproduction and underproduction of a particular hormone. If the regulating mechanisms do not operate properly, and hormonal levels are excessive or deficient, then several disorders develop.
The Hypothalamus – Pituitary Gland
The hypothalamus is a small region of the brain, located below the two lobes of the thalamus. It is the major link between the Nervous System and the Endocrine System. It is the main organ for hormones, synthesizing at least nine different hormones.
The pituitary gland is a pea-sized structure that lies below the hypothalamus and is attached by a thin stalk called the infundibulum. It is separated into the anterior lobe and the posterior lobe. Both the lobes also secrete hormones.
Draw a Diagram of the Hypothalamus and Pituitary Glands
Specific Hormonal Control – Thyroxin and ADH
Control of Thyroxin
Thyroxin is produced and secreted by the thyroid gland. The thyroid gland is located just below the larynx, with left and right lobes on either side of the trachea. Thyroxin increases the metabolic rate and heat production in the body.
The release of thyroxin is done together with the hypothalamus, pituitary gland and the thyroid, using negative feedback.
- The hypothalamus produces Thyroid Releasing Hormone.
- This hormone stimulates the anterior pituitary to produce and release Thyroid Stimulating Hormone, or TSH.
- Thyroxin is released when the TSH gets to the thyroid.
- The thyroxin increases the metabolic rate and the basal temperature increases, along with the other factors previously mentioned in maintaining Homeostasis.
(See the Summary of Temperature Regulation)
Control of ADH (Anti Diuretic Hormone) – More detail in our Excretion Unit
Another hormone that is also controlled by negative feedback is ADH, or Anti Diuretic Hormone. It is also called Vasopressin.
The posterior lobe of the pituitary does not synthesize hormones, but it does store and release ADH.
The main function of ADH is to decrease urinary output and maintain water levels. In the absence of ADH, normal urine output of 1-2 L would increase to 25 L per day. ADH is also released if there is a loss of blood volume, diarrhea and excess sweating.
Water potential is a measure of the tendency of water to move between regions. In practice it is a force, acting on water molecules in solution when separated from pure water by membrane permeable to water only. We will talk more about water potential in excretion.
How it works
The kidneys are the organs used for maintaining the water potential in the mammal. The large amounts of water that pass through the kidneys is mostly reabsorbed. The exact amount of water is controlled by the ADH.
- ADH is produced in the hypothalamus and secreted from the posterior pituitary gland.
- The hypothalamus contains osmoreceptors, which detect changes in the blood solute potential.
- If the water concentration is lower than usual in the blood, the osmoreceptors detect the high osmotic pressure (low water concentration).
- The ADH is synthesized and released, by nerve impulses acting on the posterior pituitary. It is carried by the blood to the kidneys. The kidneys respond by decreasing urine output, and more water is retained. The ADH also decreases the rate of perspiration. ADH can also increase blood pressure caused by the constriction of arterioles.
- If the water concentration in the blood is higher than normal, the osmoreceptors will detect the low osmotic pressure, and the ADH secretion is reduced or stopped. The kidneys put out a large amount of urine.
An example is when you feel thirsty. The mucous membranes in your mouth are drier than normal. This is caused by the lower amount of water in your body and your response will be to drink some fluid.
By the way, alcohol inhibits ADH secretion and increases urine output. You get dehydrated and may cause both the thirst and the banger of a headache of a typical hangover!!
Draw a flow chart for ADH secretion.
These are some examples of hormones. Some work by positive feedback as well, and act on different parts of the body. The following pages outline some of the more common hormones and functions.