This video describes the main endocrine glands and their main functions, as well as water-soluble and fat-soluble hormones.
FUNCTIONS OF THE ENDOCRINE SYSTEM
The endocrine system is one of the two major communication systems of the body. The nervous system communicates via neurotransmitters released by neurons, and the endocrine system communicates via hormones released by particular cells into the bloodstream. Both the nervous system and the endocrine system are key components of homeostasis. The nervous system responds to stimuli in the environment through various receptors (such as those for pain, temperature, pH, pressure), and neurotransmitters target other neurons, muscle cells (skeletal, smooth, or cardiac), or glands (including adrenal glands, salivary glands). The nervous system response time from stimulus to target cells is extremely rapid and only lasts for a brief period of time. Think of a moment when something startled you and you experienced an instant electrical feeling flood your body; that is a nervous system response. The lingering effects are due the epinephrine released from the adrenal gland into the bloodstream as a hormone.
The endocrine system involves many glands that produce hormones that can target any cell type in the body. Generally, the hormone response is initiated more slowly than the nervous system response, but the effects last much longer. It is essential that the nervous system and the endocrine system cooperate to ensure proper reactions to changes in the external environment. The major endocrine glands of the body include the hypothalamus, pituitary gland, pineal gland, thyroid and parathyroid glands, thymus, pancreas, adrenal glands, testes and ovaries (Figure 17.1). Many cells that are not considered endocrine glands also produce hormones.
As a point of comparison, exocrine glands are part of the epithelial tissues that secrete substances (enzymes, sweat, oil, mucus) through a duct into a hollow cavity or to the surface of the body. For example, digestive enzymes are released into the small intestine, and sweat glands secrete sweat onto the surface of the skin.
Functions of Hormones
Hormones have many functions in the body, depending on the cell type they bind to and what signalling processes occur within the particular cell. General functions of hormones include the following:
- Homeostatic regulation of blood sugar levels, water balance, blood calcium levels, blood pressure, thirst, hunger, blood cell production, as well as the regulation of other hormones in the blood
- Growth, metabolism, and energy production
- Reproductive functions such as secondary sex characteristics, lactation, childbirth, and development of sperm or eggs
- Stress response
- Regulation of digestion
- Regulation of circadian rhythms such as sleep-wake cycles
TYPES OF HORMONES
For hormones to exert an effect they must bind to a specific receptor either on the cell membrane or inside the target cell. All hormones initiate a signalling pathway that either directly or indirectly affects transcription and translation of a gene or multiple genes. For example, insulin is a hormone secreted by the pancreas in response to an increase in blood sugar after we eat food. Insulin binds to very specific receptors on all cells to initiate the transcription and translation of the glucose-transporter protein, which then embeds into cell membranes so glucose can enter the cells.
There are two categories of hormones, water soluble and fat soluble. Water-soluble hormones cannot diffuse across membranes, so the specific receptor is located on the extracellular side of the membrane: only on cells that the hormone is meant to target. Water-soluble hormones elicit their response indirectly through a second messenger system (Figure 17.2). A second messenger relays a signal from the membrane receptor to a target molecule inside the cell, and this alters the activity of the cell. Fat-soluble hormones can easily cross cell membranes to bind to their receptor inside the cell, and usually they directly affect transcription and function as transcription factors.
Water-soluble hormones are modified amino acids, small peptides, or proteins. The four most common categories of water-soluble hormones are the following:
- Catecholamines—dopamine, epinephrine, and norepinephrine
- Pancreatic hormones—insulin and glucagon
- Pituitary hormones
- Hypothalamic hormones
Fat-soluble hormones are steroids produced from cholesterol (Figure 17.4), except thyroid hormones, which are fat-soluble but are made from tyrosine. Recall from Chapter 2 the many important functions of cholesterol. The five most common fat-soluble hormones are the following:
- Thyroid hormones—T3 and T4
- Cortisol
- Aldosterone
- Androgens—testosterone, DHEA, estrogen, and progesterone
- Calcitriol (active Vitamin D3)
Did You Know?
We make some very important molecules from amino acids other than proteins. Tryptophan and tyrosine are the amino acids used to produce hormones. Tryptophan is an essential amino acid, so it has to be acquired through the diet (mostly poultry, eggs, and dairy, but also fish and beef), and it is used to produce serotonin, which is both a hormone and a neurotransmitter and involved in feeling happy and content. Tyrosine is the amino acid used to produce dopamine, thyroid hormones, and the catecholamines. Tyrosine is not an essential amino acid, so our cells can produce it from other amino acids.
