Week 2 Bioscience Flashcards
Endocrine System
To maintain homeostasis, the activities of body cells are constantly changing, and these changes are regulated by the body’s two communication systems: the nervous system and the endocrine system. Although these two systems work together to regulate body functions, they act in very different ways.
Hormone Production and Secretion
Hormones are produced and secreted by the various endocrine glands located throughout the body (see figure below), as well as the kidneys, stomach and heart as these organs contain small populations of endocrine cells.
Hormones are secreted in response to a stimulus, and the stimuli that trigger endocrine glands to secrete their hormone(s) can be grouped into three main categories:
1. Hormonal stimulus - one hormone stimulates the secretion of another
2. Humoral stimulus - changes in ion or nutrient blood levels, e.g. glucose, Ca2+, Na+, K+.
3. Neural stimulus - signals from the nervous system
Chemical Classes of Hormones
Steroid hormones:
○ are made from cholesterol
○ are lipid-soluble and can easily diffuse across the plasma membrane
○ bind to receptors inside a cell - i.e. intracellular receptors
Amino acid-based hormones:
○ vary in size – can be single amino acids, peptides or proteins
- are lipid insoluble and cannot easily diffuse across the plasma membrane
- bind to receptors embedded in the plasma membrane – i.e. plasma membrane receptors
Hormone Actions
The many effects of hormone action include:
* activating or deactivating enzymes - e.g. enzymes required for metabolic reactions)
* changing the permeability of the plasma membrane by opening or closing ion channels
* stimulating protein synthesis - e.g. structural proteins for growth and cell division, enzymes for metabolic reactions, protein channels (e.g. aquaporins) for transportation
* inducing cell secretory activity - e.g. stimulates hormone secretion
* stimulating cell division - e.g. for body growth and development
Hypothalamus-Posterior Pituitary
As the posterior pituitary is a down growth of the hypothalamus it is composed of neural tissue. It does not produce hormones, but rather secretes two of the hormones (i.e oxytocin and ADH) produced by the hypothalamus as follows:
1. The cell bodies of neurons within the hypothalamus produce the hormones oxytocin and antidiuretic hormone (ADH).
2. The axons of these neurons form the hypothalamic-hypophyseal tract, which transports these hormones through the infundibulum to the posterior pituitary, where they are stored in the axon terminals.
3. When these hypothalamic neurons are stimulated the stored hormones are secreted from the posterior pituitary into the bloodstream.
Antidiuretic Hormone (ADH)
Stimulus for secretion: ADH is secreted by the posterior pituitary when blood Na+ levels increase above the normal range and/or blood volume and blood pressure decrease below the normal range.
Main Action: ADH primarily targets the kidneys, and as the name “antidiuretic” suggests ADH decreases urine output by stimulating the kidneys to return more water to the blood (i.e. ADH increases the reabsorption of water from the urine being produced into the blood).
This effectively:
* dilutes the blood plasma, restoring normal blood Na+ levels
* increases blood volume and pressure to normal levels
* maintains normal blood volume and pressure
Oxytocin
Stimulus for secretion: The secretion of oxytocin is stimulated by the stretching of the uterus during labour and the suckling action of the infant during breastfeeding.
Main actions: Oxytocin targets the uterus to stimulate smooth muscle contractions during labour and the mammary glands to stimulate the ejection of milk during breastfeeding.
Hypothalamus-Anterior Pituitary
Hypothalamic neurons also produce and secrete releasing and inhibiting hormones (see table below) and these hormones regulate (stimulate or inhibit) the secretion of anterior pituitary hormones.
FSH
Stimulates:
- the development of oocyte containing follicles
- production and secretion of oestrogen
LH
stimulates:
- ovulation
- production and secretion of progesterone and oestrogen
Prolactin (PRL)
Stimulus for secretion: Prolactin-inhibiting hormone (PIH) primarily controls the secretion of PRL from the anterior pituitary. Decreased PIH secretion leads to an increase in PRL secretion.
Main action: PRL targets the mammary glands and stimulates milk production.
Adrenocorticotropic Hormone (ACTH)
Stimulus for secretion: Corticotropin-releasing hormone (CRH) stimulates the secretion of ACTH from the anterior pituitary.
Main action: ACTH targets the adrenal cortex (outer part of the adrenal glands) and stimulates the adrenal cortex to secrete the hormone cortisol.
Thyroid-stimulating Hormone (TSH)
Stimulus for secretion: Thyrotropin-releasing hormone (TRH) stimulates the secretion of TSH from the anterior pituitary.
Main action: TSH targets the thyroid gland and stimulates the thyroid gland to secrete the thyroid hormones T3 (triiodothyronine) and T4 (thyroxine).
Growth Hormone (GH)
Stimulus for secretion: The secretion of GH is controlled by two hypothalamic hormones:
* Growth hormone-releasing hormone (GH-RH) - stimulates the secretion of GH
* Growth hormone-inhibiting hormone (Somatostatin) - inhibits the secretion of GH
Main action: As its name suggests, GH promotes the growth of body tissues, especially skeletal muscle and bone. GH stimulates cells in the liver, skeletal muscles and bones to secrete insulin-like growth factors (IGFs), which stimulate the actions required for growth - i.e. cell division and protein synthesis.
Thyroid hormones (TH).
Stimulus for secretion: Thyroid-stimulating hormone (TSH) stimulates the secretion of TH from the thyroid gland.
Main actions: TH affect virtually every cell in the body and primarily:
* increases basal metabolic rate (BMR) - i.e. the amount of energy (ATP) required by body cells to carry out all metabolic reactions at rest
