Endocrine System Flashcards
Endocrine Glands
- Two kinds of glands:
- Exocrine glands: secrete into a duct that carries the secretion to the body surface, or one of the body cavities.
- For example, sweat glands, mucous glands and salivary glands.
- Endocrine glands: secrete hormones into the extracellular fluid that surrounds the cells making up the gland. The secretion usually passes into the capillaries to be transported by the blood.
- Examples: hypothalamus, thymus, pituitary, adrenal glands, etc.
Hormones
- Hormones are the secretion of an endocrine gland.
- They may be proteins, steroids or amines.
- They travel through the blood and affect either: all the cells, target cells or target organs.
- Hormones are only able to influence a cell if it has the specific receptor.
- Saturation can occur.
- There are three types of hormones:
- Protein and amine hormones attach to receptor molecules in the cell membrane.
- This starts a chain reaction resulting in a secondary messenger substance (for example releasing Ca2+ or cAMP) to diffuse through the cell and activate particular enzymes.
- Steroid hormones (being fat soluble) diffuse through the bi-lipid membrane and combine with a receptor inside the cell.
- The hormone-receptor complex activates the genes controlling the formation of particular proteins.
- Hormones may:
- Activate certain genes in the nucleus so that a particular enzyme or structural protein is produced.
- Change the rate of production of an enzyme or structural protein by changing the rate of transcription or translation during protein synthesis.
- Change the shape or structure of an enzyme so that it is turned “on” or “off”.
- Enzyme amplification
- One hormone can activate thousands of enzyme molecules by enzyme amplification through a chain reaction that amplifies at each stage.
- Hormone clearance
- To turn off the effect of the hormone, it is broken down, either in the target cell or more likely, the liver and kidneys.
- The degraded hormones are excreted in the bile or in the urine.
- Feedback control of hormone secretion
- It is important to regulate the production of hormones, as any over-secretion or under-secretion will cause the body to function abnormally.
- It is generally controlled through negative feedback loops.
- The hypothalamus regulates the pituitary gland by secreting releasing factors or inhibiting factors, which control the secretion of hormones by the pituitary.
The Hypothalamus and Pituitary Gland
- The hypothalamus regulates many basic body functions such as body temperature, water balance and heart rate.
- The pituitary gland lies just under the hypothalamus and is joined to the hypothalamus by a stalk called the infundibulum.
- The anterior and posterior lobes of the pituitary gland function separately.
- The anterior lobe has no nerves but a complex network of capillaries connecting it to the hypothalamus.
- The hormones produced in the hypothalamus are carried by the blood to the anterior lobe of the pituitary where they stimulate or inhibit the release of hormones made in the anterior lobe.
- Other hormones pass along the nerve fibres to the posterior lobe of the pituitary where they are then secreted.
Anterior Lobe of the Pituitary
Follicle stimulating hormone (FSH):
- Target: Ovaries Or Testes
- Action: Stimulates the development of follicles in the ovaries - Or - The production of sperm in the testes
Luteinising hormone (LH):
- Target: Ovaries Or Testes
- Action: Bring about ovulation and forms the corpus luteum in females - Or - Stimulates the production of testosterone in males
Growth hormone (GH):
- Target: All body cells (particularly osteocytes)
- Action: Stimulates body growth, particularly of the skeleton.
It increases the rate at which amino acids are taken up by cells and built into proteins
Thyroid stimulating hormone (TSH):
- Target: Thyroid
- Action: Stimulates production and release of hormones from the thyroid gland
Adrenocorticotropic hormone (ACTH):
- Target: Adrenal Cortex
- Action: Controls the production and release of some of the hormones from the cortex of the adrenal glands
Prolactin (PRL):
- Target: Mammary glands
- Action: Works with other hormones to initiate and maintain milk secretion in females
Posterior Lobe of the Pituitary
- Hormones stored by the posterior lobe are created in the hypothalamus, and then with the conduction of nerve impulses are released into the bloodstream.
Oxytocin (OT):
- Target: Uterus and mammary glands
- Action: Stimulates the contraction of smooth muscle of the uterus during labour and the contraction of cell in the mammary glands to release milk during breast feeding.
Antidiuretic hormone (ADH):
- Target: DCT and PCD of nephrons
- Action: Causes the nephrons (kidneys) to remove water from the filtrate in the nephrons and return it to the bloodstream, therefore retains fluid in the body. At higher concentrations, ADH can cause vasoconstriction of arterioles
The pineal gland
- The pineal gland in children is about the size of a pea and is found deep inside the brain.
- After about the age of seven the gland begins to shrink and by adulthood is just a tiny lump of fibrous tissue.
- It does secrete hormones, but its role remains something of a mystery
- It produces melatonin, a hormone that affects the modulation of wake/sleep patterns and seasonal functions.
- Activated by dim light (inactivated in bright light – screen time)
The thyroid gland
- Is located in the neck just below the larynx.
- It consists of two lobes that lie on either side of the trachea and are joined by a narrow piece of tissue that lies across the front of the trachea.
- The main hormone secreted is thyroxine, constantly manufactured in the thyroid from iodine and an amino acid.
- Thyroxine control body metabolism by regulating catabolic and anabolic reactions.
- The overall effect of thyroxine is to bring about the release of energy (catabolic reactions) and maintain body temperature (via energy released as heat)
- Thyroxine is secreted in response to TSH from anterior pituitary.
The parathyroid gland
- There are four parathyroid glands, each about the size of small pea embedded behind the thyroid gland.
- They secrete parathyroid hormone (PTH) or parahormone, which increases calcium level in the blood and decreases phosphate levels in the blood.
The thymus
- Located in the chest just above the heart.
- It is largest in infants and then declines after puberty.
- The thymus secretes a group of hormones called thymosins, which influence the maturation of disease-fighting cells T lymphocytes.
- There are two adrenal glands, situated above each kidney.
- Each adrenal gland has an inner adrenal medulla and an outer adrenal cortex, with very different structure and functions.
The Adrenal Medulla
- Hormones:
- Adrenaline: helps to prepare the body for reactions to a threatening situation (fight-or-flight responses). These effects are similar to the sympathetic division of the autonomic nervous system.
- Noradrenaline: has effects similar to adrenaline. It increases the rate and force of the heartbeat.
The Adrenal Cortex
- More than 20 different hormones are produced in the adrenal cortex, and they are collectively known as corticosteroids.
- The two main ones are:
- Aldosterone: which acts on the kidney to reduce the amount of sodium and increase the amount of potassium in the urine. Pulls sodium out of the urine and into the blood which means the osmotic pressure increases (dehydration) and increases water reabsorption.
- Cortisol: which promotes normal metabolism. This helps the body to withstand stress and also helps repair damaged tissues.
- Androgens: androgens are the collective name given to the male sex hormones, for example testosterone. Although we normally associate androgens with the testes, they are produced in small amounts in the adrenal cortex. In females, they play important roles in the energy levels and bone density etc.
The Pancreas
- Lies just below the stomach and alongside the duodenum.
- It is both an exocrine and endocrine gland.
- The exocrine part secretes digestive enzymes into the small intestine via the pancreatic duct.
- The endocrine part includes clusters of special cells called islets of Langerhans that secrete two important hormones:
- Insulin: reduces the amount of glucose in the blood (blood sugar level), by promoting the uptake of glucose into the cells - Beta Cells.
- In the liver and skeletal muscles, insulin causes glucose to convert into glycogen and in adipose tissue to converts glucose into fat.
- Glucagon: acts in the opposite way to insulin. It increases the blood sugar levels by breaking down glycogen and fat into glucose, in the liver and adipose tissues - Alpha cells.
The gonads
- Oestrogens and progesterone: are the female sex hormones produced by the ovaries which stimulate the development and maintenance of the female sexual characteristics.
- Together with gonadotropic hormones they also regulate the menstrual cycle and are involved in the changes that occur during pregnancy.
Comparison of the Nervous and Endocrine System: Nervous System
Nature of message: Electrical impulse and neurotransmitters (electrochemical)
Transport of message: Along the membrane of neurons
Cells affected: Muscle and gland cells; other neurons/specific
Type of response: Usually local and specific
Time taken to respond: Rapid – within milliseconds
Duration of response: Brief – stops quickly when the stimulus stops
Comparison of the Nervous and Endocrine System: Endocrine System
Nature of message: Hormones (chemical)
Transport of message: By the bloodstream
Cells affected: All body cells - receptor
Type of response: May be very general and widespread
Time taken to respond: Slower - from seconds to days
Duration of response: Longer lasting – response may continue long after the stimulus has stopped
