The Endocrine System

Question 1

What is homeostasis?

Answer 1

• maintaining a constant internal environment despite external changes

Question 2

What is an exocrine gland?

Answer 2

• it is a gland that secretes molecules into a duct

* the duct takes the product directly to target cells/organ

Question 3

What is an endocrine gland?

Answer 3

• it is a gland that secretes hormones directly into the blood
• they have no ducts
• the hormone has to travel to target cells
- it will bind to glycoprotein receptors on target cells which have a complementary shape

Question 4

What is a hormone?

Answer 4

• They are molecules that are released by endocrine glands directly into the blood
• they act as chemical messengers, carrying a signal from the endocrine gland to a specific target organ or tissue

Question 5

What is a target tissue?

Answer 5

• target tissues consist of target cells the possess specific receptors on their plasma membrane
• the shape of the receptor is complementary to the shape of the hormone molecule
• only these cells will respond to the specific hormone

Question 6

What is the hypothalamus?

Answer 6

• it is a small area of the brain situated beneath the two lobes of the thalamus
• it receives information from many areas of the brain and from body organs and structures, in response to this information it alters functions by either nerve impulses or by releasing its own hormones
• it produces hormones that act on the pituitary gland
  - -> these hormones act by increasing or decreasing the hormonal secretion of the pituitary gland
• the hypothalamus also produces its own hormones in specialised nerves, neuroendocrine cells, which are transported rapidly to the anterior lobe of the pituitary gland by a network of blood vessels (pituitary portal system)

Question 7

What is the pituitary gland?

Answer 7

• it is situated at the base of the brain in the hypophyseal fossa of the sphenoid bone
• consists of an anterior and posterior lobe
• vital role in the maintenance of homeostasis, connected to the hypothalamus by the pituitary stalk through which many nerve fibres run

Question 8

Describe the function of the anterior lobe of the pituitary gland and some of the hormones release from it

Answer 8

• hormones are released in response to the action of the hypothalamus
• the hypothalamus monitors the levels of many chemicals in the body, if any of these levels become too high or too low it releases hormones that act on the pituitary gland which then produces hormones to stimulate the target gland to increase or decrease its production (negative feedback)
• gonadotrophins
  - -> released in response to gonadotrophic-releasing hormone (GnRH) from the hypothalamus
  - -> FSH is a gonadotrophin released from the anterior pituitary in the female after puberty to stimulate the ovary to produce a Graafian follicle containing a mature ovum
  - -> LH is a second gonadotrophin released to trigger ovulation and to maintain the corpus luteum in the ovary
• prolactin
  - -> produced during the puerperium to initiate and maintain the production of breast milk in response to prolactin-releasing hormone
• growth hormone
  - -> produced in response to growth hormone releasing hormone (GHRH) from the hypothalamus
  - -> stimulates the growth of many tissues and organs in the body
• thyroid-stimulating hormone (TSH)
  - -> affect the development and production of hormones by the thyroid gland
  - -> it is released in response to thryotrophin releasing hormone
• adrenocorticotrophic hormone (ACTH)
  - -> released in response to corticotrophin-releasing hormone
  - -> stimulates blood flow to the adrenal glands and the production of steroids by them
• melanocyte-stimulating hormone (MSH)
  - -> affects skin pigmentation
  - -> released in response to corticotrophin-releasing hormone

Question 9

Describe the function of the posterior lobe of the pituitary gland and some of the hormones release from it

Answer 9

• composed of specialised secretory cells and nerve fibres from the hypothalamus and other regions of the brain
• two hormones are secreted by the posterior lobe but neither is produced here, they are produced in the hypothalamus and then passed along the nerve fibres to the pituitary where they are stored and released when required
• oxytocin
  - -> involved in uterine contraction and in the contraction of the myoepithelial cells surrounding the alveoli of the breasts during lactation
• antidiuretic hormone
  - -> released when a decrease in fluid volume in the blood is detected by the hypothalamus
  - -> acts on the nephrons of the kidney to reabsorb water resulting in less being lost in urine
  - -> it is also a powerful vasoconstrictor and plays an important role in the maintenance of blood pressure

Question 10

Describe the function of the pineal gland and some of the hormones release from it

Answer 10

• very small gland attached to the roof of the third ventricle of the brain
• many sympathetic nerve pathways terminate in the pineal gland
• main hormone released is melatonin
  - -> associated with bio-rhythms and possibly by inhibiting gonadotrophins

Question 11

Describe the function of the thyroid gland and some of the hormones release from it

Answer 11

• situated in the neck in front of the larynx and trachea
• consists of two conical lobes joined by a narrow band called the isthmus
• the gland is composed of follicular cells containing colloid a thickened fluid in which thyroid hormones are produced and stored
• and parafollicular cells scattered around the follicular cells that secrete calcitonin
• thyroxine (T4) and triiodothyronine (T3)
  - -> require iodine from the diet for their manufacture
  - -> once formed they are stored in combination with colloid in the follicular cells
  - -> when stimulated by TSH from the anterior pituitary they are secreted
  - -> they have an effect on regulation of the basal metabolic rate, the metabolism of carbohydrates, fats and proteins and on the normal functioning of the cardiac, nervous and reproductive systems
• calcitonin
  - -> secreted by the parafollicular cells in response to high levels of calcium and phosphate in the blood
  - -> together with the parathyroid glands it controls the levels of these minerals
  - -> it causes a reduction in the amount of calcium and phosphate released by a bone and an increase in the amount of calcium and phosphate secreted by the renal tubules

Question 12

Describe the function of the parathyroid glands and some of the hormones release from it

Answer 12

• 4 tiny parathyroid glands lie on the posterior surface of the thyroid gland
• they secrete parathyroid hormone which with calcitonin from the thyroid gland maintains the homeostasis of calcium and phosphate
  - -> it functions as an antagonist to calcitonin
  - -> it releases calcium and phosphate from the bone and decreases secretion from the kidneys when blood levels of these minerals are low
  - -> it also promotes the formation in the kidneys of calcitriol which is an active form of Vitamin D, it is involved in the absorption of calcium, phosphate and magnesium in the gastrointestinal tract

Question 13

Describe the function of the thymus gland and some of the hormones release from it

Answer 13

• situated beneath the thyroid gland in the neck

- function is to activate lymphocytes, it does this by secreting the hormones thymopoietin and the thymosins

Question 14

What are the adrenal glands?

Answer 14

• situated on the superior pole of each kidney

- consist of an outer cortex and an inner medulla

Question 15

Describe the function of the adrenal cortex in the adrenal gland and some of the hormones release from it

Answer 15

• glucocorticoids
  - -> include cortisol and corticosterone, hormones that are essential for life
  - -> release is stimulated by ACTH from the anterior pituitary or through the hypothalamus by stress
  - -> have a wide range of effects on the body by all resulting in the formation of ATP
  - -> therefore they are involved in the metabolism of glucose and carbohydrates and also in sodium and water reabsorption from the renal tubules
  - -> they also cause blood vessels to be sensitive to chemicals that cause vasoconstriction and so contribute to the maintenance of blood pressure
  - -> can also function as anti-inflammatories and can suppress the immune response when necessary
• mineralocorticoids
  - -> include the hormone aldosterone
  - -> this hormone is involved in the reabsorption of sodium in the distal convoluted tubule of the kidney and in the removal of hydrogen ions into urine
  - -> also involved in the renin-angiotensin control of blood pressure
• gonadocorticoids
  - -> include the sex hormones oestrogen and androgen
  - -> involved in female sexual behaviour

Question 16

Describe the function of the adrenal medulla in the adrenal glands and some of the hormones release from it

Answer 16

• enclosed by the cortex and is a component of the sympathetic nervous system
• adrenaline and noradrenaline secreted in response to ‘fight of flight’ response
• they help the body deal with stressful conditions by increasing metabolic processes, muscular contraction, cardiovascular and respiratory function while decreasing digestion

Question 17

Describe some of the other endocrine cells found in the body

Answer 17

• ovaries secrete oestrogen and progesterone (responsible for the reproductive cycle, the development and maintenance of female characteristics, continuation of pregnancy and the preparation of the breasts for lactation), inhibin (inhibits secretion of FSH and LH), relaxin (relaxes pelvic ligaments during pregnancy and plays a role in cervical dilatation)
• placenta releasing human chorionic gonadotropin initially to maintain the corpus luteum until the placenta is fully functional

Question 18

Describe the process of pregnancy testing

Answer 18

• once implanted in the uterine lining, a human embryo starts secreting a pregnancy hormone called human chorionic gonadotrophin (hCG)
• a pregnancy test will test for hCG
• antibodies on the pregnancy test will have a complementary shape to the hCG
• hCG will bind to the antibody
• the antibody has a coloured bead attached to it
• it forms a hCG-antibody complex which moves up the strip until it stick so a band of immobilised antibodies
• as a result all the antibodies carrying a coloured bead and attached to hCG are held in one place forming a coloured line
• there is always one control coloured line to use for comparison and a second coloured line indicates pregnancy

Question 19

Describe the changes that occur during pregnancy

Answer 19

• human chorionic gonadotrophin (hCG) is produced by the placental syntiotrophoblast and cytotrophoblast cells and by the pituitary gland
  - -> rescues corpus luteum from involution so that it can continue to produce progesterone which maintains the decidua
• relaxin is produced by the corpus luteum
  - -> contributes to the process of decidualization and the vasodilatation of healthy pregnancy
• human placental lactogen secreted into maternal circulation by the synctiotrophoblast
  - -> regulates maternal carbohydrate, lipid and protein metabolism and fetal growth
  - -> increases maternal glucose uptake and glycogen synthesis along with glucose oxidation and insulin secretion
  - -> promotes the growth of breast tissue in preparation for lactation
• the placenta secretes over 20 different oestrogens into the blood (main ones oestradiol, oestrone and oestriol)
  - -> increase uterine blood flow and facilitates placental oxygenation and nutrition to fetus
  - -> prepares the breasts for lactation
  - -> act on liver to release proteins and cholesterol
  - -> promote the growth of the endometrium
• progesterone produced by corpus luteum in early pregnancy and then the placenta in later pregnancy
  - -> promotes decidualization
  - -> inhibits smooth muscle contractility
  - -> maintains myometrial quiescence and prevents the onset of uterine contractions
• maternal pituitary gland enlarges due to hypertrophy and hyperplasia of prolactin secreting cells
  - -> gonadotrophs decrease in number as the raised oestrogen concentration inhibits release of FSH and LH
  - -> production of ACTH increases resulting in increased adrenal activity
  - -> MSH synthesis increases so hyperpigmentation may occur
  - -> towards the second half of pregnancy oxytocin production increases in posterior pituitary
• the adrenal gland increases in both size and activity
  - -> oestrogen stimulates adrenal cortisol production by inhibiting the metabolism of cortisol and increasing the synthesis of cortisol-binding protein
  - -> progesterone increases tissue resistance to cortisol
• levels of T4 and T3 increase, the increase in thyroid activity increases the basal metabolic rate causing a higher consumption of oxygen and metabolic fuels