Chemical Coordination and Integration: common doubts, answered
The questions students ask most often about Chemical Coordination and Integration, each with a short answer. For the full chapter, read the Chemical Coordination and Integration notes.
Endocrine glands and hormones
Read this section in the notes →What is a hormone and what are endocrine glands?
Hormones are non-nutrient chemicals that act as intercellular messengers and are produced in trace amounts. Endocrine glands are ductless, so they release hormones directly into the blood or tissue fluid. Neural and chemical coordination together integrate the activities of the organs, so that the body functions as a single unit.
The human endocrine system and hypothalamus
Read this section in the notes →What is the role of the hypothalamus in endocrine control?
The hypothalamus makes hormones that regulate the synthesis and secretion of pituitary hormones. Releasing hormones stimulate pituitary secretion, as with GnRH for gonadotrophins, while inhibiting hormones, such as somatostatin, which inhibits growth hormone release, switch it off. These hormones reach the anterior pituitary through a portal circulation.
The pituitary gland
Read this section in the notes →Are oxytocin and vasopressin made in the posterior pituitary?
No. Both hormones are made in the hypothalamus, in neurosecretory cells, and carried along axons to the posterior pituitary, the pars nervosa, which stores and releases them. The posterior pituitary is therefore under direct neural control and does not make hormones itself, which is why it is called neurohypophysis.
What is the difference between gigantism, acromegaly and pituitary dwarfism?
All three relate to growth hormone. Too much of it in childhood causes gigantism, abnormally high growth of the body. Too much in adults causes acromegaly, with disfigured bones of the face and extremities. Too little in childhood causes pituitary dwarfism, which results in stunted growth.
What is the difference between diabetes insipidus and diabetes mellitus?
Diabetes insipidus results from impaired ADH function, so the kidney cannot conserve water, and the body loses a lot of water in urine and becomes dehydrated. Diabetes mellitus is a condition of prolonged high blood glucose due to a lack of insulin or its action, in which glucose and ketone bodies appear in urine.
Which hormones of the anterior pituitary control other endocrine glands?
TSH controls the thyroid, ACTH controls the adrenal cortex, and the gonadotrophins LH and FSH control the gonads. Growth hormone and prolactin act more directly on tissues. All of these come from the pars distalis, whereas the pars intermedia releases only melanocyte-stimulating hormone.
Pineal and thyroid glands
Read this section in the notes →What does melatonin do?
Melatonin, secreted by the pineal gland, helps maintain the normal rhythms of the 24-hour, or diurnal, cycle, such as the sleep-wake cycle and body temperature. It also influences metabolism, pigmentation and the menstrual cycle, and defence capability of the body. It is a hormone made in a gland of the brain.
Why does iodine deficiency cause goitre?
Thyroid hormones, thyroxine and triiodothyronine, need iodine for their synthesis, so when iodine is lacking in food the thyroid cannot make enough hormone. The gland then enlarges, which is simple goitre. Hypothyroidism during pregnancy can cause defective development and maturation of the baby, called cretinism.
What is exophthalmic goitre?
Exophthalmic goitre, also called Graves' disease, is a form of hyperthyroidism. The thyroid is enlarged and overactive, the eyeballs protrude, the basal metabolic rate rises and the person loses weight. Simple goitre is a different condition: there the gland enlarges because iodine deficiency leaves it unable to make enough hormone, so the person is hypothyroid rather than hyperthyroid.
Parathyroid gland and thymus
Read this section in the notes →What is the difference between the roles of PTH and calcitonin?
Parathyroid hormone raises blood calcium, so it is hypercalcemic, by increasing bone resorption, reabsorption in the kidneys and absorption from food. Thyroid calcitonin lowers blood calcium by reducing bone resorption. The two act in opposition, and together they maintain calcium balance in the body.
How many parathyroid glands are there and where are they?
There are four parathyroid glands, on the back side of the thyroid gland, with one pair in each of its lobes. The thymus, a lobular gland behind the sternum, secretes thymosins, which play a major role in differentiation of T-lymphocytes for cell-mediated immunity. The thymus degenerates with age.
Adrenal gland
Read this section in the notes →Which layers does the adrenal cortex have?
From the outside inward, the adrenal cortex has three layers, the zona glomerulosa, zona fasciculata and zona reticularis. Inside the cortex lies the adrenal medulla. The order G-F-R is easy to remember, and wrongly placing the reticularis outermost is a typical mistake.
What is the difference between glucocorticoids and mineralocorticoids?
Glucocorticoids, mainly cortisol, act on carbohydrate metabolism: they stimulate gluconeogenesis, lipolysis and proteolysis, and ACTH from the anterior pituitary stimulates their release. Mineralocorticoids, mainly aldosterone, act on the kidney tubules to reabsorb sodium and water and to excrete potassium and phosphate ions. Aldosterone therefore maintains electrolyte balance, body fluid volume and blood pressure, while cortisol is the one linked to glucose.
Why are adrenaline and noradrenaline called emergency hormones?
They are released from the adrenal medulla during stress, anger or fear and are therefore called hormones of fight or flight. They increase alertness, heart beat, the strength of contraction, and breathing rate, cause pupils to dilate and sweating, and raise blood glucose by breaking down glycogen and fat.
What is Addison's disease?
Addison's disease is caused by underproduction of the hormones of the adrenal cortex. The shortage disturbs carbohydrate metabolism, and the person suffers from acute weakness and fatigue. Remember that it belongs to the cortex, the outer part of the gland that makes the corticoids, and not to the medulla, which makes adrenaline and noradrenaline for the emergency response.
Pancreas
Read this section in the notes →Why is glucagon hyperglycemic but insulin hypoglycemic?
Glucagon from the alpha cells of the islets of Langerhans acts mainly on liver cells, stimulating glycogenolysis and gluconeogenesis, so blood glucose rises and it is hyperglycemic. Insulin from the beta cells acts mainly on liver cells and fat cells, increasing their uptake and use of glucose and promoting glycogen formation, so blood glucose falls. The two hormones therefore have opposite effects on blood sugar.
Testis and ovary
Read this section in the notes →What is the difference between Leydig cells, follicles and corpus luteum as hormone sources?
Leydig cells of the testis make androgens such as testosterone. The growing ovarian follicles make estrogen. The corpus luteum, which forms from the ruptured follicle after ovulation, secretes mainly progesterone. These sex steroids control the reproductive organs and secondary sexual characters.
Hormones of heart, kidney and gut
Read this section in the notes →Which organs make hormones even though they are not endocrine glands?
The heart makes atrial natriuretic factor, the kidney makes erythropoietin, and the gastrointestinal tract makes gastrin, secretin, cholecystokinin and gastric inhibitory peptide. These organs have other primary functions, but their tissues also secrete hormones, so the list of endocrine tissues is longer than the classical glands.
What does atrial natriuretic factor do?
ANF is secreted by the atrial walls of the heart when blood pressure rises. It causes dilation of the blood vessels, which lowers blood pressure, and it checks the renin-angiotensin mechanism. Erythropoietin from the kidney, on the other hand, stimulates the formation of red blood cells.
Mechanism of hormone action
Read this section in the notes →How do steroid hormones and protein hormones differ in the way they act?
Steroid hormones and iodothyronines pass into the cell and bind intracellular receptors, and the complex acts on genes to change their expression. Protein hormones bind receptors on the cell membrane and do not enter, working through second messengers such as cAMP, IP₃ and calcium ions to change cell metabolism.
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