Learn first: The nervous system
The body’s activity is controlled in two ways: nervous (fast and precisely targeted) and humoral (through chemicals in the blood — slower but longer-lasting). The main tool of humoral control is hormones, produced by the endocrine glands. The exam often asks you to match "hormone — gland — disease" correctly and to identify a disorder from its symptoms.
Properties of hormones: they act in very small amounts; their action is specific (on particular target cells); they act far from where they are made; they are broken down quickly, so they are produced continuously.
Nervous and humoral control form a single neurohumoral system. They are linked by the hypothalamus of the diencephalon: it controls the pituitary, and the pituitary controls many other glands.
The pituitary is a small gland at the base of the brain, often called the "master gland".
Growth hormone: excess in childhood — gigantism; deficiency in childhood — dwarfism (proportional body, normal intelligence); excess in adults — acromegaly (enlarged hands, feet, nose and jaw).
Lack of vasopressin causes diabetes insipidus: a very large volume of urine (several litres a day) and strong thirst, but normal blood glucose.
The pineal gland in the diencephalon produces melatonin, which takes part in regulating daily rhythms and sleep.
The thyroid gland lies in the neck, in front of the larynx. Its hormones, thyroxine and triiodothyronine, contain iodine. They raise the metabolic rate, oxygen use and heat production, and affect growth and the development of the nervous system.
The parathyroid glands lie on the back of the thyroid (usually four). Parathyroid hormone raises blood calcium. When it is lacking, calcium falls, nerve and muscle excitability rises and convulsions occur.
A patient has bulging eyes and a rapid heartbeat, is losing weight despite a good appetite and is easily irritated. Which gland is affected, and how?
The adrenal glands have two layers:
The endocrine part of the pancreas is the islets of Langerhans:
Insulin (beta cells) lowers blood glucose: it moves glucose into cells and converts it into glycogen in the liver and muscles.
Glucagon (alpha cells) raises blood glucose: it breaks glycogen down into glucose.
Lack of insulin causes diabetes mellitus: high blood glucose, sugar in the urine, large volumes of urine, thirst, weakness. It is treated with insulin injections (insulin is a protein, so if swallowed it would be digested).
Gonads: the testes produce testosterone; the ovaries produce oestrogens and progesterone. They control puberty and the development of secondary sexual characteristics.
The thymus lies in the chest and is well developed in childhood; it takes part in the maturation of T lymphocytes.
Hormone levels are regulated by feedback: when a hormone increases, the signal stimulating its production weakens, and vice versa.
Blood thyroxine has fallen. How does the hypothalamus–pituitary–thyroid system respond?
By chemical structure, hormones fall into three groups:
Protein hormones cannot cross the cell membrane and bind to receptors on its surface; steroids are fat-soluble, cross the membrane and act through receptors inside the cell. Protein hormones are broken down by digestive enzymes — which is why insulin is given by injection, not as a tablet.
Blood glucose balance. In a healthy adult, fasting blood glucose is about 3.3–5.5 mmol/l. Several hormones keep it there together:
Only insulin lowers glucose.
Glucose is raised by glucagon, adrenaline, glucocorticoids (cortisol) and growth hormone.
After a meal glucose rises → beta cells release insulin → glucose enters cells and is stored as glycogen in the liver. During fasting or physical work glucose falls → alpha cells release glucagon → liver glycogen is broken down. This too is feedback: the change a hormone causes reduces its own secretion.
Stress is the body’s general response to strong challenges (cold, pain, danger, intense excitement). First comes the fast sympathetic–adrenal response: adrenaline speeds up the heart, raises blood glucose and increases blood flow to muscles — preparation for “fight or flight”. Then a slower response: via the hypothalamus and pituitary, the adrenal cortex releases cortisol, which mobilises the body’s energy reserves. If stress lasts a long time, persistently high cortisol weakens immunity, raises blood pressure and paves the way for a number of diseases.
A student comes to an exam without breakfast and is very anxious. Which hormones keep his blood glucose up?
A patient passes 6–8 litres of urine a day and is always thirsty; glucose in blood and urine is normal. Which hormone is lacking?