Hormonal coordination in humans

What is a hormone and how does it reach its target?

A hormone is a chemical secreted by a gland directly into the bloodstream. The blood carries it to a target organ or cell, where it produces an effect.

How do hormonal responses compare with nervous responses?

Hormone effects are slower than nervous responses but act for longer, so hormones control functions that do not need instant responses.

Why is the pituitary gland called the 'master gland'?

It secretes several hormones into the blood in response to body conditions, and these hormones act on other glands to stimulate the release of further hormones.

Which gland produces each of these: thyroxine, insulin, adrenaline, testosterone, oestrogen?

Thyroxine – thyroid; insulin – pancreas; adrenaline – adrenal glands; testosterone – testes; oestrogen – ovaries.

What is adrenaline and what is it for?

Adrenaline is the 'fight or flight' hormone, produced by the adrenal glands in situations where the body may be in danger. It prepares the body for movement.

Give three effects of adrenaline on the body.

It increases heart rate and breathing rate, diverts blood flow towards muscles (away from non-essential parts such as the alimentary canal), and raises blood glucose concentration for increased respiration in muscle cells.

How does adrenaline raise blood glucose concentration?

It causes stored glycogen to be broken down to glucose in the liver and muscle cells, and glucose released by the liver is transported to active muscle cells.

What does thyroxine do and where is it released from?

Thyroxine is released from the thyroid gland and stimulates the basal metabolic rate – the speed of chemical reactions in the body at rest.

How is the level of thyroxine controlled?

By negative feedback. If thyroxine is normal or too high, TRH and TSH release are inhibited so less thyroxine is made; if thyroxine falls, TRH and then TSH increase, stimulating the thyroid to release more thyroxine.

What is negative feedback?

A mechanism that keeps conditions in the body within an optimal narrow range: a change away from normal is detected and triggers a response that brings the level back to normal.

What happens when blood glucose concentration is too high?

The pancreas secretes insulin, which stimulates liver and muscle cells to take up glucose and convert excess glucose into glycogen for storage, lowering blood glucose back to normal.

What happens when blood glucose concentration is too low?

The pancreas produces glucagon, which causes stored glycogen in the liver to be converted into glucose and released into the blood, raising blood glucose back to normal.

What is the difference between glucagon and glycogen?

Glucagon is a hormone produced by the pancreas; glycogen is the polysaccharide that glucose is stored as in the liver and muscles.

Compare Type 1 and Type 2 diabetes.

In Type 1 the pancreas fails to produce sufficient insulin, so it is treated with insulin injections. In Type 2 the body cells no longer respond to insulin; it is treated with a carbohydrate-controlled diet and exercise, and obesity is a risk factor.

What is ovulation and when does it occur in a 28-day cycle?

Ovulation is the release of a mature egg from the ovary, occurring around the middle of the cycle, about day 14.

What are the four stages of the menstrual cycle?

1) Menstruation – loss of the uterus lining; 2) the lining starts to thicken; 3) ovulation around day 14; 4) the lining is maintained ready to accept a fertilised egg.

What is the role of FSH in the menstrual cycle?

Follicle-stimulating hormone is released by the pituitary gland and causes maturation of an egg in the ovary. It also stimulates the ovaries to release oestrogen.

What is the role of LH in the menstrual cycle?

Luteinising hormone is released from the pituitary gland when oestrogen peaks. It stimulates the release of the egg (ovulation) and stimulates the ovary to produce progesterone.

What does oestrogen do in the menstrual cycle?

Oestrogen, made by the ovaries, causes growth and repair of the uterus lining and inhibits the production of FSH. When it reaches a high enough level it stimulates the release of LH.

What does progesterone do in the menstrual cycle?

Progesterone, produced by the corpus luteum, maintains the uterus lining. If the egg is not fertilised the corpus luteum breaks down, progesterone levels drop and menstruation occurs.

What happens to progesterone and the uterus lining if fertilisation occurs?

The corpus luteum continues to produce progesterone, preventing the uterus lining from breaking down. Once the placenta develops it secretes progesterone throughout the pregnancy.

How do oral contraceptives prevent pregnancy?

They contain hormones (progesterone alone, or oestrogen and progesterone) that inhibit FSH production so that no eggs mature, and maintain the uterus lining so no egg is released.

How do barrier methods such as condoms and diaphragms prevent pregnancy?

They physically prevent sperm from reaching an egg. Condoms also protect against sexually transmitted infections.

How do intrauterine devices (IUDs) work?

An IUD releases hormones that thicken the mucus in the cervix so sperm cannot swim into the uterus, and thins the uterus lining so a fertilised egg cannot implant. It also interferes with the passage of sperm.

Which contraceptive methods are usually permanent?

Surgical methods: vasectomy (the sperm ducts are cut) and female sterilisation (the oviducts are cut or tied off). Both are very effective but difficult to reverse.

What is the 'rhythm method' of contraception and why is it unreliable?

Avoiding sexual intercourse during the fertile period when ovulation occurs, worked out by monitoring body temperature and cervical mucus. It is the least reliable method and is better used to plan conception.

How is clomifene therapy used to treat infertility?

Clomifene is given to women to stimulate egg maturation and ovulation by causing more FSH and LH to be released. Several eggs can be released at once, increasing the chance of multiple births.

Describe the stages of IVF.

The mother is given FSH and LH to mature several eggs; eggs are collected and fertilised with sperm in the laboratory; the fertilised eggs develop into embryos; one or two embryos are inserted into the mother's uterus.

What are the main issues with fertility treatments such as IVF?

The risk of multiple births is high, the success rate is low (about 30%), failures are emotionally and physically stressful, and there are ethical concerns about unused embryos and genetic testing.

How is the water level of the body controlled?

The pituitary gland releases ADH when the blood is too concentrated. ADH makes the kidney tubules more permeable so more water is reabsorbed into the blood. This is controlled by negative feedback.

What is deamination and why is it needed?

The liver breaks down excess amino acids from protein digestion. The amino part is converted to ammonia, which is toxic, so it is immediately converted to urea for safe excretion by the kidneys.

How do the kidneys produce urine?

By filtration of the blood under high pressure, then selective reabsorption of useful substances such as all the glucose and some ions and water. What is not reabsorbed forms urine.