B4.5. Homeostasis and Response

What is homeostasis?

The regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes.

Why is homeostasis important?

It maintains optimum conditions for enzyme action and all cell functions.

What conditions are controlled by homeostasis in humans?

Blood glucose concentration, body temperature and water levels.

What are the three main components of a control system?

Receptors, coordination centres and effectors.

What is a receptor?

A cell that detects a stimulus.

What is a stimulus?

A change in the environment.

What is a coordination centre?

A part of the body such as the brain, spinal cord or pancreas that receives and processes information from receptors.

What is an effector?

A muscle or gland that brings about a response.

What is the nervous system?

A system that enables humans to react to their surroundings and coordinate their behaviour.

What is the central nervous system (CNS)?

The brain and spinal cord.

How is information transmitted through the nervous system?

As electrical impulses along neurones.

What is a sensory neurone?

A neurone that carries electrical impulses from receptors to the CNS.

What is a relay neurone?

A neurone in the CNS that connects sensory neurones to motor neurones.

What is a motor neurone?

A neurone that carries electrical impulses from the CNS to an effector.

What is a synapse?

A junction between two neurones.

How does an impulse cross a synapse?

Chemicals are released from the first neurone, diffuse across the synapse and cause an electrical impulse in the next neurone.

What is a reflex action?

A rapid, automatic response to a stimulus.

Why are reflex actions important?

They provide rapid responses that can protect the body from harm.

What is a reflex arc?

The pathway taken by an impulse during a reflex action.

What is the pathway of a reflex arc?

Stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response.

Why are reflex actions rapid?

They are automatic and do not involve the conscious part of the brain.

What does the receptor do in a reflex arc?

It detects the stimulus.

What does the sensory neurone do?

It carries the electrical impulse from the receptor to the CNS.

What does the relay neurone do?

It passes the impulse between the sensory and motor neurones in the CNS.

What does the motor neurone do?

It carries the electrical impulse from the CNS to the effector.

What does the effector do?

It produces the response, such as a muscle contracting or a gland secreting a hormone.

What is the endocrine system?

A system of glands that secretes hormones directly into the bloodstream.

What is a hormone?

A chemical messenger carried in the blood to a target organ where it produces an effect.

How does hormonal coordination compare with nervous coordination?

Hormonal responses are slower but their effects usually last longer.

What is a target organ?

An organ that responds to a particular hormone.

What is the pituitary gland?

A gland in the brain that secretes several hormones and acts as a 'master gland' controlling other glands.

Which endocrine glands must you be able to identify on a diagram?

The pituitary gland, pancreas, thyroid, adrenal glands, ovaries and testes.

Where is the pituitary gland?

In the brain.

Where is the pancreas?

In the abdomen, behind the stomach.

Where is the thyroid gland?

In the neck.

Where are the adrenal glands?

Above the kidneys.

Where are the ovaries?

In the female reproductive system.

Where are the testes?

In the male reproductive system.

What controls blood glucose concentration?

The pancreas.

What happens when blood glucose concentration is too high?

The pancreas produces insulin.

What does insulin do?

It causes glucose to move from the blood into cells.

What happens to excess glucose in liver and muscle cells?

It is converted into glycogen for storage.

How does insulin lower blood glucose concentration?

It causes cells to take up glucose and causes excess glucose to be converted into glycogen.

What is glycogen?

A storage form of glucose found mainly in the liver and muscles.

What is Type 1 diabetes?

A disorder in which the pancreas fails to produce sufficient insulin.

How is Type 1 diabetes normally treated?

With insulin injections.

What is Type 2 diabetes?

A disorder in which body cells no longer respond effectively to insulin.

How can Type 2 diabetes be treated?

A carbohydrate-controlled diet and an exercise regime are common treatments.

What is a risk factor for Type 2 diabetes?

Obesity.

What is the difference between Type 1 and Type 2 diabetes?

Type 1 involves insufficient insulin production, while Type 2 involves body cells becoming resistant to insulin.

What should you be able to interpret about insulin?

Graphs showing the effect of insulin on blood glucose levels in people with and without diabetes.

What happens when blood glucose concentration is too low?

The pancreas produces glucagon.

What does glucagon do?

It causes glycogen to be converted into glucose and released into the blood.

How do insulin and glucagon work together?

They have opposite effects and form a negative feedback system that controls blood glucose concentration.

What is negative feedback?

A control mechanism in which a change triggers responses that reverse the original change.

What happens during puberty?

Reproductive hormones cause secondary sexual characteristics to develop.

What is oestrogen?

The main female reproductive hormone produced by the ovaries.

What happens to eggs during puberty?

Eggs begin to mature and approximately one is released every 28 days.

What is ovulation?

The release of a mature egg from an ovary.

What is testosterone?

The main male reproductive hormone produced by the testes.

What does testosterone stimulate?

Sperm production.

What is FSH?

Follicle-stimulating hormone.

What does FSH do?

It causes an egg to mature in the ovary.

What is LH?

Luteinising hormone.

What does LH do?

It stimulates the release of an egg from the ovary.

What are the roles of oestrogen and progesterone in the menstrual cycle?

They are involved in maintaining the uterus lining.

How does FSH interact with oestrogen?

FSH causes an egg to mature and stimulates the ovary to produce oestrogen.

How does oestrogen affect FSH?

Oestrogen inhibits the production of FSH.

How does high oestrogen affect LH?

High oestrogen levels stimulate the release of LH, causing ovulation.

What does progesterone do?

It helps maintain the uterus lining and inhibits FSH and LH.

What happens to progesterone if pregnancy does not occur?

Progesterone levels fall, causing the uterus lining to break down.

What should you be able to interpret about the menstrual cycle?

Graphs showing changes in FSH, oestrogen, LH and progesterone levels.

What is contraception?

A method used to prevent pregnancy.

What are the two broad types of contraception?

Hormonal and non-hormonal methods.

How do oral contraceptives work?

They contain hormones that inhibit FSH production so that eggs do not mature.

How can slow-release progesterone be used as contraception?

It inhibits the maturation and release of eggs.

How do barrier methods such as condoms and diaphragms work?

They prevent sperm from reaching an egg.

How do intrauterine devices work?

They can prevent implantation of an embryo or release a hormone.

What do spermicidal agents do?

They kill or disable sperm.

How can abstaining from intercourse be used as contraception?

Intercourse can be avoided when an egg may be in the oviduct.

What is sterilisation as a contraceptive method?

A surgical method of preventing reproduction in males or females.

How can FSH and LH be used to treat infertility?

They can be given as fertility drugs to stimulate egg maturation.

What is IVF?

In vitro fertilisation, where an egg is fertilised by sperm in a laboratory and the resulting embryo is placed into the uterus.

What hormones are used during IVF?

FSH and LH are given to stimulate the maturation of several eggs.

What happens to the eggs during IVF?

They are collected from the mother and fertilised by sperm from the father in a laboratory.

What happens after fertilisation during IVF?

The fertilised eggs develop into embryos, and one or two embryos are inserted into the mother's uterus.

What are some disadvantages of IVF?

It can be emotionally and physically stressful, success rates are not high, and it can result in multiple births that pose risks to the mother and babies.

What is adrenaline?

A hormone produced by the adrenal glands during fear or stress.

What does adrenaline do?

It increases heart rate and boosts the delivery of oxygen and glucose to the brain and muscles.

Why does adrenaline prepare the body for 'fight or flight'?

It increases the supply of oxygen and glucose to the brain and muscles, providing more energy for rapid action.

What is thyroxine?

A hormone produced by the thyroid gland that stimulates the basal metabolic rate.

What is the role of thyroxine?

It stimulates basal metabolic rate and plays an important role in growth and development.

How are thyroxine levels controlled?

By negative feedback.

What is the purpose of negative feedback in thyroxine control?

It keeps thyroxine levels within suitable limits.

What is a required practical investigation for this topic?

An investigation into the effect of a factor on human reaction time.

What could be investigated in a reaction-time experiment?

The effect of a factor such as distraction on human reaction time.

What is the independent variable in a reaction-time investigation?

The factor deliberately changed by the investigator.

What is the dependent variable in a reaction-time investigation?

Human reaction time.

What variables should be controlled in a reaction-time investigation?

Factors other than the independent variable that could affect reaction time, such as the testing conditions and method.

How can you improve the reliability of a reaction-time investigation?

Repeat measurements and calculate a mean.