B5: Homeostasis and Response

Homeostasis

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

Why homeostasis is important

It maintains a constant internal environment (e.g. blood glucose, body temperature, water levels) needed for enzyme action and cell function to work properly.

What homeostasis controls in the human body

Blood glucose concentration, body temperature, and water/ion content.

Automatic control systems

Systems in the body (nervous and hormonal) that may involve nervous or chemical responses and always include: receptors, coordination centres, and effectors.

Receptors

Cells that detect stimuli (changes in the environment).

Coordination centres

Include the brain, spinal cord and pancreas; they receive and process information from receptors and coordinate a response.

Effectors

Muscles or glands that bring about a response to restore optimum levels (muscles contract, glands secrete hormones).

Negative feedback

A mechanism where a change in a system triggers a response that counteracts the change, returning the system to its normal/optimum level.

CNS

The Central Nervous System, consisting of the brain and spinal cord.

Function of the nervous system

Allows humans to react to their surroundings and coordinate their behaviour by using electrical impulses.

Reflex arc

The pathway of a reflex action: stimulus, receptor, sensory neurone, relay neurone (in CNS), motor neurone, effector, response.

Sensory neurone

Carries electrical impulses from receptors to the central nervous system (CNS).

Relay neurone

Neurone found in the CNS that connects sensory neurones to motor neurones.

Motor neurone

Carries electrical impulses from the CNS to effectors (muscles or glands).

Synapse

The junction/gap between two neurones; a chemical (neurotransmitter) diffuses across it to transmit the nerve signal to the next neurone.

Reflex action

An automatic, rapid response to a stimulus that does not involve conscious thought or the brain, helping to prevent injury.

Why reflexes are important

They are fast (protective) and don't need conscious brain processing, reducing reaction time to protect the body from harm.

Cerebral cortex

The outer, highly folded part of the brain; responsible for consciousness, intelligence, memory and language.

Cerebellum

Part of the brain that controls muscle coordination and balance.

Medulla

Part of the brain (brainstem) that controls unconscious/involuntary activities such as heart rate and breathing.

How scientists study brain function

Using MRI scanning, studying patients with brain damage, and electrically stimulating parts of the brain, to map regions to functions.

Why investigating/treating brain disorders is difficult

The brain is complex, delicate, easily damaged, and not fully understood; the skull makes physical access difficult, and treatment carries high risk.

Sclera

The tough, white outer layer of the eye that supports the structure of the eyeball and protects it.

Cornea

The transparent layer at the front of the eye that refracts (bends) light into the eye.

Iris

Contains muscles that contract or relax to control the diameter of the pupil and how much light enters the eye.

Pupil

The hole in the centre of the iris through which light enters the eye.

Lens (eye)

Focuses light onto the retina; its shape is changed by the ciliary muscles and suspensory ligaments (accommodation).

Retina

Contains light receptor cells (rods and cones) sensitive to light intensity and colour.

Optic nerve

Carries electrical impulses from the retina to the brain.

Accommodation

The process by which the eye changes the shape of the lens to focus light on the retina for near or distant objects.

Accommodation to focus on a near object

Ciliary muscles contract, suspensory ligaments loosen, the lens becomes fatter/more curved, refracting light strongly.

Accommodation to focus on a distant object

Ciliary muscles relax, suspensory ligaments are pulled tight, the lens becomes thin, refracting light only slightly.

Myopia (short-sightedness)

A condition where the eyeball is too long or the lens is too curved, so light is focused in front of the retina; distant objects appear blurry.

Correcting myopia

Using a concave (diverging) lens to spread out light rays before they enter the eye, or laser/surgical correction.

Hyperopia (long-sightedness)

A condition where the eyeball is too short or the lens is too thin, so light is focused behind the retina; near objects appear blurry.

Correcting hyperopia

Using a convex (converging) lens to bring light rays together before they enter the eye, or laser/surgical correction.

Other treatments for eye problems

Cataract removal and replacement with an artificial lens; laser eye surgery to change the shape of the cornea.

Thermoregulatory centre

Located in the brain; monitors and controls body temperature using receptors sensitive to blood temperature, and receives input from skin temperature receptors.

Response to body temperature too high

Vasodilation (blood vessels near skin surface widen) and increased sweating (evaporation cools the skin).

Response to body temperature too low

Vasoconstriction (blood vessels near skin surface narrow), shivering (muscle contractions release heat), less sweat produced, hairs stand up to trap insulating air.

Vasodilation

Widening of blood vessels near the skin's surface, allowing more blood to flow near the surface so more heat is lost by radiation.

Vasoconstriction

Narrowing of blood vessels near the skin's surface, reducing blood flow near the surface so less heat is lost.

Endocrine system

A collection of glands that secrete hormones directly into the bloodstream to be carried to target organs, producing a slower but longer-lasting effect than the nervous system.

Hormone

A chemical messenger secreted by a gland, carried in the blood, that alters the activity of one or more target organs.

Pituitary gland

A "master gland" in the brain that secretes several hormones into the blood which act on other glands to stimulate them to release hormones to bring about effects.

Comparing nervous and hormonal responses

Nervous responses are very fast, act for a short time, and act on a precise area via electrical impulses; hormonal responses are slower, act for a longer time, and act more generally via chemicals in the blood.

Pancreas

Gland that monitors and controls blood glucose concentration by producing the hormones insulin and glucagon.

Insulin

Hormone produced by the pancreas that causes glucose to move from the blood into cells, and liver/muscle cells to convert glucose to glycogen for storage, lowering blood glucose level.

Glucagon

Hormone produced by the pancreas that causes glycogen in the liver to be converted back into glucose, which is released into the blood, raising blood glucose level.

Control of blood glucose by negative feedback

Blood glucose rises after eating, pancreas releases insulin, glucose moves into cells/liver stores glycogen, blood glucose falls; blood glucose falls (e.g. exercise), pancreas releases glucagon, liver converts glycogen to glucose, blood glucose rises.

Type 1 diabetes

A condition in which the pancreas produces little or no insulin, usually starting in childhood; treated with insulin injections and monitoring carbohydrate intake.

Type 2 diabetes

A condition in which body cells no longer respond properly to insulin (insulin resistance), often linked to obesity; controlled through diet, exercise, and monitoring, sometimes medication.

Obesity and Type 2 diabetes

A person's body mass index (BMI) can be used to assess whether they are obese, which is a risk factor strongly linked to developing Type 2 diabetes.

FSH (Follicle Stimulating Hormone)

Produced by the pituitary gland; causes maturation of an egg in the ovary and stimulates the ovaries to produce oestrogen.

Oestrogen

Produced by the ovaries; inhibits the release of FSH, and stimulates the release of LH, causing the uterus lining to build up.

LH (Luteinising Hormone)

Produced by the pituitary gland; stimulates the release of an egg (ovulation) at day 14 of the menstrual cycle.

Progesterone

Produced by the corpus luteum in the ovary; maintains the uterus lining during the second half of the menstrual cycle and inhibits FSH and LH.

Menstrual cycle - Day 1

Menstruation begins - the uterus lining breaks down for about 4 days.

Menstrual cycle - Days 4 to 14

The uterus lining builds up again (repairs) in preparation for the possible implantation of an embryo, controlled by oestrogen.

Menstrual cycle - Day 14

Ovulation occurs - an egg is released from the ovary, triggered by a surge in LH.

Menstrual cycle - Days 14 to 28

The uterus lining is maintained by progesterone, ready to receive a fertilised egg; if no fertilisation occurs, progesterone falls and the cycle restarts.

Hormonal contraception

Contains oestrogen and/or progesterone; can inhibit FSH production so no eggs mature, e.g. the contraceptive pill, injection, implant, or patch.

Combined oral contraceptive pill

Contains oestrogen and progesterone to inhibit FSH release and stop egg maturation; must be taken regularly and can have side effects.

Non-hormonal methods of contraception

Include condoms (barrier, also prevents STIs), diaphragms (barrier), intrauterine devices (prevent implantation/release copper), spermicides, natural methods (avoiding intercourse near ovulation), and surgical sterilisation.

Using hormones to treat infertility

FSH and LH can be given as fertility drugs to stimulate egg maturation and release in women with low FSH levels who don't ovulate.

IVF (In Vitro Fertilisation)

A fertility treatment where FSH and LH are given to stimulate the maturation of several eggs, which are collected and fertilised by sperm in a laboratory, then grown into embryos and transferred to the mother's uterus.

Advantages of IVF

Allows infertile couples to have a baby that is genetically their own.

Disadvantages/risks of IVF

Emotionally and physically stressful, low success rate, multiple births carry health risks to mother and babies, and it raises ethical concerns.

Plant hormones - Auxin

A plant hormone produced in the tips of shoots and roots that controls growth in response to light (phototropism) and gravity (gravitropism) by stimulating cell elongation.

Phototropism

Growth of a plant in response to light; shoots are positively phototropic (grow towards light), roots are negatively phototropic.

How auxin causes phototropism in shoots

Auxin accumulates on the shaded side of the shoot, causing cells there to elongate more, so the shoot bends towards the light.

Gravitropism (geotropism)

Growth of a plant in response to gravity; roots are positively gravitropic (grow downwards), shoots are negatively gravitropic (grow upwards).

How auxin causes gravitropism in roots

Auxin accumulates on the lower side of the root; in roots, high auxin concentration inhibits cell elongation, so the lower side grows slower and the root bends downwards.

Gibberellins

Plant hormones important in initiating seed germination and stem growth.

Ethene

A plant hormone that controls cell division and ripening of fruits.

Commercial uses of plant hormones

Auxins used as weedkillers (selective, kill broadleaved weeds) and rooting powders; gibberellins used to end seed dormancy, promote flowering, and increase fruit size; ethene used to control ripening of fruit during transport/storage.