Animal Coordination, Control and Homeostasis

What you'll learn

This topic covers how animals detect and respond to changes in their environment through nervous and hormonal coordination. You'll study homeostasis—the maintenance of constant internal conditions—including temperature regulation, blood glucose control, water balance, and how organisms respond to stimuli. These processes are essential for GCSE exam success and form 10–15% of your Paper 2 content.

Key terms and definitions

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

Stimulus — a change in the environment that is detected by receptors and triggers a response

Effector — a muscle or gland that produces a response to a stimulus

Negative feedback — a control mechanism that responds to a change in conditions by initiating responses that reverse the change, returning conditions to the optimum

Hormone — a chemical messenger secreted by an endocrine gland, transported in the blood plasma to target organs where it produces an effect

Synapse — the junction between two neurones where neurotransmitter chemicals diffuse across a gap to transmit the nerve impulse

Insulin — a hormone produced by the pancreas that causes glucose to be taken up by cells and converted to glycogen in the liver, lowering blood glucose concentration

Glucagon — a hormone produced by the pancreas that causes glycogen to be converted to glucose and released into the blood, raising blood glucose concentration

Core concepts

The nervous system and reflex actions

The nervous system allows rapid responses to stimuli through electrical impulses transmitted along neurones. The system comprises:

• Central nervous system (CNS): brain and spinal cord
• Peripheral nervous system: sensory and motor neurones connecting receptors and effectors to the CNS

Receptors detect stimuli and convert them into electrical impulses:

• Eyes contain light receptors (rods and cones in the retina)
• Ears contain sound receptors and balance receptors
• Tongue and nose contain chemical receptors for taste and smell
• Skin contains pressure, temperature and pain receptors

The reflex arc is an automatic, rapid response that doesn't involve conscious control. The pathway is:

Stimulus → Receptor → Sensory neurone → Relay neurone (in CNS) → Motor neurone → Effector → Response

Example reflex: The knee-jerk reflex or withdrawing your hand from a hot object.

Synapses transmit impulses between neurones:

1. Electrical impulse reaches the end of the first neurone
2. Chemical neurotransmitters are released into the synaptic gap
3. Neurotransmitters diffuse across the gap (approximately 20 nm wide)
4. Neurotransmitters bind to receptors on the second neurone
5. This triggers a new electrical impulse in the second neurone

Synapses ensure impulses travel in one direction only and allow multiple pathways for nerve impulses.

The endocrine system

The endocrine system produces hormones that coordinate long-term changes. Unlike the nervous system, hormonal responses are slower but longer-lasting.

Major endocrine glands and their hormones:

Adrenaline effects include:

• Increased heart rate and breathing rate
• Conversion of glycogen to glucose for respiration
• Diversion of blood to muscles
• Dilation of pupils

These changes prepare the body for increased physical activity.

Control of blood glucose concentration

Blood glucose concentration must be maintained within narrow limits because cells require constant glucose for respiration. The pancreas monitors and controls blood glucose through negative feedback.

When blood glucose is too high (e.g., after eating):

1. Pancreas detects the rise
2. Pancreas secretes insulin
3. Insulin causes liver and muscle cells to take up glucose from blood
4. Glucose is converted to glycogen for storage (glycogenesis)
5. Blood glucose concentration decreases to normal

When blood glucose is too low (e.g., during exercise):

1. Pancreas detects the fall
2. Pancreas secretes glucagon
3. Glucagon causes liver cells to convert glycogen to glucose (glycogenolysis)
4. Glucose is released into the blood
5. Blood glucose concentration increases to normal

This is negative feedback because the response counteracts the original change.

Type 1 and Type 2 diabetes

Type 1 diabetes:

• Pancreas produces little or no insulin
• Usually develops in childhood
• Caused by an autoimmune response that destroys pancreatic cells
• Blood glucose concentration can rise to fatally high levels
• Treatment: insulin injections (usually before meals), careful diet monitoring carbohydrate intake, regular exercise

Type 2 diabetes:

• Body cells no longer respond properly to insulin (insulin resistance)
• Usually develops in adults, increasingly in younger people
• Risk factors: obesity, lack of exercise, diet high in simple sugars, family history, ethnicity (higher risk in people of African-Caribbean, Black African, Chinese and South Asian descent)
• Treatment: diet control (reducing simple carbohydrates), weight loss, regular exercise; medication or insulin if diet/exercise insufficient

Type 2 diabetes prevalence is increasing globally, particularly in developed countries, correlating with increased obesity rates.

Thermoregulation

The human body maintains core temperature at approximately 37°C for optimal enzyme function. The thermoregulatory centre in the brain (hypothalamus) monitors blood temperature and receives impulses from skin temperature receptors.

When body temperature is too high:

• Vasodilation: blood vessels near skin surface dilate (widen), increasing blood flow to skin surface, increasing heat loss by radiation
• Increased sweating: sweat glands produce more sweat; evaporation of water requires heat energy, cooling the skin
• Hairs lie flat (in humans this has minimal effect)
• Reduced metabolic rate

When body temperature is too low:

• Vasoconstriction: blood vessels near skin surface constrict (narrow), reducing blood flow to skin surface, reducing heat loss
• Shivering: rapid muscle contractions require respiration, which releases heat energy
• Reduced sweating
• Hairs stand up to trap insulating air layer
• Increased metabolic rate

These mechanisms work through negative feedback to maintain constant core temperature.

Water and nitrogen balance (osmoregulation)

The body must balance water and mineral ion intake against losses through urine, sweat and breathing out. The kidneys regulate water content.

Antidiuretic hormone (ADH) controls water reabsorption in kidneys:

When blood water concentration is too low (dehydration):

1. Hypothalamus detects change
2. Pituitary gland releases more ADH into blood
3. ADH increases permeability of kidney tubules
4. More water is reabsorbed from kidney tubules into blood
5. Small volume of concentrated urine is produced
6. Blood water concentration increases

When blood water concentration is too high:

1. Hypothalamus detects change
2. Pituitary gland releases less ADH
3. Kidney tubules become less permeable
4. Less water is reabsorbed
5. Large volume of dilute urine is produced
6. Blood water concentration decreases

The kidneys also remove urea (produced from breakdown of excess amino acids in the liver) and regulate mineral ion levels.

The menstrual cycle and hormonal control

The menstrual cycle typically lasts 28 days and is controlled by four hormones working together:

FSH (follicle-stimulating hormone): produced by pituitary gland

• Causes an egg to mature in a follicle in the ovary
• Stimulates ovaries to produce oestrogen

Oestrogen: produced by ovaries

• Causes the uterus lining to thicken
• Stimulates production of LH
• Inhibits further FSH production

LH (luteinising hormone): produced by pituitary gland

• Triggers ovulation (release of egg) around day 14

Progesterone: produced by empty follicle (corpus luteum) after ovulation

• Maintains the uterus lining
• Inhibits FSH and LH production

If fertilisation doesn't occur, progesterone levels drop, the uterus lining breaks down (menstruation), and the cycle repeats.

Contraceptive methods using hormones:

• Oral contraceptive pill: contains oestrogen and progesterone to inhibit FSH production, preventing egg maturation
• Contraceptive injection, implant or skin patch: slow release of progesterone
• Intrauterine device (IUD): releases progesterone

Fertility treatments:

• Clomifene therapy: stimulates FSH production, causing eggs to mature
• IVF (in vitro fertilisation): FSH and LH given to stimulate egg production; eggs collected and fertilised in laboratory; embryos inserted into uterus

Worked examples

Example 1: Explain why a reflex action is faster than a conscious response. (3 marks)

Answer:

• The impulse bypasses the conscious parts of the brain (1 mark)
• The pathway involves fewer synapses / shorter pathway (1 mark)
• The response is automatic / doesn't require processing/decision making (1 mark)

Example 2: A student measures their urine output over several days. On Monday they drink 3 litres of water and produce 1.8 litres of pale urine. On Tuesday they drink 1 litre of water and produce 0.4 litres of dark yellow urine. Explain these observations using your knowledge of ADH. (4 marks)

Answer:

• On Monday, water concentration in blood is high / too high (1 mark)
• Less ADH is released by the pituitary gland (1 mark)
• On Tuesday, water concentration in blood is low / too low (1 mark)
• More ADH is released, causing more water reabsorption in kidney tubules, producing concentrated urine (1 mark)

Example 3: People with Type 2 diabetes are often advised to lose weight and exercise regularly. Explain how this helps control the condition. (3 marks)

Answer:

• Exercise uses glucose (in respiration), lowering blood glucose concentration (1 mark)
• Weight loss / exercise increases sensitivity of cells to insulin / reduces insulin resistance (1 mark)
• So less insulin is required to control blood glucose / cells respond better to insulin (1 mark)

Common mistakes and how to avoid them

• Confusing hormones with neurones: Remember hormones are chemicals transported in blood (slower, longer-lasting effects), whereas neurones transmit electrical impulses (faster, short-lived effects). Don't say hormones are "sent along neurones."

• Mixing up insulin and glucagon: Insulin lowers blood glucose (think: insulin IN to cells); glucagon raises blood glucose. Many students reverse these functions under exam pressure.

• Vasodilation vs vasoconstriction confusion: Vasodilation = vessels widen = more blood to skin = more heat loss (when too hot). Vasoconstriction = vessels narrow = less blood to skin = less heat loss (when too cold). The blood vessels change diameter; blood doesn't move to/from the skin.

• Incomplete reflex arc sequences: Always include all components in order: stimulus → receptor → sensory neurone → relay neurone → motor neurone → effector → response. Examiners penalise incomplete sequences.

• Confusing negative feedback with positive feedback: Negative feedback reverses the change (returning to optimum). Don't describe the initial change as negative feedback—describe the corrective response.

• Type 1 vs Type 2 diabetes characteristics: Type 1 = no insulin produced, usually childhood onset, requires insulin injections. Type 2 = cells don't respond to insulin, usually adult onset (increasingly younger), linked to obesity, treated initially with diet/exercise.

Exam technique for "Animal Coordination, Control and Homeostasis"

• "Explain" questions require reasons: When asked to "explain" hormonal control or reflex actions, don't just describe what happens—state why each step occurs and link cause to effect. Each causal link typically earns one mark.

• Use correct sequence words: For processes like reflex arcs or negative feedback, use "which causes," "leading to," "resulting in" to show clear causal relationships. Bullet points alone may not demonstrate your understanding of the sequence.

• Compare nervous vs endocrine carefully: Six-mark questions often ask you to compare these systems. Structure your answer clearly: speed of response, duration of effect, transmission method, and specificity of target. Use a table format if it helps you organise thoughts.

• Calculate marks-per-point: A three-mark "explain" question typically requires three distinct points. If you've only written one sentence, you're likely missing marks. Aim for one developed point per mark available.

Quick revision summary

Homeostasis maintains constant internal conditions through negative feedback mechanisms. The nervous system uses electrical impulses in neurones for rapid responses; reflexes bypass conscious control via reflex arcs. The endocrine system uses hormones (chemical messengers in blood) for slower, sustained responses. Blood glucose is regulated by insulin and glucagon from the pancreas. Body temperature is controlled by the hypothalamus through vasodilation/vasoconstriction and sweating/shivering. Water balance is regulated by ADH affecting kidney function. The menstrual cycle is controlled by FSH, LH, oestrogen and progesterone.