SBI4U · Unit 4 test review

Homeostasis: Test Review

Everything for the Unit 4 test on one page: the key facts from all 7 lessons, the mistakes that cost the most marks, and a practice test that points you back to the lessons you need.

Key facts

Homeostasis, Feedback and Thermoregulation

Feedback loop: stimulus $\to$ receptor $\to$ control centre $\to$ effector $\to$ response, which is sensed again.
Negative feedback reverses a change (stabilizing); positive feedback amplifies it and needs an end point (childbirth, clotting).
Hypothalamus = thermostat. Hot: vasodilation, sweating. Cold: vasoconstriction, shivering, piloerection, more thyroxine.
Bicarbonate buffer: $CO_{2} + H_{2} O ⇌ H_{2} CO_{3} ⇌ H^{+} + HCO_{3}^{-};$ normal blood pH is 7.35–7.45.
pH defence speeds: buffer (seconds), lungs exhale $CO_{2}$ (minutes), kidneys secrete $H^{+}$ and reabsorb $HCO_{3}^{-}$ (hours to days).
Choice chamber: change one factor, control all others, run a no-difference control trial, replicate and rotate.

The Nervous System

Resting potential ≈ −70 mV: the Na⁺/K⁺ pump moves 3 Na⁺ out and 2 K⁺ in per ATP, and K⁺ leaks out through open channels.
Threshold ≈ −55 mV. Depolarization: voltage-gated Na⁺ channels open, Na⁺ in, peak ≈ +35 mV. Repolarization: K⁺ channels open, K⁺ out.
All-or-none: stimulus strength is coded by the frequency of action potentials, not their size.
Saltatory conduction: in myelinated axons the impulse jumps between nodes of Ranvier, which is faster and uses less ATP.
Synapse: action potential → Ca²⁺ enters → vesicles release neurotransmitter → binds receptors → removed by enzyme (acetylcholinesterase for ACh) or reuptake.
Reflex arc: receptor → sensory neuron → interneuron → motor neuron → effector.

The Brain, Spinal Cord and Reflexes

Meninges from outside in: dura mater, arachnoid mater, (CSF in the subarachnoid space), pia mater. CSF is made by the choroid plexus and cushions, floats and cleans the CNS.
Lobes: frontal (planning, motor cortex, Broca's speech), parietal (touch, body position), temporal (hearing, Wernicke's comprehension, hippocampus memory), occipital (vision). Cerebellum: coordination and balance. Medulla: breathing and heart rate.
Contralateral control: each hemisphere controls and senses the opposite side of the body.
Spinal cord: grey matter inside (synapses, interneurons), white matter outside (tracts). Sensory in through the dorsal root, motor out through the ventral root.
Reflex arc: receptor → sensory neuron → (interneuron) → motor neuron → effector. Stretch reflexes are monosynaptic; withdrawal reflexes are polysynaptic with reciprocal inhibition.
EEG measures electrical activity (good timing); fMRI measures blood oxygenation (good location).

The Endocrine System

Steroid hormones are lipid-soluble, bind intracellular receptors and change gene expression; peptide hormones are water-soluble, bind membrane receptors and act through second messengers such as cAMP.
Hypothalamus releasing hormones → anterior pituitary (TSH, ACTH, FSH, LH, GH, prolactin). Posterior pituitary stores and releases ADH and oxytocin made in the hypothalamus.
TRH → TSH → thyroxine; thyroxine inhibits the hypothalamus and pituitary (negative feedback). Iodine deficiency → high TSH → goitre.
High glucose → beta cells → insulin → uptake and glycogen storage. Low glucose → alpha cells → glucagon → liver releases glucose.
Adrenal medulla: adrenaline (short-term stress). Adrenal cortex: cortisol and aldosterone (long-term).
Menstrual cycle: FSH → follicle → estrogen → LH surge (positive feedback) → ovulation → corpus luteum → progesterone → negative feedback.

The Excretory System and Water Balance

Nitrogenous wastes: ammonia (toxic, needs lots of water), urea (humans; moderate water and energy), uric acid (least water, most energy).
Nephron path: glomerulus → Bowman's capsule → proximal tubule → loop of Henle → distal tubule → collecting duct.
Filtration (by size, pressure-driven) → reabsorption (useful substances back to blood) → secretion (blood to filtrate: H⁺, K⁺, drugs).
Loop of Henle: ascending limb pumps out salt (impermeable to water); descending limb loses water. This builds a salty medulla.
ADH (high blood solute concentration) → aquaporins in the collecting duct → more water reabsorbed. Aldosterone (low blood pressure, via renin–angiotensin) → more Na⁺ and water reabsorbed.
Dialysis: blood and dialysate separated by a selectively permeable membrane; urea diffuses out because dialysate has none.

Supplements, Drugs and Performance

Excess of a substance is excreted (extra liver and kidney load), stored (toxic build-up) or causes negative feedback that switches off the body's own production.
Fat-soluble vitamins (A, D, E, K) are stored in the liver and fat and can reach toxic levels; water-soluble vitamins (B, C) are mostly excreted in urine.
Excess amino acids are deaminated in the liver; the nitrogen becomes urea, which the kidneys must excrete with extra water.
EPO raises red blood cell production; doping with EPO or transfusions raises hematocrit and blood viscosity, increasing clot, stroke and heart attack risk.
Anabolic steroids suppress GnRH, LH and FSH by negative feedback; the axis is slow to recover after stopping.
Alcohol inhibits ADH (more urine, dehydration); stimulants mimic the sympathetic response and mask fatigue and heat signals.

Systems Working Together

Real disturbances trigger several loops at once; when loops compete for the same resource (such as blood volume), the body protects blood pressure and brain supply first.
Exercise in heat: sweating and skin vasodilation (temperature), ADH and thirst (concentration), renin–angiotensin–aldosterone (blood pressure), glucagon and cortisol (glucose).
DKA: no insulin → hyperglycemia and ketone production → acidic blood; glucose in urine causes water loss; deep rapid breathing removes CO₂ to raise pH.
Fight or flight has three tiers: sympathetic nerves (seconds), adrenaline (seconds to minutes), cortisol via CRH and ACTH (minutes to hours).
A feedback flowchart: stimulus → receptor → control centre → messenger → effector → response, with a minus-sign arrow back to the stimulus.
pH is logarithmic: a fall of 0.3 pH units doubles $[H^{+}]$ , since $1 0^{0.3} \approx 2$ .

Where marks get lost

Saying the receptor "decides" the response. Receptors only detect; the control centre (often the hypothalamus) compares the input with the set point and sends the command. (Homeostasis, Feedback and Thermoregulation)
Calling any feedback that "helps" the body positive feedback. Positive and negative refer to the direction of the response relative to the change, not whether it is good. Sweating when hot is negative feedback because it lowers the temperature that rose. (Homeostasis, Feedback and Thermoregulation)
Writing that blood vessels "move closer to the skin" when you are hot. Vessels do not move; arterioles supplying the skin dilate, so more blood flows through the surface capillaries. (Homeostasis, Feedback and Thermoregulation)
Saying hyperventilation makes blood more acidic. Breathing out more CO₂ shifts the bicarbonate equilibrium to the left, removing H⁺, so blood becomes more alkaline. (Homeostasis, Feedback and Thermoregulation)
Running a choice-chamber test with no control trial and only one group of animals. Without a both-sides-equal control and replication, a lamp or draft could explain the result. (Homeostasis, Feedback and Thermoregulation)
Saying the Na⁺/K⁺ pump causes repolarization. Repolarization is caused by voltage-gated K⁺ channels opening; the pump works slowly in the background to maintain the gradients over many action potentials. (The Nervous System)
Thinking a stronger stimulus makes a bigger action potential. Action potentials are all-or-none; stronger stimuli produce more of them per second. (The Nervous System)
Writing that the action potential "jumps" over the myelin because it is electricity in a wire. Ions cannot cross myelinated membrane; depolarization spreads internally to the next node, where a new action potential is generated. (The Nervous System)
Saying neurotransmitters travel down the axon. The action potential travels down the axon; neurotransmitter is released only at the terminal and diffuses across the cleft. (The Nervous System)
Mixing up sympathetic and parasympathetic effects on digestion: the sympathetic system slows digestion (blood goes to muscles); the parasympathetic system stimulates it. (The Nervous System)
Saying the brain processes a spinal reflex. The arc is completed in the spinal cord; the brain is informed afterward, which is why pain is felt after the movement. (The Brain, Spinal Cord and Reflexes)
Putting grey matter on the outside of the spinal cord. In the spinal cord grey matter is the inner butterfly and white matter is outside; in the brain the cortex is grey matter on the outside. (The Brain, Spinal Cord and Reflexes)
Saying the left hemisphere controls the left side of the body. Most pathways cross over, so the left hemisphere controls the right side. (The Brain, Spinal Cord and Reflexes)
Saying fMRI records nerve impulses. It detects changes in blood oxygenation that follow neural activity; EEG is the method that records electrical activity. (The Brain, Spinal Cord and Reflexes)
Assuming a normal CT or MRI rules out a concussion. Concussion is a functional injury to axons and metabolism and usually does not show on standard scans; it is diagnosed from symptoms. (The Brain, Spinal Cord and Reflexes)
Saying insulin "turns glucose into glycogen in the blood". Glycogen is made and stored inside liver and muscle cells; insulin signals those cells to take up glucose and store it. (The Endocrine System)
Mixing up glucagon (the hormone that raises blood glucose) with glycogen (the storage polysaccharide in liver and muscle). (The Endocrine System)
Saying the posterior pituitary makes ADH and oxytocin. They are made in the hypothalamus and only stored in and released from the posterior pituitary. (The Endocrine System)
Thinking steroid hormones use a second messenger. Steroids are lipid-soluble, enter the cell and act on genes directly; second messengers are for water-soluble hormones that cannot cross the membrane. (The Endocrine System)
Saying type 2 diabetes means the pancreas makes no insulin. In type 2, insulin is usually present but target cells respond poorly (insulin resistance); little or no insulin is type 1. (The Endocrine System)
Saying the glomerulus filters out only wastes. Filtration is by size, so glucose, amino acids, salts and water are filtered too; the tubule must reabsorb them. (The Excretory System and Water Balance)
Mixing up reabsorption and secretion. Reabsorption moves substances from filtrate back into blood; secretion moves substances from blood into the filtrate. (The Excretory System and Water Balance)
Saying ADH makes you urinate more. ADH is antidiuretic: it increases water reabsorption, so urine volume falls. Alcohol increases urine because it inhibits ADH. (The Excretory System and Water Balance)
Claiming that glucose in the urine means the kidneys have failed. In diabetes mellitus, healthy kidneys simply cannot reabsorb glucose once blood levels exceed their transport capacity. (The Excretory System and Water Balance)
Saying dialysis "filters" blood by pressure the way the glomerulus does. Dialysis relies on diffusion down concentration gradients across a membrane, which is why dialysate composition matters. (The Excretory System and Water Balance)
Assuming water-soluble vitamins can never cause harm. They are excreted faster, but megadoses of B6, niacin and vitamin C still cause nerve damage, liver strain or kidney stones. (Supplements, Drugs and Performance)
Saying the body stores extra protein as muscle. Excess amino acids are deaminated; the nitrogen becomes urea and the rest is burned or stored as fat. Muscle grows from training plus adequate, not excessive, protein. (Supplements, Drugs and Performance)
Thinking EPO doping is dangerous only because it is cheating. The physiological danger is raised hematocrit and blood viscosity, which cause clots, strokes and heart attacks. (Supplements, Drugs and Performance)
Expecting steroid side effects to stop the day the drug stops. Negative feedback has suppressed the body's own axis, which takes weeks to months to recover. (Supplements, Drugs and Performance)
Saying alcohol warms you up. It dilates skin blood vessels, which feels warm but speeds heat loss from the core. (Supplements, Drugs and Performance)
Describing each system separately in an integrated question. Show the links: sweating (integumentary) lowers blood volume (circulatory), which triggers ADH and aldosterone (endocrine) acting on the nephron (excretory). (Systems Working Together)
Writing "the brain" as the control centre. Name the region and receptors, such as osmoreceptors in the hypothalamus or chemoreceptors in the medulla. (Systems Working Together)
Thinking more water is always safer in endurance events. Drinking far more plain water than is lost dilutes blood sodium and can cause hyponatremia. (Systems Working Together)
Saying the fast breathing in DKA is caused by low oxygen. It is a response to acidic blood: exhaling CO₂ shifts the bicarbonate equilibrium to remove H⁺. (Systems Working Together)
Forgetting to close the loop. A feedback flowchart must show the response reducing the original stimulus, or it is just a list of events. (Systems Working Together)

Practice test

21 questions from across the unit. Each one shows the lesson it comes from.

Homeostasis, Feedback and Thermoregulation
1
Put the parts of a homeostatic feedback loop in order.
1. 1Response
2. 2Effector
3. 3Control centre
4. 4Stimulus
5. 5Receptor
Homeostasis, Feedback and Thermoregulation
2
Evaporating 1 g of sweat from skin absorbs about 2.4 kJ of heat. How much heat, in kilojoules, is removed when 250 g of sweat evaporates? Give two significant figures.
kJ
Homeostasis, Feedback and Thermoregulation
3
Name the organ that removes excess H⁺ from the body and restores bicarbonate over hours to days.
The Nervous System
4
Put the events of an action potential in order, starting from rest.
1. 1K⁺ flows out and the membrane repolarizes
2. 2Resting potential restored
3. 3Brief hyperpolarization below −70 mV
4. 4Membrane at rest at about −70 mV
5. 5Stimulus depolarizes the membrane to threshold (about −55 mV)
6. 6Voltage-gated Na⁺ channels open and Na⁺ rushes in
7. 7Na⁺ channels inactivate and voltage-gated K⁺ channels open
The Nervous System
5
During an action potential the membrane potential goes from −70 mV (rest) to +35 mV (peak). What is the total change in potential, in millivolts?
mV
The Nervous System
6
Which part of the brainstem controls breathing rate and heart rate?
The Brain, Spinal Cord and Reflexes
7
Put these layers in order from the outside of the head to the surface of the brain.
1. 1Pia mater
2. 2Subarachnoid space containing CSF
3. 3Arachnoid mater
4. 4Skull
5. 5Dura mater
The Brain, Spinal Cord and Reflexes
8
Suppose the choroid plexus makes 500 mL of CSF per day and the CNS holds 150 mL of CSF at any moment. How many times is the CSF replaced each day? (1 decimal place)
times per day
The Brain, Spinal Cord and Reflexes
9
Which statement correctly contrasts the somatic and autonomic motor pathways?
The Endocrine System
10
Why do steroid hormones generally take longer to act than peptide hormones?
The Endocrine System
11
A person's menstrual cycle is regularly 26 days long. Assuming a luteal phase of 14 days, on which day of the cycle would ovulation most likely occur?
day
The Endocrine System
12
Why were male fish in lakes receiving synthetic estrogen found to be feminized?
The Excretory System and Water Balance
13
Put the parts of the nephron in the order that filtrate passes through them.
1. 1Distal convoluted tubule
2. 2Ascending limb of the loop of Henle
3. 3Collecting duct
4. 4Bowman's capsule
5. 5Proximal convoluted tubule
6. 6Descending limb of the loop of Henle
The Excretory System and Water Balance
14
Why do humans excrete most of their nitrogenous waste as urea rather than ammonia?
The Excretory System and Water Balance
15
Which hormone, released by the adrenal cortex, increases Na⁺ reabsorption in the distal tubule and collecting duct?
Supplements, Drugs and Performance
16
Which vitamin is most likely to reach toxic levels if taken in large doses for months?
Supplements, Drugs and Performance
17
Match each substance to its main effect on homeostasis.
Alcohol
Anabolic steroids
Prednisone
Recombinant EPO
High-dose stimulants
Supplements, Drugs and Performance
18
A supplement ad says "Clinically proven! Thousands of satisfied customers!" Which question best tests the quality of the evidence?
Systems Working Together
19
Put the events of diabetic ketoacidosis in order, starting from a missed insulin dose.
1. 1Deep, rapid breathing exhales CO₂ and removes H⁺
2. 2Chemoreceptors detect the low pH
3. 3The liver makes acidic ketone bodies and blood pH falls
4. 4Insulin is absent, so cells cannot take up glucose
5. 5Unopposed glucagon raises blood glucose and cells break down fat
Systems Working Together
20
Late in a marathon on a hot day, a dehydrated runner's skin becomes less flushed and sweating decreases, yet core temperature keeps rising. What best explains this?
Systems Working Together
21
What is the medical term for low blood sodium caused by drinking far more plain water than is lost?

Endless drill

Fresh numbers every time, for the calculation questions in this unit.

Endless drillnew numbers every time

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