Unit 10 Sense organs and homeostatic mechanisms
Key unit competence: Relate the structure of sense organs to their functions and explain how the body maintains a constant internal environment.
Key ideas
- Sense organs contain receptors that change a stimulus into nerve impulses: eye (light), ear (sound and balance), tongue (taste), nose (smell), skin (touch, pressure, pain, temperature).
- The eye: conjunctiva (protects the front), cornea (transparent, refracts most light), iris (coloured muscle that controls the size of the pupil), lens (fine focusing), ciliary muscles and suspensory ligaments (change lens shape), retina (contains rods: dim light, black and white; and cones: colour, bright light), fovea (most cones, sharpest vision), blind spot (no receptors, where the optic nerve leaves), choroid (dark, absorbs stray light, blood supply), sclera (tough white coat).
- Pupil reflex: in bright light the circular muscles of the iris contract and the pupil becomes smaller, protecting the retina; in dim light the radial muscles contract and the pupil widens.
- Accommodation: for near objects, ciliary muscles contract, suspensory ligaments slacken, the lens becomes thicker (more convex). For distant objects, ciliary muscles relax, ligaments are tight, the lens becomes thinner.
- Eye defects: short sight (myopia): distant objects blurred, image forms in front of the retina; corrected with a concave (diverging) lens. Long sight (hyperopia): near objects blurred, image behind the retina; corrected with a convex (converging) lens. Cataract: cloudy lens (surgery). Astigmatism: uneven cornea (cylindrical lens).
- The ear: outer ear (pinna and ear canal) collects sound; eardrum vibrates; middle ear ossicles (hammer, anvil, stirrup) amplify vibrations to the oval window; the Eustachian tube equalises pressure; the inner ear cochlea converts vibrations into impulses sent along the auditory nerve; the semicircular canals detect head movement (balance).
- The tongue has taste buds that detect sweet, salty, sour, bitter and umami; the nose has olfactory receptors that detect chemicals in the air; much "taste" is really smell. The skin has receptors for touch, pressure, pain, heat and cold.
- Homeostasis is keeping the internal environment constant (body temperature about 37 °C, blood glucose, water and salt levels, pH) so that enzymes and cells work well. It works mainly by negative feedback: a change is detected and the body acts to reverse it. (Positive feedback increases a change, e.g. contractions during childbirth.)
- Thermoregulation by the skin: when hot: sweating (evaporation cools), vasodilation (arterioles widen, more blood near the surface loses heat), hairs lie flat. When cold: shivering (muscle contraction makes heat), vasoconstriction (less blood near surface), hairs stand up, less sweat. The hypothalamus detects blood temperature and controls these responses.
Common misconceptions
- "The pupil is a structure that changes colour." No: the pupil is a hole; the iris controls its size.
- "Short-sighted people cannot see near objects." No: they see near objects clearly but distant objects are blurred.
- "Blood vessels move closer to the skin when hot." No: vessels do not move; arterioles dilate so more blood flows near the surface.
- "Sweat cools the body because it is cold." No: cooling happens when sweat evaporates, taking heat from the skin.
Worked examples
Example 1. Explain what happens in the eye when a learner looks up from a book to the blackboard.
Solution
To focus on a distant object, the ciliary muscles relax, the suspensory ligaments become tight, pulling the lens thinner (less convex), so light is refracted less and focused on the retina. This is accommodation.
Example 2. Explain how the skin helps to cool the body on a hot day.
Solution
The hypothalamus detects a rise in blood temperature. Sweat glands produce more sweat, which evaporates, removing heat. Arterioles in the skin dilate (vasodilation), so more blood flows near the surface and loses heat by radiation. This is negative feedback.
Practice questions
State the functions of: (a) cornea; (b) iris; (c) retina; (d) optic nerve.
Show answer
(a) Refracts light into the eye. (b) Controls the size of the pupil and amount of light. (c) Contains light receptors (rods and cones) that produce impulses. (d) Carries impulses to the brain.
Common mistake: Saying the iris focuses light.
Explain the pupil reflex in bright light.
Show answer
Light receptors detect bright light; impulses cause the circular muscles of the iris to contract and radial muscles to relax; the pupil becomes smaller, letting in less light and protecting the retina.
Common mistake: Saying the pupil gets bigger.
Describe accommodation for a near object.
Show answer
Ciliary muscles contract, suspensory ligaments slacken, the elastic lens becomes thicker (more convex), refracting light more to focus it on the retina.
Common mistake: Saying ciliary muscles relax.
Describe short sight and its correction.
Show answer
Distant objects appear blurred because the image forms in front of the retina (eyeball too long or lens too strong); corrected with a concave (diverging) lens.
Common mistake: Correcting with a convex lens.
Describe how sound is transmitted from the pinna to the brain.
Show answer
Pinna collects sound; ear canal leads it to the eardrum, which vibrates; ossicles amplify and pass vibrations to the oval window; fluid in the cochlea vibrates; hair cells produce impulses carried by the auditory nerve to the brain.
Common mistake: Leaving out the ossicles.
Define homeostasis and give three conditions it controls.
Show answer
Maintaining a constant internal environment; body temperature, blood glucose, water and salt balance, pH, CO₂ level.
Common mistake: Defining it as excretion only.
Explain how the body responds to cold.
Show answer
Shivering produces heat; vasoconstriction reduces blood flow near the skin; hairs stand up to trap air; sweating decreases; metabolic rate may rise.
Common mistake: Saying vessels move deeper.
Name the receptors in the skin.
Show answer
Touch, pressure, pain, heat and cold receptors.
Common mistake: Naming sweat glands as receptors.
Matching and drag-and-drop
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A mechanism that reverses a change is: a. throat (pharynx) 2. The Eustachian tube connects the middle ear to the: b. negative feedback 3. The part of the brain that detects blood temperature is the: c. hypothalamus 4. Receptors for colour vision are: d. cones Show answer
1 → b (negative feedback) · 2 → a (throat (pharynx)) · 3 → c (hypothalamus) · 4 → d (cones)
- It restores the normal level.
- It equalises pressure.
- It controls thermoregulation.
- They work in bright light.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
accommodation · positive feedback · thicker and more convex · cochlea- The part of the ear that changes vibrations into nerve impulses is the .
- To focus on a near object, the lens becomes .
- Contractions of the uterus during childbirth get stronger and stronger. This is an example of .
Show answer
1: cochlea · 2: thicker and more convex · 3: positive feedback
- It contains hair cells.
- Ciliary muscles contract.
- The change is increased, not reversed.
Matching
Match each item in column A with the correct answer in column B. Write the letter next to each number.
Column A Column B 1. A cloudy lens that blurs vision is a: a. retina 2. The semicircular canals detect: b. cataract 3. In bright light the pupil becomes: c. movement and balance 4. The light-sensitive layer of the eye is the: d. smaller Show answer
1 → b (cataract) · 2 → c (movement and balance) · 3 → d (smaller) · 4 → a (retina)
- It is treated by surgery.
- They contain fluid.
- This protects the retina.
- It contains rods and cones.
Drag and drop
Choose the correct word or phrase from the box for each gap. One word in the box is not needed.
hammer, anvil and stirrup · convex lens · semicircular canals · rods- The ear bones (ossicles) are the .
- Receptors that work in dim light are .
- Long sight is corrected with a .
Show answer
1: hammer, anvil and stirrup · 2: rods · 3: convex lens
- They amplify vibrations.
- They give black and white vision.
- It converges light.