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3.5.5Additional lessons

Control Mechanisms — IB Biology SL Revision Notes & Topical Questions

5 min read 4 parts Lesson 69 of 142

📚 IB Biology SL Hub / Topical Revision
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Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
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Part 1 of 4Hypothalamus and Pituitary Gland

Hypothalamus
  • Hypothalamus It monitors blood passing through the brain and links nervous control with hormone control.
Homeostatic roles
  • Homeostatic roles It helps regulate body temperature and the water content of blood, releasing ADH when blood is too concentrated.
Anterior pituitary
  • Anterior pituitary It makes hormones, some of which stimulate other glands to release their own hormones.
Posterior pituitary
  • Posterior pituitary It stores and releases hormones such as ADH and oxytocin that are made in the hypothalamus.
ADH
  • ADH It increases water reabsorption in the kidneys, so less water is lost in urine.

Part 2 of 4Negative Feedback

Most control mechanisms in the body work by negative feedback.

1
A sensor detects that a variable, such as blood pH or temperature, has moved away from its set point.
2
The sensor sends a signal to a control centre, which compares the value with the set point.
3
The control centre activates effectors that act to reverse the change.
4
The variable returns towards the set point, the stimulus decreases and the response is switched off.

Part 3 of 4Control of Heart Rate

Control of Heart Rate
Medulla oblongata
  • Medulla oblongata The cardioregulatory centre contains an acceleratory centre and an inhibitory centre, both linked to the SAN.
Sinoatrial node
  • Sinoatrial node This group of cells in the right atrium is the pacemaker that sets the rhythm of heart contraction.
Speeding up
  • Speeding up Sympathetic neurones release norepinephrine at the SAN, which increases the frequency of its signals.
Slowing down
  • Slowing down Parasympathetic neurones in the vagus nerve release acetylcholine at the SAN, which reduces heart rate.
Sensors
  • Sensors Chemoreceptors detect a fall in blood pH from raised CO₂, and baroreceptors detect pressure changes.
ConditionDetected byResponse
Exercise raises CO₂ and lowers pHChemoreceptorsHeart rate increases
Blood pressure too highBaroreceptorsHeart rate decreases
Stress or fearBrain and adrenal medullaEpinephrine raises heart rate

Part 4 of 4Exam Checklist & Tips

3.5.5 – Key Points for the Exam

  • Negative feedback A response that reverses a change and keeps a variable near its set point.
  • Hypothalamus The brain region linking the nervous and endocrine systems.
  • Pituitary gland A gland below the hypothalamus that releases hormones controlling other glands.
  • Pacemaker (SAN) The group of cells that sets the heart's rhythm.
  • ADH A hormone that increases water reabsorption in the kidney.

Common Mistakes

  • Saying the posterior pituitary makes ADH; it stores and releases ADH made in the hypothalamus.
  • Describing negative feedback as making the change bigger.
  • Forgetting that heart rate is controlled by the autonomic nervous system, not conscious thought.
  • Stating that exercise lowers CO₂; respiration raises CO₂ and lowers blood pH.

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