System integration and levels of organization
System integration = coordination of component parts so they collectively perform an overall function.
Cells → tissues → organs → body systems form a hierarchy of subsystems in multicellular organisms.
Integration between subsystems produces emergent properties.
Example: a cheetah is an effective predator because muscles, nerves, circulation, respiration and sensory systems work together.
In animals, integration depends on nervous signalling, hormonal signalling, and transport of materials and energy in the blood.
Nervous system = usually rapid, targeted, short-term coordination.
Endocrine system = usually slower, chemical, longer-lasting, more widespread coordination.
Blood links organs by transporting hormones, nutrients, respiratory gases, wastes and heat.
Brain, spinal cord and nerves
The brain is the main information integration organ.
The brain combines information from multiple inputs and is involved in learning and memory.
The spinal cord is an integrating centre for unconscious processes.
Sensory neurons carry impulses from receptors to the central nervous system (CNS).
Motor neurons carry impulses from the CNS to effectors, especially muscles.
Cerebral hemispheres send output to skeletal muscles through motor neurons.
Nerves are bundles of nerve fibres containing both sensory and motor neurons.
In transverse section, a nerve shows a protective sheath plus myelinated and unmyelinated nerve fibres.
Reflex arcs and involuntary responses
A reflex arc is a rapid, automatic, involuntary response to a stimulus.
IB example: pain reflex arc with skeletal muscle as the effector.
In the hand, a free sensory nerve ending acts as a pain receptor.
Pathway: receptor → sensory neuron → single interneuron in grey matter of spinal cord → motor neuron → skeletal muscle.
Reflexes are protective because they produce a response before conscious decision-making.
The spinal cord integrates the response, while the brain becomes aware of pain afterward.
Distinguish conscious processes (brain-led awareness/decision) from unconscious processes (automatic integration in spinal cord/brainstem).
Coordination of movement and balance
The cerebellum coordinates skeletal muscle contraction.
It is important for balance, posture and smooth, coordinated movement.
Damage to cerebellar function would reduce precision of movement rather than muscle strength itself.
Effective movement requires integration of sensory input, motor output and feedback.
Endocrine control and hormonal integration
The hypothalamus and pituitary gland control much of the endocrine system.
The hypothalamus links the nervous system and endocrine system.
The pituitary gland acts as a major control gland under hypothalamic influence.
Hormones coordinate responses across multiple organs because they are transported in the blood.
Endocrine signalling is especially important for longer-lasting or whole-body responses.
This image shows the adrenal glands and relates to secretion of epinephrine (adrenaline) during short-term stress. It helps connect hormone release to widespread body responses that prepare muscles for vigorous activity. Source
Melatonin and circadian rhythms
Melatonin is secreted by the pineal gland.
Melatonin secretion follows a diurnal pattern as part of circadian rhythms.
Melatonin helps establish the sleep–wake cycle.
Typically, melatonin secretion is higher in darkness/night and lower in daylight.
This hormone helps coordinate body systems with environmental light–dark cycles.
This image shows how melatonin levels rise and fall over 24 hours, linking hormone secretion to circadian rhythm and the sleep–wake cycle. Use it to remember that melatonin is a timing signal, not simply a “sleep chemical.” Source
Epinephrine (adrenaline) and preparation for vigorous activity
Epinephrine (adrenaline) is secreted by the adrenal glands.
Its role is to prepare the body for vigorous activity.
Effects are widespread and help support intense skeletal muscle contraction.
Think of this as rapid whole-body integration during emergency or stress.
Likely body-wide effects include increased delivery of oxygen and energy substrates to muscles.
This is a key example of how the endocrine system coordinates many organs at once.
Feedback control of heart rate
Heart rate is regulated by negative feedback.
Sensory input comes from baroreceptors and chemoreceptors.
Baroreceptors monitor blood pressure.
Chemoreceptors monitor blood pH and concentrations of oxygen and carbon dioxide.
The medulla coordinates the response.
The medulla sends nerve impulses to the heart to alter heart rate and stroke volume.
This maintains stable internal conditions despite changing body demands.
This image shows the baroreceptor reflex, linking receptors that detect blood pressure changes to the medulla and then to the heart. It is useful for exam answers explaining negative feedback control of cardiovascular function. Source
Feedback control of ventilation rate
Ventilation rate is also regulated by negative feedback.
Sensory input comes from chemoreceptors in the brainstem.
These receptors detect changes related to blood pH.
A fall in blood pH is linked to increased carbon dioxide, because dissolved CO2 contributes to acidity.
The response is coordinated by the CNS and sent to the diaphragm and intercostal muscles.
Changing ventilation rate helps restore blood pH and gas balance.
Be able to link higher CO2 → lower pH → increased ventilation.
Control of peristalsis in the digestive system
Peristalsis is controlled by both the central nervous system (CNS) and the enteric nervous system (ENS).
Swallowing is initiated voluntarily under CNS control.
Egestion of faeces is also under voluntary CNS control.
Between these points, movement through the gut is mainly involuntary and coordinated by the ENS.
The ENS ensures passage of material through the digestive tract is coordinated.
This is a good example of local nervous integration within one organ system.
Checklist: can you do this?
Compare the roles of the nervous system, endocrine system and blood transport in integration.
Explain a pain reflex arc using the sequence receptor → sensory neuron → interneuron → motor neuron → effector.
Interpret negative feedback in heart rate and ventilation rate using baroreceptor/chemoreceptor input.
Distinguish between conscious and unconscious control, and between CNS and ENS roles in peristalsis.
Apply the idea of emergent properties to how body systems work together in a multicellular organism.
HL only: tropisms and phytohormones
Tropic responses in seedlings can be observed and recorded using qualitative data such as diagrams.
They can also be measured quantitatively by recording the angle of curvature.
Positive phototropism = growth of plant shoots towards lateral light.
Phytohormones are signalling chemicals controlling growth, development and responses to stimuli in plants.
Plants use a variety of phytohormones, not just one.
This image shows positive phototropism in a shoot, with auxin redistributed to the shaded side, causing greater elongation there and bending towards the light. It is ideal for explaining how a concentration gradient produces a directional growth response. Source
HL only: auxin transport and phototropism mechanism
Auxin efflux carriers help maintain concentration gradients of phytohormones.
Auxin can diffuse into plant cells, but cannot freely diffuse out.
Auxin efflux carriers are positioned on one side of a cell membrane.
Coordinated positioning of carriers across tissues causes directional transport of auxin.
This produces uneven auxin distribution in plant tissues.
Auxin promotes cell growth by stimulating hydrogen ion secretion into the apoplast.
This acidifies the cell wall, loosens cross-links between cellulose molecules, and promotes cell elongation.
In shoots, unequal auxin distribution causes unequal growth, producing bending towards light.
This image shows how auxin accumulates unevenly in a shoot exposed to one-sided light, causing cells on one side to elongate more than the other. It is useful for linking auxin transport, cell elongation, and phototropic curvature in one mechanism. Source
HL only: interaction of auxin, cytokinin and ethylene
Root tips produce cytokinin, which is transported to shoots.
Shoot tips produce auxin, which is transported to roots.
Interactions between auxin and cytokinin help integrate root and shoot growth.
Ethylene stimulates the changes that occur during fruit ripening.
Ripening also stimulates increased ethylene production.
This is positive feedback.
Benefit: fruit ripening becomes rapid and synchronized.
Exam traps and quick links
Do not confuse nervous signalling with hormonal signalling: nerves are generally faster and more specific, hormones are slower and more widespread.
Do not say reflexes are controlled by the brain first; the classic pain reflex is integrated in the spinal cord.
For heart rate control, mention medulla, baroreceptors, chemoreceptors, heart rate and stroke volume.
For ventilation rate control, link CO2, pH, chemoreceptors, brainstem, diaphragm and intercostal muscles.
For HL plant responses, focus on auxin gradients, efflux carriers, cell wall acidification, elongation, and positive phototropism.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.
Shubhi is a seasoned educational specialist with a sharp focus on IB, A-level, GCSE, AP, and MCAT sciences. With 6+ years of expertise, she excels in advanced curriculum guidance and creating precise educational resources, ensuring expert instruction and deep student comprehension of complex science concepts.