Integration of body systems

📚The biology

System integration

In any system made of parts, the parts must be coordinated for the system to perform its overall function. In living organisms, this integration is essential: organs working independently, at their own pace, would not keep an organism alive.

A multicellular organism is a hierarchy of subsystems: cells form tissues, tissues form organs, and organs work together in organ systems, all integrated into one organism. At each level, interactions between the parts produce emergent properties that the parts do not have alone. A cheetah becomes an effective predator only through the integration of its systems: the nervous system detects prey and coordinates movement, muscles and skeleton produce speed, the respiratory and circulatory systems supply oxygen, and adrenaline prepares the whole body for the chase.

Nerves, hormones and the blood

The blood system integrates organs by transporting materials between them. For example: glucose absorbed in the small intestine is carried to the liver for storage and to muscles for respiration; oxygen from the lungs is carried to every tissue; urea made in the liver is carried to the kidneys for excretion; hormones made in glands are carried to their targets. Energy, in the form of heat from active muscles and the liver, is also distributed by the blood.

The brain

The brain is the central organ that integrates information. It receives input from many sensory receptors at once, combines it — sight, sound, balance, memory of past events — and processes it to decide on responses. It is also where learning and memory happen, through changes in the strength and number of synaptic connections between neurons.

The spinal cord

The spinal cord is an integrating centre for unconscious processes. Conscious processes are those we are aware of and can control, such as deciding to pick up a cup; they involve the cerebral hemispheres of the brain. Unconscious processes happen without our awareness or decision, such as reflexes; many are coordinated by the spinal cord without needing the brain.

Sensory neurons, motor neurons and nerves

The pain reflex arc

A reflex is a rapid, involuntary response to a stimulus. The pathway is a reflex arc. Taking your hand away from a hot object:

1. A free nerve ending of a sensory neuron in the skin of the hand acts as a pain receptor and is stimulated.
2. Impulses pass along the sensory neuron to the spinal cord, entering through the dorsal root.
3. In the grey matter of the spinal cord, the sensory neuron synapses with a single interneuron (relay neuron).
4. The interneuron synapses with a motor neuron, which carries impulses out of the spinal cord to the arm.
5. The effector, a skeletal muscle (the biceps), contracts, pulling the hand away.

Because the pathway goes through the spinal cord and has few synapses, the response is very fast and does not wait for a conscious decision. Impulses are also sent up to the brain, so you feel pain a moment later.

The cerebellum

The cerebellum, at the back of the brain, coordinates skeletal muscle contraction and balance. It does not start movements, but it receives information about the position of the body and the movements being made, and fine-tunes the timing and strength of contractions so that movements are smooth, accurate and balanced. Damage to the cerebellum causes jerky, uncoordinated movement.

Melatonin and sleep

Melatonin is a hormone secreted by the pineal gland in the brain. Its secretion follows a diurnal (daily) pattern: it is low during daylight and rises in the evening as it gets dark, peaking in the night. Light detected by the eyes inhibits melatonin release. Rising melatonin causes drowsiness and helps initiate sleep; falling melatonin in the morning helps waking. It is part of the body’s circadian rhythm, a roughly 24-hour cycle, keeping the sleep–wake cycle synchronized with day and night. Travel across time zones disrupts this pattern, causing jet lag.

Adrenaline and vigorous activity

The adrenal glands secrete adrenaline in response to stress, fear or excitement. It has widespread effects that together prepare the body for intense muscle contraction (“fight or flight”):

The hypothalamus and pituitary gland

The hypothalamus, a region at the base of the brain, links the nervous and endocrine systems. It receives information from many parts of the body and brain, and controls the pituitary gland directly below it.

• The hypothalamus secretes releasing hormones that stimulate or inhibit the pituitary’s secretion of hormones; neurons from the hypothalamus also extend into the pituitary and release hormones there.
• The pituitary secretes hormones such as growth hormone, TSH, FSH, LH and ADH, many of which control other endocrine glands, such as the thyroid, the ovaries and the testes.

This makes the hypothalamus and pituitary the control centre of the endocrine system (D3.1, D3.3).

Feedback control of heart rate

Both send impulses to the cardiovascular centre in the medulla of the brainstem, which coordinates the response and sends impulses along nerves to the heart’s pacemaker:

• During exercise, CO₂ rises and pH falls. The medulla sends more impulses along the sympathetic nerve, increasing heart rate and stroke volume.
• If blood pressure rises too high, baroreceptors send more impulses, and the medulla sends impulses along the vagus nerve, slowing the heart.

Each response reduces the change that caused it: negative feedback.

Feedback control of ventilation rate

During exercise, muscles produce more carbon dioxide. CO₂ dissolves in blood plasma to form carbonic acid, which releases hydrogen ions, so blood pH falls. (Lactate from anaerobic respiration lowers pH further.) Chemoreceptors in the brainstem (and in the aorta and carotid arteries) detect the fall in pH. The respiratory centre in the brainstem sends more frequent impulses to the diaphragm and intercostal muscles, increasing the rate and depth of ventilation. More CO₂ is removed, and pH returns towards normal.

Control of peristalsis

Peristalsis is the wave of muscle contraction that moves food along the gut.

• Swallowing is initiated voluntarily, under control of the central nervous system.
• From the oesophagus to the rectum, peristalsis is involuntary, controlled by the enteric nervous system — a network of neurons in the wall of the gut itself. It ensures that contractions are coordinated so material passes along steadily.
• Egestion of faeces is again under voluntary control by the CNS.

Tropisms AHL

A tropism is a directional growth response of a plant to a directional stimulus. You should have observed tropic responses in seedlings, recording:

• qualitative data — descriptions and labelled diagrams of the direction of growth;
• quantitative data — for example the angle of curvature of each seedling, measured with a protractor or from photographs using image software.

Precision is limited by how exactly angles can be read and where the curve is judged to begin; accuracy by systematic errors such as parallax. Using image analysis, measuring many seedlings, and repeating the experiment improve precision, accuracy and reliability.

Phototropism AHL

Positive phototropism is growth of a plant shoot towards light coming from one side. It allows shoots to position leaves to capture more light for photosynthesis.

Phytohormones AHL

Plants have no nervous system; they coordinate growth, development and responses to stimuli with phytohormones (plant hormones). A variety of chemicals act as phytohormones, including auxin, cytokinins, gibberellins, abscisic acid and ethylene.

Auxin efflux carriers AHL

Auxin is made in shoot tips. It can diffuse freely into plant cells, but once inside it becomes charged and cannot diffuse out. It leaves only through auxin efflux carriers, membrane proteins that actively transport it out of the cell.

A cell can position its efflux carriers on one side of the cell. When all the cells in a tissue concentrate their carriers on the same side, auxin is pumped from cell to cell in one direction, and it becomes concentrated in one part of the plant. In a shoot lit from one side, efflux carriers are repositioned so that auxin moves towards the shaded side.

How auxin promotes growth AHL

Auxin stimulates target cells to secrete hydrogen ions into the apoplast (the cell walls and spaces outside the membrane). The cell wall becomes more acidic, which loosens the cross-links between cellulose molecules. The wall becomes more flexible, so as the cell takes in water by osmosis it elongates.

In phototropism, more auxin on the shaded side means cells there elongate faster than cells on the lit side. The difference in growth rate bends the shoot towards the light.

Auxin and cytokinin AHL

Root tips produce cytokinin, which is transported up to the shoots; shoot tips produce auxin, which is transported down to the roots. Each tells the other part of the plant how it is growing. A vigorously growing root system sends more cytokinin, stimulating shoot growth; a vigorously growing shoot sends more auxin, stimulating root growth. These interactions keep root and shoot growth integrated, so that neither outgrows the other’s ability to supply it with water and minerals or with sugars.

Positive feedback in fruit ripening AHL

Ethylene (IUPAC name ethene) is a gaseous phytohormone that stimulates the changes of ripening: softening, conversion of starch to sugars, colour change and aroma. Ripening itself stimulates more ethylene production. This is positive feedback: once ripening begins, it accelerates. Because ethylene is a gas, it also spreads to neighbouring fruit and triggers them too.

The benefit is that ripening is rapid and synchronized across a plant, so that a crop of ripe fruit is available at once, attracting animals that eat the fruit and disperse the seeds. It is also why one ripe banana in a bag makes the rest ripen faster.