C3.1 Integration of body systems

Any system with many parts needs integration: the parts must be coordinated so that together they carry out an overall function. In a car, the engine, gears and brakes must work in step; in a multicellular organism, millions of cells, many tissues and several organ systems must do the same. Integration needs communication between the parts and a way to control what each part does.

A hierarchy of subsystems

Multicellular organisms are organized as a hierarchy of nested subsystems:

• Cells of the same type, such as muscle fibres, are grouped into tissues.
• Different tissues are combined into organs, such as a muscle, which contains muscle tissue, connective tissue, blood vessels and nerves.
• Organs that work together form organ systems, such as the musculoskeletal, nervous, circulatory and respiratory systems.
• Organ systems together make up the organism.

Each level is a system in its own right, and each is also a component of the level above.

Emergent properties

When subsystems are integrated, the whole has emergent properties: abilities that none of the parts has on its own. A cheetah is an effective predator because its body systems work together. Its brain integrates information from its eyes to track prey; its nervous system coordinates powerful contractions of its leg muscles, acting on a long, flexible skeleton; its lungs, heart and blood deliver oxygen and glucose to the muscles and remove CO₂ and heat. No muscle, bone or neuron can sprint at more than 90 km/h, but the integrated animal can.

Organs in animals are integrated in two ways: by signals, which coordinate what organs do, and by transport of materials and energy between them.

Two signalling systems

The two systems work together. The hypothalamus, in the brain, controls much of the endocrine system, and some nerves stimulate glands directly: sympathetic nerves trigger the release of epinephrine from the adrenal glands. Nerves and hormones are covered in more detail in C2.2 Neural signalling and C2.1 Chemical signalling.

Transport by the blood

The blood system links all organs. It carries materials from where they are absorbed or produced to where they are used or removed:

• oxygen from the lungs to respiring tissues, and CO₂ back to the lungs;
• glucose, amino acids and other products of digestion from the small intestine to the liver and then to all tissues;
• urea from the liver, where it is made, to the kidneys, where it is excreted;
• hormones from endocrine glands to their target cells.

Blood also distributes energy: chemical energy in glucose and fatty acids, and heat from active organs such as the liver and muscles to the skin, where it can be lost. See B3.2 Transport for the structure of blood vessels.

The central nervous system (CNS) is the brain and spinal cord. It receives information from receptors, processes it and sends signals to effectors.

The brain as an integrating organ

The cerebral hemispheres (cerebrum) are the main centre for integrating information. They receive impulses from many sensory inputs at once and combine them. When you catch a ball, for example, information from your eyes about the ball’s movement is combined with information about the position of your arm and hand and with memories of earlier catches, and the brain then sends a coordinated set of signals to your muscles. The brain also stores information as memories, and learning happens when experience changes the connections between neurons, so a later response can be different.

Conscious and unconscious processes

• Conscious processes are ones we are aware of and can control, such as deciding to pick up a cup. They are coordinated in the cerebral hemispheres.
• Unconscious processes happen without our awareness or decision, such as a withdrawal reflex, the knee-jerk reflex, or the control of heart rate.

The spinal cord acts as an integrating centre for unconscious processes. Reflexes are processed in its grey matter (cell bodies and synapses of relay and motor neurons), while its outer white matter consists of myelinated axons carrying impulses up to and down from the brain.

The cerebellum

The cerebellum is at the back of the brain, below the cerebral hemispheres. It coordinates skeletal muscle contraction and balance. It receives information about the movement the brain intends to make, together with feedback from stretch receptors in muscles and joints, from the balance organs of the inner ear and from the eyes. It compares what is happening with what was intended and fine-tunes the timing and force of muscle contractions, so movements are smooth and accurate and posture is maintained. Skills such as riding a bicycle become automatic as the cerebellum learns them. Damage to the cerebellum causes clumsy, jerky movements and an unsteady walk, although the muscles themselves are not weakened.

Input: sensory neurons

Receptors detect stimuli. Some are specialized receptor cells, such as the rods and cones of the eye; others are the endings of the sensory neurons themselves, such as the free nerve endings that detect pain. Sensory neurons carry nerve impulses from receptors into the CNS: through the dorsal roots of spinal nerves into the spinal cord, or through cranial nerves directly into the brain. From the spinal cord the information is relayed up to the cerebral hemispheres, where sensations are perceived. A sensory neuron’s cell body lies in a ganglion just outside the CNS.

Output: motor neurons

To make a voluntary movement, the cerebral hemispheres send impulses down the spinal cord, where they are passed to motor neurons. A motor neuron’s cell body is in the grey matter of the spinal cord (or brainstem); its long axon runs out through a ventral root and a nerve to a skeletal muscle. At the motor end plate the motor neuron releases a neurotransmitter that stimulates the muscle fibres to contract.

Nerves

A nerve is a bundle of many nerve fibres. Most nerves are mixed: they contain the fibres of both sensory neurons (carrying impulses towards the CNS) and motor neurons (carrying impulses away from it). In a transverse section you can see:

• an outer protective sheath of connective tissue (the epineurium);
• several bundles (fascicles) of fibres, each wrapped in its own sheath (the perineurium);
• myelinated fibres: axons surrounded by a myelin sheath, which looks like a ring with a dark rim and a pale centre;
• unmyelinated fibres: smaller axons, often grouped within a Schwann cell, with no myelin ring;
• small blood vessels in the connective tissue.

Myelinated fibres conduct impulses much faster than unmyelinated ones (see C2.2 Neural signalling).

A reflex is a rapid, automatic, involuntary response to a stimulus. The pathway the impulses follow is a reflex arc. The pain withdrawal reflex protects the body from damage, and the IB example has skeletal muscle as the effector.

The five parts

1. Receptor: free nerve endings of a sensory neuron in the skin of the hand act as pain receptors. Heat or pressure strong enough to damage tissue stimulates them, and nerve impulses begin.
2. Sensory neuron: carries the impulses along its axon, in a spinal nerve, through the dorsal root into the spinal cord. Its cell body is in the dorsal root ganglion.
3. Relay neuron (interneuron): lies entirely within the grey matter of the spinal cord. The sensory neuron synapses with it, and it synapses with the motor neuron. In this example there is a single relay neuron.
4. Motor neuron: its cell body is in the ventral part of the grey matter; its axon leaves the cord through the ventral root and runs in the same spinal nerve back to the arm.
5. Effector: the motor neuron stimulates a skeletal muscle, here the biceps, to contract. The arm bends at the elbow and the hand is pulled away.

Why it is fast and involuntary

Only three neurons and two synapses in the spinal cord are involved, and the response does not wait for the brain. The sensory neuron also passes impulses to neurons that carry them up the white matter to the brain, so a moment later the pain is felt. The brain can then decide what to do next, and in some cases (for example holding on to a hot plate to avoid dropping it) can override the reflex.

The hypothalamus and pituitary gland

The hypothalamus, at the base of the brain, links the nervous and endocrine systems. It receives impulses from other parts of the brain and monitors the blood, and it controls the pituitary gland, which hangs just below it. Some hormones are made by neurosecretory cells in the hypothalamus and released from the pituitary; others are made by the pituitary itself in response to releasing hormones from the hypothalamus. Many pituitary hormones control other endocrine glands: for example TSH stimulates the thyroid gland, and FSH and LH act on the ovaries and testes. When the hormone from the target gland rises, it inhibits the hypothalamus and pituitary: negative feedback keeps hormone levels stable. (You need only this general account, not the differences between the anterior and posterior pituitary.)

Melatonin and the sleep–wake cycle

The pineal gland secretes melatonin in a diurnal (daily) pattern. Light detected by the retina inhibits secretion, through the suprachiasmatic nucleus (SCN) in the hypothalamus, which acts as the body’s master clock. So melatonin concentration is low during the day, rises in the evening as it gets dark, peaks in the middle of the night and falls towards morning. High melatonin promotes sleepiness and lowers body temperature, helping to establish a regular circadian rhythm of sleeping and waking of about 24 hours. Jet lag happens because the melatonin rhythm takes several days to shift to a new time zone, and bright screens late in the evening can delay melatonin secretion.

Epinephrine and vigorous activity

In danger, stress or intense exercise, sympathetic nerves stimulate the adrenal glands, on top of the kidneys, to secrete epinephrine (adrenaline) into the blood. It has widespread effects, which together prepare the body for vigorous activity (“fight or flight”):

• heart rate and stroke volume increase, so more blood reaches the muscles;
• bronchioles widen and ventilation rate increases, so more oxygen is absorbed;
• the liver breaks down glycogen and releases glucose into the blood;
• arterioles supplying skeletal muscles widen, while those supplying the gut and skin narrow;
• pupils dilate and alertness increases.

The result is a greater supply of oxygen and glucose to skeletal muscle, so it can respire faster and contract more intensely.

Heart rate is controlled by negative feedback: a change is detected and a response is made that reverses it. The sinoatrial (SA) node sets the heartbeat (see B3.2 Transport), but the cardiovascular centre in the medulla oblongata changes its rate.

The receptors and where they are

• Baroreceptors detect blood pressure by the stretch of the artery wall. They are in the walls of the aortic arch and the carotid sinuses (swellings of the carotid arteries in the neck).
• Chemoreceptors detect blood pH and the concentrations of CO₂ and O₂. They are in the carotid bodies and aortic bodies, small structures close to the baroreceptors; there are also chemoreceptors in the medulla.

The response

The medulla sends impulses along two kinds of nerve to the SA node:

• sympathetic (cardiac accelerator) nerves increase heart rate. They also act on the ventricular muscle, increasing the force of contraction, so stroke volume rises;
• the vagus nerve (parasympathetic) decreases heart rate.

Why blood pH changes

Blood pH is normally kept between about 7.35 and 7.45. The main cause of change is carbon dioxide. CO₂ from cell respiration dissolves in the plasma and, mostly inside red blood cells (where carbonic anhydrase speeds up the reaction), reacts with water to form carbonic acid, which dissociates:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

So when respiration speeds up, for example during exercise, more CO₂ enters the blood, more H⁺ is released and pH falls. In very intense exercise, lactic acid from anaerobic respiration lowers pH further. When CO₂ is removed faster than it is produced, pH rises.

The control loop

1. Chemoreceptors in the brainstem (medulla) detect the fall in pH; chemoreceptors in the carotid and aortic bodies detect pH, CO₂ and O₂ in arterial blood.
2. They send impulses to the respiratory centre in the medulla.
3. The respiratory centre sends impulses more often along nerves to the diaphragm (the phrenic nerves) and the external intercostal muscles, which contract more often and more strongly.
4. Ventilation rate and tidal volume (depth) increase, so CO₂ is removed from the blood faster.
5. Blood CO₂ falls and pH rises back towards normal. When pH returns to normal, impulses from the chemoreceptors decrease and ventilation slows: negative feedback.

In the simulation above, tick the thought-experiment box to block the chemoreceptor signals. Ventilation can no longer rise, and in hard exercise CO₂ builds up and pH falls far outside the normal range: a dangerous acidosis.

Peristalsis is the wave of muscle contraction that moves food along the gut. The gut wall has two layers of smooth muscle: an inner circular layer and an outer longitudinal layer. Behind the bolus of food the circular muscle contracts, narrowing the gut and squeezing the food forwards; in front of it the circular muscle relaxes, so the gut is wider there.

Two levels of control

• Swallowing is started voluntarily. The tongue (skeletal muscle under the control of the brain) pushes food to the back of the mouth. From there, swallowing continues automatically and the food enters the oesophagus.
• Peristalsis is involuntary. From the oesophagus, through the stomach and the small and large intestines, waves of contraction are coordinated by the enteric nervous system (ENS): networks of neurons within the gut wall, between and beneath the muscle layers. The ENS detects stretching of the gut wall by food and coordinates contraction behind the bolus and relaxation in front of it, so material passes along in the right direction and at a suitable rate. A section of gut removed from the body can still carry out peristalsis, showing the ENS can work without the brain (though the CNS can speed it up or slow it down).
• Egestion is under voluntary control. Faeces collect in the rectum. The anus is closed by sphincter muscles, one of which is skeletal muscle that the CNS can keep contracted until it is convenient to egest. Babies do not yet have this control; it is learned in early childhood.