C3.1 Integration of body systemsIB Biology SL: Revision notes

Living things are built as a hierarchy of subsystems: cells → tissues → organs → body systems → organism. For the organism to perform an overall function, the parts must be coordinated — this is system integration.

Integration produces emergent properties: abilities the whole organism has that none of its parts has alone. A cheetah is an effective predator because its nervous, muscular, skeletal, circulatory, respiratory and endocrine systems work together; no single muscle or neuron can hunt.

Organs are integrated in three ways: nervous signalling, hormonal (endocrine) signalling and transport of materials and energy in the blood.

• Nervous system: electrical impulses along neurons to specific effectors — very fast, targeted, short-lived.
• Endocrine system: hormones secreted into the blood reach every tissue, but only cells with the right receptors respond — slower, widespread, longer-lasting.
• Blood system: carries glucose from the liver and gut, oxygen from the lungs, urea to the kidneys, hormones from glands, and distributes heat.

The brain is the central information-integration organ: it combines information from several inputs and is where learning and memory happen. Conscious processes (such as deciding to move) involve the cerebral hemispheres.

The spinal cord is an integrating centre for unconscious processes such as reflexes.

• Sensory neurons carry impulses from receptor cells into the spinal cord and cerebral hemispheres.
• Motor neurons carry impulses from the cerebral hemispheres (via the spinal cord) to muscles, stimulating them to contract.
• A nerve is a bundle of nerve fibres of both sensory and motor neurons, surrounded by a protective sheath. In transverse section it contains both myelinated and unmyelinated fibres.

A reflex is a rapid, involuntary response. In the pain reflex arc:

1. A free nerve ending of a sensory neuron in the skin of the hand acts as a pain receptor.
2. The sensory neuron carries impulses into the spinal cord.
3. A single interneuron in the grey matter of the spinal cord passes the impulse on.
4. A motor neuron carries it to the effector, a skeletal muscle, which contracts and pulls the hand away.

Impulses also travel up to the brain, so we become aware of the pain — but only after the hand has moved.

The cerebellum coordinates skeletal muscle contraction and balance, giving smooth, controlled movements of the body.

• Melatonin is secreted by the pineal gland in a diurnal pattern: secretion rises in the evening as it gets dark and falls in the morning light. High melatonin promotes sleep, so it helps set the circadian rhythm of sleeping and waking.
• Epinephrine (adrenaline) is secreted by the adrenal glands to prepare the body for vigorous activity. Its effects are widespread: faster heart rate and greater stroke volume, faster ventilation, breakdown of glycogen to glucose, and diversion of blood from the gut to skeletal muscles. Together these supply the oxygen and glucose needed for intense muscle contraction.
• The hypothalamus controls the pituitary gland, which secretes hormones that regulate many other endocrine glands. This links the nervous system to the endocrine system.

Heart rate: baroreceptors in the walls of the aorta and carotid arteries monitor blood pressure. Chemoreceptors in the aorta, carotid arteries and medulla monitor blood pH and the concentrations of oxygen and carbon dioxide. The medulla coordinates the response and sends nerve impulses to the heart to change heart rate and stroke volume — negative feedback that keeps blood pressure and blood gases stable.

Ventilation rate: carbon dioxide in the blood reacts with water to form carbonic acid, which releases H⁺ ions and lowers pH. Chemoreceptors in the brainstem detect the fall in pH, and the brainstem sends impulses to the diaphragm and intercostal muscles to increase the rate and depth of breathing. More carbon dioxide is exhaled and pH rises back towards normal.

Movement of food through the gut is controlled by two systems:

• Swallowing food at the start and egestion of faeces at the end are under voluntary control by the central nervous system (CNS).
• Peristalsis in between is involuntary, controlled by the enteric nervous system (ENS), a network of neurons in the gut wall. The ENS coordinates waves of smooth muscle contraction so material passes along the gut in order.