Homeostasis and response, at a glance

Homeostasis maintains a relatively constant internal environment. Negative feedback produces a response that opposes a change.

• Stimulus -> receptor -> coordination -> effector -> correction.
• The response reverses the original deviation.
• A smaller deviation reduces the corrective response.

Keep in mind: Conditions fluctuate within a narrow range. Homeostatic responses limit the deviations; they do not stop all change.

The hypothalamus coordinates changes in heat loss and heat production to oppose a change in body temperature.

• Warm: more sweating and more blood flow near the skin.
• Cold: less surface blood flow and shivering.
• Evaporation removes heat; respiration releases heat.

Keep in mind: Blood vessels stay in place. Dilation of supplying arterioles increases the flow of blood through capillaries near the skin.

A hormone is a chemical substance produced by a gland, carried in the blood and affecting specific target organs.

• Endocrine gland -> hormone in blood -> specific target.
• Endocrine glands are ductless.
• The pancreatic islets secrete insulin and glucagon.

Keep in mind: A hormone affects suitable target cells with the appropriate receptors. Circulation and response are different things.

Insulin lowers blood glucose by promoting uptake and storage; glucagon raises it by promoting glucose release from the liver.

• Insulin: blood glucose falls; glucose uptake and glycogen formation increase.
• Glucagon: liver glycogen breaks down; blood glucose rises.
• Glucose is a sugar; glycogen is its storage polymer.

Keep in mind: Both are hormones that signal changes in target-cell activity. Glucose and glycogen are the substances interconverted during storage and release.

Type 2 diabetes involves persistently high blood glucose because cells respond less effectively to insulin, insulin supply is insufficient, or both.

• Insulin resistance is reduced responsiveness of target cells.
• Persistent high blood glucose differs from a brief normal rise.
• Risk factors affect likelihood; management depends on the individual.

Keep in mind: Insulin resistance and/or insufficient insulin are involved. Insulin may be present, and the balance of these problems varies between individuals.

More ADH increases the water permeability of kidney collecting ducts, so more water is reabsorbed and a smaller volume of concentrated urine is produced.

• Low blood water potential -> more ADH -> more water reabsorbed.
• More reabsorption -> less, more concentrated urine.
• ADH is released from the pituitary; the kidney is a target.

Keep in mind: ADH is made in the hypothalamus and released by the posterior pituitary. It changes kidney permeability; water moves by osmosis.

A reflex is a rapid, automatic response. A reflex arc links a receptor to an effector through sensory, relay and motor neurones.

• Central nervous system = brain and spinal cord.
• Sensory neurone: towards CNS. Motor neurone: towards effector.
• Withdrawal: receptor -> sensory -> spinal relay -> motor -> muscle.

Keep in mind: Withdrawal can be coordinated by the spinal cord while the brain also receives information. Other reflexes, including the pupil reflex, are coordinated through the brain.

Light is refracted by the cornea and lens, then detected by receptors in the retina. The optic nerve carries impulses to the brain.

• Pupil = opening in the iris; retina = receptor layer.
• Cornea refracts strongly; lens adjusts focus.
• Fovea: sharp vision. Blind spot: no receptors.

Keep in mind: The pupil is an opening. Light reaches the retina; nerve impulses then travel along the optic nerve.

For a near object, ciliary muscles contract, suspensory ligaments slacken and the lens becomes more convex, refracting light more strongly.

• Near: ciliary contract, ligaments slack, lens thicker.
• Distant: ciliary relax, ligaments taut, lens thinner.
• Accommodation changes focus; the pupil reflex changes light entry.

Keep in mind: Contraction reduces tension in the suspensory ligaments. The lens becomes more convex for near vision.

Bright light makes circular iris muscles contract and radial muscles relax, narrowing the pupil and reducing light entry.

• Bright: circular contract, radial relax, pupil smaller.
• Dim: radial contract, circular relax, pupil larger.
• The brain coordinates this reflex; conscious choice is unnecessary.

Keep in mind: Pupil size controls light entry. Ciliary muscles and suspensory ligaments control lens shape for accommodation.