14.4 Homeostasis

Learning objectives

Homeostasis is the maintenance of a constant internal environment.

Internal conditions are kept within narrow limits so cells and enzymes can function effectively even when external conditions change.

Homeostasis keeps conditions within limits; it does not mean that every internal value is perfectly fixed.

Insulin decreases blood glucose concentration.

It is released when blood glucose concentration is too high and helps return the concentration towards its normal level.

Insulin lowers blood glucose concentration; glucagon raises it.

A set point is the normal level around which an internal condition is regulated. Negative feedback reverses a deviation from that set point.

Stage	Job
1	a condition moves above or below its set point
2	receptors detect the change
3	a control system activates effectors
4	the response opposes the original change
5	the condition returns towards the set point

Negative feedback opposes a change; it does not amplify the deviation from the set point.

The pancreas detects blood-glucose changes and uses insulin or glucagon to control the liver by negative feedback.

Blood glucose	Pancreatic hormone	Liver response	Result
too high	more insulin	takes up glucose and converts glucose to glycogen	blood glucose falls
too low	more glucagon	converts glycogen to glucose and releases glucose	blood glucose rises

The pancreas secretes the hormones; the liver stores or releases carbohydrate in response. Insulin and glucagon have opposite effects.

Type 1 diabetes is treated by replacing missing insulin and actively managing blood glucose concentration.

Treatment action	Purpose
insulin injections or an insulin pump	supplies insulin
regular blood-glucose monitoring	guides insulin dose and detects unsafe levels
controlled carbohydrate intake and regular meals	limits large glucose changes
appropriate exercise	supports glucose control

Insulin controls Type 1 diabetes but does not cure the destroyed insulin-producing cells.

Identify skin structures by both their shape and their connections.

Structure	Identification cue
hair	shaft projecting through skin surface
hair erector muscle	sloping muscle attached to a hair follicle
sweat gland	coiled gland with a duct to the surface
receptor	sensory ending in the skin
sensory neurone	nerve fibre leading from a receptor
blood vessels	tubes and capillary networks in the skin
fatty tissue	layer of large fat cells beneath the skin

A sweat gland is a coiled secretory structure; a receptor detects change, while its sensory neurone carries impulses.

The brain coordinates responses that balance heat loss and heat production around normal body temperature.

Condition	Response	Effect
too hot	sweating	evaporation removes heat
too cold	shivering	muscle contraction releases heat
cold surroundings	hairs raised and fatty tissue insulates	trapped air and fat reduce heat loss

Temperature receptors provide information to the brain, which coordinates the appropriate effectors.

Sweating cools only when sweat evaporates; shivering generates heat through repeated muscle contractions.

Arterioles supplying skin-surface capillaries change diameter to alter heat transfer from the blood.

Condition	Arterioles	Blood near skin surface	Heat loss
too hot	vasodilate	more	increases
too cold	vasoconstrict	less	decreases

Vasodilation and vasoconstriction describe changes in arteriole diameter, not capillaries moving closer to or farther from the skin.