13.1 - Homeostasis

Homeostasis and the Internal Environment

Homeostasis: Regulation of physical and chemical factors of the internal environment within normal ranges for cells to function in optimum conditions.
Homeostasis keeps the internal environment constant despite large changes in the external environment.
A constant internal environment ensures that cell activities continue at the optimum level.

Physical factors: Temperature, blood osmotic pressure and blood pressure.
Chemical factors: pH value, mineral concentration and blood sugar concentration.

A deviation from the normal range triggers the homeostatic mechanism through negative feedback.
A factor above the normal range is reduced to the normal range.
A factor below the normal range is increased to the normal range.

Body temperature: Integumentary system (skin and sweat glands), nervous system, circulatory system, muscle system and endocrine system.
Blood sugar levels: Endocrine system, circulatory system and digestive system.
Partial pressure of carbon dioxide in the blood: Respiratory system, circulatory system and nervous system.
Blood pressure: Circulatory system and nervous system.

A fixed body-temperature range allows enzyme-catalysed cell metabolism reactions to occur at optimum levels.
A temperature that is too high denatures enzymes.
A temperature that is too low slows cell metabolic activity and prevents cell processes from supporting cell survival.
Thermoreceptors in the skin and hypothalamus detect changes in body temperature.

Physical methods:
- Arterioles in the skin dilate (vasodilation); more blood flows to the skin surface and more heat is lost through radiation.
- Erector muscles relax; fine hair lowers towards the skin surface, trapping a thin layer of air so heat is released quickly.
- Sweat glands produce more sweat; heat is absorbed to evaporate sweat, cooling the skin.
- Skeletal muscles contract and relax less; the body does not shiver.
Chemical methods:
- Adrenal glands secrete less adrenaline; the metabolic rate decreases.
- The thyroid gland secretes less thyroxine; the metabolic rate decreases and no excess heat is generated.

Physical methods:
- Arterioles in the skin constrict (vasoconstriction); less blood flows to the skin surface and less heat is lost through radiation.
- Erector muscles contract; fine hair stands erect, trapping a thick insulating layer of air that prevents heat loss from the skin.
- Sweat glands are not stimulated; sweating does not occur.
- Skeletal muscles contract and relax; shivering generates heat because muscle contraction requires energy.
Chemical methods:
- Adrenal glands secrete more adrenaline; glycogen is converted to glucose, the metabolic rate increases, and glucose oxidation releases heat.
- The thyroid gland secretes more thyroxine; the metabolic rate increases and more heat is generated.

The pancreas maintains blood sugar (glucose) levels within the normal range of 75–110 mg/100 ml.
Langerhans cells in the pancreas continuously produce and secrete insulin and glucagon into the bloodstream.

Beta ( $β$ ) cells in pancreatic Langerhans cells secrete insulin into the blood.
Insulin stimulates liver cells and muscle cells to use glucose in cell respiration.
Insulin stimulates excess glucose to be converted to glycogen for storage in liver cells and muscle cells.
Insulin converts excess glucose to fat in adipose cells.
Negative feedback returns blood sugar levels to normal.

Alpha ( $α$ ) cells in pancreatic Langerhans cells secrete glucagon into the blood.
Glucagon stimulates liver cells to convert glycogen to glucose.
Glucagon promotes fat breakdown, releasing fatty acids that can be metabolised to produce energy.
Negative feedback returns blood sugar levels to normal.

Failure in insulin production, secretion or intake by target cells can cause diabetes mellitus.
Blood sugar levels are usually high and unstable after a meal; the patient feels thirsty, tired and fatigued, and loses weight.
Diabetes mellitus can be controlled through insulin injections, pills that lower blood sugar levels and a proper diet.

Breathing is an involuntary action regulated by the respiratory control centre in the medulla oblongata.
During vigorous activity, cellular respiration increases the partial pressure of carbon dioxide.
Carbon dioxide dissolves in blood plasma to form carbonic acid, which breaks down into hydrogen ions and bicarbonate ions.
The pH of blood and cerebrospinal fluid decreases.
Central chemoreceptors in the medulla oblongata and peripheral chemoreceptors in the carotid body and aortic body detect the pH change.
Nerve impulses are sent to the respiratory control centre and cardiovascular control centre in the medulla oblongata.
Intercostal muscles, the diaphragm and cardiac muscles contract and relax quickly.
Breathing rate, heart rate and ventilation rate increase; more carbon dioxide is expelled from the lungs.
The partial pressure of carbon dioxide and blood pH return to normal.

Baroreceptors in the aortic arch and carotid artery detect blood pressure and continuously send impulses to the cardiovascular control centre in the medulla oblongata.