Key internal conditions controlled in humans

Homeostasis and response • Homeostasis

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Describe the response to an increase in core body temperature.

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Core temperature increase → thermoregulatory centre stimulates sweating and vasodilation → increased heat loss → core temperature decreases .

Key concepts

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Blood glucose concentration - cause → effect

A rise in blood glucose after a meal is detected by the pancreas, which increases insulin secretion. Insulin causes liver and muscle cells to convert glucose to glycogen and increases cellular uptake of glucose; blood glucose concentration falls as a result. A fall in blood glucose during fasting or exercise triggers the pancreas to release glucagon. Glucagon causes glycogen to break down into glucose in the liver and raises blood glucose concentration back to normal . The insulin–glucagon system operates by negative feedback. Excessive or insufficient control leads to disorders such as diabetes, where blood glucose regulation becomes impaired and blood glucose remains too high despite regulatory attempts .

Body temperature - cause → effect

Temperature receptors in the skin and brain detect changes from the normal core temperature and send impulses to the thermoregulatory centre in the brain. When core temperature rises, the centre increases sweating and causes vasodilation of skin arterioles; evaporative cooling and increased heat loss cause core temperature to fall. When core temperature falls, the centre stimulates shivering and vasoconstriction; muscle contractions generate heat and reduced skin blood flow conserves heat, causing core temperature to rise . Limiting factors include extreme environmental temperatures and failures of effectors. Fever, hypothermia and hyperthermia occur when regulatory responses cannot restore normal temperature quickly enough or are overwhelmed by external conditions .

Water balance - cause → effect

Changes in blood water concentration are detected by osmoreceptors in the brain and processed by the osmoregulatory centre adjacent to the pituitary gland. Low blood water concentration causes increased release of antidiuretic hormone (ADH) from the pituitary. ADH increases water reabsorption in the kidney nephrons, reducing urine volume and conserving water; blood water concentration rises as a result. High blood water concentration causes reduced ADH release, decreased water reabsorption in the kidneys and increased dilute urine output, which lowers blood water concentration . Kidney filtration, selective reabsorption and urine concentration act together to maintain water and ion balance. Failure of kidney function or severe fluid loss overwhelms these mechanisms and disturbs homeostasis .

Key notes

Important points to keep in mind

Homeostasis maintains internal variables within narrow limits for enzyme function .

Negative feedback reverses deviations from normal and prevents runaway changes .

High blood glucose → insulin → glycogen formation → blood glucose falls; low blood glucose → glucagon → glycogen breakdown → blood glucose rises .

Core temperature rises → vasodilation and sweating → increased heat loss; core temperature falls → vasoconstriction and shivering → heat conservation and production .

Low blood water concentration → increased ADH → kidneys reabsorb more water → urine volume decreases .

The nephron filters blood, selectively reabsorbs useful substances and excretes excess water, salts and urea .

Failure of organs involved in regulation (pancreas, kidneys, thermoregulatory centre) disrupts homeostasis and causes clinical conditions .

Enzyme activity limits temperature tolerance; extreme temperatures produce fever, hypothermia or hyperthermia .

Blood-borne hormones act more slowly but often have longer-lasting effects than nerve impulses .

Regular revision of cause → effect chains improves recall for regulation processes (sensor → control centre → effector → response).