Maarg Manthan · Topic 3.5

Osmoregulation, ADH and Formation of Urine; Thermoregulation

Science & Technology › Human Physiology – Organ Systems · Topic 3.5

Osmoregulation is the control of the water and salt content of body fluids, and thermoregulation is the control of body temperature. In humans, the kidneys and the hypothalamus work together to keep water balance. Antidiuretic hormone, ADH, also called vasopressin, is made in the hypothalamus and released by the posterior pituitary. It makes the collecting ducts of the kidney more permeable to water, so more water is reabsorbed and urine becomes concentrated. Low ADH causes diabetes insipidus, with large amounts of dilute urine. Body temperature in humans is kept near 37 degrees Celsius by the hypothalamus, which acts as a thermostat and triggers sweating, shivering and changes in blood flow in the skin.

5 min read · UPSC Prelims · Science & Technology · Human Physiology – Organ Systems

At a Glance

ADH From posterior pituitary; saves water
Low ADH Diabetes insipidus
Aldosterone Adrenal cortex; saves sodium
Normal temperature About 37 degrees Celsius
Control centre Hypothalamus
Exam link UPSC Prelims, Science & Technology: Human Physiology – Organ Systems

Where Does It Fit?

This is the fifth topic of the macro on human physiology and organ systems. It builds on kidney function in the previous topic and adds control of body heat. Previous topic: Excretory System: Excretion in Plants and Animals. Next topic: Digestive System and Human Dietary Patterns.

How Do Hormones Control Water and Salt?

Hormone	Action
ADH (vasopressin)	Made in the hypothalamus and stored in the posterior pituitary. Released when blood is too concentrated, and makes the collecting duct reabsorb more water
Aldosterone	From the adrenal cortex. Increases reabsorption of sodium and loss of potassium, and water follows sodium
Renin-angiotensin system	When blood pressure falls, the kidney releases renin, which leads to angiotensin and aldosterone and raises blood pressure
Atrial natriuretic factor	Released by the heart when blood pressure is high. Increases excretion of sodium and water
Thirst centre	Hypothalamus signals thirst when water is short
Diabetes insipidus	Caused by low ADH or kidneys that do not respond, giving very large volumes of dilute urine and thirst

What Does Urine Show About Health?

Composition: normal urine is about 95 per cent water with about 2.5 per cent urea and small amounts of salts, creatinine and uric acid.
Glucose in urine: called glycosuria and a sign of diabetes mellitus, where blood sugar exceeds the amount the kidney can reabsorb.
Protein in urine: proteinuria points to kidney damage.
Blood and ketones: blood suggests infection, stones or injury, and ketone bodies appear in uncontrolled diabetes and starvation.
Colour: dark urine suggests loss of water, and orange or brown urine may suggest liver disease.
Diuretics: substances such as caffeine and alcohol increase urine output, and some medicines are used to treat high blood pressure and swelling.

How Is Body Temperature Controlled?

Hypothalamus: receives signals from temperature sensors in the skin and blood and acts as the thermostat.
In heat: sweat glands release sweat, which cools the body when it evaporates, and the blood vessels of the skin widen, called vasodilation, to lose heat.
In cold: muscles shiver to produce heat, blood vessels narrow, called vasoconstriction, hair stands up and metabolic rate rises, and the thyroid hormones raise heat production.
Endotherms and ectotherms: birds and mammals are warm-blooded and keep a constant temperature, while fishes, amphibians and reptiles are cold-blooded and depend on the surroundings.
Hibernation and aestivation: some animals sleep through the cold winter or the hot dry season to save energy, such as bears and some frogs and snails.
Fever: a rise in the set point of the hypothalamus during infection, which helps the body to fight microbes.
Disorders: heat stroke when temperature control fails in extreme heat, and hypothermia when body temperature falls below about 35 degrees Celsius.

What Else Should You Remember and Why Does It Matter?

Osmoregulators in water: freshwater fishes take in water and excrete dilute urine, while marine bony fishes lose water and drink seawater and excrete salt through gills.
Link to hormones: ADH and aldosterone are described in Endocrine System: Hypothalamus, Pineal Body, Pituitary Gland, Thyroid, Adrenal Glands, Reproductive Glands.
Climate link: heat waves raise the risk of heat stroke, and the topic connects to heat action plans for health.
Dehydration and ORS: loss of water and salts in diarrhoea is treated by oral rehydration solution, a low-cost intervention that saves many lives.
Exam pattern: hormone-organ matching, the cause of diabetes insipidus and examples of warm-blooded animals.
Antidiuretic hormone and alcohol: alcohol blocks the release of antidiuretic hormone, which is why it increases urine output and can cause dehydration.
Sweat: sweat is about 99 per cent water with salts such as sodium chloride, and an adult can lose a litre or more an hour in hot conditions.
Brown fat: babies and hibernating animals have brown fat, which burns fuel to make heat instead of ATP.
Temperature scales: normal body temperature of 37 degrees Celsius equals about 98.6 degrees Fahrenheit, and it varies slightly through the day.

As an analytical point, osmoregulation and thermoregulation are two forms of homeostasis, the keeping of a steady internal environment. Hormones and nerves work together as feedback loops to correct changes.

Why Is This Topic Important?

Facts Hormone actions asked
Disease Diabetes insipidus
Link Heat and dehydration

What Came Next?

The next topic covers the digestive system and human dietary patterns. Next topic: Digestive System and Human Dietary Patterns.

Key People and Terms

Osmoregulation Control of water and salts
Homeostasis Steady internal environment
Vasodilation Widening of blood vessels
Endotherm Warm-blooded animal
Hibernation Winter dormancy
Glycosuria Glucose in urine

Exam Corner

Points to Remember

ADH from the posterior pituitary.
Low ADH: diabetes insipidus.
Aldosterone: adrenal cortex.
Body temperature centre: hypothalamus.
Normal temperature 37 degrees Celsius.

Do Not Confuse With

Diabetes insipidus and diabetes mellitus Low ADH versus low insulin action
Hibernation and aestivation Winter versus summer dormancy
Vasodilation and vasoconstriction Widening versus narrowing

Memory Hook

ADH holds water, aldosterone holds salt; hypothalamus thermostat, sweat to cool and shiver to melt.

Mains Angle

Questions ask about homeostasis and its disorders. These points add depth.

Water ADH and kidney
Heat Hypothalamus
Disorders Diabetes insipidus, heat stroke

A Question You May Face

An original practice question, not a past paper question.

“Explain how the human body regulates water balance and body temperature, with the role of hormones.”

How to Answer

Homeostasis Concept.
Water ADH, aldosterone.
Heat Hypothalamus.
Disorders Examples.

GS Relevance

Prelims: hormones and their action, diabetes insipidus, temperature control, endotherms and ectotherms. GS Paper 3: science and technology, health.

Frequently Asked Questions

What is ADH?

Antidiuretic hormone, also called vasopressin, is made in the hypothalamus and released by the posterior pituitary gland. It acts on the collecting ducts of the kidney so that they reabsorb more water, which makes the urine concentrated and conserves water, and a lack of it causes diabetes insipidus.

What is diabetes insipidus?

Diabetes insipidus is a disorder in which the body does not make enough ADH or the kidneys do not respond to it, so large volumes of dilute urine are passed and the person is very thirsty. It is different from diabetes mellitus, which is caused by a lack of insulin or its action and raises blood sugar.

How does the body control its temperature?

The hypothalamus acts as a thermostat. In heat it triggers sweating and widening of skin blood vessels to lose heat, and in cold it triggers shivering, narrowing of blood vessels and higher metabolism to produce and keep heat. Normal human body temperature is about 37 degrees Celsius.

What is the difference between endotherms and ectotherms?

Endotherms, such as birds and mammals, make their own body heat and keep a nearly constant temperature, so they are called warm-blooded. Ectotherms, such as fishes, amphibians and reptiles, depend mainly on the temperature of the surroundings, so they are called cold-blooded, and their activity changes with the weather.

What does glucose in urine show?

Glucose in urine, called glycosuria, usually shows that blood sugar is too high for the kidney to reabsorb it all, as in diabetes mellitus. It is a reason for further tests, since healthy kidneys reabsorb all the glucose in the filtrate so that normal urine contains none.

PYQ Practice — Statement Analysis

1 ADH is released by the posterior pituitary.

True

It conserves water.

2 Diabetes insipidus is caused by lack of insulin.

False

It is caused by low ADH or its action.

3 The hypothalamus controls body temperature.