It is the process by which living things keep a stable internal environment, even when the external environment changes. Its important processes are thermoregulation, osmoregulation and excretion.

What is the difference between poikilotherms and homeotherms?

A poikilotherm cannot keep its body temperature within narrow limits. It varies with the environment. A homeotherm keeps a fairly constant temperature whatever the surroundings.

What is the difference between an osmoconformer and an osmoregulator?

An osmoconformer lets its body fluids match its environment and stays isotonic with it. An osmoregulator keeps its internal concentrations steady even when the environment changes.

Why do marine fish drink seawater?

Water leaves their bodies by osmosis through the gills and skin. They drink seawater to make up the loss. They also make a small amount of very concentrated urine.

Which nitrogenous waste needs the least water?

Uric acid. One gram needs about 1 mL of water. One gram of urea needs 50 mL and one gram of ammonia needs 500 mL.

FSc Notes Punjab Board: 2nd Year Biology Thermoregulation and Osmoregulation

Homeostasis keeps the inside of the body steady while the outside changes. Three processes carry it out: thermoregulation for heat, osmoregulation for water and salts, and excretion for wastes. Learn the control loop first. Then every example in Chapter 13 fits into it.

This note follows our lessons Homeostasis, Osmoregulation, Excretory Products and Habitat and Thermoregulation. They follow the new PECTAA Biology 12 book (2026-27), Chapter 13. Keep your own book open too.

What to know

Homeostasis is a vital process by which living organisms keep a stable internal environment, despite changes in the external environment. The book lists three important processes: thermoregulation, osmoregulation and excretion.

Homeostasis works through three elements:

Receptors are specialised structures that detect changes inside or outside the body. They send the information to the control centre. Example: receptor cells in the skin detect temperature change.
Control centre receives the information, processes it and sends a message to the effectors. It keeps conditions near the set points. The brain, spinal cord and endocrine glands act as control centres.
Effectors respond to the message. Muscles contract. Glands release their secretion. Example: when body temperature drops below the set point, effectors in the skin and muscles cause shivering and vasoconstriction.

Feedback mechanisms are the foundation of homeostasis. The output of a system is used to control the system itself.

Negative feedback: the output reduces the activity of the system. It is the most common form. Examples: body temperature, blood glucose, hormone secretion. If body temperature goes above 37 °C, thermoreceptors in the hypothalamus detect it. The control centre in the hypothalamus signals the sweat glands. Sweat lowers the temperature, and the receptors stop sending signals.
Positive feedback: the output increases the activity of the system. It is less common. Examples: blood clotting, labour and childbirth. In childbirth, uterine contractions send impulses to the hypothalamus. It signals the pituitary gland to release oxytocin. Oxytocin causes more contractions. The loop continues until the baby is born.
Unregulated positive feedback can harm. In severe infection, cytokines push the immune response too far. This causes too much inflammation and tissue damage.

Key points: osmoregulation

Osmoregulation is the maintenance of the balance of water and solutes in body fluids. It stops the fluids from becoming too dilute or too concentrated. Osmosis always moves water from a more dilute (hypotonic) solution to a less dilute (hypertonic) solution.

Osmoconformers let their body fluids match the environment. They remain isotonic to it. The book lists all marine invertebrates, some freshwater invertebrates, and some marine vertebrates such as hagfishes, sharks and rays.
Osmoregulators keep constant internal concentrations despite changes outside. They use active transport of salts, accumulation of salts and water conservation. Most vertebrates are osmoregulators.

Freshwater animals are generally hypertonic to their surroundings. Water keeps entering through permeable surfaces like gills. Salts such as Na⁺ and Cl⁻ diffuse out. They make large volumes of dilute urine, actively take salts back in, and rarely drink water.

Marine animals are mostly hypotonic to the sea. Water leaves through the gills and skin. They drink seawater and make very small amounts of highly concentrated urine. Chloride cells in the gills use ATP to pump salts out. The kidneys also remove salt. Some marine fishes have rectal glands that move salt into the gut, to leave with the faeces.

Terrestrial animals lose water by evaporation from respiratory surfaces and skin, and in waste. They must match intake (drinking, food and metabolic water) to loss.

Impermeable outer layers: a waxy chitin exoskeleton in insects, keratin scales in reptiles, and dead keratinised skin cells plus oil glands in mammals and birds.
Kidneys retain water.

Key points: nitrogenous wastes

Waste	Toxicity	Water needed per gram	Typical animals
Ammonia	highly toxic	500 mL	most fishes, protozoans, sponges (ammonotelic)
Urea	less toxic	50 mL	terrestrial animals, humans (ureotelic)
Uric acid	least toxic, less soluble	1 mL	insects, birds, most reptiles (uricotelic)

Ammonia forms when the amino group is removed from amino acids, and the liver combines it with H⁺. Terrestrial animals convert it to urea in the liver. Uric acid takes a lot of energy to make.

Key points: thermoregulation

Thermoregulation is the homeostatic process that keeps internal body temperature in a tolerable range. It matters because enzymes work best at an optimal temperature. Too cold, and metabolic rates drop. Too hot, and enzymes denature. Cell membranes also lose their selective permeability when hot, and become rigid and brittle when cold.

Ectotherms gain most body heat from outside. Their metabolism is low. Fishes, amphibians, reptiles and most invertebrates are ectotherms. They bask in the sun, sit on a warm rock, seek shade or burrow.
Endotherms make most of their heat by internal metabolism. Birds and mammals are endotherms. They may shiver or burn brown adipose tissue. Hair, feathers and fat insulate them.
Poikilotherms cannot keep body temperature within narrow limits. All invertebrates, fishes, amphibians and reptiles are poikilotherms.
Homeotherms keep a relatively constant temperature. Mammals and birds are homeotherms. The hypothalamus works as the thermostat.

Humans are endothermic homeotherms and use negative feedback.

Overheating: above 37 °C. Vasodilation widens the capillaries under the epidermis, so more blood flows and heat is lost. Sweat evaporates and cools the body. The thyroid may release less thyroxine, which lowers the basal metabolic rate.
Cooling: below 37 °C. Vasoconstriction reduces blood flow to the skin. Shivering produces heat. Goosebumps trap air as insulation. The metabolic rate rises.

Worked examples

Example 1. Describe the negative feedback that controls body temperature.

The temperature rises above the set point of 37 °C.
Thermoreceptors in the hypothalamus detect the rise.
The control centre in the hypothalamus signals effectors, the sweat glands.
Sweat lowers the temperature. The receptors stop sending signals.

Example 2. Explain how a marine fish keeps its water balance.

It is hypotonic to seawater, so water leaves through gills and skin.
It drinks seawater and makes a small volume of concentrated urine.
Chloride cells in the gills pump salt out using ATP.

Example 3. Why can a desert bird not excrete ammonia?

Ammonia needs 500 mL of water per gram. This is too costly.
The bird converts it to uric acid, which needs just 1 mL per gram.

Common mistakes in the exam

Saying "negative" feedback is harmful. It means the output reduces the activity. It is the most common kind.
Confusing the roles. Receptors detect. The control centre decides. Effectors act.
Mixing up poikilotherm and ectotherm. One is about how steady the temperature is. The other is about where the heat comes from.
Calling every sea animal an osmoconformer. The book names only some marine vertebrates (hagfishes, sharks and rays) as osmoconformers. Most vertebrates are osmoregulators.

Practice questions

Which term covers thermoregulation, osmoregulation and excretion? Answer: Homeostasis.
Name the control centre for human thermoregulation. Answer: The hypothalamus.
How do marine fishes make up for water loss? Answer: They drink seawater.
Where does ammonia change to urea? Answer: In the liver.
Which nitrogenous waste needs the most water? Answer: Ammonia.
Name one response to cooling in humans. Answer: Any one: vasoconstriction, shivering, goosebumps or a higher metabolic rate.

Quick revision

Receptor, control centre, effector.
Negative feedback reduces. Positive feedback increases.
Freshwater: large dilute urine. Marine: drink seawater, small concentrated urine.
Water needed per gram: ammonia 500 mL, urea 50 mL, uric acid 1 mL.

Study the lessons Homeostasis (/learn/fsc-2/biology/homeostasis), Osmoregulation (/learn/fsc-2/biology/osmoregulation), Excretory Products and Habitat (/learn/fsc-2/biology/excretion) and Thermoregulation (/learn/fsc-2/biology/thermoregulation).

Ready to practise this? Continue in 2nd Year Biology.