NIOS Biology Chapter 18 Homeostasis: The Steady State

NIOS Class 12 Biology Homeostasis Solutions help students understand how organisms maintain internal balance. The chapter covers the regulation of temperature, water, and salts in the body. It explains mechanisms of feedback and stability in living systems.

Solved terminal exercises provide answers for easy revision. This chapter is important for linking physiology with survival strategies. It ensures that learners prepare thoroughly for exams while building strong conceptual clarity in NIOS Class 12 Biology.

NIOS Class 12 Biology Homeostasis Solutions

1. List the three conditions necessary for the body cells to function properly.

Homeostasis is necessary because the body cells need to have suitable conditions around them for proper functioning. These conditions include the presence of proper concentration of chemicals, proper temperature, and a suitable pH (degree of salinity or acidity), etc., inside its cells.

2. When do we pass out more concentrated urine–during hot summers or cold winters?

During hot summers, the urine passed out is more concentrated despite drinking lots of water. This is because the body loses more water through perspiration, and to maintain the requisite amount of water, the body withholds water, and only a smaller amount of water is excreted through urine. 

3. How does our body deal with any extra sugar absorbed into the blood after meals?

The liver converts the extra sugar in the blood into glycogen and stores it. 

4. What is our normal RBC count per cubic millimetre? Will it go up or go down if a plain dweller shifts to a mountain or hill?

A normal human adult possesses about 5 million red blood corpuscles (RBCs) per

cubic millimetre of blood. If a pain dweller shifts to a mountain or hill, the amount of oxygen carried by the normal number of RBCs is insufficient, so the body adds more RBCs into the blood. When the person returns to the plains, the RBC level will be back to normal in a day or two. 

5. In which temperature range do the enzymes in our body act best?

The enzymes in living organisms can act best only within a certain range of temperature of about 35-45° C.

6. Name the two terms often used synonymously for ectotherms.

Two other terms often used synonymously with ectotherms are

• Poikilotherms (poikilo: changing/varying, therm: heat) refer to acquiring the body temperature from that of the surroundings.
• Cold-blooded (the oldest term and seldom used now) means animals that are cold when touched. If you hold a frog in your hand or feel the touch of a cockroach, they are always colder than your body.

7. Name any two animals that tolerate the intense heat of the deserts by promoting heat loss.

Camels, desert rats, and several other species tolerate the intense heat of the tropical deserts mainly by promoting heat loss.

8. Which kind of feedback mechanism–the positive or the negative, normally operates in bringing about water-salt balance in our body?

It is a negative feedback mechanism. 

9. How is the enzymatic activity affected upon cooling?

On cooling. At temperatures lower than the optimum temperature, the enzymes become less and less efficient. At freezing temperatures, the enzymes may turn totally inactive.

10. How do honeybees fight cold during an intense winter?

Honey bees, during cold winter nights, huddle together inside the hive to conserve body heat collectively. During hot summers, they even operate a kind of ‘desert cooler’ by sprinkling some water on the honeycombs and fanning with their wings to cool the honeycombs.

11. Differentiate between the two terms homeotherms and poikilotherms.

Homoeotherms refer to organisms that keep the same or constant (warm) body temperature. Poikilotherms are those animals whose body temperature can not be regulated but varies according to environmental temperature.

12. Give any two examples of preventing loss of body heat by postural behaviour in humans.

By posture. At times when we feel cold,

• We hold our arms cross-folded tightly over our chest while standing or sitting.
• While sleeping in bed, we often hold our arms and legs closely folded near the body in a curved posture.

Such postures reduce the exposed body surface for heat radiation.

13. List the components of homeostasis in their proper sequence.

Homeostasis of any kind involves four components:

Norm → Sensor → Integrating centre → Effectors

• 1. Set point or the norm – This is the normal level of any factor in the body. The set point may have a small or large range. For example, the normal set point of human body temperature is approximately 37o °C (with 0.5o °C plus or minus).
• 2. Sensor – This consists of the sensory part that perceives the change in the set point. The sensor in thermoregulation comprises the heat receptors in (i) the skin and (ii) the hypothalamus, the part of the brain which perceives the temperature of the flowing blood.
• 3. Integrating centre – The integrating centre is the part that receives the information about the change in the set point of the particular state, interprets it, and then sends the command for correction. In thermoregulation, the integrating centre is the hypothalamus plus some adjoining parts of the brain.
• 4. Effectors – The effectors are the agencies that act to restore the set point. For example,
  • (i) Sweat glands, which pour out the sweat to produce cold by evaporation,
  • (ii) Skin blood vessels, which widen (vasodilate) to bring more blood to the body surface for radiating out heat and
  • (iii) Skeletal muscles, which vigorously contract (shivering) to produce heat.

14. Differentiate between positive and negative feedback mechanisms

Positive Feedback	Negative Feedback
Causes the required change in the body	Reverses the deviated change to bring it back to the normal condition.
Not concerned with homeostasis	Maintain homeostasis
E.g.. Blood coagulation	Eg. Thermoregulaiton

15. Explain the role of the following in thermoregulation in humans:

(i) Sweat glands

(ii) Skeletal muscles

(iii) Blood vessels in the skin

• (i) Sweat glands pour out the sweat to produce cold by evaporation.
• (ii) Skeletal muscles vigorously contract (shivering) to produce heat.
• (iii) Blood vessels in the skin widen (vasodilate) to bring more blood to the body surface for radiating out heat.

16. What is meant by the feed-back mechanism? What are its two types? Which one of these is applicable to thermoregulation and why?

The feedback in the living systems is of two types: negative to reverse a condition, and positive to continue in the direction of the change.

• In thermoregulation, the kind of feedback mechanism operating is of the negative type. Any deviation from the set point has to be reversed to bring it back to the normal condition. Therefore, a command has to be given to the organs concerned to function in a manner so that the deviation is corrected and brought back to the normal state.
• Positive feedback is very rare in living systems. One such example is that of the coagulation of blood. This process includes several steps in succession. The first feedback does not revive the set point, so it is not a negative feedback; instead, it produces the next and the third, and so on until the last one completes the process by plugging the cut in the blood vessel. All the feedback mechanisms in blood coagulation are of the positive type.

17. Why is thermoregulation required in our body?

The process of regulating internal body temperature is called thermoregulation. The living organisms can normally survive only within a certain range of temperature of about 0-45° C. However, organisms tend to adjust if they happen to be in places of higher or lower temperature.

When the internal body temperature is high, it destroys enzymes, denatures proteins, breaks down the plasma membrane, etc. It happens because enzymes require a specific range of temperature to be functional. Thus, thermoregulation is necessary for the body. 

18. Differentiate between endotherms and ectotherms. Which ones of these do you think can survive better if there is a sudden change in environmental temperature?

• Endotherms are organisms that maintain a steady body temperature irrespective of the temperature of the surroundings.  E.g.. Birds and mammals. They are homoiotherms and warm-blooded.
• Ectotherms are animals whose body temperature rises and falls with the rise and fall of the surrounding temperature. E.g., Frogs, fish, lizards, etc. These are poikilotherms and cold-blooded.

Endotherms are the ones that can survive better if there is a sudden change in environmental temperature. 

19. Differentiate between physiological and behavioral responses for thermoregulation in humans.

• Physiological regulation: Changes in blood circulation like vasodilation or vasoconstriction, sweating or not sweating, an increase or decrease in cell metabolism, shivering, etc. All these adjustments are not under the control of will.
• Behavioral regulation. It includes the conscious and subconscious acts, such as moving to a cooler place when the temperature increases or wrapping up in a blanket when the temperature drops.

20. Explain the role of the hypothalamus during heat regulation in humans.

The principal heat-regulating centre is located in the hypothalamus, a part of the

forebrain. This part acts like a thermostat.

• When the body has to face cooling below the normal temperature, it ‘switches on’ or speeds up the heat-producing processes and simultaneously ‘shuts off’ the heat-losing ones.
• When the body faces overheating during summer or after intense physical exercise, it accelerates the cooling process and ‘switches off’ the heat-producing ones.

21. Explain the relationship between the sensor and the integrating centre during any one kind of homeostasis.

The sensor consists of the sensory part that perceives the change in the set point. The integrating centre is the part that receives the information about the change in the set point of the particular state, interprets it, and then sends the command for correction. The hypothalamus acts as both a sensor and an integrating centre.  

The hypothalamus senses or detects a reduction in the normal body temperature. 

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The integrating centre in the brain commands vasoconstriction and shivering.

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This causes vasoconstriction and rapid muscle contractions or shivering. 

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This results in the warming up of the body

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Temperature returns to normal.

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