Biology Class 11 NCERT Solutions: Chapter 17 Breathing and Exchange of Gases Part 1

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Q: 1. Define vital capacity. What is its significance?


Vital capacity is the maximum volume of air that can be exhaled after a maximum inspiration. It is about litres in the human body. It promotes the act of supplying fresh air and getting rid of foul air, thereby increasing the gaseous exchange between the tissues and the environment.

Q: 2. State the volume of air remaining in the lungs after a normal breathing.


The volume of air remaining in the lungs after a normal expiration is known as functional residual capacity (FRC). It includes expiratory reserve volume (ERV) and residual volume (RV). ERV is the maximum volume of air that can be exhaled after a normal expiration. It is about is the volume of air remaining in the lungs after maximum expiration. It is about .

Functional residual capacity of the human lungs is about .

Q: 3. Diffusion of gases occurs in the alveolar region only and not in the other parts of respiratory system. Why?


Each alveolus is made up of highly permeable and thin layers of squamous epithelial cells. Similarly, the blood capillaries have layers of squamous epithelial cells. Oxygen-rich air enters the body through the nose and reaches the alveoli. The deoxygenated (carbon dioxide-rich) blood from the body is brought to the heart by the veins. The heart pumps it to the lungs for oxygenation. The exchange of and takes place between the blood capillaries surrounding the alveoli and the gases present in the alveoli.

Thus, the alveoli are the sites for gaseous exchange. The exchange of gases takes place by simple diffusion because of pressure or concentration differences. The barrier between the alveoli and the capillaries is thin and the diffusion of gases takes place from higher partial pressure to lower partial pressure. The venous blood that reaches the alveoli has lower partial pressure of and higher partial pressure of as compared to alveolar air. Hence, oxygen diffuses into blood. Simultaneously, carbon dioxide diffuses out of blood and into the alveoli.

Q: 4. What are the major transport mechanisms for ? Explain.


Plasma and red blood cells transport carbon dioxide. This is because they are readily soluble in water.

Image of the carbon dioxide transport

Image of the Carbon Dioxide Transport

Image of the carbon dioxide transport

(1) Through plasma:

About of is carried in a dissolved state through plasma. Carbon dioxide combines with water and forms carbonic acid.

Since the process of forming carbonic acid is slow, only a small amount of carbon dioxide is carried this way.

(2) Through RBCs:

About of is transported by the red blood cells as carbaminohemoglobin. Carbon dioxide binds to the amino groups on the polypeptide chains of haemoglobin and forms a compound known as carbaminohemoglobin.

(3) Through sodium bicarbonate:

About of carbon dioxide is transported as sodium bicarbonate. As diffuses into the blood plasma, a large part of it combines with water to form carbonic acid in the presence of the enzyme carbonic anhydrase. Carbonic anhydrase is a zinc enzyme that speeds up the formation of carbonic acid. This carbonic acid dissociates into bicarbonate and hydrogen ions .

Q: 5. What will be the p02 and pC02 in the atmospheric air compared to those in the alveolar air?

(i) lesser, higher

(ii) higher, lesser

(iii) higher, higher

(iv) lesser, lesser


(ii) higher, lesser

The partial pressure of oxygen in atmospheric air is higher than that of oxygen in alveolar air. In atmospheric air, is about . In alveolar air, it is about .

The partial pressure of carbon dioxide in atmospheric air is lesser than that of carbon dioxide in alveolar air. In atmospheric air, is about . In alveolar air, it is about .