1. Human body is designed for breathing through the nose with nostrils and respiratory pathways well organized.
2. Nasal breathing increases circulation, blood oxygen and carbon dioxide levels and improves overall lung volumes.
3. Mouth respiration is only for emergency and causes problems such as snoring.

1. During hard physical work, we require more energy.
2. The metabolism of food increases. This increases the amount of CO₂ in the blood.
3. The increase of CO₂ concentration in the blood stimulates the respiratory centre of the brain.
4. The brain sends stimulus through nerves for the increase in the rate of respiration. Thus we breathe faster during any kind of hard physical work.

Definition: It is an additional volume of air a person can inspire by forceful inspiration.
Normal value: 2500 - 3000 mL

Smoking is inhaling the smoke from burning tobacco. There are thousands of known chemicals which includes nicotine, tar, carbon monoxide, ammonia, sulphur- dioxide and even small quantities of arsenic. Carbon monoxide and nicotine damage the cardiovascular system and tar damages the gaseous exchange system. Nicotine is the chemical that causes addiction and is a stimulant which makes the heart beat faster and the narrowing of blood vessel results in raised blood pressure and coronary heart diseases. Presence of carbon monoxide reduces oxygen supply. Lung cancer, Cancer of the mouth and larynx is more common in smokers Smoking also causes cancer of the stomach, pancreas and bladder and lowers sperm count in men. Smoking can cause lung diseases by damaging the airways and alveoli and results in emphysema and chronic bronchitis. These two diseases along with asthma are often referred as Chronic Obstructive Pulmonary Disease (COPD). When a person smokes, nearly 85% of the smoke released is inhaled by the smoker himself and others in the vicinity, called passive smokers, are also affected. Guidance or counselling should be done in such users to withdraw this habit.

The oxygen is poorly soluble in water so, only 3% of the oxygen is transported in the haemoglobin in a reversible manner to form oxyhaemoglobin (HbO₂).