Gaseous Exchanges in Humans

Question: Write a note on gaseous exchanges in human body?

Answer: Gaseous exchange in humans: Human beings have an efficient respiratory system adopted well for the terrestrial mode of life. This system ensures smooth exchange of gases between human body and environment.

The human respiratory system or ventillatory system consists of specific specific organs and structures used for process of respiration. It is involved in the intake and exchange of oxygen and carbon dioxide between man and environment. Gas exchange system of man is divided into two main components which are as follows.

  • Air passage way.
  • Lungs

Air passage way: The air passageway consists of the parts through which the outside air comes in the lungs and after the exchange of gases it goes out. This passage of air consists of the following parts.


A) Nose and nostrils: Nose present at the ventral side of the head consists of two nostrils. Nostrils allows the air from the outside to nasal cavities. Nostrils are the external openings of the nose.


B) Nasal cavities: The inhaled air passes through the nose to into the nasal cavity. Bone partition separate the nasal cavity into two chambers. Each portion is lined by fine hairs (cilia) and mucous which filter the dust particles from the air. The mucous also moistens and warms the incoming air and keeps its temperature nearly equal to that of the body.

C) Pharynx: The nasal cavity opens into the pharynx by means of two small openings called internal nostrils. Pharynx is a muscular passage 4.5 inches long and is common to both food and air.

Pharynx is lined by mucous membrane. It contain tonsils and adenoids which are organs of lymphatic tissues used to trap and filter microorganisms. Pharynx is extends to larynx and esophagus.

D) Larynx: The air goes from the pharynx into the larynx. We know that glottis (cavity of larynx) is a narrow opening at the floor of pharynx which leads into larynx.

The larynx is a box, made of cartilage. It is present between pharynx and trachea. It is also called the voice box. Two pairs of fibrous bands called vocal cords are stretched across the larynx. The vocal cords vibrate when the air passes through them. This vibration produces sounds.

E) Trachea: larynx opens into the trachea or windpipe, which is a tubular structure. It is about 12 cm long tube which lies in front of the esophagus. There are 16 to 20 C-shaped cartilaginous rings in the wall of trachea. The cartilages keep the trachea from collapsing even when there is no air in it. The trachea is ciliated and has mucous membrane.

F) Bronchi: On entering the chest cavity, the trachea divides into two smaller tubes called bronchi (Singular: bronchus). The bronchi also have cartilaginous plates in their walls. Each bronchus enters into the lung of its side and then divides into smaller branches.

G) Bronchioles or Broncheoli: The bronchioles are smaller air pipes formed by continuous division of bronchi. They have diameter of 1 millimeter or less than one millimeter. The bronchioles progressively lose the cartilages as they become narrower.

The bronchioles end as fine tubules called the alveolar ducts. Each alveolar duct opens into a cluster of pouches called alveoli. The alveoli form the respiratory surface in human body. Each alveolus is a sac-like structure lined by a single layer of epithelial cells. It is bound on the outside by a network of capillaries.

The pulmonary artery from the heart containing deoxygenated blood enters the lungs and branches into arterioles and then into capillaries which surround the alveoli. These then joins together to form the venules which form pulmonary vein. The pulmonary vein carries the oxygenated blood back to the heart.

Lungs and alveoli (alveolus means small cavity):

All the alveoli on one side constitute a lung. Alveoli are the functional unit of lungs. First gaseous exchanges occur in the alveoli of lungs through the process of diffusion. There is a pair of lungs in the thoracic cavity. The chest wall is made up of 12 pairs of ribs and the rib muscles called inter-coastal muscles. A thick muscular structure, called diaphragm, is present below the lungs.

The left lung is slightly smaller and has two lobes and the right lung is bigger with three lobes. They are spongy and elastic organs. The lungs also have blood vessels that are the branches of the pulmonary arteries and veins.

Each lung is enclosed by two membranes called the outer pleural membrane and the inner pleural membrane. The membranes enclose a fluid (pleural effusion) which provides lubrication for the free expanding and contracting of the lungs.


The vibrations in vocal cords and the movements of lips, cheeks, tongue and jaws produce specific sounds which result in speech. Speech is an ability that only humans are gifted with and this is one of the characteristics which has put human beings superior to all.


The trachea and the bronchi are also lined with ciliated and glandular cells. The glandular cells secrete mucus which moistens the air and also traps any fine particles of dust or bacteria that have escaped from the nasal cavity. The cilia beat with an upward motion so that the foreign particles along the mucus are sent to the oral cavity from where it may be either swallowed or coughed out.


Question: Write down the steps of inhalation and exhalation?

Answer: The Mechanism of Breathing: Mechanism of breathing is basically the process by which the lungs expand to take in air and then contract to expel it. The physical movements associated with the gaseous exchange are called breathing.

Phases of breathing: There are two phases of breathing.


  1. Inspiration or Inhalation

During inspiration, the rib muscles contract and ribs are raised. At the same time the dome-shaped diaphragm contracts and is lowered. These movements increase the area of the thoracic cavity, which reduces the pressure on lungs. As a result, the lungs expand and the air pressure within them also decreases. The air from outside rushes into the lungs to equalize the pressure on both sides.


  1. Expiration or Exhalation

After the gaseous exchange in the lungs, the impure air is expelled out in exhalation. The rib muscles relax bringing the ribs back to the original position. The diaphragm muscles also relax and it gets its raised dome shape. This reduces the space in the chest cavity and increases the pressure on lungs. The lungs contract and the air is expelled out of them. The duration between inspiration and expiration is called pause.


Question: Briefly describe how breathing rate can be controlled by brain?

Answer: Humans breathe 16 -20 times per minute in normal circumstances i.e. at rest. The rate of breathing is controlled by the respiratory centre in the brain. The respiratory centre is sensitive to the concentration of carbon dioxide in the blood.

When we do exercise or some hard job our muscle cells carry out cellular respiration a greater rate. It results in the production of more carbon dioxide which is released in the blood. This greater than normal concentration of carbon dioxide stimulates the respiratory centre of brain.

The respiratory centre sends messages to the rib muscles and diaphragm to increase the rate of breathing so that the excess carbon dioxide present in blood can be removed out of body. During exercise or other hard physical works the breathing rate may increase up to 30-40 times per minute.


For your information: The breathing movements are involuntary to a large extent. However, we can control the rate of breathing but not for a long time.


Question: Draw a comparison chart of the composition of inspired and expired air?


Table: Comparison between the inspired and expired air
Feature Inspired Air Expired Air
Amount of oxygen 21% 16%
Amount of carbon dioxide 0.04% 4%
Amount of nitrogen 79% 79%
Amount of water vapours Variable Saturated
Amount of dust particles Variable Almost none
Temperature Variable Almost equal to body temperature

Question: State the signs and symptoms, causes and treatments of bronchitis, emphysema and pneumonia.

Answer: Respiratory disorders: Sometime normal functioning of the respiratory system is disrupted due to certain factors which can lead to serious respiratory disorders. Some of these disorders are discussed below.

  1. Bronchitis: Bronchitis is the inflammation of the bronchi or bronchioles.

Symptoms and effects of bronchitis:

* Cough.

* Production of excessive amount of mucus (sputum), which can be clear, white, yellowish-gray or green in color — rarely, it may be streaked with blood. Large amount of mucous results in narrowing and swelling of bronchial tubes.

* Fatigue.

* Shortness of breath (especially when doing hard job).

* Slight fever and chills.

* Chest discomfort.

* Mild wheezing,


Causes: It is caused by viruses, bacteria or exposure to chemical irritants or pollutants (e.g. tobacco smoke).

Types of Bronchitis: There are two major types of bronchitis i.e. acute and chronic. The acute bronchitis usually lasts about two weeks and patients recover with no permanent damage to the bronchi or bronchioles. In chronic bronchitis, the bronchi develop chronic inflammation. It usually lasts for three months to two years.


Prevention and Treatment of bronchitis: Getting rest, avoid smoke, use of cough syrup like hydrilline. When caused by bacteria antibiotics are used. Aspirin and acetaminophen are used for adults


The majority of people diagnosed with chronic bronchitis are 45 years of age or older


  1. Emphysema:

Emphysema is the destruction of the walls of the alveoli due to extensive cough.

Symptoms and Effects: It results in larger sacs but with less surface area for gaseous exchange. As lung tissue breaks down, the lungs do not come back to their original shape after exhalation. So air cannot be pushed out and is trapped in the lungs.

The symptoms of emphysema include shortness of breath, fatigue, recurrent respiratory infections and weight loss. By the time the symptoms of emphysema appear, the patient has usually lost 50% to 70% of his / her lung tissue.

The level of oxygen in blood may get so low that it causes serious complications. Effects of emphysema are permanent and irreversible, however if person stop smoking soon enough further damage might be reduced.

Causes: Environmental pollution and cigarette smoke can cause emphysema.

Treatment: There is no drug treatment for emphysema. Glucocorticoid and antibiotics are used. Sometime vaccines are also used.


  1. Pneumonia: Pneumonia is an infectious disease of one or both lungs that is characterized by inflammation of lung tissues.

Types: Pneumonia is of two types. If pneumonia affects one lung then it is called single pneumonia. And if this affect both lungs then it is called double pneumonia.

Causes: The most common cause of pneumonia is a bacterium, Streptococcus pneumoniae. Some viral (influenza virus) and fungal infections may also lead to pneumonia.

Symptoms and effects: When the causative organisms enter the alveoli, they settle there and grow in number. They break the lung tissues and the area becomes filled with fluid and pus.

The symptoms of pneumonia include a cold that is followed by a high fever, shivering, sneezing and a cough with sputum production. Patient may become short of breath. The patient’s skin colour may change and become dusky or purplish. It is due to poor oxygenation of blood.

Treatment: Vaccines are available to prevent pneumonia caused by S. pneumoniae. Antibiotics are used in the treatment of this type of pneumonia.


Prior to the discovery of antibiotics, one-third of pneumonia patients died from the infection.


  1. Asthma:

Asthma is a form of allergy, in which there is inflammation of the bronchi, more mucous production and narrowing of the airways.


Affects and symptoms: In asthma patients, the bronchi and bronchioles become sensitive to different allergens (allergy causing factors) e.g. dust, smoke, perfumes, pollen’s etc.

When exposed to any of such allergens, the sensitive airways show immediate and excessive response of constriction. In this condition, the patient feels difficulty in breathing.

The symptoms of asthma vary from person to person. The major symptoms include shortness of breath (especially with exertion or at night), wheezing (whistling sound when breathing out), cough and chest tightness.

Treatment: The chemicals with ability to dilate the bronchi and bronchioles are used in the treatment of asthma. Such medicine is given in the form of inhalers. Natural things like cod fish, salmon fish is also used in the treatment.

  1. Lung Cancer: Lung cancer is a disease of uncontrolled cell divisions in the tissues of the lung.

Affects and symptoms: In lung cancer the cells continue to divide without any control and form tumor. The cellular growth may also invade adjacent tissues beyond the lungs. The most common symptoms are shortness of breath,coughing (including coughing up blood) , pleural chest pain and weight loss.


Causes: The main causes of any cancer include carcinogens (such as those in cigarette smoke), ionizing radiation and viral infection. Smoking is the main cause of lung cancer. This risk of lung cancer is significantly lower in non smokers. Cigarette smoke contains over 50 known carcinogens.

Passive smoking (the inhalation of smoke from another,s smoking) is also a cause of lung cancer. The smoke from the burning end of a cigarette is more dangerous than the smoke from the filter end.


Prevention and treatment: Eliminating tobacco smoking is a primary goal in the prevention of lung cancer. The World Health Organization has called for governments to stop tobacco advertising to prevent young people from taking up smoking.

Treatment of lung cancer depends on type and stages. Lung cancer may be treated through radiations, surgery, etc. In surgery targeted cancerous tissues are removed or along the cancerous tissues nearer tissues are also removed or entire lung is removed.


Question : What are the bad effects of smoking? How does the tobacco smoke damage the respiratory system?


Answer: Bad Effects of Smoking:  J Smoking is harmful due to the chemicals in cigarettes and smoke. Tobacco smoke contains over 4,000 different chemicals, out of which at least 50 are carcinogens and many are poisonous.


* Smokers are at greater risk of developing kidneys, oral cavity, lungs and breast cancer


* Cigarette smoke affects the body from head to toe. Smokers have a much higher risk of developing a number of life threatening diseases. Like Emphysema and other respiratory disorders.


* Effects on circulatory system: Smoking also has effects on the circulatory system. The carbon monoxide present in tobacco smoke lessens the oxygen-carrying capacity of hemoglobin. Many other chemicals in smoke increase the production of blood platelets. When platelets are more than the normal numbers, they make the blood viscous and it can lead to arteriosclerosis.


* Infections in lungs: Smokers are at greater risk of developing infections, particularly in the lungs. For example, smoking increases the risk of tuberculosis by two to four times, and of pneumonia by four times.


* Weakening and staining of the teeth: Smoking is also responsible for weakening and staining the teeth. Tooth loss is 2 to 3 times higher in smokers than in non-smokers.


If a person stops smoking, the chance to develop cancer decreases as damage to the lungs is repaired and contaminant particles are gradually removed.
Nicotine is a powerful poison and was widely used as an insecticide in the past. When inhaled through tobacco smoking, it reaches our circulatory system and not only hardens the walls of the arteries but also damages the brain tissues.
According to the WHO, the rates of smoking have declined in the developed world. In the developing world, however, it is rising by 3.4% per year as of 2002.


For your information: The World No Tobacco Day is celebrated on the 31st of May every year.

Non-smokers who are exposed to second-hand smoke (passive smoke) at home or work increase their heart disease risk by 25-30% and their lung cancer risk by 20-30%.

Smoking also affects the social life of a person. Smokers may face social un-acceptance because other people may not want to be exposed to other’s smoke.


Question: How does the tobacco smoke damage the respiratory system?

 Answer: The effects of tobacco smoke on respiratory system include irritation of the trachea and larynx, reduced lung function and breathlessness due to swelling and narrowing of the lung airways and excess mucus in the lung passages.

Hot gases and particulate matter inhaled during cigarette smoke results in coughing and wheezing due to inflammation of air passage way. Cigarette smoke destroys the cilia in nose so dust particles inter into the air passage way and block it. It also fills the bronchi from phlegm and leads to bronchitis. Tobacco smoke breaks the alveoli and the alveoli fail to transfer oxygen. This severe disease is called emphysema.

Smokers are at greater risk of developing infections, particularly in the lungs. For example, smoking increases the risk of tuberculosis by two to four times, and of pneumonia by four times.


Question: Differentiate between breathing and cellular respiration.

Answer: Difference between breathing and cellular respiration:


Cellular respiration Breathing
1) Cellular respiration is the complex process by which food is broken down and energy is released. 1) Breathing is the taking in of oxygen and letting out of carbon dioxide by animals.
2) It involves biochemical as well as mechanical processes. 2) It involves only mechanical processes.
3) Respiration has three phases, i.e. glycolysis, krebs cycle and electron transport chain. 3) Breathing consist of two phases, i.e. Expiration and inspiration.


Question: Trace the path of air from the nasal cavity to the alveoli.

Answer: Nostril > Nasal cavity > Pharynx > Larynx > Trachea > Bronchi > Bronchioles (Lungs).


Alveolar duct: Fine tubules at the end of bronchioles; open into alveoli.

Alveolus: A sac-like structure present next to the alveolar duct in lungs.

Asthma: An inflammation of the bronchi that causes swelling and narrowing of the airways.

Breathing: The process through which animals take air in their bodies to get oxygen and then give out the air for getting rid of carbon dioxide.

Bronchus: The part of air passageway; formed by the division of the trachea.

Bronchioles: Fine tubules formed by the division of the bronchi.


Diaphragm: The muscular structure that forms the floor of the chest cavity;

present below lungs

Emphysema: A disease in which the walls of the alveoli are destroyed.

Exhalation: The phase of breathing in which air is expelled from the lungs.

Gaseous exchange: Taking in and giving out of gas (oxygen and carbon dioxide) by organism.

Inhalation: The phase of breathing in which air is drawn into the lungs.

Larynx: The part of the air passageway between pharynx and the trachea.

Vocal cords: Two pairs of fibrous bands in the larynx; vibrate when the air passes through them and produce sounds.

Nasal cavity: Hollow space in the nose; opens to the outside through nostrils; divided into two portions by a wall.

Nostril : Nostrils are the external openings of the nose that allows the air from the outside to nasal cavities.

Pneumonia: The infection of one or both lungs; caused by specific bacteria, viruses or fungi; the infected part of the lung becomes filled with fluid and pus.

Trachea or Windpipe: Part of the air passageway between larynx and bronchi.

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