Respiration is essential for life because it provides energy for carrying out all the life processes which are necessary to keep the organisms alive.



All living cells need a constant supply of oxygen to enable them to carry out the essential biochemical reactions of their metabolism. This oxygen supply is provided by the blood which also removes the CO2 and other waste products. Where does the blood get the oxygen, and what does it do with the CO2 ? The oxygen comes from the outside air, into which also the CO2 is discharged. The respiratory system provides the means of doing this.

It is the synthetic or constructive metabolism. In this case smaller molecules unite to form larger molecules. e.g., photosynthesis.

It is the destructive metabolism which involves the breaking down of large organic molecules. This is of ten accompanied with the liberation of energy e.g., respiration.

The sum total of the constructive (anabolism) and the destructive (catabolism) chemical  changes occurring in living beings.

The process involving inspiration (intake of air or oxygen) and expiration (removal of air or carbon dioxide) is called breathing. No enzymes are involved in this process.

The process of releasing energy from food is called respiration.
ATP. It refers to a nitrogenous compound, Adenosine Tri-Phosphate. The energy released during cellular respiration is immediately used to synthesise a molecule called ATP from ADP and inorganic phosphate as

ATP is used to fuel all other activities in the cell. Therefore, it is said to be the energy currency for most cellular processes.


Type of RespirationType of Respiration

It is a type of respiration which occurs in the presence of oxygen. The organisms showing aerobic respiration are called aerobes.

{C_6}{H_{12}}{O_6} + 6{O_2} - \to 6C{O_2} + 6{H_2}O + 2830kj(686Kcal)

Break down of glucose occurs in various steps which can be summarized as :


When food is oxidized without using oxygen is called anaerobic respiration.

(a) Fermentation
Anaerobic respiration is also called fermentation. It is found in lower organisms like anaerobic bacteria and yeasts :

{C_6}{H_{12}}{O_6}\buildrel {Bacteria} \over\longrightarrow 2{C_2}{H_5}OH + 2C{O_2} + 2ATP

SugarSoluion\buildrel {Yeast} \over\longrightarrow Ethanol + C{O_2} + Energy

(b) Temporary anaerobic respiration
may occur even in our own body in the fast working skeletal muscles, as in fast running, walking, swimming etc. The fatigue experience is due to lactic acid accumulated in the muscles in the shortage of oxygen, a condition which may be called Oxygen debt. When one rests after the exercise the lactic acid gets slowly oxidized by the oxygen later available and then the “debt” is cleared.

{C_6}{H_{12}}{O_6}\mathrel{\mathop{\kern0pt\longrightarrow}\limits_{Cytoplasm}^{Glycolysis}} PyruvicAcid\buildrel {AB{O_2}} \over\longrightarrow LacticAcid + 2ATP


Differences between Aerobic and Anaerobic respiration


Respiration in plants is simpler than the respiration in animals. Gaseous exchange occurs through:
(i) Stomata in leaves
(ii) Lenticels in stems
(iii) General surface of the roots

(i) Respiration through stomata : 

    • Stomata are small apertures found on the surface of leaf.
    • For the process of respiration, oxygen enters stomata by the process of diffusion and then into other cells of the leaf.
    • This oxygen is utilized in the break down of glucose to carbon dioxide and water.
    • This does not occur in a single step but in a series of steps.
    • When concentration of CO2 increases inside the cells, it is diffused out through stomata.


(ii) Respiration through lenticels :
Lenticels are the openings in the bark of woody stems.

(iii) Respiration through general surface of the roots :

  • Exchange of gases in roots take place by the process of diffusion, when oxygen diffuses into the root hairs and passes into the root cells, from where carbon dioxide moves out into the soil.
  • In older roots there are no root hairs present. Instead they have layer of dead cells which is protective in nature and encloses small openings. These are used for gaseous exchange between soil and inner living cells.

Difference between Respiration and Photosynthesis

Respiration in AnimalsRespiration in Animals’

It takes place with the help of some specific respiratory organs  which differs in different animal groups, according to their habitat. Aquatic animals like fish, prawns and mussels have gills as respiratory organs ; land animals like lizard, bird, human have lungs, frogs breathe both by skin and lungs and insects like grasshopper, housefly or cockroach have air lubes or trachea as their respiratory organs.


Respiration in amoeba

 Respiration in fish



This kind of respiration, where lungs are the main structures is called pulmonary respiration. Respiratory system communicates wilh the outside atmosphere through external nostrils which draw air into nasal cavities.

Respiratory organs in human beings consists of :

1. Nostrils and nasal cavity
2. Nasopharynx
3. Larynx
4. Trachea
5. Bronchi
6. Alveoli
7. Lungs
8. Diaphragm


Nostrils are two nasal openings which serve like the gateway of the respiratory system. The nasal cavity has one central septum that divides the whole cavity into two parts.  The nasal cavity secretes mucus which helps to remove the dust particles from the air and air also normalizes the air while swallowing food.

It is the junction between the nasal cavity and the larynx. It is guarded by epiglottis which closes the passage of air to body temperature.

3. LARYNX (Also known voicebox)
It is the voice box which is interposed to prevent the entry of food material in the trachea. While swallowing this part rises and falls. Larynx contains two ligamentous folds called vocal cords. Air expelled between the vocal cords vibrates them producing sound.

Trachea is about four inches longt. It is composed of 16-20 incomplete cartilagenous rings. These cartiligenous rings ensure that trachea does not collapse even when there is very less air in it. The oesophagus is situated on the back of the trachea.

The bronchi are formed by the bilateral bifurcation of trachea. Further divide into bronchioles which end into alveoli inside the lungs.

6. Alveoli 
Alveloli are lined by a layer of epithelial cells and surrounded by a network of blood capillaries. Alveoli covers about area of 80 metre square when spread out. This large surface area helps in efficient exchange of gases.

These are two spongy elastic organs formed of alveoli bronchioles, blood vessels etc. The right lung has three lobes and the left lung has two lobes.
Lung is covered by a double membran known as ‘pleura’. The visceral layer of the pleura is closely attached to the lungs. The free layer on the thoracic wall is known as parietal layer. Between two pleural layers, there is a fluid which lubricates the surface and prevents friction between the lungs and the chest wall during respiration.

It is a large dome shaped sheath of muscle which separates the thoracic cavity from the abdominal cavity. The contraction of diaphragm brings about it’s downward movement which decreases the intrathoracic pressure and increases the intra-abdominal pressure.

Human Respiratory System

Mechanism of Breathing
Lungs cannot expand or contract of their own. The contraction and expansion of lungs is brought about by diaphragm muscles and external intercostal muscles.


(A) Inspiration

It is also known as inhalation. It is as a result of combined action of the ribs and the diaphragm.

    • Ribs move upward and outward by a set of muscles known as intercostal muscles thus enlarging the thoracic cavity.
    • Diaphragm, which normally remains arched upward like a dome, towards the base of the lungs, flattens to an almost horizontal plane and thus enlarging thoracic cavity lengthwise.

As a result of above two actions of ribs and diaphragm, thoracic cavity increases in size. It leads to the decrease in pressure in the lungs as compared to the atmospheric pressure. Therefore, the atmospheric air which is at a greater pressure rushes into the lungs.

(B) Expiration
It is also known as exhalation. It is the reverse of inspiration. It again involves the action of ribs and diaphragm.

Diaphragm Depressed During Inspiration

  • Ribs move downward and inward by the action of other set of intercostal muscles.
  • Diaphragm moves upwards to form a dome shape thus, putting pressure on lungs.

As a result, there is increase in pressure in the thoracic cavity as compared to the atmospheric pressure. Lungs are therefore compressed forcing the air out into the atmosphere.


Gas Exchange in Alveoli

  • Blood rich in carbon dioxide, i.e., the deoxygenated blood enters the capillary network of alveolus.
  • CO2 diffuses into the alveolar cavity because of its higher concentration in the blood.
  • Alveolus has a higher concentration of oxygen as compared to the blood in capillaries.
  • Therefore, O2 diffuses into the capillaries and combines with haemoglobin of red blood cells to form oxyhaemoglobin to be transported throughout the body.


Gas Exchange in Tissues.

  • In the cells, continuous metabolism of glucose and other substances results in the production of CO2 and utilisation of O2.
  • The concentration of oxygen in the cells and tissue fluid decreases while the concentration of CO2 is higher than in the capillaries.
  • Therefore, oxyhaemoglobin breaks down releasing  O2 diffuses  out from the capillaries into the tissue fluid and then into each and every cell.


Lung volumes and capacities

Spirometer is the instrument used to measure lung volume & capacities.

(1) Tidal volume
Amount of air inhaled and exhaled during quiet breathing = 500 ml.
(a) Dead space
air : A small amount of air breathed in respiratory tract that does not take part in gaseous exhange.
(b) Alveolar
air : Part of tidal air takes part in gaseous exhange. It occurs at alveoli is called alveolar air. It is about = 350 ml.

(2) Inspiratory reserve volume (IRV) :
(Complemental air) Maximum amount of air that can be inhaled forcibly following normal expiration (Tidal expiration).
It is about = 3000 ml.

(3) Expiratory reserve volume (ERV) :
(Supplemental air) Maximum amount of air that can be expired forcefully after a tidal inspiration.
It is about 1000 ml.

(4) Residualvolume :
Volume of air that remains in
lung after maximal expiration.

(5) Vital capacity : Maximum volume of air that can be takes in or expelled by maximum inspiration and expiration = 4500ml ; V. C. = T. V. + I. R. V. + E. R. V.

(6) Total lung capacity : Maximum air which can at any time be taken in two lunge = 6000 ml.
TLC = V.C. + R.V.
Gas exchange occurs in alveoli. Fresh air has
high concentration of oxygen and very low concentration of carbondioxide. As a result oxygen diffuses from alveolar air to blood present in capillaries around the alveoli. Carbon dioxide diffuses from blood in to alveolar air.


  • Respiratory cycle-inspiration, expiration and respiration pause.
  • In adults respiratory rate is 12-14 breaths/min.
  • In Newborn respiratory rate is 18-22 breaths/min.
  • Increase % of CO2 in blood leads to increase respiration.
  • Slow and shallow breathing least to inadequate supply of oxygen.
  • At altitudes above 5,000 m above sea level, one could easily suffer from dizziness, blackouts accompanied by impaired vision.
  • As the altitudes increases, the pressure decreases resulting in decreases supply of oxygen.
  • Respiratory rate and cycle is maintained by respiratory centres present in Pons & Medulla.

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