[Part – 3] Respiratory System | Respiration – its Types

Respiratory System | Respiration – its Types

VISIT PART 1 : Lists Of Respiratory Diseases | Respiratory Disorders
VISIT PART 2 : Respiratory System | EXCHANGE OF GASES


  • Respiration is a biochemical process by which organic compounds are oxidized to liberate chemical energy from food in step-wise process. The organic compounds are carbohydrates, fats and proteins and the energy released is stored as the ATP molecules.
  • Cellular respiration is the use of oxygen and production of carbon dioxide at cellular level.


    #1. Anaerobic respiration :

    When food is oxidized without the use of molecular oxygen, it is called anaerobic respiration. The organism undergoing this type of respiration are termed as anaerobes. Examples are anaerobic bacteria, yeasts, many parasitic animals such as Taenia, Fasciola and Ascaris. In microorganisms, this respiration is termed as fermentation and this is
    termed after the name of the product they form, such as alcoholic fermentation and lactic acid fermentation.

    – Alcoholic fermentation occurs in yeasts, where they oxidize glucose to ethyl alcohol and carbon dioxide

    C6H12O6 —-2C2H5OH + 2CO2 + energy

    – Lactic acid fermentation occurs in some bacteria where glucose is metabolized to lactic acid

    – Anaerobic respiration occurs in cytoplasm and provides less energy (2ATP molecules)

    – In muscles and erythrocytes, glucose is metabolized to form lactic acid which enters the blood and reaches the liver, where it is converted to glycogen aerobically for further reuse. Accumulation of lactic acid in muscles causes fatigue.

    #2. Aerobic respiration :

    When oxygen is used for the oxidation of food it is termed as aerobic respiration and the organisms undergoing this process are termed as aerobes. It is a high energy yielding process.

    It is of two types:

    a. Direct respiration: It is the exchange of environmental molecular oxygen with the carbon dioxide of the body cells without any special respiratory organ and blood. It is found in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most arthropods.

    b. Indirect respiration: In this, exchange of gases takes place through special respiratory organs such as skin, gills, bucco-pharyngeal cavity and lungs. It needs blood for transporting oxygen and carbon dioxide after the exchange.

    – The respiration through organs are termed according to their names.
    Examples skin – cutaneous gills- brachial, buccopharyngeal cavity – buccopharyngeal and lung-pulmonary respiration.

    – The indirect respiration occur in two phases external respiration and internal respiration. These are preceded by a preliminary phase called breathing (ventilation)

    – Aerobic respiration occurs both in cytoplasm ( glycolysis) and in mitochondria (krebs cycle and electron transport chain) and provides much more energy (38 ATP molecules)

    – Breathing refers to the movement that sends fresh air to the respiratory organs and remove foul air from them.

    External respiration : It is intake of oxygen by the blood from air or water in the respiratory organs and elimination of carbon dioxide.

    Internal respiration : It involves 4 process.
    Uptake of oxygen by tissue cells from blood via blood tissue.

    – Oxidation of food in the tissue cells by the action of oxidizing enzymes producing carbon dioxide, water and energy. This is also termed as cell respiration.

    – Storage of energy from oxidation in the phosphate bonds of ATP.

    – Release of carbon dioxide by tissue cells into the blood via tissue fluid.


  • The surface at which exchange of gases (CO2 and O2) occurs is termed respiratory surface. This surface must have enough area of gas exchange to meet the metabolic needs of the Hrganism.
  • For the exchange to be efficient, respiratory surface should have the following features.

    i. It should be thin, large and moist.
    ii. It should be permeable to respiratory gases.
    iii. It should be highly vascular.
    iv. It must be directly or indirectly in contact with the source of oxygen.


    Respiratory system

  • The human respiratory system is divided into upper respiratory tract and lower respiratory tract. The upper respiratory tract includes external nostrils, nasal passage, internal nostril
    and pharynx. The lower respiratory system includes larynx, trachea, bronchi and bronchioles.

  • The special mammalian features of respiratory system are:
      1. Presence of nose

      2. Elongation of nasal passage and its complete separation from buccal passage through palate. So, that internal nostrils open deep into nasopharyngeal part of pharynx.

      3. Long wind pipe due to presence of well defined neck

      4. Spongy, solid lungs


    They are a pair of slit-like opening present on the lower end of nose.


    It occurs between palate and cranium. Nasal cavity is divisible into two nasal chambers by a nasal septum. Each nasal chamber has three parts.

    a) Vestibule

    It is a lower smaller part just above external naris which is lined by skin and bears hair as well as oil glands. Hair help in filtering out dust particles from incoming air.

    b) Conditioner ( Respiratory region)

    It is middle part of nasal chamber. There are three bony projections called nasal conchae or turbinates ( superior, middle, inferior) and some sinuses ( maxillary, frontal, sphenoid and ethmoid)

    – The conditioner part is reddish pinkish in coloured by ciliated pseudostratified
    columnar epithelium with mucous and serous glands. The inhaled air is
    moistened warned and cleaned.

    c) Olfactory region

    Upper part of nasal chamber and superior nasal concha are yellowish brown. They are covered by olfactory epithelium which perceives sensation of smell.


    The two nasal chambers open into nasopharynx through internal nares or choanac.


    .Nasopharynx occurs at the base of skill and has lining of ciliated stratified squamous epithelium.

  • Nasopharynx leads to oropharynx or common pathway of respiratory and digestive system.
  • Oropharynx passes into laryngopharynx which contains epiglottis and passes into larynx.


  • Larynx or voice box opens into laryngo-pharynx through a slit-like glottis which can be widened by intrinsic muscles. Glottis can be closed by a large leaf-like cartilaginous flap
    called epiglottis.

  • Larynx has C-shaped thyroid cartilage ( on sides and in front where it can be felt as Adam’s apple), a pair of triangular arytenoids ( arytaenoid) cartilages ( on back), a ring like cricoids
    cartilages and a pair of nodule like cartilages of santorini ( upper end of arytenoids cartilages). Internally larynx has ciliated columnar mucous epithelium and a pair of vocal
    cord ( attached to thyroid and arytenoids cartilages).

  • Vocal cords become thickened in adult males. Vocal cord are shorter and thinner in women produced by passage of air between vocal cords and modulations created by tongue, teeth,
    tips and nasal cavity.


  • It is 10-12 cm long tube with 2-3 cm diameter which arises from larynx and passes upto middle of thorax. Trachea is supported by 16-20 C-shaped incompleted cartilaginous rings
    and lined by ciliated pseudostratified mucous epithelium.


  • Trachea divides into right and left primary bronchi. Left bronchus is about 5 cm long while right bronchus is only 2.5 cm long. Right bronchus almost directly enters the right lung.
    Infection of right lung is more common due to this.

  • Inside the lung, the primary bronchus divides into secondary bronchi, secondary bronchi into segmental bronchi and latter into bronchioles. All bronchi are lined by ciliated and
    mucus secreting pseudostratified epithelium and supported by incomplete cartilaginous rings.

  • Bronchioles divides into terminal bronchioles, respiratory bronchioles, alveolar ducts, air sacs and alveoli. Mucus secreting cells are absent from terminal bronchioles and their branches. Epithelium is ciliated in bronchioles and terminal bronchioles. It is non-ciliated in respiratory bronchioles and their branches.


  • A pair of conical spongy elastic lunges of pinkish to slate grey colour occur inside air tight thoracic cavity. A small space called mediastinum lies in between the two lungs ( especially due to concavity called cardiac notch of left lump). It encloses heart. Each lung is covered by a pleural sac made of an outer parietal pleuron in contact with wall of thoracic cavity and inner visceral pleuran in contact with the surface of lung. A narrow pleural cavity (0.02 mm) occurs between them. It contains pleural fluid. It allows frictionless sliding of pleura during inspiration and expiration. Protection and moistening of lungs are also provided. Pleurisy is painful infection involving inflammation of pleura and over-production of pleural fluid. Normally pleural fluid is under negative pressure due to its formation from the membranous covering.
  • Left lung is slightly narrower and longer than the right one. Right lung has three lobes –right superior, right middle and right inferior. Left lung has two lobes – left superior and left inferior. It contains a cardiac notch in antero-median region for accommodating heart. Each lobe is divided internally into segments and segments into lobules. A lobule receives a terminal bronchiole.
  • Terminal bronchiole produces a few respiratory bronchiole produces a few respiratory bronchioles. A respiratory bronchiole give rise to 2 – 11 alveolar sac or infundibulum. The latter has a number of small pouches named alveoli or air sacs. Number of alveoli in human pulmonary system is 300 – 400 million with a surface area 100 m2.
  • Each alveolus is polyhedral in outline with a thin wall made of non-ciliated squamous epithelium with a few cubical cells that secrete a lipoprotein surfactant to present collapse and sticking of alveolar walls during expiration. Life span of epithelial cells is about 3 weeks so that alveolar wall is being continuously replace. Blood capillaries occur on the surface of alveoli for gaseous exchange.


  • It is a membranous musculo – tendinous partition between thorax and abdomen. Normally it is convex with convexity towards thorax.


  • Phrenic muscles attach diaphragm to ribs and vertebral column. Contraction of muscles straighten the diaphragm to increase thoracic cavity
  • Intercoastal muscles: There are
  • i. External intercoastal
  • ii. Internal intercoastal
  • iii. External oblique
  • iv. Internal oblique muscles

    Abdominal muscles: Relaxation allows compression of abdominal organs when diaphragm straightens. Contraction presses the abdominal viscera against diaphragm to bulge it more
    upwardly ( for expiration).


    cycle of breathing inspiration and expiration
    Cycle of breathing, inspiration and expiration. respiratory system anatomy. diaphragm functions. illustration for medical, science, and educational use


  • It is the process by which the fresh atmospheric air enters into the alveoli of the lungs. It is an active process and is brought about by activity of inspiratory muscles. The main muscles of inspiration in normal quiet breathing are the external intercoastal muscles and phrenic or radial muscles of diaphragm. During difficult or deep breathing (forced inhalation) they are assisted by the muscles of abdomen.

    i. Diaphragm: When relaxed the diaphragm is dome-shaped structure which separates the thoracic cavity from the abdominal cavity. Phrenic or radial muscles extened from diaphragm to ribs and vertebral column. When these muscles contract diaphragm becomes flat, thus increases the thoracic cavity antero posteriorly. These are the principle inspiratory muscles and play about 75% role in inspiration, other muscles play 25% role in inspiration.

    ii. External intercoastal muscles: They occur between the ribs. These are 11 pairs of muscles extending between 12 pairs of ribs. Their contraction pulls the ribs and sternum upward and outward there by increasing the thoracic cavity dorso-ventrally and laterally.

    iii. Abdominal muscles: These muscles relax and allow compression of abdominal organs by diaphragm.

  • Due to simultaneous contraction of inspiratory muscles, volume of thoracic cavity increases in all directions
  • As the lungs are held tightly against thoracic wall, enlargement of thoracic cavity results in expansion of lungs
  • This decreases the intrapulmonary pressure than atmospheric pressure by -2 to -6 mmHg.
  • As it is property of gases, that they move from the place from higher pressure to place of lower pressure, fresh air rushes through respiratory passage into the lungs to equalize the pressure.
  • The movement of fresh air into lungs is called inspiration.


  • It is the process by which foul air is expelled out of the lungs. Expiration is normally a passive process and involves the relaxation of inspiration, expiratory muscles becomes active, making expiration an active energy consumed process.

    i. Diaphragm: When muscles of diaphragm relax it again becomes dome-shaped, decreasing the thoracic cavity.

    ii. External intercoastal muscles: When these muscles relax, sternum and ribs come to their original position. This also decreases thoracic cavity.

    iii. Abdominal muscles: Contraction of abdominal muscles presses the abdominal viscera against the diaphragm, bulging it further upward and thus decreasing the thoracic cavity more vertically.

    iv. Internal intercoastal muscles: Contraction of these muscles moves the ribs downward and inward and reduces the thoracic cavity laterally and dorsoventrally. The abdominal and external intercoastal muscles are called expiratory muscles.

  • Due to the action of above muscles, the overall volume of thoracic cavity decreases and the intra pleural pressure increase by +3 to +4 mmHg. Due to this increased pressure in lungs, foul air is given out of them.
  • One breath includes one inspiration and one expiration.
  • The respiratory rate is the number of breaths taken per minute. For a person breathing normally at rest, it is equal to 12-14 breath per minute.
  • Breathing through nose is healthier as it get filtered and conchal of nose warm up the air.
  • Mammals have a negative pressure breathing as it allows them to eat and breath at the same time and in human female thoracic breathing is more predominant.

    [Part - 3] Respiratory System | Respiration - its Types 3


  • Spirometry is the process of recording the changes in the volume movement of air into and out of lungs and the instrument used for this purpose is called sphirometer or respirometer.
    The graph showing the changes in the pulmonary volumes and capacities under different conditions of birthing is called spirogram.

  • The quantity of air the lungs can receive, hold or expel under different conditions are called pulmonary volumes.
  • Combinations of two or more pulmonary volumes are called pulmonary capacities.
  • Tidal volume (T.V.): Volume of air inspired or expired in relaxed or resting position -500ml. It consists of 150 ml of dead space volume and 350 ml of alveolar volume.
  • Dead space: Part of inspiratory tract not involved in gaseous exchange. ( Nose to terminal bronchi, volume 150 ml
  • Residual volume (R.V): Air left in lungs and dead space after forceful expiration. 1.1-1.2 litres. The air left in lungs is useful in uninterrupted gaseous exchange.
  • Inspiratory reserve volume ( I.R.V = complemental air): Volume of air in excess of tidal volume which can be inhaled due to forceful inspiration.
  • Expiratory reserve volume ( E.R.V = Supplemental air): Volume of air in excess of tidalvolume which can be exhaled due to forceful expiration 1 – 1.1 litres.
  • Vital capacity (v.C) : It is the total volume of air inspired and expired to a maximum level. It is the sum total of tidal volume, inspiratory reserve volume and expiratory reserve volume

    VC = T.V. + I.R.V. + E.R.V.

    It is 3.5 to 4.5 litres

    i. The vital capacity is higher in athletes mountaineers or mountain – dwellers and lower in non-athletes, people living in plains, women, old individuals, cigarette smoker.

    ii. Higher the vital capacity, higher is the amount of air exchanged in each breath.

  • Inspiratory capacity (IC) : It is the total volume of air that can be inhaled after normal expiration. It includes tidal volume and inspiratory reserve volume.

    IC = T.V. + I.R.V

    It is 2.5 to 3.0 litres

  • Expiratory capacity (E.C.) : It is the total volume of air that can be exhaled after normal inspiration. It includes tidal volume and expiratory reserve volume.

    EC = T.V. + E.R.V

    Its value is 1.5 to 1.6 litres

    Functional residual capacity (FRC) : It is the sum total o residual volume and the expiratory reserve volume.

    F.R.C. = R.V. + E.R.V.

    Its value is 2.3 to 2.7 litre

    Total lung capacity (TLC) : It is the total amount of air present in the lungs and the respiratory passage after maximum inspiration. It is the sum total of vital capacity and residual volume.

    TLC = VC + RV or TLC = TV +IRY + ERV + RV

    Its value is 5 to 6 litre

    Alveolar ventilation : It is the rate at which the fresh air reaches the alveoli and adjoining areas like alveolar ducts, alveolar sacs and respiratory bronchioles. It is calculated as.

    – Alveolar ventilation per minute

    = Rate of respiration × (TV – dead space volume)

    = 12 × (500 – 150)
    = 12 × 350
    = 4.2 litres / minute

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