Respiratory Organs

Respiration

       Respiration is a process which involves intake of oxygen from environment and to deliver it to the cells. It include stepwise oxidation of food in cells with incoming oxygen, elimination of $C{O}_2$ produced in oxidation, release of energy during oxidation and storing it in the form of ATP. It takes place in three basic steps –

       (1) Pulmonary ventilation : The first process, pulmonary (pulmo = lung) ventilation, or breathing, is the inspiration (inflow) and expiration (outflow) of air between the atmosphere and the lungs.

       (2) External (pulmonary) respiration : This is the exchange of gases between the air spaces of the lungs and blood in pulmonary capillaries. The blood gains O₂ and loses CO₂.

       (3) Internal (tissue) respiration : The exchange of gases between blood in systemic capillaries and tissue cells is known as internal (tissue) respiration. The blood loses O₂ and gains CO₂. Within cells, the metabolic reactions that consume O₂ and give off CO₂ and give off CO₂ during production of ATP are termed cellular respiration.

Respiratory surface

       The surface at which exchange of gases (CO₂ ਊnd O₂) takes place is called respiratory surface. Respiratory surface must be vascular and have enough area for gas exchange. For example – plasma membrane in protozoa, body wall (skin) in annelids, alveocapillary membrane in men.

Respiratory medium

       Oxygen is dissolved in air and water. Thus water an air are source of oxygen for animals and called respiratory medium. Water and air are external respiratory medium. Inside the body an internal respiratory medium is also found. This internal respiratory medium is tissue fluid.

       Types of respiration : It is of two types

       (1) Aerobic respiration : It occurs in the presence of molecular oxygen. The oxygen completely oxidizes the food to carbon dioxide and water, releasing large amount of energy. The organisms showing aerobic respiration, are called aerobes. It is found in most of animals and plants. Aerobic respiration is of two main types direct and indirect.

\mathop {{C_6}{H_{12}}{O_6}}\limits_{{\text{Glucose}}}#xA0+ \mathop {6{O_2}}\limits_{{\text{oxygen}}}#xA0\to \mathop {6C{O_2}}\limits_{{\text{Carbon dioxide}}}#xA0+ \mathop {6{H_2}O}\limits_{{\text{Water}}}#xA0+ \mathop {2830\,kJ}\limits_{{\text{Energy}}}

       (i) Direct respiration : It is the exchange of environmental oxygen with the carbon dioxide of the body cells without special respiratory organs and without the aid of blood. It is found in aerobic bacteria, protists, plants, sponges, coelenterates, flatworms, roundworms and most arthropods.

(ii) Indirect respiration : It involves special respiratory organs, such as skin, buccopharyngeal lining, gills and lungs, and needs the help of blood. The respiration in the skin, buccopharyngeal lining, gills and lungs is respectively called cutaneous buccopharyngeal, bronchial and pulmonary respiration. Cutaneous respiration takes place in annelids, some crustaceans, eel fish, amphibians and marine snakes. It occurs both in water and in air. Buccopharyngeal respiration is found in certain amphibians such as frog and toad. It occurs in the air. Branchial respiration is found in many annelids, most crustaceans and mollusks, some insect larvae, echinoderms, all fishes and some amphibians. It occurs in water only. Pulmonary respiration is found in snails, pila, some amphibians and in all reptiles, birds and mammals. It takes place in air only.

(2) Anaerobic respiration : It occurs in the absence of molecular oxygen and is also called fermentation. In this, the food is only partially oxidised so only a part of energy (5%) is released and of energy remains trapped in the intermediate compounds. It is found in lower organisms like bacteria and yeast. It is also found in certain parasitic worms (Ascaris, Taenia) which live in deficient medium. The organism showing anaerobic respiration, are called anaerobes. These involve one of following reactions.

\mathop {{C_6}{H_{12}}{O_6}}\limits_{{\text{Glucose}}} \mathop {\xrightarrow{{{\text{In yeasts }}}}}\limits_{{\text{(Fermentation of sugars)}}} \mathop {2{C_2}{H_5}OH}\limits_{{\text{Ethanol}}}#xA0+ 2C{O_2} + 118\,kJ

\mathop {{C_6}{H_{12}}{O_6}}\limits_{{\text{Glucose}}} \xrightarrow{{{\text{In intestinal worms}}}}\mathop {2C{H_3}CHOHCOOH}\limits_{{\text{Lactic acid}}}#xA0+ {\text{Energy}}

      ꃎrtain body tissues of even aerobes also show anaerobic metabolism e.g., during the vigorous contraction of skeletal muscle fibres. In this, the glucose is metabolised into the lactic acid in anaerobic conditions. The rapid formation and accumulation of lactic acid are responsible for muscle-fatigue. The mammalian RBCs shows anaerobic respiration as these lack the mitochondria. In lens of eye and cornea of eye respiration is anaerobic because these structures are a non vascular. Anaerobic respiration appeared first in primitive organisms because there was absence of O₂ in primitive atmosphere.

Respiratory organs

       (1) Skin : Respiration by skin is called cutaneous respiration. Skin is the only respiratory organ in most annelids (earthworm and leeches) and an additional respiratory organ in amphibians (Toads and frogs). Skin should be thin, moist, naked, permeable and well vascular for respiration. For cutaneous respiration animal should have large surface area then its volume and should have relatively inactive life to minimize the use of oxygen. Some marine annelids such as sandworms (nereis) have parapodia (locomotory appendages) for respiration. In frog 100% cutaneous respiration during hibernatin. In all marine snakes 20% respiration by skin.

               (2) Tracheae : In insects, peripatus centipedes and millipedes tracheae are found for respiration. Tracheae are complex system of whitish, shining, intercommunicating air tubules. Tracheae are ectodermal air tubes. In cockroaches, three pairs of longitudinal tracheal trunks are present all along the length of body which are further connected with each other with the help of transverse branches. The main tracheae give off smaller tracheae whose branch repeatedly form a network of trachioles throughout the body. Trachea internally lined by chitinous cuticle called intima, which spirally thickened to form taenidae. Trachioles without taenidae, trachioles lined by trachein protein. From each tracheal trunk three branches come out. The dorsal branch is supplied to the dorsal muscles where as ventral one to nerve cord and ventral muscles and middle one to the alimentary canal.

(3) Book lungs and book gills : Spiders ticks, mites and scorpion (belongs to class arachnida) have book lungs for respiration. In scorpion 4 pairs of book lungs are present. A book lung is a chamber containing a series of thin vascular, parallel lamellae arranged like the pages of book. Book gills are found in marine king crab or horse shoe crab.

(4) Gills : Aquatic animals such as prawn, unio, fishes, sea stars and tadpoles respire by gills. Respiration by gills called bronchial respiration. Gills are of two types –

(i) External gills : External gills are found in arenicola (lug worm), larvae of certain insects e.g. damsel fly and some amphibians e.g. necturus, siren, proteus, frog tadpole first develop external gills which are replaced by internal gills later.

(ii) Internal gills : The internal gills may be phyllobranch (prawn), monopectionata (pila) eulamellibrach (unio), lamellibranch, fillibranch (pisces). In all fishes, gills are hemibranch or demibranch and holobranch. In gills, gill lamellae are found which have capillary network. Water is drawn into gills $\to$ blood flowing in the capillaries of gill lamellae absorb oxygen from water and release $C{O}_2$ $\to$ water containing $C{O}_2$ is thrown out from gills. The 80% of $O_2$ of incoming water is absorbed.

(5) Buccopharyngeal lining : Frog breathes by buccopharyngeal lining of buccopharyngeal cavity. This is called buccopharyngeal respiration.

Respiratory system of human

Human respiratory system is derived from endoderm. Human respiratory system may be divided into two components –

(1) Respiratory tract or conducting portion                  

(2) Respiratory organs

(1) Respiratory tract or conducting portion : It is the passage for the air. In this part gaseous exchange does not takes place. It is also called dead air space. It is divided in following parts –

(i) Nose (Latin-Nasa) (Greek-Rhine) : Cavity of nose is called nasal cavity. Nasal cavity is divided into two parts by nasal septum called mesethmoid. Each part is called nasal chamber. Each nasal chamber opens out side by external nares. Nasal septum has two part. First part is small and is made of cartilage (hyaline). Second part is major and it is bony. Vomer is the main bone. Each nasal chamber has three region.

(a) Vestibular region : Vestibular region also known as vestibule, lined by non keratinized squamous epithelium, it is ectodermal in originਊnd have sebaceous gland, sweat gland and hair. Vestibule is also found in inner air larynx, mouth and vagina. It acts like a seive to check the entry of large dust particles and other things.

(b) Respiratory region : Middle region lined by respiratory epithelium which is ciliated pseudostratified columnar epithelium. It contains mucus and serous cells. Mucus cells produce mucus and serous cells produce watery fluid. Respiratory epithelium is highly vascular and appears pink or reddish. Respiratory region acts as a air conditioner and makes the temperature of in going air nearly equal to body. It also acts as a filter not give entry to dust particles, flies or mosquitoes.

(c) Olfactory region : It is upper region. It is lined by olfactory epithelium. This is also called Schneiderian epithelium. Olfactory region is the organ of smell and detect the odour of inspired air. Inspiration is stopped if odour of air is foul or offensive. According to new researches pheromone receptors are found in nasal cavities.

(ii) Nasal conchae : Lateral wall of nasal cavity have three shelves like structures called conchae or turbinate. 3 pairs of nasal conchae are found. Nasal conchae are covered with mucus membrane. They increase the surface of nasal chamber. Both the chambers of nasal cavity open into nasopharynx by their apertures called internal nostrils or conchae. Adjacent to internal nostril there are opening of eustachian tube. Names of these three conchae and names of the bones that form them are given below.

(a) Superior conchae : The dorsal most chochae is supported mainly by nasal bone called nasoturbinate. It is the smallest conchae.

(b) Middle conchae : Ethmoid bone called ethmoturbinate.

(c) Inferior conchae : The ventral most conchae supported by maxilla bone called maxilloturbinate. It is a separate bone itself.

(iii) Pharynx : It is the short vertical about 12 cm long tube. The food and air passages cross here. It can be divided in 3 parts –

(a) Nasopharynx : Nasopharynx is only respiratory upper part in which internal nares open. There are 5 opening in its wall two internal nares, two eustachian tube opening and opening into oropharynx.

(b) Oropharynx : Middle part is called oropharynx. In this part oral cavity open known as fauces. Two pair tonsils the palatine and lingual tonsils are found in the oropharynx.

(c) Laryngopharynx or hypopharynx : Lowest part is called laryngopharynx. It leads into two tubes. One at the front is wind pipe or trachea and one at the back is food pipe or oesophagus. Both oro and laryngo pharynx is both a respiratory and a digestive pathway.

Nasopharynx lined by ciliated pseudostratified epithelia, oropharynx and laryngopharynx lined by non keratinized epithelium. Mouth serves as an alternate route for air when nasal chambers are blocked. Foramen by which pharynx opens into larynx called glottis. In general it remains open. During swallowing it is closed. It provides passage for air. Pharyns leads into the oesophagus through an aperture called gullet. In general condition it remains closed and opens at the time of swallowing. During swallowing epiglottis closes the glottis.

(iv) Larynx or Voice box : It is found both in frogs and rabbits. Larynx does not help in respiration. It is present on tip of trachea and is made up of 9 cartilages such as thyroid (single) has a prominence called pomum admi or adam’s apple, cricoid (single), arytenoid (paired), are piece of hyaline cartilage. While epiglottis (single), carniculate (paired), cuniform (paired), santorini are piece of elastic cartilage. Clinically, the cricoid cartilage is the landmark for making an emergency air way.

Larynx is a short tubular chamber and opens into the laryngopharynx by a slit like aperture called glottis. Glottis always remains open except during swallowing. Larynx is more prominent in men than women due to male hormone. Before puberty, the larynx is inconspicuous and similar in both sexes. Larynx is a voice producing instrument. For this purpose larynx have two types of vocal cord. In birds voice producing organ is syrinx, found at lower end of tracheae.

(a) False vocal cord or vibrating fold or anterior vocal cord : These are folds of mucus membrane. Gap between them is called rema vestibuli. These are not responsible for sound production. In elephants only true vocal cords are present and are responsible for this trumpet sound.

(b) True vocal cord or posterior vocal cords : They are made up of yellow elastic fibres. Gap between them is called rema glottides or peep hole. In males the length of true vocal cord is 2.25 cm and in female is 1.75 cm. Sound produced by rabbit is called quaking. Hippopotamus lacks true vocal cords. Pitch is controlled by the tension of vocal folds.

(v) Trachea : It is a tubular structure of about 12 cm. in length and 2.5 cm in diameter. The wall of trachea is made of fibres, cartilage muscles and the mucus membrane. In middle of thorax at the level of 4^th and 5^th thoracic vertebra it divides into two branches called right 

(a) Kultchitsky cells or argentaffin cells : They secrete serotonin and histamine. Histamine dilate while serotonin constrict the bronchioles.

(b) Clara cells : They secrete a phospholipid named diapalmityl lecithin which acts as a surfactant. This surfactant prevents the collapse of bronchioles lacking cartilagenous rings. Collapsing of lungs is called atelectesis. Pottle in 1956 proved the existence of surfactant. Surfactant is formed by clara cells only at later stage of foetal life. Some times at birth some infants are devoid of surfactant so there is great respiratory difficulty because lungs refuse to expand. In this condition death may occur. This is called respiratory distress syndrome (RDS) or hyaline membrane disease (HMD) or glassy lung disease.

(c) Dust cells : They are phagocytes which eat foreign particles (dust).

Respiratory organs

In men the respiratory organ are a pair of lung. Some snakes have unpaired lungs. Respiration by lungs is called pulmonary respiration. Lungs are found in all vertebrates except fishes. In Lung fishes such as protopterus, neoceratodus and lepidosiren air bladder is found, which is modified lung. Respiration in men and rabbit is pulmonary.

Lungs : Lungs lie in thoracic cavity on both side of heart in mediasternum space. Base of lung is attached to diaphragm. Right lung is divided into 3 lobes viz. Superior, Middle, Inferior and left lung is divided into two lobes Superior and Inferior. In rabbit, the left lung is divided into two lobes left anterior and left posterior where as the right lung has four lobes anterior azygous, right anterior, right posterior and posterior azygous. Lungs of reptiles are more complex than those of amphibians. In birds lungs are supplemented by elastic air sacs which increase respiratory efficiency. The narrow superior partion of lung is termed the apex or cupula.

Each lung is enclosed in two membrane called pleura. Pleura are layers of peritonium of thorax. Inner membrane is called the visceral pleuron. It is firmly bound to surface of lungs. The outer membrane is called parietal pleuron. It is attached to chest wall or wall of thoracic cavity. A narrow space exists between the two pleura. It is called pleural cavity. In pleural cavity a watery fluid is found called pleural fluid. Pleural fluid is glycoprotein in nature and secreted by pleura. Pleural fluid lubricate the pleura so that they may slide over each other without friction. This fluids reduces friction bewteen the membrane. When the lungs expand and contract in respiration. Pressure inside pleural cavity is negative – 5 mm Hg. Plurisy is inflamation of pleura and cause collection of fluid in pleural cavity. It results painful breathing (dyspnea). The surface of lung lying against the ribs, known as coastal surface. The mediastinal (medial) surface of each lung contains a region – the hilus, through which bronchi, pulmonary blood vessels, lymphatic vessels and nerve enter and exit.

Pulmonary volumes and capacities

The apparatus commonly used to measure the volume of air exchanged during breathing and the rate of ventilation is a spirometer (spiro=breathe) or respirometer. The record is called a spirogram. There are 4 respiratory volumes and capacity.

IV. Respiratory volumes

(1) Tidal volume (TV) : Volume of air inspired or expired in relaxed or resting position – 500 ml. It consists of 150 ml of dead space volume and 350 ml of alveolar volume.

(2) Inspiratory reserve volume (IRV) : By taking a very deep breath, you can inspire a good deal more than 500 ml. This additional inhaled air, called IRV is about 3000 ml.

(3) Expiratory reserve volume (ERV) : If you inhale normally & then exhale as forcibly as possible, you should be able to push out 1100 ml. of air in addition to 500 ml. of T.V. The extra 1100 ml. is called ERV.

(4) Residual volume (RV) : Even after expiratory reserve volume is expelled, considerable air remains in the lung, this volume, which can not be measured by spirometry, it is called residual volume is about 1200 ml.

(5) Dead space : Portion of tracheobronchial tree where gaseous exchange does not occur called dead space. It is also called conductive zone. Dead space is 150 ml.

(6) Functional residual capacity (FRC) : It is the amount of air that remains in the lungs after a normal expiration. It is about 2300 ml.

FRC = ERV + RV

         = 1100 + 1200 = 2300 ml.

(7) Vital capacity (VC) : This is the maximum amount of air that can be expired forcefully from his lungs after first filling these with a maximum deep inspiration. It is about 4600 ml.

VC = IRV + TV + ERV

       = 3000+500+1100 = 4600 ml.

(8) Total lung capacity (TLC) : TLC is the sum of vital capacity (VC) and residual volume (RV). It is about 5800 ml.

TLC = VC + RV

        = 4600 + 1200 = 5800 ml.

(9) Inspiratory capacity (IC) : It is the total amount of air a person can inspire by maximum distension of his lungs.

IC = TV + IRV

     = 500 + 3000 = 3500 ml.