Respiratory System 



Respiration

 

Respiration can be defined as biochemical process by which foods are oxidized to liberate energy.

Overall respiration involves two processes.

 

a.       Exchange of respiratory gases between lungs and blood

b.      Oxidation of digested food to release energy.

 

Types of Respiration

 1. Aerobic respiration 

 It occurs in all higher forms (organisms) in this type Oxygen is necessary. 

 C₆H₁₂O₆ + 6O₂ -------- 6CO₂ + 6H₂O + 2880 K joule . 

 2. Anaerobic respiration 

 In the absence of oxygen. C₆H₁₂O₆ ----------- C₂H₅OH(ethyl alcohol) + CO₂ + 210 K joule 

example yeast C₆H₁₂O₆ ----------- C₃H₂O-3 (lactic acid)+ energy example intestinal worms  

Respiratory organs:

 a. Nose

 b. Larynx 

 c. Trachea 

 d. Bronchi 

 e. Alveoli 

 f. Lungs 


a. Nose : 

- It is a part of respiratory tract lying above mouth made by hyaline cartilage

. - It is divided into right and left nasal cavity by nasal septum. Its anterior portion cartilaginous and

 posterior portion is bony. 

- The nasal cavities open outside through nostrils or external nares. The nasal cavities open posteriorly

 into the nasopharynx through internal nares.

 - The opening of paranasal sinuses (air cavities) and nasolacrimal ducts lie in the nasal cavities. 

- Internally, nasal cavity is lined with mucous membrane and ciliated columnar epithelium.

 It contains nasal hairs. 

b. Larynx -

 Also called sound box or voice box.

 - It is situated in the anterior neck i.e. in front of esophagus. 

- It is small, thin walled, tubular part present in the neck at the apex of trachea. 

- It connects the lower part of pharynx and trachea. 

- It is composed of several irregular shaped cartilages- they are: 1 thyroid cartilage, 1 cricoid cartilage

 and 2 arytenoids cartilage. They prevent the larynx from collapsing.

 - There are two vocal folds (true vocal cords) situated in the cavity of larynx between thyroid and

 arytenoid cartilage which produces sound when vibrated by the force of air. 

- space between two vocal cord is called rima glottidis.

c. Trachea 

 - It is hollow tube of about 11-12 cm in length and 2.5 cm in diameter. 

- It extends from the base of larynx to thoracic cavity. 

- It runs in the neck in front of oesophagus. 

- It is supported by 16-20 C-shaped cartilaginous tracheal ring. These rings prevent the trachea from

 collapsing due to continuous relaxation and expansion. 

- Internally, the wall of trachea is lined by pseudostratified ciliated epithelium with mucus secreting

 goblet cells. The secretion of mucus cells keep the wall of tube moist and trap dust particles.

d. Bronchi 

 - As the trachea reaches into the thoracic cavity, it divides into two branches called bronchi- right and left bronchi. 

- Each bronchus has the structure similar to trachea. The right bronchus is wider and shorter than left

 bronchus. It is about 2.5 cm long and left bronchus is about 5 cm in length. 

- Each bronchus when enter into corresponding lungs, it divides into smaller secondary bronchi and

 then into tertiary bronchi. These bronchi progressively subdivide into smaller and smaller tube called

 bronchioles and then into terminal bronchioles.

-  Bronchioles continue to branch, and open into respiratory bronchioles which in turn branch into

 alveolar duct that lead into alveoli (microscopic air sac). 

e. Alveoli - -

 - - The bronchioles terminate into tiny air sacs called alveoli. 

-There are about 150 million alveoli in each lungs. 

- The alveoli contains a dense network of blood capillaries. 

-These are the main sites of gas exchange. 


f. Lungs 

 - - These are a pair of soft, spongy organs present in the thoracic cavity. 

-The space between two lungs is called mediastinum which is occupied by heart. 

-The right lung is larger than left lung. The right lung has 3 lobes right superior, right inferior and

 middle lobe whereas the left lung has two lobes – left superior and left inferior. 

- These are surrounded by two thin tough flexible transparent membrane called pleural membrane. 

-The space in between them is filled by pleural fluid which reduces the friction produced during breathing.

Mechanism of respiration: - -
 - Breathing is simply taking in of fresh air from atmosphere and giving out of used air from lungs. 
- It is accomplished through changes in the volume and air pressure of the thoracic cavity. 
-Change in volume and air pressure is carried out by movement of ribs, internal and external intercostals muscles, diaphragm and abdominal muscles. 
-Breathing can be divided into inhalation and exhalation. 
 - One breath includes one inspiration or inhalation and one expiration of exhalation.

Inhalation
 1.Taking in of atmospheric air
 2.Contraction of external intercostal muscle or inspiratory muscle and relaxation of internal intercostals muscle. 
 3.Rib cage moves forward and outward
 4.Diaphragm contracts and becomes flattened shaped.
 5.Increase in the volume of thoracic cavity.
 6.Decrease in air pressure (below atmospheric pressure). 
 7.Rushing in of air through nostril into alveolar sacs causing inflation of lung.

Exhalation
 1.Giving out of air from lungs.
 2.Relaxation of external intercostals muscle and contraction of intercostal muscle. 
 3.Rib cage moves downward and inward.
 4.Diaphragm relaxes and become dome 
 5.Decrease in volume of thoracic cavity. 
 6.Increase in the air pressure.
 7.Expulsion of air from lungs into atmosphere causing deflation of lungs. 

Exchange of gases in lungs: - - - - - -
 The air inhaled by inspiration comes to alveoli. All around the alveoli there is a network of blood capillaries. 
These capillaries are extremely fine tubes with only one layered wall. 
 The deoxygenated blood collected from different parts of the body is at first brought to the heart, and from here pumped out to the lungs.
 This blood which may also be called venous blood is sent to the lungs where it has to pass through the network of capillaries around the alveoli.
 The oxygen from the alveolus diffuses out into the blood capillary due to difference in partial pressures of oxygen and is picked up by the hemoglobin molecules present inside the red blood corpuscles.
 Again, carbon dioxide, which is in greater amount in the venous blood, comes from the capillary into the alveolus. 

Physiology of respiration:
Physiology of respiration can be studied under following headings. 

 External respiration: It is uptake of O₂ and release of CO₂. It takes place in the lungs called breathing.

 Transport of O2 by blood: 
RBC of blood contain hemoglobin as respiratory pigment.
 Human blood contains nearly 150 gram of Hb per 100 ml. The maximum amount of oxygen that a normal human blood can absorb is 20ml/100 ml of blood. 
 The hemoglobin has high affinity with oxygen and combines forming oxyhemoglobin. 
One haemoglobin contains 4 heme groups so one Hb can combine with 4 molecules of oxygen.
 Hb + 4O₂ → Hb(4O₂) 
 Oxyhaemoglobin is then transported to tissues.

Internal respiration / tissue respiration.

It includes two steps.
 Dissociation of oxyhaemoglobin : in tissue oxyhaemoglobin dissociates into free oxygen and Hb in tissue where O2 enters into tissue and Hb returns back to RBC to pick up more oxygen. 

 Oxidation of food : the free oxygen oxidizes glucose in the presence of respiratory enzymes and liberates energy, water and CO2.

Transport of CO₂:
 Carbon dioxide is one of the poisonous gas which by any means should be eliminated from the body. The transportation of CO2 takes place by 

 1.In the form of carbonic acid:
CO₂ dissolve in water of blood plasma to form carbonic acid, about 7 % of total CO₂ carried in this way.

 2.In the form of bicarbonates: - 
about 70% of CO2 is transported in the form of sodium and potassium bicarbonates. 
 Large amount of H2CO3 dissociates into H+ and HCO3- where most of HCO3- diffuses from RBC into plasma. 
To maintain electro neutrality equal amount of Cl- diffuses from plasma into RBC called chloride shift or Hamburger phenomenon

. H2CO3 ------------------ H+ + HCO3
- In plasma HCO3- fuses with Na + and K+ forming sodium and potassium carbonates
. Na+ + HCO3- -------- NaHCO3

 3.By RBC in the form of carbamino compounds:
Amino acid present in RBC gets oxidized to release amino group and carboxyl group. Carbon dioxide combines with amino group to form carbamino compounds. About 23 % CO2 is transported in this form.

Oxygen dissociation curve or Bohr effect :
The graph showing the percentage saturation of hemoglobin with oxygen at different values of PO2 at constant pH is known as Oxygen dissociation curve.



 Christian Bohr discovered this effect, with an increase partial pressure of CO2 the Oxygen dissociation curve is shifted to right and this is known as Bohr effect. 
 Higher the partial pressure of CO2 in blood lower is the percentage saturation of oxygen with haemoglobin,
Similarly, lower the partial pressure of CO2, higher is the percentage saturation of Oxygen with haemoglobin.