Human Respiratory System

Nasal Cavity-

It is located at the back of nostrils just above the mouth cavity. It has specific nasal epithelium by which air is filtered (by hair) moistened (by mucus) warmed by capillary network. Mucus traps the dust and other fine particle olfactory region is also situated for different smell.

Nasopharynx-

It is located at the posterior part of pharynx. Air enters into nasopharynx through two internal opening.

Larynx-

Also called as voice box. It is a cartilaginous structure at the opening of trachea and has fibro elastic ligaments called vocal chords. Different sounds can be produced using tension caused in vocal chords.

Glottis-

It is the opening to larynx through which air passes into trachea.

Epiglottis-

It is a triangular flap of cartilage present at glottis

Trachea-

It runs through the neck in front of oesophagus and extends into the thoracic cavity. It has C shaped rings of cartilage that prevents the collapse of trachea during respiration. The open part of C shaped rings lies towards oesophagus. It is lined with ciliated pseudostratified columnar epithelium that keeps the unwanted particles away from the lungs by breathing the ciliary towards buccal cavity. Mucus secreted by goblet cells of epithelium traps dust particle and microbes.

Bronchi-

At the end of trachea, it is divided into two bronchi (left bronchi and right bronchi) right bronchi divides into three bronchioles and left bronchi divides into two bronchioles which enters the two lobes.

Bronchioles-

They are network of branching tubes in lungs. Some wide bronchioles with inner diameter more than 1mm have cartilage rings but fine branches have only smooth muscles. Bronchioles branch into respiratory bronchioles.

Respiratory bronchioles-

They are finest and smallest tube only 0.5 mm in width lack cartilage rings. Each one divides into number of alveolar tubes.

Alveolar tubes-

They are the fine branching which leads to alveoli. Lined with cuboidal epithelium.

Alveoli-

They are thin walled sack like structure surrounded by network of capillary. There are 300 million of alveoli in one lung which enhances the lungs capacity to 110m2. 

 





Respiratory volume and capacity-

Tidal Volume- (TV)

It is the volume of air which is breathed into and out during normal breathing or in each respiratory cycle. i.e., Tidal volume (TV) is equal to 0.5 litres.

Expiratory reserved volume-(ERV)

It is the volume of air breathed out above and over the tidal volume during forced expiration and a normal expiration. i.e., expiratory reserved volume (ERV) is equal to 1 litres to 1.5 litres.

Inspiratory reserved volume- (IRV)

It is the volume of air breathed in above and over the tidal volume during forced inspiration. i.e, inspiratory reserved volume (IRV) is equal to 2 litres to 2.5 litres

Inspiratory capacity- (IC)

It is the sum total of tidal volume and inspiratory reserved volume i.e.; inspiratory capacity (IC) is equal to sum to tidal volume and inspiratory reserved volume. (IC=TV+IRV=0.5 L +2.5 L = 3 L)

Expiratory Capacity- (EC)

It is the sum total of tidal volume and expiratory reserved volume. (EC= TV+ERV=0.5+1.5+2L)

Vital capacity- (VC)

It is the air breathed out after a forced inspiration followed by forced expiration (VC=TV+IRV+ERV=0.5L+2.5L+1.5L=4.5L)

Residual volume- (RV)

It is the volume of air left inside the lungs even after forced expiration. It can’t be excelled out. (RV=1.2L)

Functional residual volume-(FRV)

It is the volume of air left inside the lungs after normal expiration. (FRV=RV+ERV=1.2L+1.5L=2.7L)

TOTAL LUNG CAPACITY- (TLC)

Volume of air present after forced inspiration is called total lungs capacity. (TLC=TV+ERV+IRV+RV=0.5L+1.5L+2.5L+1.2L=5.7L)

 

 

 

 

Exchange of gases-

Alveoli’s are the primary site of exchange of gases. Exchange of gases occurs between blood and tissue. O2 and CO2 are exchanged in these sites by simple diffusion mainly based on pressure/ concentration gradient.

In atmosphere-

Partial pressure for oxygen O2 (pO2) =158mmHg.

Partial pressure for carbon dioxide CO2 (pCO2) =0.3mmHg.

Exchange of air in between alveoli and venous blood

Alveoli    

Venous blood

(RV + Atm. Air)

 

pO2=100mmHg

pO2=40mmHg

pCO2=40mmHg

pCO2=45mmhg

                                                            

                                                

                                                

 

 

Exchange of air in between arterial blood and cells.

Arterial blood

Cell

pO2=95mmHg

pO2=40mmHg

pCO2=40mmHg

pCO2=46mmHg

                                                   

                                                  

                                                 

 

Diffusion membrane is made up of three layers namely thin squamous epithelium of alveoli, endothelium of alveolar capillaries and basement substance in between them.

Transportation of oxygen-

Nearly 3% of oxygen is carried in dissolved state by plasma of blood, nearly 97% of oxygen is carried in haemoglobin

Haemoglobin combines with oxygen and forms oxyhaemoglobin.

Haemoglobin-----------O2------------àOxyhaemoglobin             (alveoli and veins)

Oxyhaemoglobin-----------O2------------à Haemoglobin            (arteries and cell)

Each haemoglobin molecule can carry maximum 4 oxygen atom.

In alveoli there is a high pO2, Low pCO2, Less H+ concentration and low temperature so all the factors are responsible for formation of oxyhaemoglobin.

But in tissue there is high pCO2, Low pO2, High H+ concentration and High temperature. So, all the factors are responsible for breaking of O2 from Oxy haemoglobin.

Every 100ml of oxygenated blood can deliver around 5ml of oxygen to tissue under normal physiological condition.

 

Transportation of carbon dioxide-

Nearly 20-25% of CO2 is transported by RBC where as 70% CO2 is carried as bicarbonates and above 7% of CO2 by blood plasma in dissolved state.

CO2 is carried by haemoglobin as carbamino haemoglobin in the interior of RBC in Water.

RBC contains very high concentration of enzymes carbonic anhydrase this enzyme accelerates the reaction.

CO2+H2O--------------carbonic anhydrase --à H3CO3-----Carbonic anhydrase--à HCO3- + H+.

At tissue site where pCO2 is high due to catabolism diffuses into blood and forms HCO3- and H+.

At the alveolar site where pCO2 is low reaction proceeds in backward direction leading to production of CO2 and H2O.

Thus CO2 is Trapped as bicarbonate at the tissue level and transported to alveoli is released out as CO2.

Every 100ml of deoxygenated blood delivers approximately 4ml of CO2 to alveoli.

Regulation of respiration by brain-

A specialized center present in the medulla region of brain called respiratory rhythmic center is primarily responsible for respiratory regulation. Another center present in the pons region of brain called pneumatic center can moderate the functioning of respiratory rhythmic center. Neural signal from this center can reduce the duration of inspiration and thereby alters the rate of respiration. A chemosensitivity area is situated adjacent to the rhythmic center which is highly sensitive to H+ and CO2 concentration. Increase in any of these can activate this center which in turn signals the respiratory rhythmic center to make necessary adjustment to eliminate these substances.


Location: India

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