BREATHING OR RESPIRATION PART-1: BREATHING AND HUMAN RESPIRATORY SYSTEM

In this post we are going to discuss about the process of breathing and human respiratory system in brief.

For the anatomy of digestive system post click here

Breathing or Respiration:

•Oxygen (O2) is utilised by the organisms to indirectly break down simple molecules like glucose, amino acids, fatty

acids, etc., to derive energy to perform various activities.

•Carbon dioxide (CO2) which is harmful is also released during the above catabolic reactions. 

•It is, therefore, evident that O2

 has to be continuously provided

to the cells and CO2 produced by the cells have to be released out. 

•This process of exchange of O2

 from the atmosphere with CO2

 produced by the cells is called breathing, commonly known as respiration.

For the process of digestion post click here

Respiratory Organ:

Mechanisms of breathing vary among different groups of animals depending mainly on their habitats and levels of organisation.

•Lower invertebrates like sponges, coelenterates, flatworms, etc., exchange O2

with CO2 by simple diffusion over their entire body surface. 

•Earthworms use their moist cuticle and insects have a network of tubes (tracheal

tubes) to transport atmospheric air within the body. 

•Special vascularised structures called gills (branchial respiration) are used by most of the aquatic arthropods and molluscs.

•Vascularised bags called lungs

(pulmonary respiration) are used by the terrestrial forms for the exchange of gases. 

•Among vertebrates, fishes use gills whereas amphibians, reptiles, birds and mammals respire through lungs. 

•Amphibians like frogs can

respire through their moist skin (cutaneous respiration) also.

                         Lungs 

For the absorption of digested food and diseases related to digestion post, click here

Human Respiratory System:

•We have a pair of external nostrils opening out above the upper lips.

•It leads to a nasal chamber through the nasal passage. 

•The nasal chamber opens into the pharynx, a portion of which is the common passage for food and air. 

•The pharynx opens through the larynx region into the trachea. 

•Larynx is a cartilaginous box which helps in sound production and hence called the sound box. 

•During swallowing glottis can be covered by a thin elastic cartilaginous flap called epiglottis to prevent the entry of food into the larynx. 

•Trachea is a straight tube

extending up to the mid-thoracic cavity, which divides at the level of 5th thoracic vertebra into a right and left primary bronchi.

•Each bronchi undergoes repeated divisions to form the secondary and tertiary bronchi

and bronchioles ending up in very thin terminal bronchioles.

•The tracheae, primary, secondary and tertiary bronchi, and initial bronchioles are supported by incomplete cartilaginous rings. 

•Each terminal bronchiole gives rise to a number of very thin, irregular-walled and vascularised bag-like structures called alveoli. 

•The branching network of bronchi, bronchioles and alveoli comprise the lungs. 

•We have two lungs which are covered by a double layered pleura, with pleural fluid between them. 

•It reduces friction on the lung-surface.

•The outer pleural membrane is in close contact with the thoracic lining whereas the inner pleural membrane is in contact with the lung

surface. 

•The part starting with the external nostrils up to the terminal bronchioles constitute the conducting part whereas the alveoli and their ducts form the respiratory or exchange part of the respiratory system.

•The conducting part transports the atmospheric air to the alveoli, clears it from foreign particles, humidifies and also brings the air to body

temperature. 

•Exchange part is the site of actual diffusion of O2 and CO2 between blood and atmospheric air.

•The lungs are situated in the thoracic chamber which is anatomically an air-tight chamber. 

•The thoracic chamber is formed dorsally by the vertebral column, ventrally by the sternum, laterally by the ribs and on the lower side by the dome-shaped diaphragm. 

•The anatomical setup of

lungs in thorax is such that any change in the volume of the thoracic cavity will be reflected in the lung (pulmonary) cavity. 

•Such an arrangement is essential for breathing, as we cannot directly alter the pulmonary volume.

Respiration involves the following steps:

(i) Breathing or pulmonary ventilation by which atmospheric air is drawn in and CO2 rich alveolar air is released out.

(ii) Diffusion of gases (O2 and CO2) across alveolar membrane.

(iii) Transport of gases by the blood.

(iv) Diffusion of O2 and CO2

 between blood and tissues.

(v) Utilisation of O2 by the cells for catabolic reactions and resultant release of CO2.

Mechanism of Respiration:

•Breathing involves two stages : -i) Inspiration during which atmospheric air is drawn in and 

ii) Expiration by which the alveolar air is released out.

•The movement of air into and out of the lungs is carried out by creating a pressure gradient between the lungs and the atmosphere. 

                    Inspiration

•Inspiration can occur if the pressure within the lungs (intra-pulmonary pressure) is less than the atmospheric pressure, i.e., there is a negative pressure in the lungs with respect to atmospheric pressure. 

•Similarly, expiration takes place when the intra-pulmonary pressure is higher than the atmospheric pressure.

•The diaphragm and a specialised set of muscles – external and internal intercostals between the ribs, help in generation of such gradients.

•Inspiration is initiated by the contraction of diaphragm which increases the volume of thoracic chamber in the antero-posterior axis. 

•The contraction of external inter-costal muscles lifts up the ribs and the sternum causing an increase in the volume of the thoracic chamber in the dorso-ventral axis.

•The overall increase in the thoracic volume causes a similar increase in pulmonary

volume. 

•An increase in pulmonary volume decreases the intra-pulmonary pressure to less

than the atmospheric pressure which forces the air from outside to move into the lungs,

i.e., inspiration. 

•Relaxation of the diaphragm and the inter-costal muscles

returns the diaphragm and sternum to their normal positions and reduce the thoracic volume and thereby the pulmonary volume.

•This leads to an increase in intra-pulmonary pressure to slightly above the atmospheric

pressure causing the expulsion of air from the lungs, i.e., expiration. 

•We have the ability to increase the strength of inspiration and expiration with the help of

additional muscles in the abdomen. 

•On an average, a healthy human breathes 12-16 times/minute. 

•The volume of air involved in

breathing movements can be estimated by using a spirometer which helps in clinical

assessment of pulmonary functions.

                     Expiration