Respiratory system in humans Neet

Respiration

• Respiration is a process by which oxygen reaches the body cells and is utilized by them in metabolism and carbon-di-oxide formed as a waste the product gets eliminated.
• The real function of respiration is to provide the energy needed by body cells. Cells obtain energy by metabolizing glucose utilising oxygen. Hence they require a constant supply of oxygen.
• In addition, the waste products of the metabolic process, namely carbon-di-oxide must be carried away from the cells. 
• The transport of oxygen and carbon-di-oxide between lungs and body cells takes place by an efficient cardio-vascular system.

Process of pulmonary respiration 

Respiration includes several processes which are listed below:

(i). Ventilation is the breathing in of air with more oxygen into the lungs (inspiration). It is followed by the expulsion of air with more carbon-di-oxide (expiration).

(ii). Diffusion of oxygen from the alveoli into the blood inside surrounding capillaries. 

(iii). Transport of oxygen by the blood to the heart through the pulmonary vein.

(iv). Distribution of oxygen by various arteries and their capillary network to all cells of the body. As the blood passes through tissue capillaries, it gives up oxygen (and nutrients such as glucose) to the body, tissues and picks up the waste products of cellular respiration (Carbon-dioxide and water). 

(v). Exchange of the oxygen and carbon-di-oxide between the blood and body cells. In body cells, glucose and oxygen take part in a complex series of reactions that provide energy to power the cells. During this cellular respiration, glucose is converted to carbon-di-oxide and water. (Enzymatic oxidation).

(vi). Transporting blood with carbon-di-oxide. Carbon-di-oxide is carried back in the blood to the heart then to the lungs where it diffuses into the alveoli and is breathed out of the body (External respiration). 

(vii) Exchanging of carbon-di-oxide with oxygen at the alveolar surface.

(viii) Expiration of air with carbon-di-oxide from the lungs.

Mechanism of Breathing

• The process of inspiration and expiration happens due to pressure changes in the thoracic cavity. 
• The thorax is an airtight compartment bounded by the sternum in front, the vertebral column at the back, the ribs encircling the sides and the diaphragm found below. 
• The rib bones are provided with two sets of muscles namely external and internal intercostal muscles.
• By the contraction and expansion of these muscles, the volume of the thoracic cavity is reduced or increased. 
• The floor of the thoracic cavity is completely closed by the diaphragm. 
• The act of breathing is performed by expansion and contraction of the thoracic cavity.

Inspiration 

• Inspiration is the process by which fresh air is drawn into the lungs. 
• It is an active process. 
• The muscles participating in the inspiration the process is external - intercostal muscles present in between the ribs and the diaphragm.

• During quiet respiration, contraction of external intercostal muscle causes the ribs to move anteriorly and outwardly. 
• This movement enlarges the cavity of the thorax by increasing it side to side and in dorso-ventral dimensions.
• The contraction of radial muscles of the diaphragm leads to flattening of the inelastic, dome-shaped central part of the diaphragm. 
• As a result of these muscular movements, the volume of the thoracic cavity is increased. 
• This causes the air pressure within the lungs to fall below the atmospheric pressure. 
• So air (tidal air) from outside passes through the air passage into the lungs to equalize the pressure.  

Expiration

• Inspiration is followed by expiration. It is a passive process. 
• Expiration is the process by which air is exhaled or blown out from the lungs. 
• The expiration results when the volume of the thoracic cavity is decreased and air pressure in the lung is increased. 
• The expiratory process involves the following movements.

1. The diaphragm relaxes and rises to resume the original dome shape. 

2. The ribs take their original position as a result of the contraction of the internal intercostal muscles.

Gaseous exchange in the alveoli 

• Once the air is within the lungs the process of gaseous exchange begins.
• Capillaries of the pulmonary artery remain close to the wall of the alveoli. 
• This enhances the exchange of gases.

• Oxygen and carbon-di-oxide are exchanged across the alveolar membrane by diffusion from the site of higher to low partial pressure until the partial pressure of the two regions is equal.
• This process is a simple physical one which does not involve any secretary or active transport mechanism.
• In the atmospheric air there is a high concentration of oxygen 20-95% (PO2 140mm Hg) while the proportion of carbon dioxide is low (0.04%). 
• The alveolar PO2 is about 100mm Hg and the PO2 of venous blood is about 40mm Hg. 
• This pressure gradient is sufficient for the transfer of O2 . 
• The PCO2 of venous blood is 46mm.Hg and that of alveolar air is only 6mm.Hg (1/10th of O2 ), it is adequate for CO2 transfer by diffusion. 
• CO2 diffuses 20 times faster than O2 .

Regulation of respiration

• In the brain, the medulla oblongata contains a respiratory centre. 
• This controls breathing. The respiratory centre consists of an inspiratory centre and an expiratory centre. 
• The axons from the nerve cells of these centres lead to the intercostals muscle through the intercostals nerves and the diaphragm via the phrenic nerves. 
• These nerve fibres transmit impulses to the external intercostal muscles and internal intercostal muscles alternately. 
• The walls of the alveoli have sense endings which are stimulated by changes in the tension of alveolar walls.
• When the alveoli are stretched at the height of inspiration the receptors send stimuli to the expiratory centre of the medulla through the vagus nerve which inhibits further inspiration. This sequence of events is called Herring – Breuer reflex. 
• In addition, the medulla contains a pneumotaxic centre which is connected to the breathing centre and helps to ensure rhythmic breathing. 
• During inspiration, the inspiratory part of the respiratory centre sends impulses to the pneumotaxic centre which response by sending impulses to the expiratory part of the respiratory centre. 
• The expiratory centre is then activated and so the inspiratory centre is inhibited reflexly, respiratory rhythm is controlled in this manner by these centres in the brain.