The process of exchange of O2 from the atmosphere with CO2 produced by the cell is called Respiration. It occurs in two stages of inspiration and expiration. During inspiration air enters the lungs from the atmosphere and during expiration air leaves the lungs.

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Anatomy of Respiratory System

• • The human respiratory system consisted of a pair of nostrils openings out above the upper lips.
  • The nasal passage starts from the nostrils and ends in the nasal chamber.
  • This passage opens into the pharynx. It is the common passage for food and air and it is connected with the mouth and nose.
  • This pharynx is connected with the trachea via Larynx.
  • The larynx or soundbox, consisting of two vocal cords, is a cartilaginous box that is responsible for sound production.
  • During the time of swallowing glottis is covered by the epiglottis, a thin elastic cartilaginous flap and it prevents the entry of food particles into the larynx.
  • Trachea, a straight tube-like structure extending up to the middle of the thoracic cavity.
  • This trachea then divides at the level of the 5th thoracic vertebra into a right and left primary bronchi.
  • Each bronchus is then further divided into repeated divisions to form the secondary and tertiary.
  • Those tertiary bronchi then finally end up in very thin terminal bronchioles.
  • Many incomplete cartilaginous rings are present in the passage between the trachea and the initial bronchioles to support the structure.
  • Alveoli, very thin, irregular-walled and vascularized saclike structures are present at the end of each terminal bronchiole.
  • The branching network of bronchi, bronchioles and alveoli comprise the lungs.
  • Humans have two lungs, the left and the right lung. Both are covered by a double-layered membrane, called Pleura. Between the double layer membrane, a jelly-like fluid is also present to lower the friction on the lung surface.
  • The lungs are situated in the air-tight chamber known as the thoracic chamber.
  • The vertebral column is present in the back or dorsal position of the thoracic chamber and the sternum is present on the front side, the ribs forming the lateral walls and on the lower side, it is guarded by the dome-shaped diaphragm, a muscular membrane.

There are two main parts of the human respiratory system:-

• • The conducting part: From the external nostrils up to the terminal bronchioles. It transports the atmospheric air to the alveoli, removes the foreign particles, humidifies the air and brings the air to body temperature.
  • The exchange part: The alveoli and their ducts. In this part the actual diffusion of O2 and CO2 between blood and atmospheric air takes place.

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Steps in Respiration

• • Pulmonary ventilation or Breathing by which atmospheric air is drawn in and CO2 rich alveolar air is released out. Diffusion of gases (O2 and CO2) across the alveolar membrane.
  • Transport of gases by the blood.
  • Diffusion of O2 and CO2 between blood and tissues.
  • Utilization of O2 by the cells for catabolic reactions like cellular respiration and release of CO2 as a waste product

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Mechanism of Respiration

• • Breathing involves inspiration and expiration. During inspiration atmospheric air is drawn in and during expiration, alveolar air is released out.
  • The movement of air in and out takes place due to differences in pressure gradients.
  • Inspiration occurs when the pressure inside the lung is less than atomic pressure and expiration occurs when the pressure inside the lung is more than atomic pressure.
  • The diaphragm and external and internal intercostal muscles between the ribs help in developing pressure gradients due to change in volume.
  • The contraction of intercostal muscles lifts the ribs and sternum causing an increase in the volume of the thoracic cavity that results in a decrease in pressure than the atmospheric pressure. This causes inspiration.
  • Relaxation of the diaphragm and intercostal muscles reduce the thoracic volume and increase the pressure causing expiration.
  • The volume of air involved in breathing movements is estimated by using a spirometer for the clinical assessment of pulmonary functions and volume.

Exchange of Gases

• • The exchange of gases takes place at two sites
  • Alveoli to blood
  • Between blood and tissues.
  • Exchanges of gases occur by simple diffusion due to pressure/ concentration gradient, the solubility of the gases and thickness of the membrane.
  • Pressure contributed by an individual gas in a mixture of gas is known as partial pressure represented by pCO2 and pO2.
  • The partial pressure of Oxygen and carbon dioxide at different parts involved in diffusion varies from one part to another and moves from higher partial pressure to lower partial pressure.
  • The solubility of CO2 is 20-25 times more than the solubility of O2, so CO2 diffuses much faster through the membrane.
  • The diffusion membrane is three-layered thick, that is the alveolar squamous epithelium, endothelium of alveolar capillaries, and basement substance between them.

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Chloride Shift

• • The chloride shift is defined as the movement of chloride ions from the plasma into RBCs as the blood undergoes the transition from arterial to venous gas.
  • The chloride shift is also known as the Hamburger shift. It is the process that takes place in a cardiovascular system where the exchange of bicarbonate HCO−3 and chloride Cl– across the membrane of RBCs.
  • Following are the times when does the chloride shift occur:
    • • When the chloride moves into erythrocytes and bicarbonate moves out into the venous blood.
      • When chloride moves out and bicarbonate moves into the pulmonary capillaries.

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Regulation of Respiration

• The central nervous system controls Breathing.
• Respiratory centres are situated in the medulla oblongata and pons Varolii and they regulate the rate and depth of breathing. This is done by controlling the contraction of the diaphragm and other respiratory muscles.
• The dorsal portion of the medulla oblongata contains the inspiration centre and the ventral portion contains the expiration centre connected with the vagus nerve that innervates the lungs.
• Pons varollii contains pneumotaxic centre that can moderate the functions of the respiratory rhythm centre and apneustic centre that operates in association with the depth of the inspiration
• The neural signal from the pneumotaxic centre can reduce the duration of inspiration and thus the respiratory rate.
• A chemosensitive area is highly sensitive to CO2 and hydrogen ions. It is situated near the rhythm centre. An increase in CO2 and hydrogen ions activates this centre. It signals the rhythm centre to make necessary adjustments in the respiration process.

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