Process of Respiration in Humans – Diagram

What is Respiration?

The process of respiration involves four stages –  ventilation which we know as breathing (inhalation or inspiration and exhalation  or expiration), exchange of gases between the air in the lungs and blood stream (pulmonary diffusion), transport of gases in the blood (perfusion) and exchange of gases between the blood and tissues (peripheral diffusion).

Breathing is only one part of the respiration process where the lungs take in air for oxygen absorption and passes out air thereby expelling carbon dioxide. This is an essential process to maintain the oxygen and carbon dioxide levels and expel some of the waste substances in the blood. Difficulty with breathing, like shortness of breath (dyspnea), is  only symptom of a disturbance in the respiration process.

Understanding the process of respiration is essential for identifying possible causes of trouble with breathing.

Respiration Process

  • The muscles of respiration contract thereby expanding the chest cavity.
  • This causes a negative pressure within the pleural cavity (where the lungs are housed) which forces the lungs to expand.
  • The expansion of the lungs reduces the air pressure in the lungs.
  • This draws air from the environment which is at a higher pressure. Air will flow from an area of high pressure to low pressure.
  • Air is taken in through the nose and the air is ‘filtered’ and heated in the nasal cavity.
  • It then passes down the throat and enters the trachea where it rushes into the bronchi.
  • The bronchi divides the air flow between the two lungs.
  • The air then passes into smaller air tubes known as bronchioles and empty into the lungs.
  • The air enters the tiny air sacs within the lungs, called alveoli, where oxygen crosses into the blood and carbon dioxide empties into the lung.
  • The respiratory muscles relax and the chest cavity contracts.
  • The elastic lungs recoil and pushes air out through the air passages where it is emptied into the environment.

However, this does not occur in isolation. The blood carries the oxygen to tissues by binding it to hemoglobin in the red blood cells. The heart pumps this blood throughout the body ensuring that oxygen reaches all cells. Simultaneously, carbon dioxide is carried away by red blood cells towards the lungs where it can be expelled.

Oxygen and carbon dioxide levels in the blood are monitored by the respiratory center in the brainstem. This in turns control the rate of respiration – the speed of breathing. In addition, carbon dioxide can change the pH (acid-base balance) of the blood and this also serves as a signal for the respiratory center.

Any trouble with breathing may indicate a host of causes affecting the airways, lungs, gas exchange, blood gas transport and heart function and control of breathing by the respiratory center.