Physiology|Respiratory Function

For tissue to be oxygenated the body has to control the respiratory process. Breathing or pulmonary ventilation depends on the volume and changes in the thoracic cavity. During inspiration, the diaphragm lowers which cause the rib cage to rise. The area inside the thorax increases, the same amount of air in a large space means air will flow in the lungs.



Respiratory Volumes 

  • During quiet breathing about 500mls of air moves in and out the lungs, this is called tidal volume.
  • The amount of air that can be forcibly inspired beyond tidal volume is called inspiratory reserve volume, which is usually 2100-3200mls. While the expiratory reserve volume is the amount of air that can be expired beyond the tidal volume, which is the region of 1000-1200mls
  • Residual reserve volume; even after the strongest of expiration 1200mls of air remains in the lungs, this keeps the alveolar open and prevents the lungs from collapsing.

Partial pressure is the pressure exerted by each gas in a mixture of gases. Gases dissolve in the liquid in proportion to its partial pressure. The partial pressures of oxygen and carbon dioxide drive the diffusion of gasses across the respiratory membrane and between the tissue and capillaries.

Oxygen has limited solubility in plasma. Therefore it must be transported by the red blood cells. 98.5% of the oxygen that is carried in the blood is bound to haemoglobin. As the oxygen partial pressure increase, more oxygen is bound to RBC and if the partial pressures decrease oxygen is released by the haemoglobin. The rate at which haemoglobin binds or releases oxygen is regulated by:

  • The partial pressure of oxygen
  • Temperature
  • Blood acidity
  • partial pressures of carbon dioxide
  • 2,3 BPGs

If the factors listed above are altered, they change the shape of the haemoglobin, thereby affecting its affinity to oxygen. Fetal haemoglobin has a higher affinity for oxygen because the blood arriving at the placenta has a low partial pressure of carbon dioxide.

Carbon dioxide transport 

Active body cells produce 200mls of carbon dioxide each minute, Transport from the tissue to lungs is in five forms.

  1. Dissolved in plasma
  2. As Carbonic acid
  3. Combined with plasma proteins and phosphates
  4. Bound to proteins
  5. As Bicarbonate ions in the plasma

Pulmonary Ventilation 

This is the amount of the air that moves in and out of the lungs in a single respiratory cycle. There are three factors that affect pulmonary ventilation; air resistance, alveolar surface tension and lung compliance.

Air Resistance 

  • The diameter of the first conducting part is larger therefore resistance is low.  At the terminal, Bronchioles resistance is low as well because of the high collecting diameter of all bronchioles. The resistance is high in the medium-sized bronchioles where you have a lower collecting diameter. Asthma is the bronchoconstriction that almost stops pulmonary ventilation.

Alveolar surface tension 

  • The liquid film that lines the alveolar walls is made of water, which has very high surface tension. The water is always acting to reduce the alveolar to the smallest possible size and would collapse alveolar between breaths.  Surfactants that are products of the type II alveolar cells reduce this surface tension.

Lung Compliance 

  • The Lungs have the amazing ability to stretch and the compliance the easier it is to expand the lungs. High compliance is determined by,  how stretchy the lung tissue is, alveolar surface tension and thoracic wall compliance.

Pulmonary Function Tests 

  1. Spirometry 
  2. Vitagraphy 
  3. Forced vital capacity 
  4. Forced expiratory capacity


Chemoreceptors are stimulated by the levels of PCO2, PH and PO are outside the normal range. This cause the depth of rate of breathing to increase, the change in PO2 has little effect, but small changes in PCO2 has a large influence on the respiratory centers.  Carbon dioxide levels regulate activities under normal condition. Changes in PH that happens when exercising helps stimulate respiratory activity.

When there is an increase in the levels of Carbon dioxide (Hypercapnia) in the arteries, chemoreceptors in the arteries and the central nervous system are stimulated. Carbon dioxide will cross the blood-brain barrier stimulating the chemoreceptors of the medulla. This then stimulates the respiratory centres to increase the depth and work of breathing. Removes the carbon dioxide and homeostasis is restored.

Hypocapnia happens when the pCO2 of arterial blood reduces, this reduces the stimulation of chemoreceptors in the arteries and CNS. Less CO2 in the CSF thus decreased stimulation in the Medulla. This then causes reduced work and depth of breathing, this removes less CO2 thereby restoring homeostasis.

Clinical terms

  • Hypoxia – Inadequate oxygen delivery to the tissue
  • Anaemic hypoxia – this happens due to too few RBC or haemoglobin
  • Ischemic hypoxia – reduced oxygen delivery when blood supply is impaired or blocked, seen in a heart attack.
  • Histotoxic Hypoxia – Happens when oxygen is available but can be used due to metabolic poison, for example, cyanide poisoning.
  • Hypoxemic hypoxia – reduce arterial partial pressures of oxygen


The respiratory system involves four processes, pulmonary ventilation, external respiration, transport of respiratory gasses and both the respiratory and cardiovascular system are involved in the process.


Sharma, J. (2017). Facilitating Respiratory System Through Pranayama.

Marieb, N.(2010). Respiratory function


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