External Respiration

*Overall: External respiration describes the flow of air into and out of the lungs. It refers to the exchange of gases between the lungs and the bloodstream (the transfer of oxygen and carbon dioxide into the bloodstream through diffusion. The components of external respiration include alveolar surface area, ventilation and perfusion matching, and partial pressure gradients.

*Ventilation and perfusion matching

-Ventilation and perfusion matching is vital for ensuring continuous delivery of oxygen and removal of carbon dioxide from the body.

-Ventilation

+Is the circulation and exchange of gases in the lungs.

+Alveolar ventilation rate (AVR) can be calculated by multiplying the subtraction between the tidal volume (the amount of air that you are just normally breathing in and out which is 500ml/min) and the dead space volume (the amount of air in the alveoli that does not contribute to the gas exchange, which is 150 ml/min) with the respiratory rate (12 breaths/min).Thus, the average AVR is around 4200 ml/min.

-Perfusion

+Is the amount of blood that flows through the pulmonary capillaries (500 ml/min).

-Thus, the normal ratio between ventilation and perfusion should be around 0.8 to ensure the normal exchange of gases during respiration.

*How to maintain this ventilation perfusion ratio?

-Good ventilation 

• Is when the partial pressure of oxygen increases while the partial pressure of carbon dioxide decreases.
• Since the amount of oxygen is high, it will come into the epithelial cell and activate a specific mechanism to produce nitrogen dioxide which will then cause muscle cells to relax (vasodilation).
• Therefore, since bloods are dilating, the amount of blood that flows through the pulmonary capillaries also increases.
• Since we both observe the increase of ventilation and perfusion, the ratio will still be roughly 0.8.

-Poor ventilation 

• Is when the partial pressure of oxygen decreases and the partial pressure of carbon dioxide increases.
• Since the amount of oxygen that comes to the epithelial cell decreases, the production of nitrogen dioxide will also decrease which then causes muscle cells to construct (vasoconstriction).
• As a result, the amount of blood that flows through the pulmonary capillaries also decreases.
• Hence, the ratio will still be the same.

-Good perfusion:

• When we exercise, play sports,…
• Our cardiac output will increase and the amount of carbon dioxide flowing through the blood also increases. 
• These carbon dioxide will make smooth muscles dilating which could increase the flow of oxygen into the lungs.
• The ratio will still be the same.

-Low perfusion

• Happens when there is a pulmonary embolism in the blood flow area which causes the perfusion to decrease.
• For this reason, a little amount of carbon dioxide will move into the alveoli and the partial pressure of carbon dioxide is lower than normal.
• As a result, this partial pressure will then action bronchial smooth muscles, causing constriction. Thus, there will be a small amount of oxygen flowing in and the ratio will remain the same despite the drop of carbon dioxide volume.

*Thickness and surface area

-Respiratory membrane contains alveolar cell (simple squamous and cuboidal) and basal lamina (basement membrane)

-The average thickness of respiratory membrane should be between 0.5-1 micrometers

+If the layer is too thick, there will be a large distance for gas exchange to move across, leading to a decrease of oxygen partial pressure and an increase of carbon dioxide partial pressure in the blood——> Decrease gas exchange

+If the layer is too thin, more oxygen will move into the blood and more carbon dioxide move across the respiratory membrane, leading to an increase of oxygen partial pressure and a decrease of carbon dioxide partial pressure in the blood—-> Increase gas exchange

-The amount of alveoli is directly proportional to the surface area of the lungs

-Inside our body, there is an enzyme called elastase which is inhibited by a molecule called alpha 1-antitrypsin. Cancer, smoking, or genetic factors might deactivate this molecule and cause the build up of lactase which destroys alveolar membrane and decreases surface membrane

+Large surface area will bring a larger flow of oxygen into our body and carry more carbon dioxide outside—->Increase gas exchange. In return, small surface area will cause a decrease in gas exchange, leading to many health problems.

*Partial pressure and solubility

-Partial pressures:

+Partial pressure of oxygen in the alveoli is around 104 mmHg, and that of carbon dioxide is 40 mmHg

+Partial pressure oxygen in the blood is just around 40 mmHg, and carbon dioxide’s partial pressure is 45 mmHg

+Gas will flow from high pressure to low pressure. Therefore, oxygen will flow into the blood plasma and carbon dioxide reversely move into the alveoli——>Gas exchange

-Solubility of the gas in the hemoglobin 

+Deoxyhemoglobin will have low affinity for oxygen and high affinity for carbon dioxide and proton

• Carbon dioxide will bind onto the hemoglobin chain and
• 20% of carbon dioxide will transport from the tissues to the alveoli in that form while 70% of carbon dioxide will form bicarbonate
• The remaining carbon dioxide dissolved in blood plasma

+When oxygen starts to bind on the hemoglobin chain, deoxyhemoglobin will transform into oxyhemoglobin (high affinity for oxygen to distribute them throughout the bloodstream and low affinity for carbon dioxide with proton)——>Haldane effect

• Carbon dioxide on the hemoglobin chain will flow into the alveoli
• Bicarbonate will combine with proton to form carbonic acid which then dissociates into carbon dioxide and water. After that, the product will flow into alveoli with the help of enzyme called carbonic anhydrase—-> Gas exchange