Changes

*Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by ''diffusion''. The oxygen must first dissolve before passing through the [[Respiratory Epithelium - Anatomy & Physiology|respiratory epithelium]].

*Gas exchange between the air within the alveoli and the pulmonary capillaries occurs by ''diffusion''. The oxygen must first dissolve before passing through the [[Respiratory Epithelium - Anatomy & Physiology|respiratory epithelium]].

*Gas moves from a region of high partial pressure to a region of low partial pressure, down a '''partial pressure gradient'''. Partial pressure is a term used to measure gases.

*Gas moves from a region of high partial pressure to a region of low partial pressure, down a '''partial pressure gradient'''. Partial pressure is a term used to measure gases. 'P' is the symbol used for this term.

*The distance between the air within the alveoli, and the blood is approx 0.7micrometers. This distance is decreased during [[Ventilation - Anatomy & Physiology#Inhalation|inhalation]], as the [[Lungs - Anatomy & Physiology|lung]] distends. This tiny distance allows extremely fast and efficient diffusion.

*The distance between the air within the alveoli, and the blood is approx 0.7micrometers. This distance is decreased during [[Ventilation - Anatomy & Physiology#Inhalation|inhalation]], as the [[Lungs - Anatomy & Physiology|lung]] distends. This tiny distance allows extremely fast and efficient diffusion.

===Oxygen===

===Oxygen===

*The PO2 is always lower in the alveoli compared to the external environment due to the oxygen diffusing across the alveolar wall continuously, and the CO2 entering the alveoli which has the effect of 'diluting' the oxygen.

*The PO2 is always lower in the alveoli compared to the external environment due to the oxygen diffusing across the alveolar wall continuously, and the CO2 entering the alveoli which has the effect of 'diluting' the oxygen, as it is travelling in the opposite direction as the O2.

*The PO2 in the alveoli is still higher than that in the capillaries, so oxygen diffuses into the blood.

*The PO2 in the alveoli is still higher than that in the capillaries, so oxygen diffuses into the blood.

*Once through the alveolar and capillary walls, the Oxygen combines with '''Haemoglobin''' to form '''Oxyhaemoglobin''' and is transported within the bloodstream.

*Once through the alveolar and capillary walls, the oxygen combines with '''haemoglobin''' to form '''oxyhaemoglobin''' and is transported within the bloodstream.

===Carbon Dioxide===

===Carbon Dioxide===

*Carbon Dioxide enters the red blood cell as a waste product from cells. In the red blood cell it reacts with water to form '''Carbonic Acid''', CA.

*Carbondioxide enters the red blood cell as a waste product from cells. In the red blood cell it reacts with water to form '''carbonic acid''', CA.

*CA dissociates to '''Bicarbonate''' ions and '''Hydrogen''' ions.

*CA dissociates to '''bicarbonate''' ions and '''hydrogen''' ions. These diffuse into plasma, where H+ are buffered by haemoglobin.

*These diffuse into plasma, where H+ are buffered by haemoglobin.

**Approx 5% of the total body CO2 dissolves in the plasma.

**Approx 5% of the total body CO2 dissolves in the plasma.

**Approx 5% of the total body CO2 is carried as carboxyhaemoglobin on proteins.

**Approx 5% of the total body CO2 is carried as carboxyhaemoglobin on proteins.