The diffusion process is a molecule random movement which crisscrosses their paths in both directions through the respiratory membrane and adjacent liquids. For diffusion to take place a source of energy must exist and this is furnished by the molecules’ kinetic movement. However, a gas net diffusion towards a direction has its origin in what we know as concentration gradient. In other words: one side of the membrane possesses more molecules than the other side.
Net gas diffusion is equal to the molecules amount going towards anterogradic direction minus the ones going in the opposite direction.
Pressures Pressure begins through the constant impact of molecules in movement against a surface. It is directly proportional to molecules’ concentration within a gas. In respiratory physiology we work with gas mixture (O2, N2, and CO2). Diffusion rate of each one is proportional to that determinate gas originated pressure which is denominated as Gas Partial Pressure.
Air is made as follows: Nitrogen (79%) and Oxygen (21%). At sea level, this mixture’s total pressure is of 780 mmHg. Thus N2 partial pressure is of 600 and oxygen’s 160 mmHg.
Gases dissolved in water or corporal tissues also exercise pressure.
In solution, a gas pressure is not only determined by its concentration but also by the gas solubility coefficient.
Pressure equals Concentration over Solubility Coefficient.
Solubility coefficients
Oxygen
0.024
Carbon Dioxide
0.57
Carbon Monoxide
0.018
Nitrogen
0.012
Helium
0.008
Net Diffusion is given by the difference of partial pressures.
Once air enters respiratory ducts, water evaporates and they are humidified. These water molecules exercise a Water steam pressure which at 37 º C is of 47 mmHg and this will represent water’s partial pressure within this gaseous mixture.
Alveolar and Atmospheric Air
Alveolar air is only partially substituted by atmospheric air with each breathe. As a matter of fact, the substituted volume represents only one seventh of the total. Therefore, during normal respiration, gas eliminates itself every 34 seconds. This alveolar air slow renovation is the key to avoid sudden variations of blood gas concentrations.
During normal alveolar ventilation, alveolar PCO 2 is of 40 mmHg.
Then, the factors which determine a gas pass speed through a membrane are: Membrane thickness, Membrane area, Gas diffusion coefficient in membrane’s substance and both sides’ pressure difference.
Diffusion Capacity
Gas volume which diffuses through a membrane per minute for a pressure difference of 1 mmHg. In a medium aged man, diffusion capacity of resting O2 is of 21 ml/min/mmHg. Medium difference pressure through the respiratory membrane is of 11 mmHg. Thus, every minute, some 230 ml of O2 diffuse through the respiratory membrane. This is our body’s oxygen consumption. During some vigorous exercise, this might reach 65 ml/min/mmHg. This is due to:
a) Opening of the previously inactive capillaries and additional dilation of those which were already open.
b) Better adjustment of the ventilation/perfusion relationship.
In the case of smoking persons, this ventilation-perfusion relationship will be very much deteriorated.
GAS TRANSPORTATION FROM AND TO THE CELLS
Pressures
Both O2 and CO2 diffuse themselves from alveoli to blood and from blood to tissues due to Partial Pressure Gradients.
O2 P in alveolus: 104 mmHg
O2 P in venous blood: 40 mmHg
O2 P in cells: 23 mmHg
During exercise any organism might require twenty times the usual oxygen. Besides, due to cardiac requests, blood permanency in capillary could reduce by half. However, there is a security factor: O2 diffusion capacity raises three times during exercise since the area (capillary surface) also rises and the ventilation-perfusion relationship is closest to the ideal.
CO2 might diffuse twenty times faster than O2. This is why pressure differences needed in order to diffuse gas are lower than oxygen’s.
Intracellular PCO2: 46 mmHg
Insterticial PCO2: 45 mmHg
Arterial PCO2 arterial which penetrates tissues: 40 mmHg
Venous PCO2 which abandons tissues: 45mmHg
Lung capillary PCO2: 45 mmHg
Alveolar PCO2: 40 mmHg
Transportation
97% of oxygen is carried by erythrocytes combined with hemoglobin. The rest (3%) travels dissolved in plasma and cell water. O2 joins hemoglobin in a reversible and lax manner.
O2 Saturation of systemic arterial blood: 97%.
O2 saturation of venous blood: 75%.
Every 100 ml of blood some 5 ml of O2 are carried to the tissues. During intense exercise this figure might raise up to 15 ml. If we add the rise of the cardiac effort, we obtain a 20 times rise of the O2 transportation towards the tissues.
Carbon Monoxide combines with hemoglobin at the very same point O2 does and its joining strength is 250 times superior. A 0.1% CO concentration in air, thus, evenly competes with oxygen.
During normal conditions 4 ml of CO2 every 100 blood ml are transported from tissues to the lungs. A seven per cent travels dissolved. A 70% combines with H2O and dissociates with H+ (hydrogen ions) and HCO 3 (bicarbonate ion) and so it travels. Twenty per cent travels as carboxyl- hemoglobin.
REGULATION
The nervous system adjusts the alveolar ventilation system rate almost exactly to the organism demands. The respiratory system is composed by several groups of neurons bilaterally located within the rachides bulb and within the protuberance.
The vague and the gloso-pharyngeal nerve transmit to the respiratory center sensitive signals of: Peripheral chemic-receptors (in carotid and aortic bodies), baro receptors, and distension and contraction receptors.
When O2 concentration in the alveoli lowers below normal (especially below 70%), adjacent blood vessels slowly constrict raising by five the vessel resistance. The opposed effect occurs during systemic circulation where vessels dilate if oxygen is low. This means that the blood flow redistributes itself to places where its efficiency is maximized.
During exercise lung’s blood flow rises 4 to seven times mainly due to three reasons.
A) The amount of open capillary rises.
B) Capillary distend and flow rate through them also rises.
C) Lung arterial pressure rises.
Oxygen 
