42 - Physical bases of gas exchange. Solubility, diffusion coefficient and diffusion capacity of gases. Gas composition of the air, lungs and blood. Diffusion of gases across the alveolocapillary membrane. Ventilation-perfusion ratio.

sections

physical bases of gas exchange

solubility

diffusion coefficient

diffusion capacity

gas composition of air

gas composition of lungs

gas composition of blood

diffusion of gases through alveolar capillary membrane

ventilation-perfusion ratio

physical bases of gas exchange

• diffusion of gas from high concentration area to low concentration.

• the pressure of gas acting on a surface of the respiratory passages and alveoli is directly proportional to the concentration of the gas molecule.

• The rate of diffusion of each gas is directly proportional to the pressure caused by the gas alone, which is called the partial pressure.

• The partial pressure of a gas : determined by concentration, and solubility coefficient.

  • Pp=concentration of dissolved gas / Solubility coefficient

  • When the partial pressure of a particular gas in the lung alveoli, too high, it will diffuse down its partial pressure gradient and is forced into the pulmonary capillaries

solubility

• Solubility is the ability for a solute to dissolve in a solid, liquid or gaseous solvent to form a solution.

• Factors affecting solubility:

  • Temperature: solubility in water increases with rising temperature

  • Pressure: solubility increases with increase in pressure

  • Molecule size and polarity- co2 is about 20 times more soluble in blood than o2 because it binds with water to form HCO3-

diffusion coefficient

a physical quantity that describes how quickly molecules, atoms, or particles spread out through a medium (such as a gas, liquid, or solid) due to random thermal motion.

It is defined by ficks first law of diffusion - describes how a substance moves from an area of high concentration to an area of low concentration under steady-state conditions (meaning the concentration profile doesn't change with time).

diffusion capacity

• the volume of a gas that will diffuse through the membrane each minute for a partial pressure difference of 1 mmHg.

• Factors that affect the rate of gas diffusion in a liquid:

  • The pressure difference

  • The solubility of the gas in the liquid

  • The cross sectional area of the fluid

  • The distance through which the gas must diffuse

  • The molecular weight of the gas

  • The temperature of the fluid.

gas composition of air

• 78% nitrogen

• 21% oxygen

• 0.04% dioxide

• other gases

• water carbon vapor between 0.4-1%

• Atmospheric P02 = 21% * 760 mm Hg = 160 mm Hg

• 7821007

gas composition of lungs

• Inhaled dry air contains: 21% oxygen (160mmHg), 79% nitrogen (600mmHg)

• Tracheal air: 20% oxygen (150mmHg), 6% water (47mmHg), 74% nitrogen (573mmHg)

• Alveolar air: 13% oxygen (100mmHg), 5% carbon dioxide (40mmHg), 6% water (47mmHg), 74% nitrogen (573mmHg)

• Exhaled dry air: 15% oxygen (110mmHg), 4% carbon dioxide (28mmHg), 6% water (47mmHg),

• Percentage of nitrogen remains relativey the same

• Atmospheric P02 = 21% * 760 mm Hg = 160 mm Hg

gas composition of blood

• Arterial blood: 11-13% oxygen (75-100mmHg), 4.7-6% dioxide (35-45mmHg) carbon

• Total amount of carbon dioxide: 23-30 mmol/L (dissolved in blood, bound to hemoglobin or modified to bicarbonate ion)

• Total amount of oxygen: 206,3 mmol/L → 3 mmol/L dissolved in blood and 203,3 mmol/L bound to haemoglobin

• Atmospheric P02 = 21% * 760 mm Hg = 160 mm Hg

diffusion of gases through alveolar capillary membrane

• high to low

• Alveolar cappilary membrane is composed of different layers:

  • a layer of fluid containing surfactant, lines the alveolus and reduces surface tension

  • the alveolar epithelium

  • the capillary basal membrane

  • capillary endothelial membrane

• Factors affecting the rate of diffusion:

  • thickness of the membrane

  • Total amount of oxygen: 206,3 mmol/L → 3 mmol/L dissolved in blood and 203,3 mmol/L bound to haemoglobin • surface area of membrane

  • diffusion coefficient of the gas

  • partial pressure difference of the gas between the two sides of the membrane

ventilation-perfusion ratio

• ventilation perfusion ratio (V/Q ratio)

• Ventilation (V): the air that reaches the alveoli

• Perfusion (Q): the blood that reaches the alveoli

• VQ is the ratio of air reaching the alveoli to the amount of blood reaching the alveoli • When a subject is station in an upright position –the apex of the lung shows a higher V/Q ratio

• at the base of the lung the ratio is lower . this is because ventilation and perfusion both increase when going from the apex to the lung however perfusion increases more greatly and so the ration is reduced. This is due to gravity

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