Respiratory Physiology
Surface tension - Liquids have a surface tension that derives from the intermolecular cohesive forces.
A surface molecule is subject to a net inward force that is resisted by compression of the molecules
below. This means that work has to be done to move a molecule to the surface so that surface
molecules have a higher energy than those in the bulk phase. Liquids therefore try and minimise
their surface area and this means they act as if they are surrounded by an elastic membrane. The
result is that the surface has the property of surface tension, a force per unit length acting to try and
reduce the surface area. A circumstance in which surface tension and Laplace’s law play a role is in
the lungs. Alveoli are interconnected and have elastic properties, partly as a result of the surface
tension of the aqueous film that lines their surface. Laplace’s law says that the pressure in the
smaller alveolus will be higher than the pressure in the larger alveolus so that it should collapse and
further inflate its larger neighbour. This would lead to a very unstable situation. In fact there is a
surfactant in the aqueous layer that lowers the surface tension and makes the surface tension
dependent on alveolar surface area. This counteracts Laplace’s law and makes your lungs stable.
Absence of the surfactant is fatal.
,
, Gas Laws
Boyles Law. The flow of air in and out of the lungs is caused by changes in pressure which are in turn
caused by changes in volume. Boyles Law states that the pressure of a fixed amount of a gas at
constant temperature is inversely proportional to the volume of the gas. So if the volume increases
(lowering of diaphragm), the pressure drops and air flow in, or if the volume decreases (raising the
diaphragm), the pressure increases and air flow out.
Dalton’s Law. In a mixture of non-reactive gases, the pressure exerted by each individual gas is
known as the partial pressure of that gas and total pressure is the sum of all individual partial
pressures (Ptotal = PN2 + PO2 + PCO2 etc).
Henry’s Law. When a gas diffuses between the air and a liquid, the amount of that gas dissolved in
the liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid. In
other words, at equilibrium, the partial pressure of the gas in the liquid is the same as the partial
pressure of that gas in the air. The driving force for the diffusion of a gas through a liquid or through
a sheet of tissue is the partial pressure difference (P1 – P2). Short - Amount of a gas dissolved in a
liquid is directly proportional to the practical pressure of that gas in equilibrium with that liquid
Some lung volumes and capacities can be measured directly using a Spirometer
Vt – Tidal volume ~ 500ml (The volume of air inspired and expired with each breath during normal,
quiet breathing. Quiet breathing is often referred to as tidal breathing. Tidal volume averages 500ml
for men and 400ml for women.)
IRV – Inspiratory Reserve Volume ~3000ml (This is the amount of air that can be inspired over and
above the normal tidal volume. Another 3000ml for men and 2100ml for women, can be added to
the lungs after a normal tidal inspiration)
ERV – Expiratory Reserve Volume ~1200ml (This is the amount of air that can be forcefully expired
after a normal tidal expiration. As much as an additional 1200ml for men and 900ml for women can
be removed from the lungs after normal tidal expiration.)
RV – Residual Volume ~1200ml (Since there is always a negative pressure in the pleural cavities, the
lungs never completely collapse. Thus ethere is always a volume of air in the lungs which cannot be
expired. This volume is the residual volume, which averages about 1200ml in young men and about
1000ml in young women.)
IC – Inspiratory Capacity = IRV + Vt ~3500ml (The tidal volume is added to the inspiratory reserve to
determine the inspiratory capacity. This volume is 3500ml for males and 2500 for females and
represents the total amout of inspired air during a forceful inspiration.)
VC – vital capacity = IRV + Vt + ERV ~4700ml (This is the maximum amount of air that can be expired
after a forceful inspiration. In young adults, vital capacity approximates 4700 for men and 3300 for
women.)
FRC – Functional Residual Capacity = ERV + RV ~2400ml (The sum of the expiratory reserve and the
residual volume equals the functional residual capacity. In young adults, this capacity averages
2400ml for men and 1800ml for women)
TLC – Total Lung Capacity = IRV + Vt + ERV + RV ~5900ml (This is the total volume contained in the
lungs with the greatest possible inspiration, including the residual volume. In young adults, the total
lung capacity averages about 5900ml for men and about 4500ml for women.)
Surface tension - Liquids have a surface tension that derives from the intermolecular cohesive forces.
A surface molecule is subject to a net inward force that is resisted by compression of the molecules
below. This means that work has to be done to move a molecule to the surface so that surface
molecules have a higher energy than those in the bulk phase. Liquids therefore try and minimise
their surface area and this means they act as if they are surrounded by an elastic membrane. The
result is that the surface has the property of surface tension, a force per unit length acting to try and
reduce the surface area. A circumstance in which surface tension and Laplace’s law play a role is in
the lungs. Alveoli are interconnected and have elastic properties, partly as a result of the surface
tension of the aqueous film that lines their surface. Laplace’s law says that the pressure in the
smaller alveolus will be higher than the pressure in the larger alveolus so that it should collapse and
further inflate its larger neighbour. This would lead to a very unstable situation. In fact there is a
surfactant in the aqueous layer that lowers the surface tension and makes the surface tension
dependent on alveolar surface area. This counteracts Laplace’s law and makes your lungs stable.
Absence of the surfactant is fatal.
,
, Gas Laws
Boyles Law. The flow of air in and out of the lungs is caused by changes in pressure which are in turn
caused by changes in volume. Boyles Law states that the pressure of a fixed amount of a gas at
constant temperature is inversely proportional to the volume of the gas. So if the volume increases
(lowering of diaphragm), the pressure drops and air flow in, or if the volume decreases (raising the
diaphragm), the pressure increases and air flow out.
Dalton’s Law. In a mixture of non-reactive gases, the pressure exerted by each individual gas is
known as the partial pressure of that gas and total pressure is the sum of all individual partial
pressures (Ptotal = PN2 + PO2 + PCO2 etc).
Henry’s Law. When a gas diffuses between the air and a liquid, the amount of that gas dissolved in
the liquid is directly proportional to the partial pressure of that gas in equilibrium with the liquid. In
other words, at equilibrium, the partial pressure of the gas in the liquid is the same as the partial
pressure of that gas in the air. The driving force for the diffusion of a gas through a liquid or through
a sheet of tissue is the partial pressure difference (P1 – P2). Short - Amount of a gas dissolved in a
liquid is directly proportional to the practical pressure of that gas in equilibrium with that liquid
Some lung volumes and capacities can be measured directly using a Spirometer
Vt – Tidal volume ~ 500ml (The volume of air inspired and expired with each breath during normal,
quiet breathing. Quiet breathing is often referred to as tidal breathing. Tidal volume averages 500ml
for men and 400ml for women.)
IRV – Inspiratory Reserve Volume ~3000ml (This is the amount of air that can be inspired over and
above the normal tidal volume. Another 3000ml for men and 2100ml for women, can be added to
the lungs after a normal tidal inspiration)
ERV – Expiratory Reserve Volume ~1200ml (This is the amount of air that can be forcefully expired
after a normal tidal expiration. As much as an additional 1200ml for men and 900ml for women can
be removed from the lungs after normal tidal expiration.)
RV – Residual Volume ~1200ml (Since there is always a negative pressure in the pleural cavities, the
lungs never completely collapse. Thus ethere is always a volume of air in the lungs which cannot be
expired. This volume is the residual volume, which averages about 1200ml in young men and about
1000ml in young women.)
IC – Inspiratory Capacity = IRV + Vt ~3500ml (The tidal volume is added to the inspiratory reserve to
determine the inspiratory capacity. This volume is 3500ml for males and 2500 for females and
represents the total amout of inspired air during a forceful inspiration.)
VC – vital capacity = IRV + Vt + ERV ~4700ml (This is the maximum amount of air that can be expired
after a forceful inspiration. In young adults, vital capacity approximates 4700 for men and 3300 for
women.)
FRC – Functional Residual Capacity = ERV + RV ~2400ml (The sum of the expiratory reserve and the
residual volume equals the functional residual capacity. In young adults, this capacity averages
2400ml for men and 1800ml for women)
TLC – Total Lung Capacity = IRV + Vt + ERV + RV ~5900ml (This is the total volume contained in the
lungs with the greatest possible inspiration, including the residual volume. In young adults, the total
lung capacity averages about 5900ml for men and about 4500ml for women.)
