Gaseous Diffusion Flashcards
What is dead space?
Airway volume with no gas exchange
What is anatomical dead space?
All except the alveolar and respiratory bronchioles
What is physiological dead space?
All except the alveolar and respiratory bronchioles as well as areas where gas exchange is dysfunctional
What is barometric pressure?
Sum of all the partial pressures of each gas including water vapour
What happens to barometric pressure at altitude?
Reduced, fraction of oxygen does not change but pressure does
What is Pb?
Barometric pressure
What does the partial pressure of a gas in a liquid depend on?
The conc of the liquid
the solubility of a gas in the liquid
Can solubility be increased?
No, it is a constant
What does water vapour pressure depend on?
Temperature (mainly) and saturation
Why is the water vapour pressure constant in the lungs?
The alveolar air temperature is constant at 37 degrees (Water vapour pressure, 6.3kPa, 47mmHg)
What is the main determinant of alveolar PO2?
Oxygen inspired and how much CO2 there is
Alveolar Gas Equation
Inspired Oxygen - (Arterial CO2 divided by R)
What is the pressure gradient driving diffusion for alveolar-capillary membrane?
Alveolar (Pa) - Mean Pulmonary Capillary (Pc)
Examples of reduction in surface area of the alveolar capillary gas exchange surface:
Emphysema
Lung resection- lung removal
Decreased venous return (heart failure)
Examples of increase in thickness of barrier membrane (larger diffusion distance):
Interstitial/alveolar fibrosis
Collagen Vascular Diseases
Congestive Heart Failure
What is the diffusion constant directly proportional to and inversely proportional to?
Direct: Solubility
Inversely: Molecular Weight
What happens to CO2 if ventilation is reduced?
Increase
What do you use to measure blood capillary O2?
Carbon monoxide
Why is carbon monoxide used as a surrogate marker for the diffusion capacity?
Haemoglobin has approx 240 times more affinity for CO than for O2
What is cyanosis?
The bluish or purplish discolouration of the skin or mucous membranes due to the tissues near the skin surface having low oxygen saturation
What is peripheral cyanosis?
Reduced blood flow to a region resulting in hypoxic tissue causing a bluish grey tinge to extremities.
Respiration is ok, arterial o2 content is ok
What are the causes of peripheral cyanosis?
cardiovascular shock
Low temp
Reduced cardiac output
Poor arterial supply
What is central cyanosis?
Arterial hypoxaemia (reduction in O2 content) Buccal mucosa and lips are best to spot this
What are the causes of central cyanosis?
Chronic Respiratory Disease
Right to left (heart) shunts
What is the Haldane Effect?
At any given PCO2 the quantity of CO2 carried is greater in partially deoxygenated blood (Venous) than in oxygenated blood (Arterial)
Why does the haldane effect occur?
Hb forms carbamino compounds more readily when deoxygenated ( can carry more CO2)
Hb binds to H+ better when deoxygenated (increases CO2 carriage)
What is hyperventilation?
Over-ventilation in proportion to metabolism
lead to a lowering of arterial PCO2 below normal= hypocapnic
Whta is Hypoventilation?
Under-ventilation in proportion to metabolism
Results in higher arterial PCO2 levels= hypercapnic