Gas Exchange Flashcards
Describe the passage of oxygen and carbon dioxide to/from the atmosphere to/from the tissues
The oxygen pathway is as follows (CO2 is reverse):
1. Atmosphere (% in air, altitude, watery environment) 2. Into the lungs by VENTILATION (tidal volume) 3. Into the blood by DIFFUSION 4. Heart. 5. Circulated to tissues in the blood 6. Into the tissues by DIFFUSION - into mitochondria (oxidative phosphorylation) - PO2 reduces as travels through system PCO2 increases as travels through system
What could affect the pathways (or partial pressures) of O2 and CO2?
Vasoconstriction
Atmospheric pressure
Altitude
Obstructions in the lungs
Mucous / inflammation / oedema
Hypovolemia
pH of blood
Describe oxygen transport through the bloodstream
Dissolved in plasma (1.5%)
Bound to haemoglobin in erythrocytes (98.5)%
Bind by cooperative binding - once one oxygen is bound it increases the affinity of the haem for oxygen
Which factors affect the haemoglobin dissociation curve
Temperature
Higher temp in metabolic active tissue
pH
Lowe pH in metabolic active tissue as increased amounts of CO2
All of which tend to be increased in respiring tissues
i.e. tissues//cells ‘doing metabolic work’ = right-ward shift in curve, = unloading of O2
(lower affinity for O2)
Describe carbon dioxide transport through the bloodstream
Physically dissolved in the blood (approx. 10%);
Carboxyhaemoglobin on proteins (20% - carbamino products) Bicarbonate ions in the plasma (70%)
CO2 + H2O ⇌ H2CO3 ⇌ HCO3- + H+
CO2 enters the RBC where it reacts with water to form CARBONIC ACID
Carbonic Acid dissociates to:
BICARBONATE IONS (transported to plasma)
HYDROGEN IONS (buffered by Hb)
Define the driving forces for oxygen and carbon dioxide transport
-Diffusion due to differences in partial pressures (drive movement of gases)
-Haemoglobin affinity for O2, CO2 and H+
-CO2 content of deoxygenated blood is higher than oxygenated blood
Explain the principles of diffusion, including Fick’s Law of diffusion and the factors affecting pulmonary perfusion of respiratory gases
Ficks law of diffusion relates the nature of the membrane, the surface area of the membrane, the partial pressure gradient and the thickness of the membrane. To give the rate of diffusion.
So the rate of diffusion is dependent on all these things and can be represented by the following equation
V′gas=D x A x ∆P/T
V’gas= rate of gas diffusion across a permeable membrane
D = diffusion co-efficient of that particular gas for that particular membrane – relates to the nature of
the membrane for that particular gas
A= Surface area of the membrane
P = Difference in partial pressure of the gas across the membrane
T = Thickness of the membrane
So rate of diffusion as well as being dependent on the surface area of the membrane and the difference in the partial pressures between the gases is inversely proportional to the thickness of the membrane, the distance over which diffusion occurs.
Outline the composition of oxygen when inspired
Air contains 21% oxygen
At altitude partial pressure (tension) decreases
- Oxygen tension (partial pressure):Is determined by barometric pressure x fraction of oxygenPO2 = Pbaro x FO2
PO2 = 760mmHg x 0.21 = 160 mm Hg
- BUT: water vapour pressure humidifies inspired air (reduces PO2)PO2 of humidified gas:
PO2 = (Pbaro - PH2O) x FIO2
PO2 = (760 - 47) x 0.21 = 149mm Hg
To account for water vapor in the air by subtracting 47 from barometric pressure of 760
Outline the composition of carbon dioxide when inspired
Alveolar carbon dioxide tension (PaCO2)
PaCO2 = K x VCO2/Va
VCO2 = Carbon dioxide production
Va = Alveolar ventilation
K= (Pbaro – PH20)
PO2 is lower in the alveolus than in inspired air because O2 & CO2 exchange occurs continually in alveolus
PaO2 = [(Pbaro-PH2O)] x FiO2 - (Pa CO2)/R
PAO2 is the alveolar oxygen partial pressure
FiO2 is the fraction of inspired oxygen
Pbaro is the barometric pressure
PH2O is the partial pressure of water (47 mm Hg)
PaCO2 is the partial pressure of carbon dioxide
R is the respiratory quotient, dependent on metabolic activity and diet and is considered to be about
0.825.
List and describe the factors governing diffusion at the tissues
Dissociation curve not fixed
Increased heat and pH shift curve to right The ‘Bohr Shift’: increased CO2 = decreased pH = unloading 2,3 Diphosphoglycerate (DPG) shifts curve to right - (each occur in metabolically active tissues)
Explain what a pulse oximeter does
Colour change-pulse oximeter
Oxy Hb absorbs more infra-red than red light
Deoxy Hb absorbs more red than infra-red
The pulse-OX makes this comparison
Draw a labelled diagram of the oxygen-haemoglobin dissociation curve
Distinguish between hypo- and hyperventilation and describe the effects of these on blood acid-base balance
Hyperventilation (Getting rid of too much CO2) INCREASED alveolar ventilation that leads to REDUCED arterial pCO2
Hypoventilation (not blowing off enough CO2) REDUCED alveolar ventilation that leads to INCREASED arterial pCO2
Outline the result and clinical signs of hyperventilation
Respiratory Alkalosis:
↓paCO2 = ↓H+ = ↑pH = respiratory alkalosis
pH paCO2 Respiratory Alkalosis (Hyperventilation) Anxiety, pain, fear ↑ ↓ Hypoxia, altitude (stimulates breathing) Excessive mechanical ventilation
Reduction in CO2 - reduction in hydrogen ion increasing pH
Outline the result and clinical signs of hypoventilation
Respiratory Acidosis:
↑paCO2 (hypercapnia) = ↑H+ = ↓pH = respiratory acidosis
pH paCO2 Respiratory Acidosis (Hypoventilation) Respiratory failure ↓ ↑ Obstructive respiratory diseases Depression of respiratory centres (e.g. trauma, drugs) Inadequate mechanical ventilation
Too much co2 - too many hydrogen ions
Decreasing pH
