Case 6 physiology Flashcards
Relative composition of inspired air
Oxygen- 20.71 %
Carbon dioxide- 0.04%
Water vapour- 1.25%
Nitrogen- 78%
Relative composition of expired air
Oxygen- 14.6%
Carbon dioxide- 3.8%
Water vapour- 6.2%
Nitrogen- 75.4%
Relative composition of alveolar air
Oxygen- 13.2%
Carbon dioxide- 5%
Water vapour- 6.2%
Nitrogen- 75.6%
How much oxygen does the body use
It uses 25% of the oxygen which is delivered to the tissues.
Oxygen saturation of venous and arteriole blood
Venous blood has an oxygen saturation of 75% whilst arteriole saturation is 98%
Factors which affect the oxygen cascade
- Permeability of the cells and lung wall
- Surface area of the cells and lung wall
- The Conc./pressure gradient of the cells and lung wall
Oxygen delivery equation (DO2)
DO2= (SaO2 x Hb x CO x 1.34) + (PaO2 x 0.003)
• DO2: oxygen delivery, works out to a 1000 ml/min
• SaO2: oxygen saturation (Hb and O binding)
• Hb: haemoglobin concentration (g/l)
• PaO: dissolved oxygen (kPa)
• Hufner’s constant: 1.34 ml/g
• Oxygen solubility coefficient: 0.003
What is oxygen consumption dependent on
The amount of aerobic respiration in the tissues
Oxygen consumption (VO2) equation
VO2 = CO x Hb x 1.34(SaO2 – SvO2)
• VO2= volume of oxygen consumed (250ml/min)
• SaO2- arterial saturation of oxygen
• SvO2- venous saturation of oxygen
Simplified oxygen consumption (VO2) equation
VO2= CO x (CaO2 – CvO2)
The amount of oxygen consumed is dependent on the difference between the arterial saturation of oxygen and the venous saturation
What is the emergency medication used in asthma
Prednisolone
Why does the composition of alveolar gas stay relatively constant
The alveoli do not change in size or volume during respiration. The ‘bulk flow’ effect of ventilation- expanding the thoracic cavity to create a negative pressure, ends at the respiratory bronchioles. So gas composition is mostly determined by random diffusion. The inputs and outputs of CO2 and O2 mostly balance out meaning the alveolar gas stays relatively constant
The layers of the diffusion barrier
Alveolar epithelium, tissue fluid, capillary endothelium, plasma and red blood cell membrane.
Right-left shunt (venous admixture)
Allows the blood to flow from the venous side of circulation and enter the arterial side without passing into functional respiratory epithelium. Meaning that normal gas exchange does not occur
An example of a right-left shunt
Bronchial circulation= The bronchial artery is supplying oxygen to the bronchial, the deoxygenated blood then flows out and mixes directly with the oxygenated blood in the pulmonary vein. So the oxygenated blood which returns in the pulmonary vein will contain a very small amount of deoxygenated blood
Pathologies which can cause a R-L shunt and the problem with it
Pneumonia or anatomical variations (Truncus arteriosis, patent foramen ovale and transposition of the great vessels). The problem with these pathologies is that they can not be treated by giving the subject 100% O2 because the shunted blood bypasses the ventilated alveoli and is not exposed to gaseous exchange.
Effects of Hypoventilation (reduced alveolar ventilation
- Causes a decreases PAO2 leading to less oxygen in the arterioles– Hypoxia.
- Increases PACO2 which increases the amount of CO2 in the arterioles- Hypercapnia.
- Hypoxaemia= decrease in arterial O2 levels which causes a decrease in the oxygen carrying capacity of the blood, this can be independent of hypoventilation.
Causes of Hypoventilation
Increased airway resistance i.e. asthma and COPD, drugs such as morphine or barbiturates. As well as paralysis of respiratory muscles
V/Q ratio at rest
V/Q = Alveolar ventilation / Cardiac output
The resting alveolar ventilation (4L/min) and the resting cardiac output (5L/min). So, the resting V/Q ratio of the lungs is 4/5=0.8
Alveolar ventilation equation
Alveolar ventilation= (Tidal volume- Dead space) x Respiratory rate
Ideal V/Q
The ideal value is 1, this is where ventilation and perfusion are matched, however, this doesnt happen in real life, it is normally 0.8. Gas in this alveolus contains a normal percentage of O2 and CO2. There will be 13.7 kPa of O2 and 5.2 kPa of CO2.
V/Q ratio in an obstructed alveoli
When ventilation is obstructed but blood flow is unchanged. The pO2 is 5.3kPa and the pCo2 is 6.1kPa, this causes the V/Q to get smaller and eventually hit zero. So, the PAO2 will fall and PACO2 will rise (alveolar). Less O2 will be taken up in the over perfused alveoli and the blood leaving these alveoli will be undersaturated and less CO2 will be removed. This is caused by airway limitation (asthma and COPD), lung collapse and loss of elastic tissue (emphysema).
V/Q ratio due to obstruction of blood flow
When perfusion is obstructed but ventilation remains unchanged. The pO2 is 20pKa whilst the pCo2 is zero, this causes the V/Q to get higher and higher till it reaches infinity. Some ventilation will be wasted and contribute to dead space. The PAO2 will rise and the PACO2 will fall (alveolar). No more O2 will be taken up by the overventilated alveoli as the haemoglobin is fully saturated and extra CO2 will be blown off. Can be caused by a pulmonary embolism, necrosis or fibrosis of the capillary bed.
V/Q ratio- emphysema
In emphysema you get destruction of alveoli and loss of capillaries. The destruction of alveoli leads to underventilation (low V/Q ratio) and the loss of capillaries leads to alveoli which are under perfused (high V/Q ratio). It therefore decreases the effectiveness of gas exchange as there will be areas of the lung with a low V/Q and other areas of the lung with a high V/Q ratio.
Hypoxic pulmonary vasoconstriction
Mechanism to reduce V/Q differences. Low alveolar pO2 due to reduced ventilation acts as a stimulus for pulmonary artery constriction, reducing blood flow. This restores the V/Q value back to normal so there is a more efficient gaseous exchange
Mechanism to reduce V/Q differences in low alveolar pCO2
Low alveolar pCo2 due to reduced blood flow causes constriction of the alveolar ducts. You therefore, reduce the ventilation and adjust the V/Q back to normal.