Lecture 11

Question 1

Describe the oxygen partial pressure cascade?

Answer 1

At sea level oxygen partial pressure is 160mmHg (21kPa) and the mitochondria can continue to perform oxidative phosphorylation at 1mmHg. The air in the upper airway becomes humidified and the pressure decreases to 150mmHg in the upper airway.

Question 2

Describe the decline of partial pressure of oxygen within the alveoli?

Answer 2

Partial pressure of oxygen has dropped to 104mmHg in the alveoli.

Question 3

Describe the decline of partial pressure of oxygen across the alveolar-capillary membrane?

Answer 3

The pressure in the capillary is now 100mmHg. A change in pressure of about 4mmHg despite the distance being 300nm, this reflects the low value of oxygen solubility.

Question 4

Describe the decline of partial pressure of oxygen due to shunts?

Answer 4

If we are unwell sometimes the blood goes through the shunt. If there was no shunt then the amount of oxygen leaving the pulmonary capillaries would be given by the product of QT and Cc’O2, the content of oxygen in the blood at the venous end of the pulmonary capillaries. However int he presence of a shunt, there are two components of blood flow: Qs and (Qt-Qs). The former (shunt) component contains blood whose oxygen content is given by that of the mixed (systemic) venous blood.

Question 5

Describe arteriovenous shunts?

Answer 5

[Blood that circulates that does not pass through the lungs.]
1. Anatomical shunts - most of the bronchial circulation and the thespian component of the coronary circulation (oxygenated blood leaks out and doesn’t get back into the pulmonary circulation) and any pulmonary arteriovenous fistula and a patent ductus arteriosus and a patent foramen ovale.
2. Physiological shunts -
Any admixture of system mixed venous blood with that form the pulmonary capillaries.

Question 6

Quantify the decline of partial pressure of oxygen due to shunts?

Answer 6

In healthy individuals you loose about 2-4mmHg. however in an ill-health individual it can be larger due to the sigmoidal shape of the oxyhemoglobin equilibrium relation. A small decrement in content of oxygen in blood leaving the lungs can have a big effect on the partial pressure.

Question 7

Describe the decline of partial pressure of oxygen due to ventilation-perfusion mismatch?

Answer 7

When the alveolar ventilation is diminished then the ventilation-perfusion ratio is diminished i.e. 1/10. When there is compromise in the capillary, so very little blood getting to the alveoli and you get mismatch ratio as well i.e. 10/1.

Question 8

Describe the decline of partial pressure of oxygen within the systemic capillaries?

Answer 8

As the cells consume oxygen the partial pressure drops to about 40mmHg.