a couple people emailed me and asked what Venous Admixture is --> here is my explaination:
VENOUS ADMIXTURE is the result of mixing of shunted non-reoxygenated blood with reoxygenated blood distal to the alveoli. The shunted blood is a result of 1) Anatomic shunts and 2) shunt-like effects.
ANATOMIC SHUNTS include:
1) Normal shunts due to the BRONCHIAL, PLEURAL, and THEBESIAN veins which flow into the Pulmonary veins that then flow into the Left Atrium, thus bypassing the lungs. This represents approximately 2-3% of cardiac output. The BRONCHIAL, PLEURAL, and THEBESIAN veins also represent an exception to general blood flow rules (as they bypass the pulmonary circuit) that students can find confusing.
2) Abnormal shunts from lung fistulae, vascular lung tumors, atrial or ventricular septa defects, and pulmonary capillary shunts caused by alveolar fluid acculmulation, consolidation, and collapse.
The SUM of #1 and #2 is referred to as ABSOLUTE or TRUE SHUNT because this blood BYPASSES the alveoli. In a heathly person only #1 is significant.
SHUNT-LIKE EFFECTS involve Ventilation-Perfusion mismatch, ie more perfusion than ventilation which causes <100% Hemoglobin saturation. This is most commonly caused by 1) Hypoventilation, 2) excessively fast pulmonary capillary blood flow, and 3) decreased air flow (increased resistance) in affected brochi-bronchioli due to bronchoconstriction, mucus clogs, etc.
Collectively, VENOUS ADMIXTURE is the sum of all of the above. Again, in a healthy person only Anatomical shunting from the BRONCHIAL, PLEURAL, and THEBESIAN veins is significant. But in sick people, the others are important. In really bad cases VA can be >50%
When the admixing occurs, the non-reoxygenated blood gains O2 at the expense of reoxygenated blood and reaches an equilibrium by the time the blood gets into the distributing arteries.
Venous Admixture explains why figures in popular A&P texts show the pO2 in the pulmonary capillaries is ~100 mmHg while the pO2 in systemic arteries is ~95 mmHg. I had to read up on this because students always ask why the pO2 decreases from the heart to the systemic arteries, and at first I didn't know. Students can also find the normal anatomical shunt veins confusing because the veins do not follow the general pattern of blood flow.
Mathematically, the amount of shunted blood is calculated by the CLASSIC SHUNT EQUATION:
Qs/Qt = (CcO2 - CaO2)/(CcO2 - CvO2)
Qs=shunted blood; Qt=cardiac output;
CcO2=oxygen content of capillary blood;
CaO2=oxygen content of arterial blood;
CvO2=oxygen content of mixed venous blood
These can be calculated by various pressures, [Hb], and FiO2(fraction inspired O2).
I now use these equations to demonstrate to students that the physiology they are learning is often based on mathematics. But of course I don't make them work math problems (I value my car tires too much). : )
hope this helps, BRENT