Final answer:
Venous admixture is the mixing of oxygenated and deoxygenated blood in the cardiovascular system.
Step-by-step explanation:
Venous admixture refers to the mixing of oxygenated and deoxygenated blood that occurs in the cardiovascular system. The main sources of venous admixture are the pulmonary veins, which carry oxygenated blood from the lungs, and the systemic veins, which carry deoxygenated blood back to the heart.
Venous admixture contributes to the A-a gradient for oxygen, which is the difference in oxygen partial pressure between the alveoli (where gas exchange occurs) and arterial blood.
When venous admixture occurs, it leads to a decrease in the partial pressure of oxygen in arterial blood compared to the alveoli. This is because the mixed blood contains a lower oxygen concentration than fully oxygenated blood. The A-a gradient for oxygen can be affected by several factors, such as ventilation-perfusion (V/Q) mismatch and shunting.