What is partial pressure of oxygen in blood?

Here's a simplified diagram if the respiratory system.

(From www.studyblue.com)

In ambient air, #P_"O₂"# = 160 mmHg.

In the alveoli

#P_"O₂"# in the alveoli is about 104 mmHg

The partial pressure in the alveoli is less than #P_"O₂"# in ambient air because of the continual diffusion of oxygen into the alveolar capillaries.

Leaving the alveolar capillaries

Oxygen diffuses from the alveoli into the alveolar capillaries. where #P_"O₂"# ≈ 100 mmHg.

In the pulmonary veins

There is no gas diffusion through veins and arteries, so #P_"O₂"# is about 100 mmHg.

Entering the systemic capillaries

Blood leaving pulmonary veins enters the left atrium and is pumped from the left ventricle into the systemic circulation.

It enters the systemic capillaries with #P_"O₂"# at 80 - 100 mmHg.

Leaving the systemic capillaries

#P_"O₂"# in the body cells is less than 40 mmHg.

Because #P_"O₂"# in the systemic capillaries is greater than the partial pressure in the body cells, oxygen diffuses from the blood and into the cells.

Leaving the systemic capillaries, #P_"O₂"# = 40 - 50 mmHg.

Entering the alveolar capillaries

Blood leaves the systemic capillaries and returns to the right atrium via veins.

The right ventricle then pumps the blood to the alveolar capillaries, with #P_"O₂"# = 20 - 40 mmHg, and the cycle starts again.

Here's an interesting animation showing the changes in #P_"O₂"# and #P_"CO₂"# as the blood moves through the body.