What is partial pressure of oxygen in blood?
Here's a simplified diagram if the respiratory system.
In ambient air,
In the alveoli
The partial pressure in the alveoli is less than
Leaving the alveolar capillaries
Oxygen diffuses from the alveoli into the alveolar capillaries. where
In the pulmonary veins
There is no gas diffusion through veins and arteries, so
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
Leaving the systemic capillaries
Leaving the systemic capillaries,
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
Here's an interesting animation showing the changes in