Final answer:
The partial pressure of carbon dioxide in venous blood is approximately 45 mm Hg. This is indicative of the venous blood's gas exchange process, where blood gains carbon dioxide from the tissues and then returns to the lungs for oxygenation.
Step-by-step explanation:
The partial pressure of carbon dioxide in venous blood is a measure of the amount of carbon dioxide gas present within the bloodstream. In the systemic capillaries, blood will lose oxygen and gain carbon dioxide due to the pressure difference between the tissues and blood. This process is a part of the gas exchange that occurs during respiration. As blood returns to the heart and then to the lungs through the pulmonary arteries, the venous partial pressure of oxygen (Po₂) is approximately 40 mm Hg, whereas the partial pressure of carbon dioxide (Pco₂) is approximately 45 mm Hg.
When blood reaches the lung capillaries, gas exchange occurs once more, with oxygen entering the blood and carbon dioxide transferred to the alveoli due to the existing pressure gradient. The partial pressure difference drives the diffusion of CO₂ out of tissue cells and into the capillaries. Therefore, the partial pressure of carbon dioxide in venous blood is approximately 45 mm Hg, making option b (45 mm Hg) the correct answer.