Why is the pH of venous blood only slightly more acidic than arterial blood?

The pH of venous blood is only slightly more acidic than that of arterial blood primarily due to the buffering capacity of deoxyhemoglobin. When hemoglobin releases oxygen to the tissues, it becomes deoxyhemoglobin and has a greater ability to bind to hydrogen ions (H+). This binding helps to buffer the acidity that comes from the increase in carbon dioxide (CO2) produced by cellular metabolism.

As CO2 diffuses into the blood, it reacts with water to form carbonic acid, which dissociates into bicarbonate and H+, contributing to a decrease in pH. However, the capacity of deoxyhemoglobin to bind these free H+ ions mitigates the decrease in pH. This buffering action is crucial because it prevents venous blood from becoming excessively acidic, which ensures a relatively stable pH despite the higher concentration of CO2.

In contrast, the other options do not accurately describe the physiological processes affecting blood pH. For instance, venous blood has a higher level of carbon dioxide compared to arterial blood due to its role in transporting waste from tissues to the lungs, rather than an absence of carbon dioxide. Although oxygen levels are lower in venous blood, this does not directly correlate with increased acidity.