During exercise, what causes the osmolarity of venous blood from active muscles to increase?

• A. Lactate concentration in plasma
• B. O2 concentration in plasma
• C. Oxyhemoglobin concentration in red blood cells
• D. Blood pressure in systemic arteries

Answer: (A)

During exercise, the increase in the osmolarity of venous blood from active muscles is primarily due to the accumulation of lactate in the plasma. As muscles work harder and engage in anaerobic metabolism (especially during high-intensity exercise when oxygen is limited), they produce lactate as a byproduct of glucose metabolism. This lactate then enters the bloodstream, raising the concentration of solutes in the blood.

High levels of lactate contribute to a higher osmolarity because osmolarity is a measure of the total concentration of solute particles in a solution. An increase in lactate, along with other metabolites that accumulate during exercise, directly contributes to this increased solute concentration. As a result, the osmolarity of venous blood from these active muscles rises during physical activity.

Other factors mentioned, such as oxygen concentration, oxyhemoglobin concentration, and blood pressure in systemic arteries, do not significantly contribute to changes in osmolarity in the same way lactate does. For instance, while oxygen levels might be reduced in venous blood because it has been utilized by tissues, this does not directly increase osmolarity. The most impactful factor remains the increase in lactate concentration from metabolic processes during exercise.