Question

During exercise, which of these statements is true? a. The arterial percent oxyhemoglobin saturation is decreased. b. The venous percent oxyhemoglobin saturation is decreased. c. The arterial \(\mathrm{PCO}_{2}\) is measurably increased. d. The arterial \(\mathrm{pH}\) is measurably decreased.

Short answer

b. The venous percent oxyhemoglobin saturation is decreased.

Step-by-step solution

Arterial percent oxyhemoglobin saturation

During exercise, the body requires more oxygen, and the heart pumps faster to circulate blood. The arterial percent oxyhemoglobin saturation generally remains the same or increases slightly, due to improved oxygen delivery to tissues. Hence, statement a is false.

Venous percent oxyhemoglobin saturation

During exercise, as muscles work harder, they consume more oxygen, which leads to a decrease in venous oxyhemoglobin saturation. Since oxygen is being extracted more efficiently from the blood by the active tissues, the venous blood returning to the lungs carries less oxygen. Hence, statement b is true.

Arterial PCO2

During exercise, the body's rate of CO2 production increases due to faster metabolism. However, the increased ventilatory rate during exercise ensures that excess CO2 is expelled from the body, preventing a measurable increase in arterial PCO2. As a result, statement c is false.

Arterial pH

During exercise, the production of lactic acid may increase as a result of anaerobic metabolism. Still, the body has efficient mechanisms to maintain the arterial pH within a narrow physiological range. The body deals with additional acid by increasing ventilation, which enhances the removal of CO2 from the body, and helping maintain the pH levels. Therefore, arterial pH is not measurably decreased during exercise, and statement d is false. Based on the analysis, the correct answer is: b. The venous percent oxyhemoglobin saturation is decreased.

Key concepts

Arterial Oxyhemoglobin Saturation

Understanding the concept of arterial oxyhemoglobin saturation is essential when diving into exercise physiology. During exercise, our bodies have to manage increased oxygen demand to fuel our muscles. The arterial oxyhemoglobin saturation refers to the percentage of hemoglobin, the protein in red blood cells responsible for transporting oxygen, that is bound to oxygen.

Oxygen is taken up in the lungs, where it binds to hemoglobin in the red blood cells, which are then distributed throughout the body via the arterial blood supply. Under normal conditions, and even during exercise, the arterial oxyhemoglobin saturation remains relatively stable or might slightly increase. This stability is a result of enhanced cardiac output and the efficient loading of oxygen onto hemoglobin in the lungs, ensuring that our tissues receive an adequate oxygen supply to meet the demands of increased activity.

Venous Oxyhemoglobin Saturation

When we switch our focus to venous oxyhemoglobin saturation, we're looking at the oxygen status of blood after it has delivered oxygen to various tissues in the body. The skeletal muscles, being the powerhouses during exercise, extract more oxygen from the blood for energy production. This increase in oxygen extraction leads to a decrease in the venous oxyhemoglobin saturation.

As a result, blood returning to the heart via the veins has less bound oxygen, hence the measure of venous oxyhemoglobin saturation typically drops. This decrease is a direct reflection of the heightened use of oxygen by the muscles and provides insight into the effectiveness of oxygen extraction during physical activity. This is the core reason why the venous oxyhemoglobin saturation decreases during exercise, making it a key indicator of the body's metabolic state.

Arterial PCO2 Changes During Exercise

Arterial PCO2, the partial pressure of carbon dioxide in arterial blood, is an important aspect to consider when assessing body function during exercise. As we engage in physical activity, the cells in our body experience a boost in metabolism, which increases the production of carbon dioxide as a byproduct.

In response, the respiratory system quickens its pace, elevating our breathing rate and volume to expel the additional CO2. This compensatory mechanism helps maintain the partial pressure of carbon dioxide (PCO2) in the arteries within a tight range, despite the upsurge in production. Thus, arterial PCO2 doesn't significantly climb during exercise, due to the body's ability to ramp up ventilation to match CO2 output.

Arterial pH Variation During Exercise

As for arterial pH, it represents the acidity level of the blood—with lower pH values indicating greater acidity. Intense physical activity can lead to a build-up of lactic acid, especially during anaerobic exercise like sprinting or lifting heavy weights. This accumulation of acid has the potential to lower pH levels.

However, the human body is equipped with robust systems to counter slight fluctuations in pH. Through processes such as augmenting breathing to eliminate CO2 (which acts as an acid in the body) and utilizing bicarbonate buffers in the blood, the body can keep the arterial pH within a narrow, healthy range. This balancing act is critical, as it ensures the continued proper function of enzymes and metabolic processes during exercise. Therefore, any variation in arterial pH during exercise is usually not drastic, highlighting the efficiency of our body's regulatory mechanisms.