Respiratory Quotient (RQ)

Respiratory Quotient (RQ)

• The respiratory quotient (RQ) is the ratio of carbon dioxide produced to oxygen consumed, represented as RQ = VCO2/VO2.
• RQ is a dimensionless value, offering insights into the substrates being metabolized by the body.
• The RQ value varies based on the primary fuel source utilized.

RQ for Different Substrates

• For carbohydrates, RQ is approximately 1.0; the oxidation of one glucose molecule requires six oxygen molecules and produces six carbon dioxide molecules (C6H12O6 + 6O2 → 6CO2 + 6H2O).
• For fats, the RQ is about 0.7, indicating that more oxygen is needed to oxidize fats relative to the carbon dioxide produced. For instance, palmitic acid (C16H32O2) oxidation needs 23 oxygen molecules and yields 16 carbon dioxide molecules.
• For proteins, the RQ is roughly 0.8 because of the complex biochemical pathways in protein metabolism, including deamination and amino acid conversion.
• A mixed diet typically leads to an RQ between 0.8 and 0.9.

Factors Affecting RQ

• Diet composition significantly affects RQ; high-carbohydrate diets raise RQ, while high-fat diets lower it.
• During intense exercise, RQ may temporarily rise to 1.0 or higher due to increased carbohydrate metabolism.
• Metabolic disorders like diabetes can influence RQ due to impaired glucose metabolism.
• Overfeeding and underfeeding can change RQ values as the body adapts its fuel utilization.
• Hormonal influences, such as insulin and thyroid hormones, can also modulate RQ.

Metabolic Rate

• Metabolic rate is the rate of energy expenditure per unit time.
• It is typically measured in kilojoules per day (kJ/day) or kilocalories per day (kcal/day).
• Metabolic rate is measured under basal conditions to find the basal metabolic rate (BMR), or during activity to find the total energy expenditure (TEE).

Basal Metabolic Rate (BMR)

• Basal Metabolic Rate (BMR) is the minimum energy expenditure rate needed to sustain life at rest.
• BMR is measured under specific conditions: after an overnight fast, in a thermally neutral environment, and while the subject is awake but resting.
• BMR accounts for about 60-75% of total daily energy expenditure in most individuals.

Factors Affecting BMR

• Age decreases BMR due to loss of lean body mass and decreased metabolic activity.
• Sex: Males generally have a higher BMR than females due to greater muscle mass.
• Individuals with more muscle mass have a higher BMR because muscle tissue is more metabolically active than fat tissue.
• Genetic factors influence BMR.
• Thyroid hormones (T3 and T4) increase BMR, while hypothyroidism decreases it.
• People in colder climates tend to have higher BMR due to increased thermogenesis.
• Prolonged starvation or very-low-calorie diets can decrease BMR.

Total Energy Expenditure (TEE)

• Total energy expenditure (TEE) is the total energy expended by an individual in a day.
• TEE includes BMR, the thermic effect of food (TEF), and the energy expended during physical activity.
• TEE is highly variable and depends on activity level, diet, and other factors.

Components of TEE

• Basal Metabolic Rate (BMR): The energy required for basic physiological functions at rest.
• Thermic Effect of Food (TEF): The increase in energy expenditure associated with the digestion, absorption, and metabolism of food; TEF typically accounts for about 10% of total energy intake.
• Physical Activity: The energy expended during voluntary movements and exercise; this component is the most variable and can range from 15% to 50% of TEE, depending on activity level.
• Non-Exercise Activity Thermogenesis (NEAT): Energy expended for activities not considered exercise, such as fidgeting, standing, and maintaining posture.

Measurement of Metabolic Rate

• Direct calorimetry measures heat production by the body in a sealed chamber, and is accurate but expensive and impractical for routine use.
• Indirect calorimetry estimates metabolic rate by measuring oxygen consumption and carbon dioxide production; it is more commonly used and can be performed using a metabolic cart or portable devices.
• Doubly labeled water (DLW) is a non-invasive method for measuring TEE over 1-2 weeks, which involves administering water labeled with stable isotopes of hydrogen and oxygen and measuring their elimination rates in urine.

Gas Exchange

• Gas exchange is the process by which oxygen is taken up from the environment and carbon dioxide is released.
• In humans, gas exchange occurs in the lungs at the alveolar-capillary interface.
• The efficiency of gas exchange is crucial for maintaining adequate oxygen supply to tissues and removing carbon dioxide.

Mechanisms of Gas Exchange

• Ventilation: The movement of air into and out of the lungs.
• Diffusion: The movement of gases across the alveolar-capillary membrane, driven by partial pressure gradients.
• Perfusion: The flow of blood through the pulmonary capillaries, which carries oxygen to the tissues and removes carbon dioxide.

Factors Affecting Gas Exchange

• Partial Pressures of Gases: Oxygen and carbon dioxide move from areas of high partial pressure to areas of low partial pressure.
• Surface Area: The large surface area of the alveoli (approximately 70 square meters) facilitates efficient gas exchange.
• Thickness of the Alveolar-Capillary Membrane: A thin membrane (about 0.5 μm) allows for rapid diffusion of gases.
• Ventilation-Perfusion Matching: Proper matching of ventilation (airflow) and perfusion (blood flow) is essential for efficient gas exchange.

Regulation of Gas Exchange

• Respiratory Control Centers: Located in the brainstem, these centers regulate the rate and depth of breathing in response to changes in blood levels of oxygen, carbon dioxide, and pH.
• Chemoreceptors: Peripheral chemoreceptors in the carotid and aortic bodies and central chemoreceptors in the medulla oblongata detect changes in blood gases and pH and signal the respiratory control centers to adjust ventilation.
• Lung Receptors: Stretch receptors in the lungs and airways provide feedback to the respiratory control centers to prevent overinflation of the lungs.

Clinical Significance

• Respiratory diseases such as asthma, chronic obstructive pulmonary disease (COPD), and pneumonia can impair gas exchange and lead to hypoxemia (low blood oxygen levels) and hypercapnia (high blood carbon dioxide levels).
• Conditions that affect ventilation, such as neuromuscular disorders and chest wall deformities, can also compromise gas exchange.
• Monitoring gas exchange through blood gas analysis (measuring partial pressures of oxygen and carbon dioxide in arterial blood) is essential in the management of critically ill patients.