Final answer:
ETCO2 levels at the end of the expiratory cycle reflect the balance of CO2 production and elimination, with various conditions such as metabolic acidosis and respiratory alkalosis affecting these levels due to changes in respiration and bicarbonate buffering.
Step-by-step explanation:
The causes of end-tidal CO2 (ETCO2) at the end of the expiratory cycle are largely based on the balance and interplay between the production and expiration of CO2, which is tightly regulated by the respiratory system. High levels of ETCO2 can be due to conditions that increase CO2 production, such as metabolic acidosis, where the body compensates by trying to blow off excess acid as CO2. Conversely, low levels of ETCO2 can occur with respiratory alkalosis, a condition often caused by hyperventilation, reducing the amount of CO2 in the blood. In a state of respiratory acidosis, the body may increase bicarbonate to buffer the excess carbonic acid, attempting to establish a normal pH balance. The respiratory rate may also increase to expel more CO2 as seen during exercise when CO2 production rises.