Final answer:
Sinus arrhythmia affects heart rate during inhalation and exhalation; inhalation increases HR, while exhalation decreases it. This physiological response ensures efficient blood flow and oxygen delivery during the respiratory cycle.
Step-by-step explanation:
In conditions of sinus arrhythmia, the heart rate (HR) is influenced by the process of inhalation and exhalation. When you inhale, there is a slight increase in HR due to a decrease in parasympathetic stimulation and a concurrent increase in sympathetic stimulation, which allows the HR to increase beyond the resting rate to cater to the body's increased demand for oxygen. On the other hand, exhaling promotes an increase in parasympathetic stimulation, which serves to slow down the HR.
This pattern of HR change is a normal physiological response and facilitates efficient blood flow during the respiratory cycle. This normal sinus rhythm is adjustable by cardiac reflexes which respond to physical activity demands by increasing blood flow as necessary. Therefore, contrary to the notion that both inhalation and exhalation decrease HR, inhalation actually increases the HR and exhalation decreases it.
However, it's important to note that in a healthy young adult, when HR increases above approximately 100 bpm due to physical demands, cardiac output may increase; however, if the HR goes beyond 160 bpm, the cardiac output tends to decrease.