Answer:
Answer is Sinoatrial node, SA.
Explanation:
The sinoatrial node, SA, is known to be a group of cells which are found at the top of the heart.
The electrical signal that is initiated by the sinoatrial node bring about or stimulates the atria and then the ventricle of the heart.
It should be noted that, the sinoatrial node are regarded as the pacemaker cells.
Answer:
Change their external environment is the correct answer.
If the partial pressure increases, then the DO concentration in water will also increase. When the altitude is increasing, the total atmospheric pressure decreases, and then the partial pressure of oxygen is also decreasing
