Since the other answer got deleted, i'll just answer it for you.
a sinoatrial node is the main pacemaker that begins each heartbeat located in the junctions of superior vena cava and the right atrium. It signals from the node that spread through the atria and determines the heart rates.
The external cell membrane. Even organisms as simple as bacteria have a cell membrane.
Prokaryotes do not have internal membranes but they still have external membranes.
Everything living thing has a cell membrane
The hydrosphere
Liquid: lakes, oceans...etc
Solids: glaciers, ice caps...etc
Gases: water vapor, clouds, fog...etc
Hope it was helpfull!!
Answer:
Chief cells in the parathyroid gland synthesize and release parathyroid hormone.
Explanation:
Parathyroid gland contains two types of cells- Chief cells and Oxyphil cells.
The chief cells in the parathyroid gland synthesize and release parathyroid hormone.
The function of the parathyroid hormone is to regulate the level of calcium in the blood. They do this by increasing the calcium levels when they are too low.
